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diff --git a/doc/protocol/rfc5246.txt b/doc/protocol/rfc5246.txt new file mode 100644 index 0000000000..869c345054 --- /dev/null +++ b/doc/protocol/rfc5246.txt @@ -0,0 +1,5827 @@ + + + + + + +Network Working Group T. Dierks +Request for Comments: 5246 Independent +Obsoletes: 3268, 4346, 4366 E. Rescorla +Updates: 4492 RTFM, Inc. +Category: Standards Track August 2008 + + + The Transport Layer Security (TLS) Protocol + Version 1.2 + +Status of This Memo + + This document specifies an Internet standards track protocol for the + Internet community, and requests discussion and suggestions for + improvements. Please refer to the current edition of the "Internet + Official Protocol Standards" (STD 1) for the standardization state + and status of this protocol. Distribution of this memo is unlimited. + +Abstract + + This document specifies Version 1.2 of the Transport Layer Security + (TLS) protocol. The TLS protocol provides communications security + over the Internet. The protocol allows client/server applications to + communicate in a way that is designed to prevent eavesdropping, + tampering, or message forgery. + +Table of Contents + + 1. Introduction ....................................................4 + 1.1. Requirements Terminology ...................................5 + 1.2. Major Differences from TLS 1.1 .............................5 + 2. Goals ...........................................................6 + 3. Goals of This Document ..........................................7 + 4. Presentation Language ...........................................7 + 4.1. Basic Block Size ...........................................7 + 4.2. Miscellaneous ..............................................8 + 4.3. Vectors ....................................................8 + 4.4. Numbers ....................................................9 + 4.5. Enumerateds ................................................9 + 4.6. Constructed Types .........................................10 + 4.6.1. Variants ...........................................10 + 4.7. Cryptographic Attributes ..................................12 + 4.8. Constants .................................................14 + 5. HMAC and the Pseudorandom Function .............................14 + 6. The TLS Record Protocol ........................................15 + 6.1. Connection States .........................................16 + 6.2. Record Layer ..............................................19 + 6.2.1. Fragmentation ......................................19 + + + +Dierks & Rescorla Standards Track [Page 1] + +RFC 5246 TLS August 2008 + + + 6.2.2. Record Compression and Decompression ...............20 + 6.2.3. Record Payload Protection ..........................21 + 6.2.3.1. Null or Standard Stream Cipher ............22 + 6.2.3.2. CBC Block Cipher ..........................22 + 6.2.3.3. AEAD Ciphers ..............................24 + 6.3. Key Calculation ...........................................25 + 7. The TLS Handshaking Protocols ..................................26 + 7.1. Change Cipher Spec Protocol ...............................27 + 7.2. Alert Protocol ............................................28 + 7.2.1. Closure Alerts .....................................29 + 7.2.2. Error Alerts .......................................30 + 7.3. Handshake Protocol Overview ...............................33 + 7.4. Handshake Protocol ........................................37 + 7.4.1. Hello Messages .....................................38 + 7.4.1.1. Hello Request .............................38 + 7.4.1.2. Client Hello ..............................39 + 7.4.1.3. Server Hello ..............................42 + 7.4.1.4. Hello Extensions ..........................44 + 7.4.1.4.1. Signature Algorithms ...........45 + 7.4.2. Server Certificate .................................47 + 7.4.3. Server Key Exchange Message ........................50 + 7.4.4. Certificate Request ................................53 + 7.4.5. Server Hello Done ..................................55 + 7.4.6. Client Certificate .................................55 + 7.4.7. Client Key Exchange Message ........................57 + 7.4.7.1. RSA-Encrypted Premaster Secret Message ....58 + 7.4.7.2. Client Diffie-Hellman Public Value ........61 + 7.4.8. Certificate Verify .................................62 + 7.4.9. Finished ...........................................63 + 8. Cryptographic Computations .....................................64 + 8.1. Computing the Master Secret ...............................64 + 8.1.1. RSA ................................................65 + 8.1.2. Diffie-Hellman .....................................65 + 9. Mandatory Cipher Suites ........................................65 + 10. Application Data Protocol .....................................65 + 11. Security Considerations .......................................65 + 12. IANA Considerations ...........................................65 + Appendix A. Protocol Data Structures and Constant Values ..........68 + A.1. Record Layer ..............................................68 + A.2. Change Cipher Specs Message ...............................69 + A.3. Alert Messages ............................................69 + A.4. Handshake Protocol ........................................70 + A.4.1. Hello Messages .....................................71 + A.4.2. Server Authentication and Key Exchange Messages ....72 + A.4.3. Client Authentication and Key Exchange Messages ....74 + A.4.4. Handshake Finalization Message .....................74 + A.5. The Cipher Suite ..........................................75 + A.6. The Security Parameters ...................................77 + + + +Dierks & Rescorla Standards Track [Page 2] + +RFC 5246 TLS August 2008 + + + A.7. Changes to RFC 4492 .......................................78 + Appendix B. Glossary ..............................................78 + Appendix C. Cipher Suite Definitions ..............................83 + Appendix D. Implementation Notes ..................................85 + D.1. Random Number Generation and Seeding ......................85 + D.2. Certificates and Authentication ...........................85 + D.3. Cipher Suites .............................................85 + D.4. Implementation Pitfalls ...................................85 + Appendix E. Backward Compatibility ................................87 + E.1. Compatibility with TLS 1.0/1.1 and SSL 3.0 ................87 + E.2. Compatibility with SSL 2.0 ................................88 + E.3. Avoiding Man-in-the-Middle Version Rollback ...............90 + Appendix F. Security Analysis .....................................91 + F.1. Handshake Protocol ........................................91 + F.1.1. Authentication and Key Exchange ....................91 + F.1.1.1. Anonymous Key Exchange ....................91 + F.1.1.2. RSA Key Exchange and Authentication .......92 + F.1.1.3. Diffie-Hellman Key Exchange with + Authentication ............................92 + F.1.2. Version Rollback Attacks ...........................93 + F.1.3. Detecting Attacks Against the Handshake Protocol ...94 + F.1.4. Resuming Sessions ..................................94 + F.2. Protecting Application Data ...............................94 + F.3. Explicit IVs ..............................................95 + F.4. Security of Composite Cipher Modes ........................95 + F.5. Denial of Service .........................................96 + F.6. Final Notes ...............................................96 + Normative References ..............................................97 + Informative References ............................................98 + Working Group Information ........................................101 + Contributors .....................................................101 + + + + + + + + + + + + + + + + + + + + +Dierks & Rescorla Standards Track [Page 3] + +RFC 5246 TLS August 2008 + + +1. Introduction + + The primary goal of the TLS protocol is to provide privacy and data + integrity between two communicating applications. The protocol is + composed of two layers: the TLS Record Protocol and the TLS Handshake + Protocol. At the lowest level, layered on top of some reliable + transport protocol (e.g., TCP [TCP]), is the TLS Record Protocol. + The TLS Record Protocol provides connection security that has two + basic properties: + + - The connection is private. Symmetric cryptography is used for + data encryption (e.g., AES [AES], RC4 [SCH], etc.). The keys for + this symmetric encryption are generated uniquely for each + connection and are based on a secret negotiated by another + protocol (such as the TLS Handshake Protocol). The Record + Protocol can also be used without encryption. + + - The connection is reliable. Message transport includes a message + integrity check using a keyed MAC. Secure hash functions (e.g., + SHA-1, etc.) are used for MAC computations. The Record Protocol + can operate without a MAC, but is generally only used in this mode + while another protocol is using the Record Protocol as a transport + for negotiating security parameters. + + The TLS Record Protocol is used for encapsulation of various higher- + level protocols. One such encapsulated protocol, the TLS Handshake + Protocol, allows the server and client to authenticate each other and + to negotiate an encryption algorithm and cryptographic keys before + the application protocol transmits or receives its first byte of + data. The TLS Handshake Protocol provides connection security that + has three basic properties: + + - The peer's identity can be authenticated using asymmetric, or + public key, cryptography (e.g., RSA [RSA], DSA [DSS], etc.). This + authentication can be made optional, but is generally required for + at least one of the peers. + + - The negotiation of a shared secret is secure: the negotiated + secret is unavailable to eavesdroppers, and for any authenticated + connection the secret cannot be obtained, even by an attacker who + can place himself in the middle of the connection. + + - The negotiation is reliable: no attacker can modify the + negotiation communication without being detected by the parties to + the communication. + + + + + + +Dierks & Rescorla Standards Track [Page 4] + +RFC 5246 TLS August 2008 + + + One advantage of TLS is that it is application protocol independent. + Higher-level protocols can layer on top of the TLS protocol + transparently. The TLS standard, however, does not specify how + protocols add security with TLS; the decisions on how to initiate TLS + handshaking and how to interpret the authentication certificates + exchanged are left to the judgment of the designers and implementors + of protocols that run on top of TLS. + +1.1. Requirements Terminology + + The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", + "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this + document are to be interpreted as described in RFC 2119 [REQ]. + +1.2. Major Differences from TLS 1.1 + + This document is a revision of the TLS 1.1 [TLS1.1] protocol which + contains improved flexibility, particularly for negotiation of + cryptographic algorithms. The major changes are: + + - The MD5/SHA-1 combination in the pseudorandom function (PRF) has + been replaced with cipher-suite-specified PRFs. All cipher suites + in this document use P_SHA256. + + - The MD5/SHA-1 combination in the digitally-signed element has been + replaced with a single hash. Signed elements now include a field + that explicitly specifies the hash algorithm used. + + - Substantial cleanup to the client's and server's ability to + specify which hash and signature algorithms they will accept. + Note that this also relaxes some of the constraints on signature + and hash algorithms from previous versions of TLS. + + - Addition of support for authenticated encryption with additional + data modes. + + - TLS Extensions definition and AES Cipher Suites were merged in + from external [TLSEXT] and [TLSAES]. + + - Tighter checking of EncryptedPreMasterSecret version numbers. + + - Tightened up a number of requirements. + + - Verify_data length now depends on the cipher suite (default is + still 12). + + - Cleaned up description of Bleichenbacher/Klima attack defenses. + + + + +Dierks & Rescorla Standards Track [Page 5] + +RFC 5246 TLS August 2008 + + + - Alerts MUST now be sent in many cases. + + - After a certificate_request, if no certificates are available, + clients now MUST send an empty certificate list. + + - TLS_RSA_WITH_AES_128_CBC_SHA is now the mandatory to implement + cipher suite. + + - Added HMAC-SHA256 cipher suites. + + - Removed IDEA and DES cipher suites. They are now deprecated and + will be documented in a separate document. + + - Support for the SSLv2 backward-compatible hello is now a MAY, not + a SHOULD, with sending it a SHOULD NOT. Support will probably + become a SHOULD NOT in the future. + + - Added limited "fall-through" to the presentation language to allow + multiple case arms to have the same encoding. + + - Added an Implementation Pitfalls sections + + - The usual clarifications and editorial work. + +2. Goals + + The goals of the TLS protocol, in order of priority, are as follows: + + 1. Cryptographic security: TLS should be used to establish a secure + connection between two parties. + + 2. Interoperability: Independent programmers should be able to + develop applications utilizing TLS that can successfully exchange + cryptographic parameters without knowledge of one another's code. + + 3. Extensibility: TLS seeks to provide a framework into which new + public key and bulk encryption methods can be incorporated as + necessary. This will also accomplish two sub-goals: preventing + the need to create a new protocol (and risking the introduction of + possible new weaknesses) and avoiding the need to implement an + entire new security library. + + 4. Relative efficiency: Cryptographic operations tend to be highly + CPU intensive, particularly public key operations. For this + reason, the TLS protocol has incorporated an optional session + caching scheme to reduce the number of connections that need to be + established from scratch. Additionally, care has been taken to + reduce network activity. + + + +Dierks & Rescorla Standards Track [Page 6] + +RFC 5246 TLS August 2008 + + +3. Goals of This Document + + This document and the TLS protocol itself are based on the SSL 3.0 + Protocol Specification as published by Netscape. The differences + between this protocol and SSL 3.0 are not dramatic, but they are + significant enough that the various versions of TLS and SSL 3.0 do + not interoperate (although each protocol incorporates a mechanism by + which an implementation can back down to prior versions). This + document is intended primarily for readers who will be implementing + the protocol and for those doing cryptographic analysis of it. The + specification has been written with this in mind, and it is intended + to reflect the needs of those two groups. For that reason, many of + the algorithm-dependent data structures and rules are included in the + body of the text (as opposed to in an appendix), providing easier + access to them. + + This document is not intended to supply any details of service + definition or of interface definition, although it does cover select + areas of policy as they are required for the maintenance of solid + security. + +4. Presentation Language + + This document deals with the formatting of data in an external + representation. The following very basic and somewhat casually + defined presentation syntax will be used. The syntax draws from + several sources in its structure. Although it resembles the + programming language "C" in its syntax and XDR [XDR] in both its + syntax and intent, it would be risky to draw too many parallels. The + purpose of this presentation language is to document TLS only; it has + no general application beyond that particular goal. + +4.1. Basic Block Size + + The representation of all data items is explicitly specified. The + basic data block size is one byte (i.e., 8 bits). Multiple byte data + items are concatenations of bytes, from left to right, from top to + bottom. From the byte stream, a multi-byte item (a numeric in the + example) is formed (using C notation) by: + + value = (byte[0] << 8*(n-1)) | (byte[1] << 8*(n-2)) | + ... | byte[n-1]; + + This byte ordering for multi-byte values is the commonplace network + byte order or big-endian format. + + + + + + +Dierks & Rescorla Standards Track [Page 7] + +RFC 5246 TLS August 2008 + + +4.2. Miscellaneous + + Comments begin with "/*" and end with "*/". + + Optional components are denoted by enclosing them in "[[ ]]" double + brackets. + + Single-byte entities containing uninterpreted data are of type + opaque. + +4.3. Vectors + + A vector (single-dimensioned array) is a stream of homogeneous data + elements. The size of the vector may be specified at documentation + time or left unspecified until runtime. In either case, the length + declares the number of bytes, not the number of elements, in the + vector. The syntax for specifying a new type, T', that is a fixed- + length vector of type T is + + T T'[n]; + + Here, T' occupies n bytes in the data stream, where n is a multiple + of the size of T. The length of the vector is not included in the + encoded stream. + + In the following example, Datum is defined to be three consecutive + bytes that the protocol does not interpret, while Data is three + consecutive Datum, consuming a total of nine bytes. + + opaque Datum[3]; /* three uninterpreted bytes */ + Datum Data[9]; /* 3 consecutive 3 byte vectors */ + + Variable-length vectors are defined by specifying a subrange of legal + lengths, inclusively, using the notation <floor..ceiling>. When + these are encoded, the actual length precedes the vector's contents + in the byte stream. The length will be in the form of a number + consuming as many bytes as required to hold the vector's specified + maximum (ceiling) length. A variable-length vector with an actual + length field of zero is referred to as an empty vector. + + T T'<floor..ceiling>; + + In the following example, mandatory is a vector that must contain + between 300 and 400 bytes of type opaque. It can never be empty. + The actual length field consumes two bytes, a uint16, which is + sufficient to represent the value 400 (see Section 4.4). On the + other hand, longer can represent up to 800 bytes of data, or 400 + uint16 elements, and it may be empty. Its encoding will include a + + + +Dierks & Rescorla Standards Track [Page 8] + +RFC 5246 TLS August 2008 + + + two-byte actual length field prepended to the vector. The length of + an encoded vector must be an even multiple of the length of a single + element (for example, a 17-byte vector of uint16 would be illegal). + + opaque mandatory<300..400>; + /* length field is 2 bytes, cannot be empty */ + uint16 longer<0..800>; + /* zero to 400 16-bit unsigned integers */ + +4.4. Numbers + + The basic numeric data type is an unsigned byte (uint8). All larger + numeric data types are formed from fixed-length series of bytes + concatenated as described in Section 4.1 and are also unsigned. The + following numeric types are predefined. + + uint8 uint16[2]; + uint8 uint24[3]; + uint8 uint32[4]; + uint8 uint64[8]; + + All values, here and elsewhere in the specification, are stored in + network byte (big-endian) order; the uint32 represented by the hex + bytes 01 02 03 04 is equivalent to the decimal value 16909060. + + Note that in some cases (e.g., DH parameters) it is necessary to + represent integers as opaque vectors. In such cases, they are + represented as unsigned integers (i.e., leading zero octets are not + required even if the most significant bit is set). + +4.5. Enumerateds + + An additional sparse data type is available called enum. A field of + type enum can only assume the values declared in the definition. + Each definition is a different type. Only enumerateds of the same + type may be assigned or compared. Every element of an enumerated + must be assigned a value, as demonstrated in the following example. + Since the elements of the enumerated are not ordered, they can be + assigned any unique value, in any order. + + enum { e1(v1), e2(v2), ... , en(vn) [[, (n)]] } Te; + + An enumerated occupies as much space in the byte stream as would its + maximal defined ordinal value. The following definition would cause + one byte to be used to carry fields of type Color. + + enum { red(3), blue(5), white(7) } Color; + + + + +Dierks & Rescorla Standards Track [Page 9] + +RFC 5246 TLS August 2008 + + + One may optionally specify a value without its associated tag to + force the width definition without defining a superfluous element. + + In the following example, Taste will consume two bytes in the data + stream but can only assume the values 1, 2, or 4. + + enum { sweet(1), sour(2), bitter(4), (32000) } Taste; + + The names of the elements of an enumeration are scoped within the + defined type. In the first example, a fully qualified reference to + the second element of the enumeration would be Color.blue. Such + qualification is not required if the target of the assignment is well + specified. + + Color color = Color.blue; /* overspecified, legal */ + Color color = blue; /* correct, type implicit */ + + For enumerateds that are never converted to external representation, + the numerical information may be omitted. + + enum { low, medium, high } Amount; + +4.6. Constructed Types + + Structure types may be constructed from primitive types for + convenience. Each specification declares a new, unique type. The + syntax for definition is much like that of C. + + struct { + T1 f1; + T2 f2; + ... + Tn fn; + } [[T]]; + + The fields within a structure may be qualified using the type's name, + with a syntax much like that available for enumerateds. For example, + T.f2 refers to the second field of the previous declaration. + Structure definitions may be embedded. + +4.6.1. Variants + + Defined structures may have variants based on some knowledge that is + available within the environment. The selector must be an enumerated + type that defines the possible variants the structure defines. There + must be a case arm for every element of the enumeration declared in + the select. Case arms have limited fall-through: if two case arms + follow in immediate succession with no fields in between, then they + + + +Dierks & Rescorla Standards Track [Page 10] + +RFC 5246 TLS August 2008 + + + both contain the same fields. Thus, in the example below, "orange" + and "banana" both contain V2. Note that this is a new piece of + syntax in TLS 1.2. + + The body of the variant structure may be given a label for reference. + The mechanism by which the variant is selected at runtime is not + prescribed by the presentation language. + + struct { + T1 f1; + T2 f2; + .... + Tn fn; + select (E) { + case e1: Te1; + case e2: Te2; + case e3: case e4: Te3; + .... + case en: Ten; + } [[fv]]; + } [[Tv]]; + + For example: + + enum { apple, orange, banana } VariantTag; + + struct { + uint16 number; + opaque string<0..10>; /* variable length */ + } V1; + + struct { + uint32 number; + opaque string[10]; /* fixed length */ + } V2; + + struct { + select (VariantTag) { /* value of selector is implicit */ + case apple: + V1; /* VariantBody, tag = apple */ + case orange: + case banana: + V2; /* VariantBody, tag = orange or banana */ + } variant_body; /* optional label on variant */ + } VariantRecord; + + + + + + +Dierks & Rescorla Standards Track [Page 11] + +RFC 5246 TLS August 2008 + + +4.7. Cryptographic Attributes + + The five cryptographic operations -- digital signing, stream cipher + encryption, block cipher encryption, authenticated encryption with + additional data (AEAD) encryption, and public key encryption -- are + designated digitally-signed, stream-ciphered, block-ciphered, aead- + ciphered, and public-key-encrypted, respectively. A field's + cryptographic processing is specified by prepending an appropriate + key word designation before the field's type specification. + Cryptographic keys are implied by the current session state (see + Section 6.1). + + A digitally-signed element is encoded as a struct DigitallySigned: + + struct { + SignatureAndHashAlgorithm algorithm; + opaque signature<0..2^16-1>; + } DigitallySigned; + + The algorithm field specifies the algorithm used (see Section + 7.4.1.4.1 for the definition of this field). Note that the + introduction of the algorithm field is a change from previous + versions. The signature is a digital signature using those + algorithms over the contents of the element. The contents themselves + do not appear on the wire but are simply calculated. The length of + the signature is specified by the signing algorithm and key. + + In RSA signing, the opaque vector contains the signature generated + using the RSASSA-PKCS1-v1_5 signature scheme defined in [PKCS1]. As + discussed in [PKCS1], the DigestInfo MUST be DER-encoded [X680] + [X690]. For hash algorithms without parameters (which includes + SHA-1), the DigestInfo.AlgorithmIdentifier.parameters field MUST be + NULL, but implementations MUST accept both without parameters and + with NULL parameters. Note that earlier versions of TLS used a + different RSA signature scheme that did not include a DigestInfo + encoding. + + In DSA, the 20 bytes of the SHA-1 hash are run directly through the + Digital Signing Algorithm with no additional hashing. This produces + two values, r and s. The DSA signature is an opaque vector, as + above, the contents of which are the DER encoding of: + + Dss-Sig-Value ::= SEQUENCE { + r INTEGER, + s INTEGER + } + + + + + +Dierks & Rescorla Standards Track [Page 12] + +RFC 5246 TLS August 2008 + + + Note: In current terminology, DSA refers to the Digital Signature + Algorithm and DSS refers to the NIST standard. In the original SSL + and TLS specs, "DSS" was used universally. This document uses "DSA" + to refer to the algorithm, "DSS" to refer to the standard, and it + uses "DSS" in the code point definitions for historical continuity. + + In stream cipher encryption, the plaintext is exclusive-ORed with an + identical amount of output generated from a cryptographically secure + keyed pseudorandom number generator. + + In block cipher encryption, every block of plaintext encrypts to a + block of ciphertext. All block cipher encryption is done in CBC + (Cipher Block Chaining) mode, and all items that are block-ciphered + will be an exact multiple of the cipher block length. + + In AEAD encryption, the plaintext is simultaneously encrypted and + integrity protected. The input may be of any length, and aead- + ciphered output is generally larger than the input in order to + accommodate the integrity check value. + + In public key encryption, a public key algorithm is used to encrypt + data in such a way that it can be decrypted only with the matching + private key. A public-key-encrypted element is encoded as an opaque + vector <0..2^16-1>, where the length is specified by the encryption + algorithm and key. + + RSA encryption is done using the RSAES-PKCS1-v1_5 encryption scheme + defined in [PKCS1]. + + In the following example + + stream-ciphered struct { + uint8 field1; + uint8 field2; + digitally-signed opaque { + uint8 field3<0..255>; + uint8 field4; + }; + } UserType; + + The contents of the inner struct (field3 and field4) are used as + input for the signature/hash algorithm, and then the entire structure + is encrypted with a stream cipher. The length of this structure, in + bytes, would be equal to two bytes for field1 and field2, plus two + bytes for the signature and hash algorithm, plus two bytes for the + length of the signature, plus the length of the output of the signing + + + + + +Dierks & Rescorla Standards Track [Page 13] + +RFC 5246 TLS August 2008 + + + algorithm. The length of the signature is known because the + algorithm and key used for the signing are known prior to encoding or + decoding this structure. + +4.8. Constants + + Typed constants can be defined for purposes of specification by + declaring a symbol of the desired type and assigning values to it. + + Under-specified types (opaque, variable-length vectors, and + structures that contain opaque) cannot be assigned values. No fields + of a multi-element structure or vector may be elided. + + For example: + + struct { + uint8 f1; + uint8 f2; + } Example1; + + Example1 ex1 = {1, 4}; /* assigns f1 = 1, f2 = 4 */ + +5. HMAC and the Pseudorandom Function + + The TLS record layer uses a keyed Message Authentication Code (MAC) + to protect message integrity. The cipher suites defined in this + document use a construction known as HMAC, described in [HMAC], which + is based on a hash function. Other cipher suites MAY define their + own MAC constructions, if needed. + + In addition, a construction is required to do expansion of secrets + into blocks of data for the purposes of key generation or validation. + This pseudorandom function (PRF) takes as input a secret, a seed, and + an identifying label and produces an output of arbitrary length. + + In this section, we define one PRF, based on HMAC. This PRF with the + SHA-256 hash function is used for all cipher suites defined in this + document and in TLS documents published prior to this document when + TLS 1.2 is negotiated. New cipher suites MUST explicitly specify a + PRF and, in general, SHOULD use the TLS PRF with SHA-256 or a + stronger standard hash function. + + First, we define a data expansion function, P_hash(secret, data), + that uses a single hash function to expand a secret and seed into an + arbitrary quantity of output: + + + + + + +Dierks & Rescorla Standards Track [Page 14] + +RFC 5246 TLS August 2008 + + + P_hash(secret, seed) = HMAC_hash(secret, A(1) + seed) + + HMAC_hash(secret, A(2) + seed) + + HMAC_hash(secret, A(3) + seed) + ... + + where + indicates concatenation. + + A() is defined as: + + A(0) = seed + A(i) = HMAC_hash(secret, A(i-1)) + + P_hash can be iterated as many times as necessary to produce the + required quantity of data. For example, if P_SHA256 is being used to + create 80 bytes of data, it will have to be iterated three times + (through A(3)), creating 96 bytes of output data; the last 16 bytes + of the final iteration will then be discarded, leaving 80 bytes of + output data. + + TLS's PRF is created by applying P_hash to the secret as: + + PRF(secret, label, seed) = P_<hash>(secret, label + seed) + + The label is an ASCII string. It should be included in the exact + form it is given without a length byte or trailing null character. + For example, the label "slithy toves" would be processed by hashing + the following bytes: + + 73 6C 69 74 68 79 20 74 6F 76 65 73 + +6. The TLS Record Protocol + + The TLS Record Protocol is a layered protocol. At each layer, + messages may include fields for length, description, and content. + The Record Protocol takes messages to be transmitted, fragments the + data into manageable blocks, optionally compresses the data, applies + a MAC, encrypts, and transmits the result. Received data is + decrypted, verified, decompressed, reassembled, and then delivered to + higher-level clients. + + Four protocols that use the record protocol are described in this + document: the handshake protocol, the alert protocol, the change + cipher spec protocol, and the application data protocol. In order to + allow extension of the TLS protocol, additional record content types + can be supported by the record protocol. New record content type + values are assigned by IANA in the TLS Content Type Registry as + described in Section 12. + + + + + +Dierks & Rescorla Standards Track [Page 15] + +RFC 5246 TLS August 2008 + + + Implementations MUST NOT send record types not defined in this + document unless negotiated by some extension. If a TLS + implementation receives an unexpected record type, it MUST send an + unexpected_message alert. + + Any protocol designed for use over TLS must be carefully designed to + deal with all possible attacks against it. As a practical matter, + this means that the protocol designer must be aware of what security + properties TLS does and does not provide and cannot safely rely on + the latter. + + Note in particular that type and length of a record are not protected + by encryption. If this information is itself sensitive, application + designers may wish to take steps (padding, cover traffic) to minimize + information leakage. + +6.1. Connection States + + A TLS connection state is the operating environment of the TLS Record + Protocol. It specifies a compression algorithm, an encryption + algorithm, and a MAC algorithm. In addition, the parameters for + these algorithms are known: the MAC key and the bulk encryption keys + for the connection in both the read and the write directions. + Logically, there are always four connection states outstanding: the + current read and write states, and the pending read and write states. + All records are processed under the current read and write states. + The security parameters for the pending states can be set by the TLS + Handshake Protocol, and the ChangeCipherSpec can selectively make + either of the pending states current, in which case the appropriate + current state is disposed of and replaced with the pending state; the + pending state is then reinitialized to an empty state. It is illegal + to make a state that has not been initialized with security + parameters a current state. The initial current state always + specifies that no encryption, compression, or MAC will be used. + + The security parameters for a TLS Connection read and write state are + set by providing the following values: + + connection end + Whether this entity is considered the "client" or the "server" in + this connection. + + PRF algorithm + An algorithm used to generate keys from the master secret (see + Sections 5 and 6.3). + + + + + + +Dierks & Rescorla Standards Track [Page 16] + +RFC 5246 TLS August 2008 + + + bulk encryption algorithm + An algorithm to be used for bulk encryption. This specification + includes the key size of this algorithm, whether it is a block, + stream, or AEAD cipher, the block size of the cipher (if + appropriate), and the lengths of explicit and implicit + initialization vectors (or nonces). + + MAC algorithm + An algorithm to be used for message authentication. This + specification includes the size of the value returned by the MAC + algorithm. + + compression algorithm + An algorithm to be used for data compression. This specification + must include all information the algorithm requires to do + compression. + + master secret + A 48-byte secret shared between the two peers in the connection. + + client random + A 32-byte value provided by the client. + + server random + A 32-byte value provided by the server. + + These parameters are defined in the presentation language as: + + enum { server, client } ConnectionEnd; + + enum { tls_prf_sha256 } PRFAlgorithm; + + enum { null, rc4, 3des, aes } + BulkCipherAlgorithm; + + enum { stream, block, aead } CipherType; + + enum { null, hmac_md5, hmac_sha1, hmac_sha256, + hmac_sha384, hmac_sha512} MACAlgorithm; + + enum { null(0), (255) } CompressionMethod; + + /* The algorithms specified in CompressionMethod, PRFAlgorithm, + BulkCipherAlgorithm, and MACAlgorithm may be added to. */ + + + + + + + +Dierks & Rescorla Standards Track [Page 17] + +RFC 5246 TLS August 2008 + + + struct { + ConnectionEnd entity; + PRFAlgorithm prf_algorithm; + BulkCipherAlgorithm bulk_cipher_algorithm; + CipherType cipher_type; + uint8 enc_key_length; + uint8 block_length; + uint8 fixed_iv_length; + uint8 record_iv_length; + MACAlgorithm mac_algorithm; + uint8 mac_length; + uint8 mac_key_length; + CompressionMethod compression_algorithm; + opaque master_secret[48]; + opaque client_random[32]; + opaque server_random[32]; + } SecurityParameters; + + The record layer will use the security parameters to generate the + following six items (some of which are not required by all ciphers, + and are thus empty): + + client write MAC key + server write MAC key + client write encryption key + server write encryption key + client write IV + server write IV + + The client write parameters are used by the server when receiving and + processing records and vice versa. The algorithm used for generating + these items from the security parameters is described in Section 6.3. + + Once the security parameters have been set and the keys have been + generated, the connection states can be instantiated by making them + the current states. These current states MUST be updated for each + record processed. Each connection state includes the following + elements: + + compression state + The current state of the compression algorithm. + + cipher state + The current state of the encryption algorithm. This will consist + of the scheduled key for that connection. For stream ciphers, + this will also contain whatever state information is necessary to + allow the stream to continue to encrypt or decrypt data. + + + + +Dierks & Rescorla Standards Track [Page 18] + +RFC 5246 TLS August 2008 + + + MAC key + The MAC key for this connection, as generated above. + + sequence number + Each connection state contains a sequence number, which is + maintained separately for read and write states. The sequence + number MUST be set to zero whenever a connection state is made the + active state. Sequence numbers are of type uint64 and may not + exceed 2^64-1. Sequence numbers do not wrap. If a TLS + implementation would need to wrap a sequence number, it must + renegotiate instead. A sequence number is incremented after each + record: specifically, the first record transmitted under a + particular connection state MUST use sequence number 0. + +6.2. Record Layer + + The TLS record layer receives uninterpreted data from higher layers + in non-empty blocks of arbitrary size. + +6.2.1. Fragmentation + + The record layer fragments information blocks into TLSPlaintext + records carrying data in chunks of 2^14 bytes or less. Client + message boundaries are not preserved in the record layer (i.e., + multiple client messages of the same ContentType MAY be coalesced + into a single TLSPlaintext record, or a single message MAY be + fragmented across several records). + + struct { + uint8 major; + uint8 minor; + } ProtocolVersion; + + enum { + change_cipher_spec(20), alert(21), handshake(22), + application_data(23), (255) + } ContentType; + + struct { + ContentType type; + ProtocolVersion version; + uint16 length; + opaque fragment[TLSPlaintext.length]; + } TLSPlaintext; + + type + The higher-level protocol used to process the enclosed fragment. + + + + +Dierks & Rescorla Standards Track [Page 19] + +RFC 5246 TLS August 2008 + + + version + The version of the protocol being employed. This document + describes TLS Version 1.2, which uses the version { 3, 3 }. The + version value 3.3 is historical, deriving from the use of {3, 1} + for TLS 1.0. (See Appendix A.1.) Note that a client that + supports multiple versions of TLS may not know what version will + be employed before it receives the ServerHello. See Appendix E + for discussion about what record layer version number should be + employed for ClientHello. + + length + The length (in bytes) of the following TLSPlaintext.fragment. The + length MUST NOT exceed 2^14. + + fragment + The application data. This data is transparent and treated as an + independent block to be dealt with by the higher-level protocol + specified by the type field. + + Implementations MUST NOT send zero-length fragments of Handshake, + Alert, or ChangeCipherSpec content types. Zero-length fragments of + Application data MAY be sent as they are potentially useful as a + traffic analysis countermeasure. + + Note: Data of different TLS record layer content types MAY be + interleaved. Application data is generally of lower precedence for + transmission than other content types. However, records MUST be + delivered to the network in the same order as they are protected by + the record layer. Recipients MUST receive and process interleaved + application layer traffic during handshakes subsequent to the first + one on a connection. + +6.2.2. Record Compression and Decompression + + All records are compressed using the compression algorithm defined in + the current session state. There is always an active compression + algorithm; however, initially it is defined as + CompressionMethod.null. The compression algorithm translates a + TLSPlaintext structure into a TLSCompressed structure. Compression + functions are initialized with default state information whenever a + connection state is made active. [RFC3749] describes compression + algorithms for TLS. + + Compression must be lossless and may not increase the content length + by more than 1024 bytes. If the decompression function encounters a + TLSCompressed.fragment that would decompress to a length in excess of + 2^14 bytes, it MUST report a fatal decompression failure error. + + + + +Dierks & Rescorla Standards Track [Page 20] + +RFC 5246 TLS August 2008 + + + struct { + ContentType type; /* same as TLSPlaintext.type */ + ProtocolVersion version;/* same as TLSPlaintext.version */ + uint16 length; + opaque fragment[TLSCompressed.length]; + } TLSCompressed; + + length + The length (in bytes) of the following TLSCompressed.fragment. + The length MUST NOT exceed 2^14 + 1024. + + fragment + The compressed form of TLSPlaintext.fragment. + + Note: A CompressionMethod.null operation is an identity operation; + no fields are altered. + + Implementation note: Decompression functions are responsible for + ensuring that messages cannot cause internal buffer overflows. + +6.2.3. Record Payload Protection + + The encryption and MAC functions translate a TLSCompressed + structure into a TLSCiphertext. The decryption functions reverse + the process. The MAC of the record also includes a sequence + number so that missing, extra, or repeated messages are + detectable. + + struct { + ContentType type; + ProtocolVersion version; + uint16 length; + select (SecurityParameters.cipher_type) { + case stream: GenericStreamCipher; + case block: GenericBlockCipher; + case aead: GenericAEADCipher; + } fragment; + } TLSCiphertext; + + type + The type field is identical to TLSCompressed.type. + + version + The version field is identical to TLSCompressed.version. + + length + The length (in bytes) of the following TLSCiphertext.fragment. + The length MUST NOT exceed 2^14 + 2048. + + + +Dierks & Rescorla Standards Track [Page 21] + +RFC 5246 TLS August 2008 + + + fragment + The encrypted form of TLSCompressed.fragment, with the MAC. + +6.2.3.1. Null or Standard Stream Cipher + + Stream ciphers (including BulkCipherAlgorithm.null; see Appendix A.6) + convert TLSCompressed.fragment structures to and from stream + TLSCiphertext.fragment structures. + + stream-ciphered struct { + opaque content[TLSCompressed.length]; + opaque MAC[SecurityParameters.mac_length]; + } GenericStreamCipher; + + The MAC is generated as: + + MAC(MAC_write_key, seq_num + + TLSCompressed.type + + TLSCompressed.version + + TLSCompressed.length + + TLSCompressed.fragment); + + where "+" denotes concatenation. + + seq_num + The sequence number for this record. + + MAC + The MAC algorithm specified by SecurityParameters.mac_algorithm. + + Note that the MAC is computed before encryption. The stream cipher + encrypts the entire block, including the MAC. For stream ciphers + that do not use a synchronization vector (such as RC4), the stream + cipher state from the end of one record is simply used on the + subsequent packet. If the cipher suite is TLS_NULL_WITH_NULL_NULL, + encryption consists of the identity operation (i.e., the data is not + encrypted, and the MAC size is zero, implying that no MAC is used). + For both null and stream ciphers, TLSCiphertext.length is + TLSCompressed.length plus SecurityParameters.mac_length. + +6.2.3.2. CBC Block Cipher + + For block ciphers (such as 3DES or AES), the encryption and MAC + functions convert TLSCompressed.fragment structures to and from block + TLSCiphertext.fragment structures. + + + + + + +Dierks & Rescorla Standards Track [Page 22] + +RFC 5246 TLS August 2008 + + + struct { + opaque IV[SecurityParameters.record_iv_length]; + block-ciphered struct { + opaque content[TLSCompressed.length]; + opaque MAC[SecurityParameters.mac_length]; + uint8 padding[GenericBlockCipher.padding_length]; + uint8 padding_length; + }; + } GenericBlockCipher; + + The MAC is generated as described in Section 6.2.3.1. + + IV + The Initialization Vector (IV) SHOULD be chosen at random, and + MUST be unpredictable. Note that in versions of TLS prior to 1.1, + there was no IV field, and the last ciphertext block of the + previous record (the "CBC residue") was used as the IV. This was + changed to prevent the attacks described in [CBCATT]. For block + ciphers, the IV length is of length + SecurityParameters.record_iv_length, which is equal to the + SecurityParameters.block_size. + + padding + Padding that is added to force the length of the plaintext to be + an integral multiple of the block cipher's block length. The + padding MAY be any length up to 255 bytes, as long as it results + in the TLSCiphertext.length being an integral multiple of the + block length. Lengths longer than necessary might be desirable to + frustrate attacks on a protocol that are based on analysis of the + lengths of exchanged messages. Each uint8 in the padding data + vector MUST be filled with the padding length value. The receiver + MUST check this padding and MUST use the bad_record_mac alert to + indicate padding errors. + + padding_length + The padding length MUST be such that the total size of the + GenericBlockCipher structure is a multiple of the cipher's block + length. Legal values range from zero to 255, inclusive. This + length specifies the length of the padding field exclusive of the + padding_length field itself. + + The encrypted data length (TLSCiphertext.length) is one more than the + sum of SecurityParameters.block_length, TLSCompressed.length, + SecurityParameters.mac_length, and padding_length. + + Example: If the block length is 8 bytes, the content length + (TLSCompressed.length) is 61 bytes, and the MAC length is 20 bytes, + then the length before padding is 82 bytes (this does not include the + + + +Dierks & Rescorla Standards Track [Page 23] + +RFC 5246 TLS August 2008 + + + IV. Thus, the padding length modulo 8 must be equal to 6 in order to + make the total length an even multiple of 8 bytes (the block length). + The padding length can be 6, 14, 22, and so on, through 254. If the + padding length were the minimum necessary, 6, the padding would be 6 + bytes, each containing the value 6. Thus, the last 8 octets of the + GenericBlockCipher before block encryption would be xx 06 06 06 06 06 + 06 06, where xx is the last octet of the MAC. + + Note: With block ciphers in CBC mode (Cipher Block Chaining), it is + critical that the entire plaintext of the record be known before any + ciphertext is transmitted. Otherwise, it is possible for the + attacker to mount the attack described in [CBCATT]. + + Implementation note: Canvel et al. [CBCTIME] have demonstrated a + timing attack on CBC padding based on the time required to compute + the MAC. In order to defend against this attack, implementations + MUST ensure that record processing time is essentially the same + whether or not the padding is correct. In general, the best way to + do this is to compute the MAC even if the padding is incorrect, and + only then reject the packet. For instance, if the pad appears to be + incorrect, the implementation might assume a zero-length pad and then + compute the MAC. This leaves a small timing channel, since MAC + performance depends to some extent on the size of the data fragment, + but it is not believed to be large enough to be exploitable, due to + the large block size of existing MACs and the small size of the + timing signal. + +6.2.3.3. AEAD Ciphers + + For AEAD [AEAD] ciphers (such as [CCM] or [GCM]), the AEAD function + converts TLSCompressed.fragment structures to and from AEAD + TLSCiphertext.fragment structures. + + struct { + opaque nonce_explicit[SecurityParameters.record_iv_length]; + aead-ciphered struct { + opaque content[TLSCompressed.length]; + }; + } GenericAEADCipher; + + AEAD ciphers take as input a single key, a nonce, a plaintext, and + "additional data" to be included in the authentication check, as + described in Section 2.1 of [AEAD]. The key is either the + client_write_key or the server_write_key. No MAC key is used. + + Each AEAD cipher suite MUST specify how the nonce supplied to the + AEAD operation is constructed, and what is the length of the + GenericAEADCipher.nonce_explicit part. In many cases, it is + + + +Dierks & Rescorla Standards Track [Page 24] + +RFC 5246 TLS August 2008 + + + appropriate to use the partially implicit nonce technique described + in Section 3.2.1 of [AEAD]; with record_iv_length being the length of + the explicit part. In this case, the implicit part SHOULD be derived + from key_block as client_write_iv and server_write_iv (as described + in Section 6.3), and the explicit part is included in + GenericAEAEDCipher.nonce_explicit. + + The plaintext is the TLSCompressed.fragment. + + The additional authenticated data, which we denote as + additional_data, is defined as follows: + + additional_data = seq_num + TLSCompressed.type + + TLSCompressed.version + TLSCompressed.length; + + where "+" denotes concatenation. + + The aead_output consists of the ciphertext output by the AEAD + encryption operation. The length will generally be larger than + TLSCompressed.length, but by an amount that varies with the AEAD + cipher. Since the ciphers might incorporate padding, the amount of + overhead could vary with different TLSCompressed.length values. Each + AEAD cipher MUST NOT produce an expansion of greater than 1024 bytes. + Symbolically, + + AEADEncrypted = AEAD-Encrypt(write_key, nonce, plaintext, + additional_data) + + In order to decrypt and verify, the cipher takes as input the key, + nonce, the "additional_data", and the AEADEncrypted value. The + output is either the plaintext or an error indicating that the + decryption failed. There is no separate integrity check. That is: + + TLSCompressed.fragment = AEAD-Decrypt(write_key, nonce, + AEADEncrypted, + additional_data) + + If the decryption fails, a fatal bad_record_mac alert MUST be + generated. + +6.3. Key Calculation + + The Record Protocol requires an algorithm to generate keys required + by the current connection state (see Appendix A.6) from the security + parameters provided by the handshake protocol. + + + + + + +Dierks & Rescorla Standards Track [Page 25] + +RFC 5246 TLS August 2008 + + + The master secret is expanded into a sequence of secure bytes, which + is then split to a client write MAC key, a server write MAC key, a + client write encryption key, and a server write encryption key. Each + of these is generated from the byte sequence in that order. Unused + values are empty. Some AEAD ciphers may additionally require a + client write IV and a server write IV (see Section 6.2.3.3). + + When keys and MAC keys are generated, the master secret is used as an + entropy source. + + To generate the key material, compute + + key_block = PRF(SecurityParameters.master_secret, + "key expansion", + SecurityParameters.server_random + + SecurityParameters.client_random); + + until enough output has been generated. Then, the key_block is + partitioned as follows: + + client_write_MAC_key[SecurityParameters.mac_key_length] + server_write_MAC_key[SecurityParameters.mac_key_length] + client_write_key[SecurityParameters.enc_key_length] + server_write_key[SecurityParameters.enc_key_length] + client_write_IV[SecurityParameters.fixed_iv_length] + server_write_IV[SecurityParameters.fixed_iv_length] + + Currently, the client_write_IV and server_write_IV are only generated + for implicit nonce techniques as described in Section 3.2.1 of + [AEAD]. + + Implementation note: The currently defined cipher suite which + requires the most material is AES_256_CBC_SHA256. It requires 2 x 32 + byte keys and 2 x 32 byte MAC keys, for a total 128 bytes of key + material. + +7. The TLS Handshaking Protocols + + TLS has three subprotocols that are used to allow peers to agree upon + security parameters for the record layer, to authenticate themselves, + to instantiate negotiated security parameters, and to report error + conditions to each other. + + The Handshake Protocol is responsible for negotiating a session, + which consists of the following items: + + + + + + +Dierks & Rescorla Standards Track [Page 26] + +RFC 5246 TLS August 2008 + + + session identifier + An arbitrary byte sequence chosen by the server to identify an + active or resumable session state. + + peer certificate + X509v3 [PKIX] certificate of the peer. This element of the state + may be null. + + compression method + The algorithm used to compress data prior to encryption. + + cipher spec + Specifies the pseudorandom function (PRF) used to generate keying + material, the bulk data encryption algorithm (such as null, AES, + etc.) and the MAC algorithm (such as HMAC-SHA1). It also defines + cryptographic attributes such as the mac_length. (See Appendix + A.6 for formal definition.) + + master secret + 48-byte secret shared between the client and server. + + is resumable + A flag indicating whether the session can be used to initiate new + connections. + + These items are then used to create security parameters for use by + the record layer when protecting application data. Many connections + can be instantiated using the same session through the resumption + feature of the TLS Handshake Protocol. + +7.1. Change Cipher Spec Protocol + + The change cipher spec protocol exists to signal transitions in + ciphering strategies. The protocol consists of a single message, + which is encrypted and compressed under the current (not the pending) + connection state. The message consists of a single byte of value 1. + + struct { + enum { change_cipher_spec(1), (255) } type; + } ChangeCipherSpec; + + The ChangeCipherSpec message is sent by both the client and the + server to notify the receiving party that subsequent records will be + protected under the newly negotiated CipherSpec and keys. Reception + of this message causes the receiver to instruct the record layer to + immediately copy the read pending state into the read current state. + Immediately after sending this message, the sender MUST instruct the + record layer to make the write pending state the write active state. + + + +Dierks & Rescorla Standards Track [Page 27] + +RFC 5246 TLS August 2008 + + + (See Section 6.1.) The ChangeCipherSpec message is sent during the + handshake after the security parameters have been agreed upon, but + before the verifying Finished message is sent. + + Note: If a rehandshake occurs while data is flowing on a connection, + the communicating parties may continue to send data using the old + CipherSpec. However, once the ChangeCipherSpec has been sent, the + new CipherSpec MUST be used. The first side to send the + ChangeCipherSpec does not know that the other side has finished + computing the new keying material (e.g., if it has to perform a + time-consuming public key operation). Thus, a small window of time, + during which the recipient must buffer the data, MAY exist. In + practice, with modern machines this interval is likely to be fairly + short. + +7.2. Alert Protocol + + One of the content types supported by the TLS record layer is the + alert type. Alert messages convey the severity of the message + (warning or fatal) and a description of the alert. Alert messages + with a level of fatal result in the immediate termination of the + connection. In this case, other connections corresponding to the + session may continue, but the session identifier MUST be invalidated, + preventing the failed session from being used to establish new + connections. Like other messages, alert messages are encrypted and + compressed, as specified by the current connection state. + + enum { warning(1), fatal(2), (255) } AlertLevel; + + enum { + close_notify(0), + unexpected_message(10), + bad_record_mac(20), + decryption_failed_RESERVED(21), + record_overflow(22), + decompression_failure(30), + handshake_failure(40), + no_certificate_RESERVED(41), + bad_certificate(42), + unsupported_certificate(43), + certificate_revoked(44), + certificate_expired(45), + certificate_unknown(46), + illegal_parameter(47), + unknown_ca(48), + access_denied(49), + decode_error(50), + decrypt_error(51), + + + +Dierks & Rescorla Standards Track [Page 28] + +RFC 5246 TLS August 2008 + + + export_restriction_RESERVED(60), + protocol_version(70), + insufficient_security(71), + internal_error(80), + user_canceled(90), + no_renegotiation(100), + unsupported_extension(110), + (255) + } AlertDescription; + + struct { + AlertLevel level; + AlertDescription description; + } Alert; + +7.2.1. Closure Alerts + + The client and the server must share knowledge that the connection is + ending in order to avoid a truncation attack. Either party may + initiate the exchange of closing messages. + + close_notify + This message notifies the recipient that the sender will not send + any more messages on this connection. Note that as of TLS 1.1, + failure to properly close a connection no longer requires that a + session not be resumed. This is a change from TLS 1.0 to conform + with widespread implementation practice. + + Either party may initiate a close by sending a close_notify alert. + Any data received after a closure alert is ignored. + + Unless some other fatal alert has been transmitted, each party is + required to send a close_notify alert before closing the write side + of the connection. The other party MUST respond with a close_notify + alert of its own and close down the connection immediately, + discarding any pending writes. It is not required for the initiator + of the close to wait for the responding close_notify alert before + closing the read side of the connection. + + If the application protocol using TLS provides that any data may be + carried over the underlying transport after the TLS connection is + closed, the TLS implementation must receive the responding + close_notify alert before indicating to the application layer that + the TLS connection has ended. If the application protocol will not + transfer any additional data, but will only close the underlying + transport connection, then the implementation MAY choose to close the + transport without waiting for the responding close_notify. No part + + + + +Dierks & Rescorla Standards Track [Page 29] + +RFC 5246 TLS August 2008 + + + of this standard should be taken to dictate the manner in which a + usage profile for TLS manages its data transport, including when + connections are opened or closed. + + Note: It is assumed that closing a connection reliably delivers + pending data before destroying the transport. + +7.2.2. Error Alerts + + Error handling in the TLS Handshake protocol is very simple. When an + error is detected, the detecting party sends a message to the other + party. Upon transmission or receipt of a fatal alert message, both + parties immediately close the connection. Servers and clients MUST + forget any session-identifiers, keys, and secrets associated with a + failed connection. Thus, any connection terminated with a fatal + alert MUST NOT be resumed. + + Whenever an implementation encounters a condition which is defined as + a fatal alert, it MUST send the appropriate alert prior to closing + the connection. For all errors where an alert level is not + explicitly specified, the sending party MAY determine at its + discretion whether to treat this as a fatal error or not. If the + implementation chooses to send an alert but intends to close the + connection immediately afterwards, it MUST send that alert at the + fatal alert level. + + If an alert with a level of warning is sent and received, generally + the connection can continue normally. If the receiving party decides + not to proceed with the connection (e.g., after having received a + no_renegotiation alert that it is not willing to accept), it SHOULD + send a fatal alert to terminate the connection. Given this, the + sending party cannot, in general, know how the receiving party will + behave. Therefore, warning alerts are not very useful when the + sending party wants to continue the connection, and thus are + sometimes omitted. For example, if a peer decides to accept an + expired certificate (perhaps after confirming this with the user) and + wants to continue the connection, it would not generally send a + certificate_expired alert. + + The following error alerts are defined: + + unexpected_message + An inappropriate message was received. This alert is always fatal + and should never be observed in communication between proper + implementations. + + + + + + +Dierks & Rescorla Standards Track [Page 30] + +RFC 5246 TLS August 2008 + + + bad_record_mac + This alert is returned if a record is received with an incorrect + MAC. This alert also MUST be returned if an alert is sent because + a TLSCiphertext decrypted in an invalid way: either it wasn't an + even multiple of the block length, or its padding values, when + checked, weren't correct. This message is always fatal and should + never be observed in communication between proper implementations + (except when messages were corrupted in the network). + + decryption_failed_RESERVED + This alert was used in some earlier versions of TLS, and may have + permitted certain attacks against the CBC mode [CBCATT]. It MUST + NOT be sent by compliant implementations. + + record_overflow + A TLSCiphertext record was received that had a length more than + 2^14+2048 bytes, or a record decrypted to a TLSCompressed record + with more than 2^14+1024 bytes. This message is always fatal and + should never be observed in communication between proper + implementations (except when messages were corrupted in the + network). + + decompression_failure + The decompression function received improper input (e.g., data + that would expand to excessive length). This message is always + fatal and should never be observed in communication between proper + implementations. + + handshake_failure + Reception of a handshake_failure alert message indicates that the + sender was unable to negotiate an acceptable set of security + parameters given the options available. This is a fatal error. + + no_certificate_RESERVED + This alert was used in SSLv3 but not any version of TLS. It MUST + NOT be sent by compliant implementations. + + bad_certificate + A certificate was corrupt, contained signatures that did not + verify correctly, etc. + + unsupported_certificate + A certificate was of an unsupported type. + + certificate_revoked + A certificate was revoked by its signer. + + + + + +Dierks & Rescorla Standards Track [Page 31] + +RFC 5246 TLS August 2008 + + + certificate_expired + A certificate has expired or is not currently valid. + + certificate_unknown + Some other (unspecified) issue arose in processing the + certificate, rendering it unacceptable. + + illegal_parameter + A field in the handshake was out of range or inconsistent with + other fields. This message is always fatal. + + unknown_ca + A valid certificate chain or partial chain was received, but the + certificate was not accepted because the CA certificate could not + be located or couldn't be matched with a known, trusted CA. This + message is always fatal. + + access_denied + A valid certificate was received, but when access control was + applied, the sender decided not to proceed with negotiation. This + message is always fatal. + + decode_error + A message could not be decoded because some field was out of the + specified range or the length of the message was incorrect. This + message is always fatal and should never be observed in + communication between proper implementations (except when messages + were corrupted in the network). + + decrypt_error + A handshake cryptographic operation failed, including being unable + to correctly verify a signature or validate a Finished message. + This message is always fatal. + + export_restriction_RESERVED + This alert was used in some earlier versions of TLS. It MUST NOT + be sent by compliant implementations. + + protocol_version + The protocol version the client has attempted to negotiate is + recognized but not supported. (For example, old protocol versions + might be avoided for security reasons.) This message is always + fatal. + + + + + + + + +Dierks & Rescorla Standards Track [Page 32] + +RFC 5246 TLS August 2008 + + + insufficient_security + Returned instead of handshake_failure when a negotiation has + failed specifically because the server requires ciphers more + secure than those supported by the client. This message is always + fatal. + + internal_error + An internal error unrelated to the peer or the correctness of the + protocol (such as a memory allocation failure) makes it impossible + to continue. This message is always fatal. + + user_canceled + This handshake is being canceled for some reason unrelated to a + protocol failure. If the user cancels an operation after the + handshake is complete, just closing the connection by sending a + close_notify is more appropriate. This alert should be followed + by a close_notify. This message is generally a warning. + + no_renegotiation + Sent by the client in response to a hello request or by the server + in response to a client hello after initial handshaking. Either + of these would normally lead to renegotiation; when that is not + appropriate, the recipient should respond with this alert. At + that point, the original requester can decide whether to proceed + with the connection. One case where this would be appropriate is + where a server has spawned a process to satisfy a request; the + process might receive security parameters (key length, + authentication, etc.) at startup, and it might be difficult to + communicate changes to these parameters after that point. This + message is always a warning. + + unsupported_extension + sent by clients that receive an extended server hello containing + an extension that they did not put in the corresponding client + hello. This message is always fatal. + + New Alert values are assigned by IANA as described in Section 12. + +7.3. Handshake Protocol Overview + + The cryptographic parameters of the session state are produced by the + TLS Handshake Protocol, which operates on top of the TLS record + layer. When a TLS client and server first start communicating, they + agree on a protocol version, select cryptographic algorithms, + optionally authenticate each other, and use public-key encryption + techniques to generate shared secrets. + + + + + +Dierks & Rescorla Standards Track [Page 33] + +RFC 5246 TLS August 2008 + + + The TLS Handshake Protocol involves the following steps: + + - Exchange hello messages to agree on algorithms, exchange random + values, and check for session resumption. + + - Exchange the necessary cryptographic parameters to allow the + client and server to agree on a premaster secret. + + - Exchange certificates and cryptographic information to allow the + client and server to authenticate themselves. + + - Generate a master secret from the premaster secret and exchanged + random values. + + - Provide security parameters to the record layer. + + - Allow the client and server to verify that their peer has + calculated the same security parameters and that the handshake + occurred without tampering by an attacker. + + Note that higher layers should not be overly reliant on whether TLS + always negotiates the strongest possible connection between two + peers. There are a number of ways in which a man-in-the-middle + attacker can attempt to make two entities drop down to the least + secure method they support. The protocol has been designed to + minimize this risk, but there are still attacks available: for + example, an attacker could block access to the port a secure service + runs on, or attempt to get the peers to negotiate an unauthenticated + connection. The fundamental rule is that higher levels must be + cognizant of what their security requirements are and never transmit + information over a channel less secure than what they require. The + TLS protocol is secure in that any cipher suite offers its promised + level of security: if you negotiate 3DES with a 1024-bit RSA key + exchange with a host whose certificate you have verified, you can + expect to be that secure. + + These goals are achieved by the handshake protocol, which can be + summarized as follows: The client sends a ClientHello message to + which the server must respond with a ServerHello message, or else a + fatal error will occur and the connection will fail. The ClientHello + and ServerHello are used to establish security enhancement + capabilities between client and server. The ClientHello and + ServerHello establish the following attributes: Protocol Version, + Session ID, Cipher Suite, and Compression Method. Additionally, two + random values are generated and exchanged: ClientHello.random and + ServerHello.random. + + + + + +Dierks & Rescorla Standards Track [Page 34] + +RFC 5246 TLS August 2008 + + + The actual key exchange uses up to four messages: the server + Certificate, the ServerKeyExchange, the client Certificate, and the + ClientKeyExchange. New key exchange methods can be created by + specifying a format for these messages and by defining the use of the + messages to allow the client and server to agree upon a shared + secret. This secret MUST be quite long; currently defined key + exchange methods exchange secrets that range from 46 bytes upwards. + + Following the hello messages, the server will send its certificate in + a Certificate message if it is to be authenticated. Additionally, a + ServerKeyExchange message may be sent, if it is required (e.g., if + the server has no certificate, or if its certificate is for signing + only). If the server is authenticated, it may request a certificate + from the client, if that is appropriate to the cipher suite selected. + Next, the server will send the ServerHelloDone message, indicating + that the hello-message phase of the handshake is complete. The + server will then wait for a client response. If the server has sent + a CertificateRequest message, the client MUST send the Certificate + message. The ClientKeyExchange message is now sent, and the content + of that message will depend on the public key algorithm selected + between the ClientHello and the ServerHello. If the client has sent + a certificate with signing ability, a digitally-signed + CertificateVerify message is sent to explicitly verify possession of + the private key in the certificate. + + At this point, a ChangeCipherSpec message is sent by the client, and + the client copies the pending Cipher Spec into the current Cipher + Spec. The client then immediately sends the Finished message under + the new algorithms, keys, and secrets. In response, the server will + send its own ChangeCipherSpec message, transfer the pending to the + current Cipher Spec, and send its Finished message under the new + Cipher Spec. At this point, the handshake is complete, and the + client and server may begin to exchange application layer data. (See + flow chart below.) Application data MUST NOT be sent prior to the + completion of the first handshake (before a cipher suite other than + TLS_NULL_WITH_NULL_NULL is established). + + + + + + + + + + + + + + + +Dierks & Rescorla Standards Track [Page 35] + +RFC 5246 TLS August 2008 + + + Client Server + + ClientHello --------> + ServerHello + Certificate* + ServerKeyExchange* + CertificateRequest* + <-------- ServerHelloDone + Certificate* + ClientKeyExchange + CertificateVerify* + [ChangeCipherSpec] + Finished --------> + [ChangeCipherSpec] + <-------- Finished + Application Data <-------> Application Data + + Figure 1. Message flow for a full handshake + + * Indicates optional or situation-dependent messages that are not + always sent. + + Note: To help avoid pipeline stalls, ChangeCipherSpec is an + independent TLS protocol content type, and is not actually a TLS + handshake message. + + When the client and server decide to resume a previous session or + duplicate an existing session (instead of negotiating new security + parameters), the message flow is as follows: + + The client sends a ClientHello using the Session ID of the session to + be resumed. The server then checks its session cache for a match. + If a match is found, and the server is willing to re-establish the + connection under the specified session state, it will send a + ServerHello with the same Session ID value. At this point, both + client and server MUST send ChangeCipherSpec messages and proceed + directly to Finished messages. Once the re-establishment is + complete, the client and server MAY begin to exchange application + layer data. (See flow chart below.) If a Session ID match is not + found, the server generates a new session ID, and the TLS client and + server perform a full handshake. + + + + + + + + + + +Dierks & Rescorla Standards Track [Page 36] + +RFC 5246 TLS August 2008 + + + Client Server + + ClientHello --------> + ServerHello + [ChangeCipherSpec] + <-------- Finished + [ChangeCipherSpec] + Finished --------> + Application Data <-------> Application Data + + Figure 2. Message flow for an abbreviated handshake + + The contents and significance of each message will be presented in + detail in the following sections. + +7.4. Handshake Protocol + + The TLS Handshake Protocol is one of the defined higher-level clients + of the TLS Record Protocol. This protocol is used to negotiate the + secure attributes of a session. Handshake messages are supplied to + the TLS record layer, where they are encapsulated within one or more + TLSPlaintext structures, which are processed and transmitted as + specified by the current active session state. + + enum { + hello_request(0), client_hello(1), server_hello(2), + certificate(11), server_key_exchange (12), + certificate_request(13), server_hello_done(14), + certificate_verify(15), client_key_exchange(16), + finished(20), (255) + } HandshakeType; + + struct { + HandshakeType msg_type; /* handshake type */ + uint24 length; /* bytes in message */ + select (HandshakeType) { + case hello_request: HelloRequest; + case client_hello: ClientHello; + case server_hello: ServerHello; + case certificate: Certificate; + case server_key_exchange: ServerKeyExchange; + case certificate_request: CertificateRequest; + case server_hello_done: ServerHelloDone; + case certificate_verify: CertificateVerify; + case client_key_exchange: ClientKeyExchange; + case finished: Finished; + } body; + } Handshake; + + + +Dierks & Rescorla Standards Track [Page 37] + +RFC 5246 TLS August 2008 + + + The handshake protocol messages are presented below in the order they + MUST be sent; sending handshake messages in an unexpected order + results in a fatal error. Unneeded handshake messages can be + omitted, however. Note one exception to the ordering: the + Certificate message is used twice in the handshake (from server to + client, then from client to server), but described only in its first + position. The one message that is not bound by these ordering rules + is the HelloRequest message, which can be sent at any time, but which + SHOULD be ignored by the client if it arrives in the middle of a + handshake. + + New handshake message types are assigned by IANA as described in + Section 12. + +7.4.1. Hello Messages + + The hello phase messages are used to exchange security enhancement + capabilities between the client and server. When a new session + begins, the record layer's connection state encryption, hash, and + compression algorithms are initialized to null. The current + connection state is used for renegotiation messages. + +7.4.1.1. Hello Request + + When this message will be sent: + + The HelloRequest message MAY be sent by the server at any time. + + Meaning of this message: + + HelloRequest is a simple notification that the client should begin + the negotiation process anew. In response, the client should send + a ClientHello message when convenient. This message is not + intended to establish which side is the client or server but + merely to initiate a new negotiation. Servers SHOULD NOT send a + HelloRequest immediately upon the client's initial connection. It + is the client's job to send a ClientHello at that time. + + This message will be ignored by the client if the client is + currently negotiating a session. This message MAY be ignored by + the client if it does not wish to renegotiate a session, or the + client may, if it wishes, respond with a no_renegotiation alert. + Since handshake messages are intended to have transmission + precedence over application data, it is expected that the + negotiation will begin before no more than a few records are + received from the client. If the server sends a HelloRequest but + does not receive a ClientHello in response, it may close the + connection with a fatal alert. + + + +Dierks & Rescorla Standards Track [Page 38] + +RFC 5246 TLS August 2008 + + + After sending a HelloRequest, servers SHOULD NOT repeat the + request until the subsequent handshake negotiation is complete. + + Structure of this message: + + struct { } HelloRequest; + + This message MUST NOT be included in the message hashes that are + maintained throughout the handshake and used in the Finished messages + and the certificate verify message. + +7.4.1.2. Client Hello + + When this message will be sent: + + When a client first connects to a server, it is required to send + the ClientHello as its first message. The client can also send a + ClientHello in response to a HelloRequest or on its own initiative + in order to renegotiate the security parameters in an existing + connection. + + Structure of this message: + + The ClientHello message includes a random structure, which is used + later in the protocol. + + struct { + uint32 gmt_unix_time; + opaque random_bytes[28]; + } Random; + + gmt_unix_time + The current time and date in standard UNIX 32-bit format + (seconds since the midnight starting Jan 1, 1970, UTC, ignoring + leap seconds) according to the sender's internal clock. Clocks + are not required to be set correctly by the basic TLS protocol; + higher-level or application protocols may define additional + requirements. Note that, for historical reasons, the data + element is named using GMT, the predecessor of the current + worldwide time base, UTC. + + random_bytes + 28 bytes generated by a secure random number generator. + + The ClientHello message includes a variable-length session + identifier. If not empty, the value identifies a session between the + same client and server whose security parameters the client wishes to + reuse. The session identifier MAY be from an earlier connection, + + + +Dierks & Rescorla Standards Track [Page 39] + +RFC 5246 TLS August 2008 + + + this connection, or from another currently active connection. The + second option is useful if the client only wishes to update the + random structures and derived values of a connection, and the third + option makes it possible to establish several independent secure + connections without repeating the full handshake protocol. These + independent connections may occur sequentially or simultaneously; a + SessionID becomes valid when the handshake negotiating it completes + with the exchange of Finished messages and persists until it is + removed due to aging or because a fatal error was encountered on a + connection associated with the session. The actual contents of the + SessionID are defined by the server. + + opaque SessionID<0..32>; + + Warning: Because the SessionID is transmitted without encryption or + immediate MAC protection, servers MUST NOT place confidential + information in session identifiers or let the contents of fake + session identifiers cause any breach of security. (Note that the + content of the handshake as a whole, including the SessionID, is + protected by the Finished messages exchanged at the end of the + handshake.) + + The cipher suite list, passed from the client to the server in the + ClientHello message, contains the combinations of cryptographic + algorithms supported by the client in order of the client's + preference (favorite choice first). Each cipher suite defines a key + exchange algorithm, a bulk encryption algorithm (including secret key + length), a MAC algorithm, and a PRF. The server will select a cipher + suite or, if no acceptable choices are presented, return a handshake + failure alert and close the connection. If the list contains cipher + suites the server does not recognize, support, or wish to use, the + server MUST ignore those cipher suites, and process the remaining + ones as usual. + + uint8 CipherSuite[2]; /* Cryptographic suite selector */ + + The ClientHello includes a list of compression algorithms supported + by the client, ordered according to the client's preference. + + enum { null(0), (255) } CompressionMethod; + + + + + + + + + + + +Dierks & Rescorla Standards Track [Page 40] + +RFC 5246 TLS August 2008 + + + struct { + ProtocolVersion client_version; + Random random; + SessionID session_id; + CipherSuite cipher_suites<2..2^16-2>; + CompressionMethod compression_methods<1..2^8-1>; + select (extensions_present) { + case false: + struct {}; + case true: + Extension extensions<0..2^16-1>; + }; + } ClientHello; + + TLS allows extensions to follow the compression_methods field in an + extensions block. The presence of extensions can be detected by + determining whether there are bytes following the compression_methods + at the end of the ClientHello. Note that this method of detecting + optional data differs from the normal TLS method of having a + variable-length field, but it is used for compatibility with TLS + before extensions were defined. + + client_version + The version of the TLS protocol by which the client wishes to + communicate during this session. This SHOULD be the latest + (highest valued) version supported by the client. For this + version of the specification, the version will be 3.3 (see + Appendix E for details about backward compatibility). + + random + A client-generated random structure. + + session_id + The ID of a session the client wishes to use for this connection. + This field is empty if no session_id is available, or if the + client wishes to generate new security parameters. + + cipher_suites + This is a list of the cryptographic options supported by the + client, with the client's first preference first. If the + session_id field is not empty (implying a session resumption + request), this vector MUST include at least the cipher_suite from + that session. Values are defined in Appendix A.5. + + compression_methods + This is a list of the compression methods supported by the client, + sorted by client preference. If the session_id field is not empty + (implying a session resumption request), it MUST include the + + + +Dierks & Rescorla Standards Track [Page 41] + +RFC 5246 TLS August 2008 + + + compression_method from that session. This vector MUST contain, + and all implementations MUST support, CompressionMethod.null. + Thus, a client and server will always be able to agree on a + compression method. + + extensions + Clients MAY request extended functionality from servers by sending + data in the extensions field. The actual "Extension" format is + defined in Section 7.4.1.4. + + In the event that a client requests additional functionality using + extensions, and this functionality is not supplied by the server, the + client MAY abort the handshake. A server MUST accept ClientHello + messages both with and without the extensions field, and (as for all + other messages) it MUST check that the amount of data in the message + precisely matches one of these formats; if not, then it MUST send a + fatal "decode_error" alert. + + After sending the ClientHello message, the client waits for a + ServerHello message. Any handshake message returned by the server, + except for a HelloRequest, is treated as a fatal error. + +7.4.1.3. Server Hello + + When this message will be sent: + + The server will send this message in response to a ClientHello + message when it was able to find an acceptable set of algorithms. + If it cannot find such a match, it will respond with a handshake + failure alert. + + Structure of this message: + + struct { + ProtocolVersion server_version; + Random random; + SessionID session_id; + CipherSuite cipher_suite; + CompressionMethod compression_method; + select (extensions_present) { + case false: + struct {}; + case true: + Extension extensions<0..2^16-1>; + }; + } ServerHello; + + + + + +Dierks & Rescorla Standards Track [Page 42] + +RFC 5246 TLS August 2008 + + + The presence of extensions can be detected by determining whether + there are bytes following the compression_method field at the end of + the ServerHello. + + server_version + This field will contain the lower of that suggested by the client + in the client hello and the highest supported by the server. For + this version of the specification, the version is 3.3. (See + Appendix E for details about backward compatibility.) + + random + This structure is generated by the server and MUST be + independently generated from the ClientHello.random. + + session_id + This is the identity of the session corresponding to this + connection. If the ClientHello.session_id was non-empty, the + server will look in its session cache for a match. If a match is + found and the server is willing to establish the new connection + using the specified session state, the server will respond with + the same value as was supplied by the client. This indicates a + resumed session and dictates that the parties must proceed + directly to the Finished messages. Otherwise, this field will + contain a different value identifying the new session. The server + may return an empty session_id to indicate that the session will + not be cached and therefore cannot be resumed. If a session is + resumed, it must be resumed using the same cipher suite it was + originally negotiated with. Note that there is no requirement + that the server resume any session even if it had formerly + provided a session_id. Clients MUST be prepared to do a full + negotiation -- including negotiating new cipher suites -- during + any handshake. + + cipher_suite + The single cipher suite selected by the server from the list in + ClientHello.cipher_suites. For resumed sessions, this field is + the value from the state of the session being resumed. + + compression_method + The single compression algorithm selected by the server from the + list in ClientHello.compression_methods. For resumed sessions, + this field is the value from the resumed session state. + + extensions + A list of extensions. Note that only extensions offered by the + client can appear in the server's list. + + + + + +Dierks & Rescorla Standards Track [Page 43] + +RFC 5246 TLS August 2008 + + +7.4.1.4. Hello Extensions + + The extension format is: + + struct { + ExtensionType extension_type; + opaque extension_data<0..2^16-1>; + } Extension; + + enum { + signature_algorithms(13), (65535) + } ExtensionType; + + Here: + + - "extension_type" identifies the particular extension type. + + - "extension_data" contains information specific to the particular + extension type. + + The initial set of extensions is defined in a companion document + [TLSEXT]. The list of extension types is maintained by IANA as + described in Section 12. + + An extension type MUST NOT appear in the ServerHello unless the same + extension type appeared in the corresponding ClientHello. If a + client receives an extension type in ServerHello that it did not + request in the associated ClientHello, it MUST abort the handshake + with an unsupported_extension fatal alert. + + Nonetheless, "server-oriented" extensions may be provided in the + future within this framework. Such an extension (say, of type x) + would require the client to first send an extension of type x in a + ClientHello with empty extension_data to indicate that it supports + the extension type. In this case, the client is offering the + capability to understand the extension type, and the server is taking + the client up on its offer. + + When multiple extensions of different types are present in the + ClientHello or ServerHello messages, the extensions MAY appear in any + order. There MUST NOT be more than one extension of the same type. + + Finally, note that extensions can be sent both when starting a new + session and when requesting session resumption. Indeed, a client + that requests session resumption does not in general know whether the + server will accept this request, and therefore it SHOULD send the + same extensions as it would send if it were not attempting + resumption. + + + +Dierks & Rescorla Standards Track [Page 44] + +RFC 5246 TLS August 2008 + + + In general, the specification of each extension type needs to + describe the effect of the extension both during full handshake and + session resumption. Most current TLS extensions are relevant only + when a session is initiated: when an older session is resumed, the + server does not process these extensions in Client Hello, and does + not include them in Server Hello. However, some extensions may + specify different behavior during session resumption. + + There are subtle (and not so subtle) interactions that may occur in + this protocol between new features and existing features which may + result in a significant reduction in overall security. The following + considerations should be taken into account when designing new + extensions: + + - Some cases where a server does not agree to an extension are error + conditions, and some are simply refusals to support particular + features. In general, error alerts should be used for the former, + and a field in the server extension response for the latter. + + - Extensions should, as far as possible, be designed to prevent any + attack that forces use (or non-use) of a particular feature by + manipulation of handshake messages. This principle should be + followed regardless of whether the feature is believed to cause a + security problem. + + Often the fact that the extension fields are included in the + inputs to the Finished message hashes will be sufficient, but + extreme care is needed when the extension changes the meaning of + messages sent in the handshake phase. Designers and implementors + should be aware of the fact that until the handshake has been + authenticated, active attackers can modify messages and insert, + remove, or replace extensions. + + - It would be technically possible to use extensions to change major + aspects of the design of TLS; for example the design of cipher + suite negotiation. This is not recommended; it would be more + appropriate to define a new version of TLS -- particularly since + the TLS handshake algorithms have specific protection against + version rollback attacks based on the version number, and the + possibility of version rollback should be a significant + consideration in any major design change. + +7.4.1.4.1. Signature Algorithms + + The client uses the "signature_algorithms" extension to indicate to + the server which signature/hash algorithm pairs may be used in + digital signatures. The "extension_data" field of this extension + contains a "supported_signature_algorithms" value. + + + +Dierks & Rescorla Standards Track [Page 45] + +RFC 5246 TLS August 2008 + + + enum { + none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5), + sha512(6), (255) + } HashAlgorithm; + + enum { anonymous(0), rsa(1), dsa(2), ecdsa(3), (255) } + SignatureAlgorithm; + + struct { + HashAlgorithm hash; + SignatureAlgorithm signature; + } SignatureAndHashAlgorithm; + + SignatureAndHashAlgorithm + supported_signature_algorithms<2..2^16-2>; + + Each SignatureAndHashAlgorithm value lists a single hash/signature + pair that the client is willing to verify. The values are indicated + in descending order of preference. + + Note: Because not all signature algorithms and hash algorithms may be + accepted by an implementation (e.g., DSA with SHA-1, but not + SHA-256), algorithms here are listed in pairs. + + hash + This field indicates the hash algorithm which may be used. The + values indicate support for unhashed data, MD5 [MD5], SHA-1, + SHA-224, SHA-256, SHA-384, and SHA-512 [SHS], respectively. The + "none" value is provided for future extensibility, in case of a + signature algorithm which does not require hashing before signing. + + signature + This field indicates the signature algorithm that may be used. + The values indicate anonymous signatures, RSASSA-PKCS1-v1_5 + [PKCS1] and DSA [DSS], and ECDSA [ECDSA], respectively. The + "anonymous" value is meaningless in this context but used in + Section 7.4.3. It MUST NOT appear in this extension. + + The semantics of this extension are somewhat complicated because the + cipher suite indicates permissible signature algorithms but not hash + algorithms. Sections 7.4.2 and 7.4.3 describe the appropriate rules. + + If the client supports only the default hash and signature algorithms + (listed in this section), it MAY omit the signature_algorithms + extension. If the client does not support the default algorithms, or + supports other hash and signature algorithms (and it is willing to + use them for verifying messages sent by the server, i.e., server + certificates and server key exchange), it MUST send the + + + +Dierks & Rescorla Standards Track [Page 46] + +RFC 5246 TLS August 2008 + + + signature_algorithms extension, listing the algorithms it is willing + to accept. + + If the client does not send the signature_algorithms extension, the + server MUST do the following: + + - If the negotiated key exchange algorithm is one of (RSA, DHE_RSA, + DH_RSA, RSA_PSK, ECDH_RSA, ECDHE_RSA), behave as if client had + sent the value {sha1,rsa}. + + - If the negotiated key exchange algorithm is one of (DHE_DSS, + DH_DSS), behave as if the client had sent the value {sha1,dsa}. + + - If the negotiated key exchange algorithm is one of (ECDH_ECDSA, + ECDHE_ECDSA), behave as if the client had sent value {sha1,ecdsa}. + + Note: this is a change from TLS 1.1 where there are no explicit + rules, but as a practical matter one can assume that the peer + supports MD5 and SHA-1. + + Note: this extension is not meaningful for TLS versions prior to 1.2. + Clients MUST NOT offer it if they are offering prior versions. + However, even if clients do offer it, the rules specified in [TLSEXT] + require servers to ignore extensions they do not understand. + + Servers MUST NOT send this extension. TLS servers MUST support + receiving this extension. + + When performing session resumption, this extension is not included in + Server Hello, and the server ignores the extension in Client Hello + (if present). + +7.4.2. Server Certificate + + When this message will be sent: + + The server MUST send a Certificate message whenever the agreed- + upon key exchange method uses certificates for authentication + (this includes all key exchange methods defined in this document + except DH_anon). This message will always immediately follow the + ServerHello message. + + Meaning of this message: + + This message conveys the server's certificate chain to the client. + + The certificate MUST be appropriate for the negotiated cipher + suite's key exchange algorithm and any negotiated extensions. + + + +Dierks & Rescorla Standards Track [Page 47] + +RFC 5246 TLS August 2008 + + + Structure of this message: + + opaque ASN.1Cert<1..2^24-1>; + + struct { + ASN.1Cert certificate_list<0..2^24-1>; + } Certificate; + + certificate_list + This is a sequence (chain) of certificates. The sender's + certificate MUST come first in the list. Each following + certificate MUST directly certify the one preceding it. Because + certificate validation requires that root keys be distributed + independently, the self-signed certificate that specifies the root + certificate authority MAY be omitted from the chain, under the + assumption that the remote end must already possess it in order to + validate it in any case. + + The same message type and structure will be used for the client's + response to a certificate request message. Note that a client MAY + send no certificates if it does not have an appropriate certificate + to send in response to the server's authentication request. + + Note: PKCS #7 [PKCS7] is not used as the format for the certificate + vector because PKCS #6 [PKCS6] extended certificates are not used. + Also, PKCS #7 defines a SET rather than a SEQUENCE, making the task + of parsing the list more difficult. + + The following rules apply to the certificates sent by the server: + + - The certificate type MUST be X.509v3, unless explicitly negotiated + otherwise (e.g., [TLSPGP]). + + - The end entity certificate's public key (and associated + restrictions) MUST be compatible with the selected key exchange + algorithm. + + Key Exchange Alg. Certificate Key Type + + RSA RSA public key; the certificate MUST allow the + RSA_PSK key to be used for encryption (the + keyEncipherment bit MUST be set if the key + usage extension is present). + Note: RSA_PSK is defined in [TLSPSK]. + + + + + + + +Dierks & Rescorla Standards Track [Page 48] + +RFC 5246 TLS August 2008 + + + DHE_RSA RSA public key; the certificate MUST allow the + ECDHE_RSA key to be used for signing (the + digitalSignature bit MUST be set if the key + usage extension is present) with the signature + scheme and hash algorithm that will be employed + in the server key exchange message. + Note: ECDHE_RSA is defined in [TLSECC]. + + DHE_DSS DSA public key; the certificate MUST allow the + key to be used for signing with the hash + algorithm that will be employed in the server + key exchange message. + + DH_DSS Diffie-Hellman public key; the keyAgreement bit + DH_RSA MUST be set if the key usage extension is + present. + + ECDH_ECDSA ECDH-capable public key; the public key MUST + ECDH_RSA use a curve and point format supported by the + client, as described in [TLSECC]. + + ECDHE_ECDSA ECDSA-capable public key; the certificate MUST + allow the key to be used for signing with the + hash algorithm that will be employed in the + server key exchange message. The public key + MUST use a curve and point format supported by + the client, as described in [TLSECC]. + + - The "server_name" and "trusted_ca_keys" extensions [TLSEXT] are + used to guide certificate selection. + + If the client provided a "signature_algorithms" extension, then all + certificates provided by the server MUST be signed by a + hash/signature algorithm pair that appears in that extension. Note + that this implies that a certificate containing a key for one + signature algorithm MAY be signed using a different signature + algorithm (for instance, an RSA key signed with a DSA key). This is + a departure from TLS 1.1, which required that the algorithms be the + same. Note that this also implies that the DH_DSS, DH_RSA, + ECDH_ECDSA, and ECDH_RSA key exchange algorithms do not restrict the + algorithm used to sign the certificate. Fixed DH certificates MAY be + signed with any hash/signature algorithm pair appearing in the + extension. The names DH_DSS, DH_RSA, ECDH_ECDSA, and ECDH_RSA are + historical. + + + + + + + +Dierks & Rescorla Standards Track [Page 49] + +RFC 5246 TLS August 2008 + + + If the server has multiple certificates, it chooses one of them based + on the above-mentioned criteria (in addition to other criteria, such + as transport layer endpoint, local configuration and preferences, + etc.). If the server has a single certificate, it SHOULD attempt to + validate that it meets these criteria. + + Note that there are certificates that use algorithms and/or algorithm + combinations that cannot be currently used with TLS. For example, a + certificate with RSASSA-PSS signature key (id-RSASSA-PSS OID in + SubjectPublicKeyInfo) cannot be used because TLS defines no + corresponding signature algorithm. + + As cipher suites that specify new key exchange methods are specified + for the TLS protocol, they will imply the certificate format and the + required encoded keying information. + +7.4.3. Server Key Exchange Message + + When this message will be sent: + + This message will be sent immediately after the server Certificate + message (or the ServerHello message, if this is an anonymous + negotiation). + + The ServerKeyExchange message is sent by the server only when the + server Certificate message (if sent) does not contain enough data + to allow the client to exchange a premaster secret. This is true + for the following key exchange methods: + + DHE_DSS + DHE_RSA + DH_anon + + It is not legal to send the ServerKeyExchange message for the + following key exchange methods: + + RSA + DH_DSS + DH_RSA + + Other key exchange algorithms, such as those defined in [TLSECC], + MUST specify whether the ServerKeyExchange message is sent or not; + and if the message is sent, its contents. + + + + + + + + +Dierks & Rescorla Standards Track [Page 50] + +RFC 5246 TLS August 2008 + + + Meaning of this message: + + This message conveys cryptographic information to allow the client + to communicate the premaster secret: a Diffie-Hellman public key + with which the client can complete a key exchange (with the result + being the premaster secret) or a public key for some other + algorithm. + + Structure of this message: + + enum { dhe_dss, dhe_rsa, dh_anon, rsa, dh_dss, dh_rsa + /* may be extended, e.g., for ECDH -- see [TLSECC] */ + } KeyExchangeAlgorithm; + + struct { + opaque dh_p<1..2^16-1>; + opaque dh_g<1..2^16-1>; + opaque dh_Ys<1..2^16-1>; + } ServerDHParams; /* Ephemeral DH parameters */ + + dh_p + The prime modulus used for the Diffie-Hellman operation. + + dh_g + The generator used for the Diffie-Hellman operation. + + dh_Ys + The server's Diffie-Hellman public value (g^X mod p). + + + + + + + + + + + + + + + + + + + + + + + +Dierks & Rescorla Standards Track [Page 51] + +RFC 5246 TLS August 2008 + + + struct { + select (KeyExchangeAlgorithm) { + case dh_anon: + ServerDHParams params; + case dhe_dss: + case dhe_rsa: + ServerDHParams params; + digitally-signed struct { + opaque client_random[32]; + opaque server_random[32]; + ServerDHParams params; + } signed_params; + case rsa: + case dh_dss: + case dh_rsa: + struct {} ; + /* message is omitted for rsa, dh_dss, and dh_rsa */ + /* may be extended, e.g., for ECDH -- see [TLSECC] */ + }; + } ServerKeyExchange; + + params + The server's key exchange parameters. + + signed_params + For non-anonymous key exchanges, a signature over the server's + key exchange parameters. + + If the client has offered the "signature_algorithms" extension, the + signature algorithm and hash algorithm MUST be a pair listed in that + extension. Note that there is a possibility for inconsistencies + here. For instance, the client might offer DHE_DSS key exchange but + omit any DSA pairs from its "signature_algorithms" extension. In + order to negotiate correctly, the server MUST check any candidate + cipher suites against the "signature_algorithms" extension before + selecting them. This is somewhat inelegant but is a compromise + designed to minimize changes to the original cipher suite design. + + In addition, the hash and signature algorithms MUST be compatible + with the key in the server's end-entity certificate. RSA keys MAY be + used with any permitted hash algorithm, subject to restrictions in + the certificate, if any. + + Because DSA signatures do not contain any secure indication of hash + algorithm, there is a risk of hash substitution if multiple hashes + may be used with any key. Currently, DSA [DSS] may only be used with + SHA-1. Future revisions of DSS [DSS-3] are expected to allow the use + of other digest algorithms with DSA, as well as guidance as to which + + + +Dierks & Rescorla Standards Track [Page 52] + +RFC 5246 TLS August 2008 + + + digest algorithms should be used with each key size. In addition, + future revisions of [PKIX] may specify mechanisms for certificates to + indicate which digest algorithms are to be used with DSA. + + As additional cipher suites are defined for TLS that include new key + exchange algorithms, the server key exchange message will be sent if + and only if the certificate type associated with the key exchange + algorithm does not provide enough information for the client to + exchange a premaster secret. + +7.4.4. Certificate Request + + When this message will be sent: + + A non-anonymous server can optionally request a certificate from + the client, if appropriate for the selected cipher suite. This + message, if sent, will immediately follow the ServerKeyExchange + message (if it is sent; otherwise, this message follows the + server's Certificate message). + + Structure of this message: + + enum { + rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4), + rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6), + fortezza_dms_RESERVED(20), (255) + } ClientCertificateType; + + opaque DistinguishedName<1..2^16-1>; + + struct { + ClientCertificateType certificate_types<1..2^8-1>; + SignatureAndHashAlgorithm + supported_signature_algorithms<2^16-1>; + DistinguishedName certificate_authorities<0..2^16-1>; + } CertificateRequest; + + certificate_types + A list of the types of certificate types that the client may + offer. + + rsa_sign a certificate containing an RSA key + dss_sign a certificate containing a DSA key + rsa_fixed_dh a certificate containing a static DH key. + dss_fixed_dh a certificate containing a static DH key + + + + + + +Dierks & Rescorla Standards Track [Page 53] + +RFC 5246 TLS August 2008 + + + supported_signature_algorithms + A list of the hash/signature algorithm pairs that the server is + able to verify, listed in descending order of preference. + + certificate_authorities + A list of the distinguished names [X501] of acceptable + certificate_authorities, represented in DER-encoded format. These + distinguished names may specify a desired distinguished name for a + root CA or for a subordinate CA; thus, this message can be used to + describe known roots as well as a desired authorization space. If + the certificate_authorities list is empty, then the client MAY + send any certificate of the appropriate ClientCertificateType, + unless there is some external arrangement to the contrary. + + The interaction of the certificate_types and + supported_signature_algorithms fields is somewhat complicated. + certificate_types has been present in TLS since SSLv3, but was + somewhat underspecified. Much of its functionality is superseded by + supported_signature_algorithms. The following rules apply: + + - Any certificates provided by the client MUST be signed using a + hash/signature algorithm pair found in + supported_signature_algorithms. + + - The end-entity certificate provided by the client MUST contain a + key that is compatible with certificate_types. If the key is a + signature key, it MUST be usable with some hash/signature + algorithm pair in supported_signature_algorithms. + + - For historical reasons, the names of some client certificate types + include the algorithm used to sign the certificate. For example, + in earlier versions of TLS, rsa_fixed_dh meant a certificate + signed with RSA and containing a static DH key. In TLS 1.2, this + functionality has been obsoleted by the + supported_signature_algorithms, and the certificate type no longer + restricts the algorithm used to sign the certificate. For + example, if the server sends dss_fixed_dh certificate type and + {{sha1, dsa}, {sha1, rsa}} signature types, the client MAY reply + with a certificate containing a static DH key, signed with RSA- + SHA1. + + New ClientCertificateType values are assigned by IANA as described in + Section 12. + + Note: Values listed as RESERVED may not be used. They were used in + SSLv3. + + + + + +Dierks & Rescorla Standards Track [Page 54] + +RFC 5246 TLS August 2008 + + + Note: It is a fatal handshake_failure alert for an anonymous server + to request client authentication. + +7.4.5. Server Hello Done + + When this message will be sent: + + The ServerHelloDone message is sent by the server to indicate the + end of the ServerHello and associated messages. After sending + this message, the server will wait for a client response. + + Meaning of this message: + + This message means that the server is done sending messages to + support the key exchange, and the client can proceed with its + phase of the key exchange. + + Upon receipt of the ServerHelloDone message, the client SHOULD + verify that the server provided a valid certificate, if required, + and check that the server hello parameters are acceptable. + + Structure of this message: + + struct { } ServerHelloDone; + +7.4.6. Client Certificate + + When this message will be sent: + + This is the first message the client can send after receiving a + ServerHelloDone message. This message is only sent if the server + requests a certificate. If no suitable certificate is available, + the client MUST send a certificate message containing no + certificates. That is, the certificate_list structure has a + length of zero. If the client does not send any certificates, the + server MAY at its discretion either continue the handshake without + client authentication, or respond with a fatal handshake_failure + alert. Also, if some aspect of the certificate chain was + unacceptable (e.g., it was not signed by a known, trusted CA), the + server MAY at its discretion either continue the handshake + (considering the client unauthenticated) or send a fatal alert. + + Client certificates are sent using the Certificate structure + defined in Section 7.4.2. + + + + + + + +Dierks & Rescorla Standards Track [Page 55] + +RFC 5246 TLS August 2008 + + + Meaning of this message: + + This message conveys the client's certificate chain to the server; + the server will use it when verifying the CertificateVerify + message (when the client authentication is based on signing) or + calculating the premaster secret (for non-ephemeral Diffie- + Hellman). The certificate MUST be appropriate for the negotiated + cipher suite's key exchange algorithm, and any negotiated + extensions. + + In particular: + + - The certificate type MUST be X.509v3, unless explicitly negotiated + otherwise (e.g., [TLSPGP]). + + - The end-entity certificate's public key (and associated + restrictions) has to be compatible with the certificate types + listed in CertificateRequest: + + Client Cert. Type Certificate Key Type + + rsa_sign RSA public key; the certificate MUST allow the + key to be used for signing with the signature + scheme and hash algorithm that will be + employed in the certificate verify message. + + dss_sign DSA public key; the certificate MUST allow the + key to be used for signing with the hash + algorithm that will be employed in the + certificate verify message. + + ecdsa_sign ECDSA-capable public key; the certificate MUST + allow the key to be used for signing with the + hash algorithm that will be employed in the + certificate verify message; the public key + MUST use a curve and point format supported by + the server. + + rsa_fixed_dh Diffie-Hellman public key; MUST use the same + dss_fixed_dh parameters as server's key. + + rsa_fixed_ecdh ECDH-capable public key; MUST use the + ecdsa_fixed_ecdh same curve as the server's key, and MUST use a + point format supported by the server. + + - If the certificate_authorities list in the certificate request + message was non-empty, one of the certificates in the certificate + chain SHOULD be issued by one of the listed CAs. + + + +Dierks & Rescorla Standards Track [Page 56] + +RFC 5246 TLS August 2008 + + + - The certificates MUST be signed using an acceptable hash/ + signature algorithm pair, as described in Section 7.4.4. Note + that this relaxes the constraints on certificate-signing + algorithms found in prior versions of TLS. + + Note that, as with the server certificate, there are certificates + that use algorithms/algorithm combinations that cannot be currently + used with TLS. + +7.4.7. Client Key Exchange Message + + When this message will be sent: + + This message is always sent by the client. It MUST immediately + follow the client certificate message, if it is sent. Otherwise, + it MUST be the first message sent by the client after it receives + the ServerHelloDone message. + + Meaning of this message: + + With this message, the premaster secret is set, either by direct + transmission of the RSA-encrypted secret or by the transmission of + Diffie-Hellman parameters that will allow each side to agree upon + the same premaster secret. + + When the client is using an ephemeral Diffie-Hellman exponent, + then this message contains the client's Diffie-Hellman public + value. If the client is sending a certificate containing a static + DH exponent (i.e., it is doing fixed_dh client authentication), + then this message MUST be sent but MUST be empty. + + Structure of this message: + + The choice of messages depends on which key exchange method has + been selected. See Section 7.4.3 for the KeyExchangeAlgorithm + definition. + + + + + + + + + + + + + + + +Dierks & Rescorla Standards Track [Page 57] + +RFC 5246 TLS August 2008 + + + struct { + select (KeyExchangeAlgorithm) { + case rsa: + EncryptedPreMasterSecret; + case dhe_dss: + case dhe_rsa: + case dh_dss: + case dh_rsa: + case dh_anon: + ClientDiffieHellmanPublic; + } exchange_keys; + } ClientKeyExchange; + +7.4.7.1. RSA-Encrypted Premaster Secret Message + + Meaning of this message: + + If RSA is being used for key agreement and authentication, the + client generates a 48-byte premaster secret, encrypts it using the + public key from the server's certificate, and sends the result in + an encrypted premaster secret message. This structure is a + variant of the ClientKeyExchange message and is not a message in + itself. + + Structure of this message: + + struct { + ProtocolVersion client_version; + opaque random[46]; + } PreMasterSecret; + + client_version + The latest (newest) version supported by the client. This is + used to detect version rollback attacks. + + random + 46 securely-generated random bytes. + + struct { + public-key-encrypted PreMasterSecret pre_master_secret; + } EncryptedPreMasterSecret; + + pre_master_secret + This random value is generated by the client and is used to + generate the master secret, as specified in Section 8.1. + + + + + + +Dierks & Rescorla Standards Track [Page 58] + +RFC 5246 TLS August 2008 + + + Note: The version number in the PreMasterSecret is the version + offered by the client in the ClientHello.client_version, not the + version negotiated for the connection. This feature is designed to + prevent rollback attacks. Unfortunately, some old implementations + use the negotiated version instead, and therefore checking the + version number may lead to failure to interoperate with such + incorrect client implementations. + + Client implementations MUST always send the correct version number in + PreMasterSecret. If ClientHello.client_version is TLS 1.1 or higher, + server implementations MUST check the version number as described in + the note below. If the version number is TLS 1.0 or earlier, server + implementations SHOULD check the version number, but MAY have a + configuration option to disable the check. Note that if the check + fails, the PreMasterSecret SHOULD be randomized as described below. + + Note: Attacks discovered by Bleichenbacher [BLEI] and Klima et al. + [KPR03] can be used to attack a TLS server that reveals whether a + particular message, when decrypted, is properly PKCS#1 formatted, + contains a valid PreMasterSecret structure, or has the correct + version number. + + As described by Klima [KPR03], these vulnerabilities can be avoided + by treating incorrectly formatted message blocks and/or mismatched + version numbers in a manner indistinguishable from correctly + formatted RSA blocks. In other words: + + 1. Generate a string R of 46 random bytes + + 2. Decrypt the message to recover the plaintext M + + 3. If the PKCS#1 padding is not correct, or the length of message + M is not exactly 48 bytes: + pre_master_secret = ClientHello.client_version || R + else If ClientHello.client_version <= TLS 1.0, and version + number check is explicitly disabled: + pre_master_secret = M + else: + pre_master_secret = ClientHello.client_version || M[2..47] + + Note that explicitly constructing the pre_master_secret with the + ClientHello.client_version produces an invalid master_secret if the + client has sent the wrong version in the original pre_master_secret. + + An alternative approach is to treat a version number mismatch as a + PKCS-1 formatting error and randomize the premaster secret + completely: + + + + +Dierks & Rescorla Standards Track [Page 59] + +RFC 5246 TLS August 2008 + + + 1. Generate a string R of 48 random bytes + + 2. Decrypt the message to recover the plaintext M + + 3. If the PKCS#1 padding is not correct, or the length of message + M is not exactly 48 bytes: + pre_master_secret = R + else If ClientHello.client_version <= TLS 1.0, and version + number check is explicitly disabled: + premaster secret = M + else If M[0..1] != ClientHello.client_version: + premaster secret = R + else: + premaster secret = M + + Although no practical attacks against this construction are known, + Klima et al. [KPR03] describe some theoretical attacks, and therefore + the first construction described is RECOMMENDED. + + In any case, a TLS server MUST NOT generate an alert if processing an + RSA-encrypted premaster secret message fails, or the version number + is not as expected. Instead, it MUST continue the handshake with a + randomly generated premaster secret. It may be useful to log the + real cause of failure for troubleshooting purposes; however, care + must be taken to avoid leaking the information to an attacker + (through, e.g., timing, log files, or other channels.) + + The RSAES-OAEP encryption scheme defined in [PKCS1] is more secure + against the Bleichenbacher attack. However, for maximal + compatibility with earlier versions of TLS, this specification uses + the RSAES-PKCS1-v1_5 scheme. No variants of the Bleichenbacher + attack are known to exist provided that the above recommendations are + followed. + + Implementation note: Public-key-encrypted data is represented as an + opaque vector <0..2^16-1> (see Section 4.7). Thus, the RSA-encrypted + PreMasterSecret in a ClientKeyExchange is preceded by two length + bytes. These bytes are redundant in the case of RSA because the + EncryptedPreMasterSecret is the only data in the ClientKeyExchange + and its length can therefore be unambiguously determined. The SSLv3 + specification was not clear about the encoding of public-key- + encrypted data, and therefore many SSLv3 implementations do not + include the length bytes -- they encode the RSA-encrypted data + directly in the ClientKeyExchange message. + + This specification requires correct encoding of the + EncryptedPreMasterSecret complete with length bytes. The resulting + PDU is incompatible with many SSLv3 implementations. Implementors + + + +Dierks & Rescorla Standards Track [Page 60] + +RFC 5246 TLS August 2008 + + + upgrading from SSLv3 MUST modify their implementations to generate + and accept the correct encoding. Implementors who wish to be + compatible with both SSLv3 and TLS should make their implementation's + behavior dependent on the protocol version. + + Implementation note: It is now known that remote timing-based attacks + on TLS are possible, at least when the client and server are on the + same LAN. Accordingly, implementations that use static RSA keys MUST + use RSA blinding or some other anti-timing technique, as described in + [TIMING]. + +7.4.7.2. Client Diffie-Hellman Public Value + + Meaning of this message: + + This structure conveys the client's Diffie-Hellman public value + (Yc) if it was not already included in the client's certificate. + The encoding used for Yc is determined by the enumerated + PublicValueEncoding. This structure is a variant of the client + key exchange message, and not a message in itself. + + Structure of this message: + + enum { implicit, explicit } PublicValueEncoding; + + implicit + If the client has sent a certificate which contains a suitable + Diffie-Hellman key (for fixed_dh client authentication), then + Yc is implicit and does not need to be sent again. In this + case, the client key exchange message will be sent, but it MUST + be empty. + + explicit + Yc needs to be sent. + + struct { + select (PublicValueEncoding) { + case implicit: struct { }; + case explicit: opaque dh_Yc<1..2^16-1>; + } dh_public; + } ClientDiffieHellmanPublic; + + dh_Yc + The client's Diffie-Hellman public value (Yc). + + + + + + + +Dierks & Rescorla Standards Track [Page 61] + +RFC 5246 TLS August 2008 + + +7.4.8. Certificate Verify + + When this message will be sent: + + This message is used to provide explicit verification of a client + certificate. This message is only sent following a client + certificate that has signing capability (i.e., all certificates + except those containing fixed Diffie-Hellman parameters). When + sent, it MUST immediately follow the client key exchange message. + + Structure of this message: + + struct { + digitally-signed struct { + opaque handshake_messages[handshake_messages_length]; + } + } CertificateVerify; + + Here handshake_messages refers to all handshake messages sent or + received, starting at client hello and up to, but not including, + this message, including the type and length fields of the + handshake messages. This is the concatenation of all the + Handshake structures (as defined in Section 7.4) exchanged thus + far. Note that this requires both sides to either buffer the + messages or compute running hashes for all potential hash + algorithms up to the time of the CertificateVerify computation. + Servers can minimize this computation cost by offering a + restricted set of digest algorithms in the CertificateRequest + message. + + The hash and signature algorithms used in the signature MUST be + one of those present in the supported_signature_algorithms field + of the CertificateRequest message. In addition, the hash and + signature algorithms MUST be compatible with the key in the + client's end-entity certificate. RSA keys MAY be used with any + permitted hash algorithm, subject to restrictions in the + certificate, if any. + + Because DSA signatures do not contain any secure indication of + hash algorithm, there is a risk of hash substitution if multiple + hashes may be used with any key. Currently, DSA [DSS] may only be + used with SHA-1. Future revisions of DSS [DSS-3] are expected to + allow the use of other digest algorithms with DSA, as well as + guidance as to which digest algorithms should be used with each + key size. In addition, future revisions of [PKIX] may specify + mechanisms for certificates to indicate which digest algorithms + are to be used with DSA. + + + + +Dierks & Rescorla Standards Track [Page 62] + +RFC 5246 TLS August 2008 + + +7.4.9. Finished + + When this message will be sent: + + A Finished message is always sent immediately after a change + cipher spec message to verify that the key exchange and + authentication processes were successful. It is essential that a + change cipher spec message be received between the other handshake + messages and the Finished message. + + Meaning of this message: + + The Finished message is the first one protected with the just + negotiated algorithms, keys, and secrets. Recipients of Finished + messages MUST verify that the contents are correct. Once a side + has sent its Finished message and received and validated the + Finished message from its peer, it may begin to send and receive + application data over the connection. + + Structure of this message: + + struct { + opaque verify_data[verify_data_length]; + } Finished; + + verify_data + PRF(master_secret, finished_label, Hash(handshake_messages)) + [0..verify_data_length-1]; + + finished_label + For Finished messages sent by the client, the string + "client finished". For Finished messages sent by the server, + the string "server finished". + + Hash denotes a Hash of the handshake messages. For the PRF + defined in Section 5, the Hash MUST be the Hash used as the basis + for the PRF. Any cipher suite which defines a different PRF MUST + also define the Hash to use in the Finished computation. + + In previous versions of TLS, the verify_data was always 12 octets + long. In the current version of TLS, it depends on the cipher + suite. Any cipher suite which does not explicitly specify + verify_data_length has a verify_data_length equal to 12. This + includes all existing cipher suites. Note that this + representation has the same encoding as with previous versions. + Future cipher suites MAY specify other lengths but such length + MUST be at least 12 bytes. + + + + +Dierks & Rescorla Standards Track [Page 63] + +RFC 5246 TLS August 2008 + + + handshake_messages + All of the data from all messages in this handshake (not + including any HelloRequest messages) up to, but not including, + this message. This is only data visible at the handshake layer + and does not include record layer headers. This is the + concatenation of all the Handshake structures as defined in + Section 7.4, exchanged thus far. + + It is a fatal error if a Finished message is not preceded by a + ChangeCipherSpec message at the appropriate point in the handshake. + + The value handshake_messages includes all handshake messages starting + at ClientHello up to, but not including, this Finished message. This + may be different from handshake_messages in Section 7.4.8 because it + would include the CertificateVerify message (if sent). Also, the + handshake_messages for the Finished message sent by the client will + be different from that for the Finished message sent by the server, + because the one that is sent second will include the prior one. + + Note: ChangeCipherSpec messages, alerts, and any other record types + are not handshake messages and are not included in the hash + computations. Also, HelloRequest messages are omitted from handshake + hashes. + +8. Cryptographic Computations + + In order to begin connection protection, the TLS Record Protocol + requires specification of a suite of algorithms, a master secret, and + the client and server random values. The authentication, encryption, + and MAC algorithms are determined by the cipher_suite selected by the + server and revealed in the ServerHello message. The compression + algorithm is negotiated in the hello messages, and the random values + are exchanged in the hello messages. All that remains is to + calculate the master secret. + +8.1. Computing the Master Secret + + For all key exchange methods, the same algorithm is used to convert + the pre_master_secret into the master_secret. The pre_master_secret + should be deleted from memory once the master_secret has been + computed. + + master_secret = PRF(pre_master_secret, "master secret", + ClientHello.random + ServerHello.random) + [0..47]; + + The master secret is always exactly 48 bytes in length. The length + of the premaster secret will vary depending on key exchange method. + + + +Dierks & Rescorla Standards Track [Page 64] + +RFC 5246 TLS August 2008 + + +8.1.1. RSA + + When RSA is used for server authentication and key exchange, a 48- + byte pre_master_secret is generated by the client, encrypted under + the server's public key, and sent to the server. The server uses its + private key to decrypt the pre_master_secret. Both parties then + convert the pre_master_secret into the master_secret, as specified + above. + +8.1.2. Diffie-Hellman + + A conventional Diffie-Hellman computation is performed. The + negotiated key (Z) is used as the pre_master_secret, and is converted + into the master_secret, as specified above. Leading bytes of Z that + contain all zero bits are stripped before it is used as the + pre_master_secret. + + Note: Diffie-Hellman parameters are specified by the server and may + be either ephemeral or contained within the server's certificate. + +9. Mandatory Cipher Suites + + In the absence of an application profile standard specifying + otherwise, a TLS-compliant application MUST implement the cipher + suite TLS_RSA_WITH_AES_128_CBC_SHA (see Appendix A.5 for the + definition). + +10. Application Data Protocol + + Application data messages are carried by the record layer and are + fragmented, compressed, and encrypted based on the current connection + state. The messages are treated as transparent data to the record + layer. + +11. Security Considerations + + Security issues are discussed throughout this memo, especially in + Appendices D, E, and F. + +12. IANA Considerations + + This document uses several registries that were originally created in + [TLS1.1]. IANA has updated these to reference this document. The + registries and their allocation policies (unchanged from [TLS1.1]) + are listed below. + + + + + + +Dierks & Rescorla Standards Track [Page 65] + +RFC 5246 TLS August 2008 + + + - TLS ClientCertificateType Identifiers Registry: Future values in + the range 0-63 (decimal) inclusive are assigned via Standards + Action [RFC2434]. Values in the range 64-223 (decimal) inclusive + are assigned via Specification Required [RFC2434]. Values from + 224-255 (decimal) inclusive are reserved for Private Use + [RFC2434]. + + - TLS Cipher Suite Registry: Future values with the first byte in + the range 0-191 (decimal) inclusive are assigned via Standards + Action [RFC2434]. Values with the first byte in the range 192-254 + (decimal) are assigned via Specification Required [RFC2434]. + Values with the first byte 255 (decimal) are reserved for Private + Use [RFC2434]. + + - This document defines several new HMAC-SHA256-based cipher suites, + whose values (in Appendix A.5) have been allocated from the TLS + Cipher Suite registry. + + - TLS ContentType Registry: Future values are allocated via + Standards Action [RFC2434]. + + - TLS Alert Registry: Future values are allocated via Standards + Action [RFC2434]. + + - TLS HandshakeType Registry: Future values are allocated via + Standards Action [RFC2434]. + + This document also uses a registry originally created in [RFC4366]. + IANA has updated it to reference this document. The registry and its + allocation policy (unchanged from [RFC4366]) is listed below: + + - TLS ExtensionType Registry: Future values are allocated via IETF + Consensus [RFC2434]. IANA has updated this registry to include + the signature_algorithms extension and its corresponding value + (see Section 7.4.1.4). + + In addition, this document defines two new registries to be + maintained by IANA: + + - TLS SignatureAlgorithm Registry: The registry has been initially + populated with the values described in Section 7.4.1.4.1. Future + values in the range 0-63 (decimal) inclusive are assigned via + Standards Action [RFC2434]. Values in the range 64-223 (decimal) + inclusive are assigned via Specification Required [RFC2434]. + Values from 224-255 (decimal) inclusive are reserved for Private + Use [RFC2434]. + + + + + +Dierks & Rescorla Standards Track [Page 66] + +RFC 5246 TLS August 2008 + + + - TLS HashAlgorithm Registry: The registry has been initially + populated with the values described in Section 7.4.1.4.1. Future + values in the range 0-63 (decimal) inclusive are assigned via + Standards Action [RFC2434]. Values in the range 64-223 (decimal) + inclusive are assigned via Specification Required [RFC2434]. + Values from 224-255 (decimal) inclusive are reserved for Private + Use [RFC2434]. + + This document also uses the TLS Compression Method Identifiers + Registry, defined in [RFC3749]. IANA has allocated value 0 for + the "null" compression method. + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Dierks & Rescorla Standards Track [Page 67] + +RFC 5246 TLS August 2008 + + +Appendix A. Protocol Data Structures and Constant Values + + This section describes protocol types and constants. + +A.1. Record Layer + + struct { + uint8 major; + uint8 minor; + } ProtocolVersion; + + ProtocolVersion version = { 3, 3 }; /* TLS v1.2*/ + + enum { + change_cipher_spec(20), alert(21), handshake(22), + application_data(23), (255) + } ContentType; + + struct { + ContentType type; + ProtocolVersion version; + uint16 length; + opaque fragment[TLSPlaintext.length]; + } TLSPlaintext; + + struct { + ContentType type; + ProtocolVersion version; + uint16 length; + opaque fragment[TLSCompressed.length]; + } TLSCompressed; + + struct { + ContentType type; + ProtocolVersion version; + uint16 length; + select (SecurityParameters.cipher_type) { + case stream: GenericStreamCipher; + case block: GenericBlockCipher; + case aead: GenericAEADCipher; + } fragment; + } TLSCiphertext; + + stream-ciphered struct { + opaque content[TLSCompressed.length]; + opaque MAC[SecurityParameters.mac_length]; + } GenericStreamCipher; + + + + +Dierks & Rescorla Standards Track [Page 68] + +RFC 5246 TLS August 2008 + + + struct { + opaque IV[SecurityParameters.record_iv_length]; + block-ciphered struct { + opaque content[TLSCompressed.length]; + opaque MAC[SecurityParameters.mac_length]; + uint8 padding[GenericBlockCipher.padding_length]; + uint8 padding_length; + }; + } GenericBlockCipher; + + struct { + opaque nonce_explicit[SecurityParameters.record_iv_length]; + aead-ciphered struct { + opaque content[TLSCompressed.length]; + }; + } GenericAEADCipher; + +A.2. Change Cipher Specs Message + + struct { + enum { change_cipher_spec(1), (255) } type; + } ChangeCipherSpec; + +A.3. Alert Messages + + enum { warning(1), fatal(2), (255) } AlertLevel; + + enum { + close_notify(0), + unexpected_message(10), + bad_record_mac(20), + decryption_failed_RESERVED(21), + record_overflow(22), + decompression_failure(30), + handshake_failure(40), + no_certificate_RESERVED(41), + bad_certificate(42), + unsupported_certificate(43), + certificate_revoked(44), + certificate_expired(45), + certificate_unknown(46), + illegal_parameter(47), + unknown_ca(48), + access_denied(49), + decode_error(50), + decrypt_error(51), + export_restriction_RESERVED(60), + protocol_version(70), + + + +Dierks & Rescorla Standards Track [Page 69] + +RFC 5246 TLS August 2008 + + + insufficient_security(71), + internal_error(80), + user_canceled(90), + no_renegotiation(100), + unsupported_extension(110), /* new */ + (255) + } AlertDescription; + + struct { + AlertLevel level; + AlertDescription description; + } Alert; + +A.4. Handshake Protocol + + enum { + hello_request(0), client_hello(1), server_hello(2), + certificate(11), server_key_exchange (12), + certificate_request(13), server_hello_done(14), + certificate_verify(15), client_key_exchange(16), + finished(20) + (255) + } HandshakeType; + + struct { + HandshakeType msg_type; + uint24 length; + select (HandshakeType) { + case hello_request: HelloRequest; + case client_hello: ClientHello; + case server_hello: ServerHello; + case certificate: Certificate; + case server_key_exchange: ServerKeyExchange; + case certificate_request: CertificateRequest; + case server_hello_done: ServerHelloDone; + case certificate_verify: CertificateVerify; + case client_key_exchange: ClientKeyExchange; + case finished: Finished; + } body; + } Handshake; + + + + + + + + + + + +Dierks & Rescorla Standards Track [Page 70] + +RFC 5246 TLS August 2008 + + +A.4.1. Hello Messages + + struct { } HelloRequest; + + struct { + uint32 gmt_unix_time; + opaque random_bytes[28]; + } Random; + + opaque SessionID<0..32>; + + uint8 CipherSuite[2]; + + enum { null(0), (255) } CompressionMethod; + + struct { + ProtocolVersion client_version; + Random random; + SessionID session_id; + CipherSuite cipher_suites<2..2^16-2>; + CompressionMethod compression_methods<1..2^8-1>; + select (extensions_present) { + case false: + struct {}; + case true: + Extension extensions<0..2^16-1>; + }; + } ClientHello; + + struct { + ProtocolVersion server_version; + Random random; + SessionID session_id; + CipherSuite cipher_suite; + CompressionMethod compression_method; + select (extensions_present) { + case false: + struct {}; + case true: + Extension extensions<0..2^16-1>; + }; + } ServerHello; + + struct { + ExtensionType extension_type; + opaque extension_data<0..2^16-1>; + } Extension; + + + + +Dierks & Rescorla Standards Track [Page 71] + +RFC 5246 TLS August 2008 + + + enum { + signature_algorithms(13), (65535) + } ExtensionType; + + enum{ + none(0), md5(1), sha1(2), sha224(3), sha256(4), sha384(5), + sha512(6), (255) + } HashAlgorithm; + enum { + anonymous(0), rsa(1), dsa(2), ecdsa(3), (255) + } SignatureAlgorithm; + + struct { + HashAlgorithm hash; + SignatureAlgorithm signature; + } SignatureAndHashAlgorithm; + + SignatureAndHashAlgorithm + supported_signature_algorithms<2..2^16-1>; + +A.4.2. Server Authentication and Key Exchange Messages + + opaque ASN.1Cert<2^24-1>; + + struct { + ASN.1Cert certificate_list<0..2^24-1>; + } Certificate; + + enum { dhe_dss, dhe_rsa, dh_anon, rsa,dh_dss, dh_rsa + /* may be extended, e.g., for ECDH -- see [TLSECC] */ + } KeyExchangeAlgorithm; + + struct { + opaque dh_p<1..2^16-1>; + opaque dh_g<1..2^16-1>; + opaque dh_Ys<1..2^16-1>; + } ServerDHParams; /* Ephemeral DH parameters */ + + + + + + + + + + + + + + +Dierks & Rescorla Standards Track [Page 72] + +RFC 5246 TLS August 2008 + + + struct { + select (KeyExchangeAlgorithm) { + case dh_anon: + ServerDHParams params; + case dhe_dss: + case dhe_rsa: + ServerDHParams params; + digitally-signed struct { + opaque client_random[32]; + opaque server_random[32]; + ServerDHParams params; + } signed_params; + case rsa: + case dh_dss: + case dh_rsa: + struct {} ; + /* message is omitted for rsa, dh_dss, and dh_rsa */ + /* may be extended, e.g., for ECDH -- see [TLSECC] */ + } ServerKeyExchange; + + enum { + rsa_sign(1), dss_sign(2), rsa_fixed_dh(3), dss_fixed_dh(4), + rsa_ephemeral_dh_RESERVED(5), dss_ephemeral_dh_RESERVED(6), + fortezza_dms_RESERVED(20), + (255) + } ClientCertificateType; + + opaque DistinguishedName<1..2^16-1>; + + struct { + ClientCertificateType certificate_types<1..2^8-1>; + DistinguishedName certificate_authorities<0..2^16-1>; + } CertificateRequest; + + struct { } ServerHelloDone; + + + + + + + + + + + + + + + + +Dierks & Rescorla Standards Track [Page 73] + +RFC 5246 TLS August 2008 + + +A.4.3. Client Authentication and Key Exchange Messages + + struct { + select (KeyExchangeAlgorithm) { + case rsa: + EncryptedPreMasterSecret; + case dhe_dss: + case dhe_rsa: + case dh_dss: + case dh_rsa: + case dh_anon: + ClientDiffieHellmanPublic; + } exchange_keys; + } ClientKeyExchange; + + struct { + ProtocolVersion client_version; + opaque random[46]; + } PreMasterSecret; + + struct { + public-key-encrypted PreMasterSecret pre_master_secret; + } EncryptedPreMasterSecret; + + enum { implicit, explicit } PublicValueEncoding; + + struct { + select (PublicValueEncoding) { + case implicit: struct {}; + case explicit: opaque DH_Yc<1..2^16-1>; + } dh_public; + } ClientDiffieHellmanPublic; + + struct { + digitally-signed struct { + opaque handshake_messages[handshake_messages_length]; + } + } CertificateVerify; + +A.4.4. Handshake Finalization Message + + struct { + opaque verify_data[verify_data_length]; + } Finished; + + + + + + + +Dierks & Rescorla Standards Track [Page 74] + +RFC 5246 TLS August 2008 + + +A.5. The Cipher Suite + + The following values define the cipher suite codes used in the + ClientHello and ServerHello messages. + + A cipher suite defines a cipher specification supported in TLS + Version 1.2. + + TLS_NULL_WITH_NULL_NULL is specified and is the initial state of a + TLS connection during the first handshake on that channel, but MUST + NOT be negotiated, as it provides no more protection than an + unsecured connection. + + CipherSuite TLS_NULL_WITH_NULL_NULL = { 0x00,0x00 }; + + The following CipherSuite definitions require that the server provide + an RSA certificate that can be used for key exchange. The server may + request any signature-capable certificate in the certificate request + message. + + CipherSuite TLS_RSA_WITH_NULL_MD5 = { 0x00,0x01 }; + CipherSuite TLS_RSA_WITH_NULL_SHA = { 0x00,0x02 }; + CipherSuite TLS_RSA_WITH_NULL_SHA256 = { 0x00,0x3B }; + CipherSuite TLS_RSA_WITH_RC4_128_MD5 = { 0x00,0x04 }; + CipherSuite TLS_RSA_WITH_RC4_128_SHA = { 0x00,0x05 }; + CipherSuite TLS_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0A }; + CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA = { 0x00,0x2F }; + CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA = { 0x00,0x35 }; + CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA256 = { 0x00,0x3C }; + CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA256 = { 0x00,0x3D }; + + The following cipher suite definitions are used for server- + authenticated (and optionally client-authenticated) Diffie-Hellman. + DH denotes cipher suites in which the server's certificate contains + the Diffie-Hellman parameters signed by the certificate authority + (CA). DHE denotes ephemeral Diffie-Hellman, where the Diffie-Hellman + parameters are signed by a signature-capable certificate, which has + been signed by the CA. The signing algorithm used by the server is + specified after the DHE component of the CipherSuite name. The + server can request any signature-capable certificate from the client + for client authentication, or it may request a Diffie-Hellman + certificate. Any Diffie-Hellman certificate provided by the client + must use the parameters (group and generator) described by the + server. + + + + + + + +Dierks & Rescorla Standards Track [Page 75] + +RFC 5246 TLS August 2008 + + + CipherSuite TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0D }; + CipherSuite TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x10 }; + CipherSuite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x13 }; + CipherSuite TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x16 }; + CipherSuite TLS_DH_DSS_WITH_AES_128_CBC_SHA = { 0x00,0x30 }; + CipherSuite TLS_DH_RSA_WITH_AES_128_CBC_SHA = { 0x00,0x31 }; + CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_SHA = { 0x00,0x32 }; + CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_SHA = { 0x00,0x33 }; + CipherSuite TLS_DH_DSS_WITH_AES_256_CBC_SHA = { 0x00,0x36 }; + CipherSuite TLS_DH_RSA_WITH_AES_256_CBC_SHA = { 0x00,0x37 }; + CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_SHA = { 0x00,0x38 }; + CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_SHA = { 0x00,0x39 }; + CipherSuite TLS_DH_DSS_WITH_AES_128_CBC_SHA256 = { 0x00,0x3E }; + CipherSuite TLS_DH_RSA_WITH_AES_128_CBC_SHA256 = { 0x00,0x3F }; + CipherSuite TLS_DHE_DSS_WITH_AES_128_CBC_SHA256 = { 0x00,0x40 }; + CipherSuite TLS_DHE_RSA_WITH_AES_128_CBC_SHA256 = { 0x00,0x67 }; + CipherSuite TLS_DH_DSS_WITH_AES_256_CBC_SHA256 = { 0x00,0x68 }; + CipherSuite TLS_DH_RSA_WITH_AES_256_CBC_SHA256 = { 0x00,0x69 }; + CipherSuite TLS_DHE_DSS_WITH_AES_256_CBC_SHA256 = { 0x00,0x6A }; + CipherSuite TLS_DHE_RSA_WITH_AES_256_CBC_SHA256 = { 0x00,0x6B }; + + The following cipher suites are used for completely anonymous + Diffie-Hellman communications in which neither party is + authenticated. Note that this mode is vulnerable to man-in-the- + middle attacks. Using this mode therefore is of limited use: These + cipher suites MUST NOT be used by TLS 1.2 implementations unless the + application layer has specifically requested to allow anonymous key + exchange. (Anonymous key exchange may sometimes be acceptable, for + example, to support opportunistic encryption when no set-up for + authentication is in place, or when TLS is used as part of more + complex security protocols that have other means to ensure + authentication.) + + CipherSuite TLS_DH_anon_WITH_RC4_128_MD5 = { 0x00,0x18 }; + CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1B }; + CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA = { 0x00,0x34 }; + CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA = { 0x00,0x3A }; + CipherSuite TLS_DH_anon_WITH_AES_128_CBC_SHA256 = { 0x00,0x6C }; + CipherSuite TLS_DH_anon_WITH_AES_256_CBC_SHA256 = { 0x00,0x6D }; + + Note that using non-anonymous key exchange without actually verifying + the key exchange is essentially equivalent to anonymous key exchange, + and the same precautions apply. While non-anonymous key exchange + will generally involve a higher computational and communicational + cost than anonymous key exchange, it may be in the interest of + interoperability not to disable non-anonymous key exchange when the + application layer is allowing anonymous key exchange. + + + + +Dierks & Rescorla Standards Track [Page 76] + +RFC 5246 TLS August 2008 + + + New cipher suite values have been assigned by IANA as described in + Section 12. + + Note: The cipher suite values { 0x00, 0x1C } and { 0x00, 0x1D } are + reserved to avoid collision with Fortezza-based cipher suites in + SSL 3. + +A.6. The Security Parameters + + These security parameters are determined by the TLS Handshake + Protocol and provided as parameters to the TLS record layer in order + to initialize a connection state. SecurityParameters includes: + + enum { null(0), (255) } CompressionMethod; + + enum { server, client } ConnectionEnd; + + enum { tls_prf_sha256 } PRFAlgorithm; + + enum { null, rc4, 3des, aes } BulkCipherAlgorithm; + + enum { stream, block, aead } CipherType; + + enum { null, hmac_md5, hmac_sha1, hmac_sha256, hmac_sha384, + hmac_sha512} MACAlgorithm; + + /* Other values may be added to the algorithms specified in + CompressionMethod, PRFAlgorithm, BulkCipherAlgorithm, and + MACAlgorithm. */ + + struct { + ConnectionEnd entity; + PRFAlgorithm prf_algorithm; + BulkCipherAlgorithm bulk_cipher_algorithm; + CipherType cipher_type; + uint8 enc_key_length; + uint8 block_length; + uint8 fixed_iv_length; + uint8 record_iv_length; + MACAlgorithm mac_algorithm; + uint8 mac_length; + uint8 mac_key_length; + CompressionMethod compression_algorithm; + opaque master_secret[48]; + opaque client_random[32]; + opaque server_random[32]; + } SecurityParameters; + + + + +Dierks & Rescorla Standards Track [Page 77] + +RFC 5246 TLS August 2008 + + +A.7. Changes to RFC 4492 + + RFC 4492 [TLSECC] adds Elliptic Curve cipher suites to TLS. This + document changes some of the structures used in that document. This + section details the required changes for implementors of both RFC + 4492 and TLS 1.2. Implementors of TLS 1.2 who are not implementing + RFC 4492 do not need to read this section. + + This document adds a "signature_algorithm" field to the digitally- + signed element in order to identify the signature and digest + algorithms used to create a signature. This change applies to + digital signatures formed using ECDSA as well, thus allowing ECDSA + signatures to be used with digest algorithms other than SHA-1, + provided such use is compatible with the certificate and any + restrictions imposed by future revisions of [PKIX]. + + As described in Sections 7.4.2 and 7.4.6, the restrictions on the + signature algorithms used to sign certificates are no longer tied to + the cipher suite (when used by the server) or the + ClientCertificateType (when used by the client). Thus, the + restrictions on the algorithm used to sign certificates specified in + Sections 2 and 3 of RFC 4492 are also relaxed. As in this document, + the restrictions on the keys in the end-entity certificate remain. + +Appendix B. Glossary + + Advanced Encryption Standard (AES) + AES [AES] is a widely used symmetric encryption algorithm. AES is + a block cipher with a 128-, 192-, or 256-bit keys and a 16-byte + block size. TLS currently only supports the 128- and 256-bit key + sizes. + + application protocol + An application protocol is a protocol that normally layers + directly on top of the transport layer (e.g., TCP/IP). Examples + include HTTP, TELNET, FTP, and SMTP. + + asymmetric cipher + See public key cryptography. + + authenticated encryption with additional data (AEAD) + A symmetric encryption algorithm that simultaneously provides + confidentiality and message integrity. + + authentication + Authentication is the ability of one entity to determine the + identity of another entity. + + + + +Dierks & Rescorla Standards Track [Page 78] + +RFC 5246 TLS August 2008 + + + block cipher + A block cipher is an algorithm that operates on plaintext in + groups of bits, called blocks. 64 bits was, and 128 bits is, a + common block size. + + bulk cipher + A symmetric encryption algorithm used to encrypt large quantities + of data. + + cipher block chaining (CBC) + CBC is a mode in which every plaintext block encrypted with a + block cipher is first exclusive-ORed with the previous ciphertext + block (or, in the case of the first block, with the initialization + vector). For decryption, every block is first decrypted, then + exclusive-ORed with the previous ciphertext block (or IV). + + certificate + As part of the X.509 protocol (a.k.a. ISO Authentication + framework), certificates are assigned by a trusted Certificate + Authority and provide a strong binding between a party's identity + or some other attributes and its public key. + + client + The application entity that initiates a TLS connection to a + server. This may or may not imply that the client initiated the + underlying transport connection. The primary operational + difference between the server and client is that the server is + generally authenticated, while the client is only optionally + authenticated. + + client write key + The key used to encrypt data written by the client. + + client write MAC key + The secret data used to authenticate data written by the client. + + connection + A connection is a transport (in the OSI layering model definition) + that provides a suitable type of service. For TLS, such + connections are peer-to-peer relationships. The connections are + transient. Every connection is associated with one session. + + Data Encryption Standard + DES [DES] still is a very widely used symmetric encryption + algorithm although it is considered as rather weak now. DES is a + block cipher with a 56-bit key and an 8-byte block size. Note + that in TLS, for key generation purposes, DES is treated as having + an 8-byte key length (64 bits), but it still only provides 56 bits + + + +Dierks & Rescorla Standards Track [Page 79] + +RFC 5246 TLS August 2008 + + + of protection. (The low bit of each key byte is presumed to be + set to produce odd parity in that key byte.) DES can also be + operated in a mode [3DES] where three independent keys and three + encryptions are used for each block of data; this uses 168 bits of + key (24 bytes in the TLS key generation method) and provides the + equivalent of 112 bits of security. + + Digital Signature Standard (DSS) + A standard for digital signing, including the Digital Signing + Algorithm, approved by the National Institute of Standards and + Technology, defined in NIST FIPS PUB 186-2, "Digital Signature + Standard", published January 2000 by the U.S. Department of + Commerce [DSS]. A significant update [DSS-3] has been drafted and + was published in March 2006. + + digital signatures + Digital signatures utilize public key cryptography and one-way + hash functions to produce a signature of the data that can be + authenticated, and is difficult to forge or repudiate. + + handshake An initial negotiation between client and server that + establishes the parameters of their transactions. + + Initialization Vector (IV) + When a block cipher is used in CBC mode, the initialization vector + is exclusive-ORed with the first plaintext block prior to + encryption. + + Message Authentication Code (MAC) + A Message Authentication Code is a one-way hash computed from a + message and some secret data. It is difficult to forge without + knowing the secret data. Its purpose is to detect if the message + has been altered. + + master secret + Secure secret data used for generating encryption keys, MAC + secrets, and IVs. + + MD5 + MD5 [MD5] is a hashing function that converts an arbitrarily long + data stream into a hash of fixed size (16 bytes). Due to + significant progress in cryptanalysis, at the time of publication + of this document, MD5 no longer can be considered a 'secure' + hashing function. + + + + + + + +Dierks & Rescorla Standards Track [Page 80] + +RFC 5246 TLS August 2008 + + + public key cryptography + A class of cryptographic techniques employing two-key ciphers. + Messages encrypted with the public key can only be decrypted with + the associated private key. Conversely, messages signed with the + private key can be verified with the public key. + + one-way hash function + A one-way transformation that converts an arbitrary amount of data + into a fixed-length hash. It is computationally hard to reverse + the transformation or to find collisions. MD5 and SHA are + examples of one-way hash functions. + + RC4 + A stream cipher invented by Ron Rivest. A compatible cipher is + described in [SCH]. + + RSA + A very widely used public key algorithm that can be used for + either encryption or digital signing. [RSA] + + server + The server is the application entity that responds to requests for + connections from clients. See also "client". + + session + A TLS session is an association between a client and a server. + Sessions are created by the handshake protocol. Sessions define a + set of cryptographic security parameters that can be shared among + multiple connections. Sessions are used to avoid the expensive + negotiation of new security parameters for each connection. + + session identifier + A session identifier is a value generated by a server that + identifies a particular session. + + server write key + The key used to encrypt data written by the server. + + server write MAC key + The secret data used to authenticate data written by the server. + + SHA + The Secure Hash Algorithm [SHS] is defined in FIPS PUB 180-2. It + produces a 20-byte output. Note that all references to SHA + (without a numerical suffix) actually use the modified SHA-1 + algorithm. + + + + + +Dierks & Rescorla Standards Track [Page 81] + +RFC 5246 TLS August 2008 + + + SHA-256 + The 256-bit Secure Hash Algorithm is defined in FIPS PUB 180-2. + It produces a 32-byte output. + + SSL + Netscape's Secure Socket Layer protocol [SSL3]. TLS is based on + SSL Version 3.0. + + stream cipher + An encryption algorithm that converts a key into a + cryptographically strong keystream, which is then exclusive-ORed + with the plaintext. + + symmetric cipher + See bulk cipher. + + Transport Layer Security (TLS) + This protocol; also, the Transport Layer Security working group of + the Internet Engineering Task Force (IETF). See "Working Group + Information" at the end of this document (see page 99). + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + + +Dierks & Rescorla Standards Track [Page 82] + +RFC 5246 TLS August 2008 + + +Appendix C. Cipher Suite Definitions + +Cipher Suite Key Cipher Mac + Exchange + +TLS_NULL_WITH_NULL_NULL NULL NULL NULL +TLS_RSA_WITH_NULL_MD5 RSA NULL MD5 +TLS_RSA_WITH_NULL_SHA RSA NULL SHA +TLS_RSA_WITH_NULL_SHA256 RSA NULL SHA256 +TLS_RSA_WITH_RC4_128_MD5 RSA RC4_128 MD5 +TLS_RSA_WITH_RC4_128_SHA RSA RC4_128 SHA +TLS_RSA_WITH_3DES_EDE_CBC_SHA RSA 3DES_EDE_CBC SHA +TLS_RSA_WITH_AES_128_CBC_SHA RSA AES_128_CBC SHA +TLS_RSA_WITH_AES_256_CBC_SHA RSA AES_256_CBC SHA +TLS_RSA_WITH_AES_128_CBC_SHA256 RSA AES_128_CBC SHA256 +TLS_RSA_WITH_AES_256_CBC_SHA256 RSA AES_256_CBC SHA256 +TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA DH_DSS 3DES_EDE_CBC SHA +TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA DH_RSA 3DES_EDE_CBC SHA +TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA DHE_DSS 3DES_EDE_CBC SHA +TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA DHE_RSA 3DES_EDE_CBC SHA +TLS_DH_anon_WITH_RC4_128_MD5 DH_anon RC4_128 MD5 +TLS_DH_anon_WITH_3DES_EDE_CBC_SHA DH_anon 3DES_EDE_CBC SHA +TLS_DH_DSS_WITH_AES_128_CBC_SHA DH_DSS AES_128_CBC SHA +TLS_DH_RSA_WITH_AES_128_CBC_SHA DH_RSA AES_128_CBC SHA +TLS_DHE_DSS_WITH_AES_128_CBC_SHA DHE_DSS AES_128_CBC SHA +TLS_DHE_RSA_WITH_AES_128_CBC_SHA DHE_RSA AES_128_CBC SHA +TLS_DH_anon_WITH_AES_128_CBC_SHA DH_anon AES_128_CBC SHA +TLS_DH_DSS_WITH_AES_256_CBC_SHA DH_DSS AES_256_CBC SHA +TLS_DH_RSA_WITH_AES_256_CBC_SHA DH_RSA AES_256_CBC SHA +TLS_DHE_DSS_WITH_AES_256_CBC_SHA DHE_DSS AES_256_CBC SHA +TLS_DHE_RSA_WITH_AES_256_CBC_SHA DHE_RSA AES_256_CBC SHA +TLS_DH_anon_WITH_AES_256_CBC_SHA DH_anon AES_256_CBC SHA +TLS_DH_DSS_WITH_AES_128_CBC_SHA256 DH_DSS AES_128_CBC SHA256 +TLS_DH_RSA_WITH_AES_128_CBC_SHA256 DH_RSA AES_128_CBC SHA256 +TLS_DHE_DSS_WITH_AES_128_CBC_SHA256 DHE_DSS AES_128_CBC SHA256 +TLS_DHE_RSA_WITH_AES_128_CBC_SHA256 DHE_RSA AES_128_CBC SHA256 +TLS_DH_anon_WITH_AES_128_CBC_SHA256 DH_anon AES_128_CBC SHA256 +TLS_DH_DSS_WITH_AES_256_CBC_SHA256 DH_DSS AES_256_CBC SHA256 +TLS_DH_RSA_WITH_AES_256_CBC_SHA256 DH_RSA AES_256_CBC SHA256 +TLS_DHE_DSS_WITH_AES_256_CBC_SHA256 DHE_DSS AES_256_CBC SHA256 +TLS_DHE_RSA_WITH_AES_256_CBC_SHA256 DHE_RSA AES_256_CBC SHA256 +TLS_DH_anon_WITH_AES_256_CBC_SHA256 DH_anon AES_256_CBC SHA256 + + + + + + + + + +Dierks & Rescorla Standards Track [Page 83] + +RFC 5246 TLS August 2008 + + + Key IV Block +Cipher Type Material Size Size +------------ ------ -------- ---- ----- +NULL Stream 0 0 N/A +RC4_128 Stream 16 0 N/A +3DES_EDE_CBC Block 24 8 8 +AES_128_CBC Block 16 16 16 +AES_256_CBC Block 32 16 16 + + +MAC Algorithm mac_length mac_key_length +-------- ----------- ---------- -------------- +NULL N/A 0 0 +MD5 HMAC-MD5 16 16 +SHA HMAC-SHA1 20 20 +SHA256 HMAC-SHA256 32 32 + + Type + Indicates whether this is a stream cipher or a block cipher + running in CBC mode. + + Key Material + The number of bytes from the key_block that are used for + generating the write keys. + + IV Size + The amount of data needed to be generated for the initialization + vector. Zero for stream ciphers; equal to the block size for + block ciphers (this is equal to + SecurityParameters.record_iv_length). + + Block Size + The amount of data a block cipher enciphers in one chunk; a block + cipher running in CBC mode can only encrypt an even multiple of + its block size. + + + + + + + + + + + + + + + + +Dierks & Rescorla Standards Track [Page 84] + +RFC 5246 TLS August 2008 + + +Appendix D. Implementation Notes + + The TLS protocol cannot prevent many common security mistakes. This + section provides several recommendations to assist implementors. + +D.1. Random Number Generation and Seeding + + TLS requires a cryptographically secure pseudorandom number generator + (PRNG). Care must be taken in designing and seeding PRNGs. PRNGs + based on secure hash operations, most notably SHA-1, are acceptable, + but cannot provide more security than the size of the random number + generator state. + + To estimate the amount of seed material being produced, add the + number of bits of unpredictable information in each seed byte. For + example, keystroke timing values taken from a PC compatible's 18.2 Hz + timer provide 1 or 2 secure bits each, even though the total size of + the counter value is 16 bits or more. Seeding a 128-bit PRNG would + thus require approximately 100 such timer values. + + [RANDOM] provides guidance on the generation of random values. + +D.2. Certificates and Authentication + + Implementations are responsible for verifying the integrity of + certificates and should generally support certificate revocation + messages. Certificates should always be verified to ensure proper + signing by a trusted Certificate Authority (CA). The selection and + addition of trusted CAs should be done very carefully. Users should + be able to view information about the certificate and root CA. + +D.3. Cipher Suites + + TLS supports a range of key sizes and security levels, including some + that provide no or minimal security. A proper implementation will + probably not support many cipher suites. For instance, anonymous + Diffie-Hellman is strongly discouraged because it cannot prevent man- + in-the-middle attacks. Applications should also enforce minimum and + maximum key sizes. For example, certificate chains containing 512- + bit RSA keys or signatures are not appropriate for high-security + applications. + +D.4. Implementation Pitfalls + + Implementation experience has shown that certain parts of earlier TLS + specifications are not easy to understand, and have been a source of + interoperability and security problems. Many of these areas have + + + + +Dierks & Rescorla Standards Track [Page 85] + +RFC 5246 TLS August 2008 + + + been clarified in this document, but this appendix contains a short + list of the most important things that require special attention from + implementors. + + TLS protocol issues: + + - Do you correctly handle handshake messages that are fragmented to + multiple TLS records (see Section 6.2.1)? Including corner cases + like a ClientHello that is split to several small fragments? Do + you fragment handshake messages that exceed the maximum fragment + size? In particular, the certificate and certificate request + handshake messages can be large enough to require fragmentation. + + - Do you ignore the TLS record layer version number in all TLS + records before ServerHello (see Appendix E.1)? + + - Do you handle TLS extensions in ClientHello correctly, including + omitting the extensions field completely? + + - Do you support renegotiation, both client and server initiated? + While renegotiation is an optional feature, supporting it is + highly recommended. + + - When the server has requested a client certificate, but no + suitable certificate is available, do you correctly send an empty + Certificate message, instead of omitting the whole message (see + Section 7.4.6)? + + Cryptographic details: + + - In the RSA-encrypted Premaster Secret, do you correctly send and + verify the version number? When an error is encountered, do you + continue the handshake to avoid the Bleichenbacher attack (see + Section 7.4.7.1)? + + - What countermeasures do you use to prevent timing attacks against + RSA decryption and signing operations (see Section 7.4.7.1)? + + - When verifying RSA signatures, do you accept both NULL and missing + parameters (see Section 4.7)? Do you verify that the RSA padding + doesn't have additional data after the hash value? [FI06] + + - When using Diffie-Hellman key exchange, do you correctly strip + leading zero bytes from the negotiated key (see Section 8.1.2)? + + - Does your TLS client check that the Diffie-Hellman parameters sent + by the server are acceptable (see Section F.1.1.3)? + + + + +Dierks & Rescorla Standards Track [Page 86] + +RFC 5246 TLS August 2008 + + + - How do you generate unpredictable IVs for CBC mode ciphers (see + Section 6.2.3.2)? + + - Do you accept long CBC mode padding (up to 255 bytes; see Section + 6.2.3.2)? + + - How do you address CBC mode timing attacks (Section 6.2.3.2)? + + - Do you use a strong and, most importantly, properly seeded random + number generator (see Appendix D.1) for generating the premaster + secret (for RSA key exchange), Diffie-Hellman private values, the + DSA "k" parameter, and other security-critical values? + +Appendix E. Backward Compatibility + +E.1. Compatibility with TLS 1.0/1.1 and SSL 3.0 + + Since there are various versions of TLS (1.0, 1.1, 1.2, and any + future versions) and SSL (2.0 and 3.0), means are needed to negotiate + the specific protocol version to use. The TLS protocol provides a + built-in mechanism for version negotiation so as not to bother other + protocol components with the complexities of version selection. + + TLS versions 1.0, 1.1, and 1.2, and SSL 3.0 are very similar, and use + compatible ClientHello messages; thus, supporting all of them is + relatively easy. Similarly, servers can easily handle clients trying + to use future versions of TLS as long as the ClientHello format + remains compatible, and the client supports the highest protocol + version available in the server. + + A TLS 1.2 client who wishes to negotiate with such older servers will + send a normal TLS 1.2 ClientHello, containing { 3, 3 } (TLS 1.2) in + ClientHello.client_version. If the server does not support this + version, it will respond with a ServerHello containing an older + version number. If the client agrees to use this version, the + negotiation will proceed as appropriate for the negotiated protocol. + + If the version chosen by the server is not supported by the client + (or not acceptable), the client MUST send a "protocol_version" alert + message and close the connection. + + If a TLS server receives a ClientHello containing a version number + greater than the highest version supported by the server, it MUST + reply according to the highest version supported by the server. + + A TLS server can also receive a ClientHello containing a version + number smaller than the highest supported version. If the server + wishes to negotiate with old clients, it will proceed as appropriate + + + +Dierks & Rescorla Standards Track [Page 87] + +RFC 5246 TLS August 2008 + + + for the highest version supported by the server that is not greater + than ClientHello.client_version. For example, if the server supports + TLS 1.0, 1.1, and 1.2, and client_version is TLS 1.0, the server will + proceed with a TLS 1.0 ServerHello. If server supports (or is + willing to use) only versions greater than client_version, it MUST + send a "protocol_version" alert message and close the connection. + + Whenever a client already knows the highest protocol version known to + a server (for example, when resuming a session), it SHOULD initiate + the connection in that native protocol. + + Note: some server implementations are known to implement version + negotiation incorrectly. For example, there are buggy TLS 1.0 + servers that simply close the connection when the client offers a + version newer than TLS 1.0. Also, it is known that some servers will + refuse the connection if any TLS extensions are included in + ClientHello. Interoperability with such buggy servers is a complex + topic beyond the scope of this document, and may require multiple + connection attempts by the client. + + Earlier versions of the TLS specification were not fully clear on + what the record layer version number (TLSPlaintext.version) should + contain when sending ClientHello (i.e., before it is known which + version of the protocol will be employed). Thus, TLS servers + compliant with this specification MUST accept any value {03,XX} as + the record layer version number for ClientHello. + + TLS clients that wish to negotiate with older servers MAY send any + value {03,XX} as the record layer version number. Typical values + would be {03,00}, the lowest version number supported by the client, + and the value of ClientHello.client_version. No single value will + guarantee interoperability with all old servers, but this is a + complex topic beyond the scope of this document. + +E.2. Compatibility with SSL 2.0 + + TLS 1.2 clients that wish to support SSL 2.0 servers MUST send + version 2.0 CLIENT-HELLO messages defined in [SSL2]. The message + MUST contain the same version number as would be used for ordinary + ClientHello, and MUST encode the supported TLS cipher suites in the + CIPHER-SPECS-DATA field as described below. + + Warning: The ability to send version 2.0 CLIENT-HELLO messages will + be phased out with all due haste, since the newer ClientHello format + provides better mechanisms for moving to newer versions and + negotiating extensions. TLS 1.2 clients SHOULD NOT support SSL 2.0. + + + + + +Dierks & Rescorla Standards Track [Page 88] + +RFC 5246 TLS August 2008 + + + However, even TLS servers that do not support SSL 2.0 MAY accept + version 2.0 CLIENT-HELLO messages. The message is presented below in + sufficient detail for TLS server implementors; the true definition is + still assumed to be [SSL2]. + + For negotiation purposes, 2.0 CLIENT-HELLO is interpreted the same + way as a ClientHello with a "null" compression method and no + extensions. Note that this message MUST be sent directly on the + wire, not wrapped as a TLS record. For the purposes of calculating + Finished and CertificateVerify, the msg_length field is not + considered to be a part of the handshake message. + + uint8 V2CipherSpec[3]; + struct { + uint16 msg_length; + uint8 msg_type; + Version version; + uint16 cipher_spec_length; + uint16 session_id_length; + uint16 challenge_length; + V2CipherSpec cipher_specs[V2ClientHello.cipher_spec_length]; + opaque session_id[V2ClientHello.session_id_length]; + opaque challenge[V2ClientHello.challenge_length; + } V2ClientHello; + + msg_length + The highest bit MUST be 1; the remaining bits contain the length + of the following data in bytes. + + msg_type + This field, in conjunction with the version field, identifies a + version 2 ClientHello message. The value MUST be 1. + + version + Equal to ClientHello.client_version. + + cipher_spec_length + This field is the total length of the field cipher_specs. It + cannot be zero and MUST be a multiple of the V2CipherSpec length + (3). + + session_id_length + This field MUST have a value of zero for a client that claims to + support TLS 1.2. + + + + + + + +Dierks & Rescorla Standards Track [Page 89] + +RFC 5246 TLS August 2008 + + + challenge_length + The length in bytes of the client's challenge to the server to + authenticate itself. Historically, permissible values are between + 16 and 32 bytes inclusive. When using the SSLv2 backward- + compatible handshake the client SHOULD use a 32-byte challenge. + + cipher_specs + This is a list of all CipherSpecs the client is willing and able + to use. In addition to the 2.0 cipher specs defined in [SSL2], + this includes the TLS cipher suites normally sent in + ClientHello.cipher_suites, with each cipher suite prefixed by a + zero byte. For example, the TLS cipher suite {0x00,0x0A} would be + sent as {0x00,0x00,0x0A}. + + session_id + This field MUST be empty. + + challenge + Corresponds to ClientHello.random. If the challenge length is + less than 32, the TLS server will pad the data with leading (note: + not trailing) zero bytes to make it 32 bytes long. + + Note: Requests to resume a TLS session MUST use a TLS client hello. + +E.3. Avoiding Man-in-the-Middle Version Rollback + + When TLS clients fall back to Version 2.0 compatibility mode, they + MUST use special PKCS#1 block formatting. This is done so that TLS + servers will reject Version 2.0 sessions with TLS-capable clients. + + When a client negotiates SSL 2.0 but also supports TLS, it MUST set + the right-hand (least-significant) 8 random bytes of the PKCS padding + (not including the terminal null of the padding) for the RSA + encryption of the ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY + to 0x03 (the other padding bytes are random). + + When a TLS-capable server negotiates SSL 2.0 it SHOULD, after + decrypting the ENCRYPTED-KEY-DATA field, check that these 8 padding + bytes are 0x03. If they are not, the server SHOULD generate a random + value for SECRET-KEY-DATA, and continue the handshake (which will + eventually fail since the keys will not match). Note that reporting + the error situation to the client could make the server vulnerable to + attacks described in [BLEI]. + + + + + + + + +Dierks & Rescorla Standards Track [Page 90] + +RFC 5246 TLS August 2008 + + +Appendix F. Security Analysis + + The TLS protocol is designed to establish a secure connection between + a client and a server communicating over an insecure channel. This + document makes several traditional assumptions, including that + attackers have substantial computational resources and cannot obtain + secret information from sources outside the protocol. Attackers are + assumed to have the ability to capture, modify, delete, replay, and + otherwise tamper with messages sent over the communication channel. + This appendix outlines how TLS has been designed to resist a variety + of attacks. + +F.1. Handshake Protocol + + The handshake protocol is responsible for selecting a cipher spec and + generating a master secret, which together comprise the primary + cryptographic parameters associated with a secure session. The + handshake protocol can also optionally authenticate parties who have + certificates signed by a trusted certificate authority. + +F.1.1. Authentication and Key Exchange + + TLS supports three authentication modes: authentication of both + parties, server authentication with an unauthenticated client, and + total anonymity. Whenever the server is authenticated, the channel + is secure against man-in-the-middle attacks, but completely anonymous + sessions are inherently vulnerable to such attacks. Anonymous + servers cannot authenticate clients. If the server is authenticated, + its certificate message must provide a valid certificate chain + leading to an acceptable certificate authority. Similarly, + authenticated clients must supply an acceptable certificate to the + server. Each party is responsible for verifying that the other's + certificate is valid and has not expired or been revoked. + + The general goal of the key exchange process is to create a + pre_master_secret known to the communicating parties and not to + attackers. The pre_master_secret will be used to generate the + master_secret (see Section 8.1). The master_secret is required to + generate the Finished messages, encryption keys, and MAC keys (see + Sections 7.4.9 and 6.3). By sending a correct Finished message, + parties thus prove that they know the correct pre_master_secret. + +F.1.1.1. Anonymous Key Exchange + + Completely anonymous sessions can be established using Diffie-Hellman + for key exchange. The server's public parameters are contained in + the server key exchange message, and the client's are sent in the + + + + +Dierks & Rescorla Standards Track [Page 91] + +RFC 5246 TLS August 2008 + + + client key exchange message. Eavesdroppers who do not know the + private values should not be able to find the Diffie-Hellman result + (i.e., the pre_master_secret). + + Warning: Completely anonymous connections only provide protection + against passive eavesdropping. Unless an independent tamper-proof + channel is used to verify that the Finished messages were not + replaced by an attacker, server authentication is required in + environments where active man-in-the-middle attacks are a concern. + +F.1.1.2. RSA Key Exchange and Authentication + + With RSA, key exchange and server authentication are combined. The + public key is contained in the server's certificate. Note that + compromise of the server's static RSA key results in a loss of + confidentiality for all sessions protected under that static key. + TLS users desiring Perfect Forward Secrecy should use DHE cipher + suites. The damage done by exposure of a private key can be limited + by changing one's private key (and certificate) frequently. + + After verifying the server's certificate, the client encrypts a + pre_master_secret with the server's public key. By successfully + decoding the pre_master_secret and producing a correct Finished + message, the server demonstrates that it knows the private key + corresponding to the server certificate. + + When RSA is used for key exchange, clients are authenticated using + the certificate verify message (see Section 7.4.8). The client signs + a value derived from all preceding handshake messages. These + handshake messages include the server certificate, which binds the + signature to the server, and ServerHello.random, which binds the + signature to the current handshake process. + +F.1.1.3. Diffie-Hellman Key Exchange with Authentication + + When Diffie-Hellman key exchange is used, the server can either + supply a certificate containing fixed Diffie-Hellman parameters or + use the server key exchange message to send a set of temporary + Diffie-Hellman parameters signed with a DSA or RSA certificate. + Temporary parameters are hashed with the hello.random values before + signing to ensure that attackers do not replay old parameters. In + either case, the client can verify the certificate or signature to + ensure that the parameters belong to the server. + + If the client has a certificate containing fixed Diffie-Hellman + parameters, its certificate contains the information required to + complete the key exchange. Note that in this case the client and + server will generate the same Diffie-Hellman result (i.e., + + + +Dierks & Rescorla Standards Track [Page 92] + +RFC 5246 TLS August 2008 + + + pre_master_secret) every time they communicate. To prevent the + pre_master_secret from staying in memory any longer than necessary, + it should be converted into the master_secret as soon as possible. + Client Diffie-Hellman parameters must be compatible with those + supplied by the server for the key exchange to work. + + If the client has a standard DSA or RSA certificate or is + unauthenticated, it sends a set of temporary parameters to the server + in the client key exchange message, then optionally uses a + certificate verify message to authenticate itself. + + If the same DH keypair is to be used for multiple handshakes, either + because the client or server has a certificate containing a fixed DH + keypair or because the server is reusing DH keys, care must be taken + to prevent small subgroup attacks. Implementations SHOULD follow the + guidelines found in [SUBGROUP]. + + Small subgroup attacks are most easily avoided by using one of the + DHE cipher suites and generating a fresh DH private key (X) for each + handshake. If a suitable base (such as 2) is chosen, g^X mod p can + be computed very quickly; therefore, the performance cost is + minimized. Additionally, using a fresh key for each handshake + provides Perfect Forward Secrecy. Implementations SHOULD generate a + new X for each handshake when using DHE cipher suites. + + Because TLS allows the server to provide arbitrary DH groups, the + client should verify that the DH group is of suitable size as defined + by local policy. The client SHOULD also verify that the DH public + exponent appears to be of adequate size. [KEYSIZ] provides a useful + guide to the strength of various group sizes. The server MAY choose + to assist the client by providing a known group, such as those + defined in [IKEALG] or [MODP]. These can be verified by simple + comparison. + +F.1.2. Version Rollback Attacks + + Because TLS includes substantial improvements over SSL Version 2.0, + attackers may try to make TLS-capable clients and servers fall back + to Version 2.0. This attack can occur if (and only if) two TLS- + capable parties use an SSL 2.0 handshake. + + Although the solution using non-random PKCS #1 block type 2 message + padding is inelegant, it provides a reasonably secure way for Version + 3.0 servers to detect the attack. This solution is not secure + against attackers who can brute-force the key and substitute a new + ENCRYPTED-KEY-DATA message containing the same key (but with normal + padding) before the application-specified wait threshold has expired. + Altering the padding of the least-significant 8 bytes of the PKCS + + + +Dierks & Rescorla Standards Track [Page 93] + +RFC 5246 TLS August 2008 + + + padding does not impact security for the size of the signed hashes + and RSA key lengths used in the protocol, since this is essentially + equivalent to increasing the input block size by 8 bytes. + +F.1.3. Detecting Attacks Against the Handshake Protocol + + An attacker might try to influence the handshake exchange to make the + parties select different encryption algorithms than they would + normally choose. + + For this attack, an attacker must actively change one or more + handshake messages. If this occurs, the client and server will + compute different values for the handshake message hashes. As a + result, the parties will not accept each others' Finished messages. + Without the master_secret, the attacker cannot repair the Finished + messages, so the attack will be discovered. + +F.1.4. Resuming Sessions + + When a connection is established by resuming a session, new + ClientHello.random and ServerHello.random values are hashed with the + session's master_secret. Provided that the master_secret has not + been compromised and that the secure hash operations used to produce + the encryption keys and MAC keys are secure, the connection should be + secure and effectively independent from previous connections. + Attackers cannot use known encryption keys or MAC secrets to + compromise the master_secret without breaking the secure hash + operations. + + Sessions cannot be resumed unless both the client and server agree. + If either party suspects that the session may have been compromised, + or that certificates may have expired or been revoked, it should + force a full handshake. An upper limit of 24 hours is suggested for + session ID lifetimes, since an attacker who obtains a master_secret + may be able to impersonate the compromised party until the + corresponding session ID is retired. Applications that may be run in + relatively insecure environments should not write session IDs to + stable storage. + +F.2. Protecting Application Data + + The master_secret is hashed with the ClientHello.random and + ServerHello.random to produce unique data encryption keys and MAC + secrets for each connection. + + Outgoing data is protected with a MAC before transmission. To + prevent message replay or modification attacks, the MAC is computed + from the MAC key, the sequence number, the message length, the + + + +Dierks & Rescorla Standards Track [Page 94] + +RFC 5246 TLS August 2008 + + + message contents, and two fixed character strings. The message type + field is necessary to ensure that messages intended for one TLS + record layer client are not redirected to another. The sequence + number ensures that attempts to delete or reorder messages will be + detected. Since sequence numbers are 64 bits long, they should never + overflow. Messages from one party cannot be inserted into the + other's output, since they use independent MAC keys. Similarly, the + server write and client write keys are independent, so stream cipher + keys are used only once. + + If an attacker does break an encryption key, all messages encrypted + with it can be read. Similarly, compromise of a MAC key can make + message-modification attacks possible. Because MACs are also + encrypted, message-alteration attacks generally require breaking the + encryption algorithm as well as the MAC. + + Note: MAC keys may be larger than encryption keys, so messages can + remain tamper resistant even if encryption keys are broken. + +F.3. Explicit IVs + + [CBCATT] describes a chosen plaintext attack on TLS that depends on + knowing the IV for a record. Previous versions of TLS [TLS1.0] used + the CBC residue of the previous record as the IV and therefore + enabled this attack. This version uses an explicit IV in order to + protect against this attack. + +F.4. Security of Composite Cipher Modes + + TLS secures transmitted application data via the use of symmetric + encryption and authentication functions defined in the negotiated + cipher suite. The objective is to protect both the integrity and + confidentiality of the transmitted data from malicious actions by + active attackers in the network. It turns out that the order in + which encryption and authentication functions are applied to the data + plays an important role for achieving this goal [ENCAUTH]. + + The most robust method, called encrypt-then-authenticate, first + applies encryption to the data and then applies a MAC to the + ciphertext. This method ensures that the integrity and + confidentiality goals are obtained with ANY pair of encryption and + MAC functions, provided that the former is secure against chosen + plaintext attacks and that the MAC is secure against chosen-message + attacks. TLS uses another method, called authenticate-then-encrypt, + in which first a MAC is computed on the plaintext and then the + concatenation of plaintext and MAC is encrypted. This method has + been proven secure for CERTAIN combinations of encryption functions + and MAC functions, but it is not guaranteed to be secure in general. + + + +Dierks & Rescorla Standards Track [Page 95] + +RFC 5246 TLS August 2008 + + + In particular, it has been shown that there exist perfectly secure + encryption functions (secure even in the information-theoretic sense) + that combined with any secure MAC function, fail to provide the + confidentiality goal against an active attack. Therefore, new cipher + suites and operation modes adopted into TLS need to be analyzed under + the authenticate-then-encrypt method to verify that they achieve the + stated integrity and confidentiality goals. + + Currently, the security of the authenticate-then-encrypt method has + been proven for some important cases. One is the case of stream + ciphers in which a computationally unpredictable pad of the length of + the message, plus the length of the MAC tag, is produced using a + pseudorandom generator and this pad is exclusive-ORed with the + concatenation of plaintext and MAC tag. The other is the case of CBC + mode using a secure block cipher. In this case, security can be + shown if one applies one CBC encryption pass to the concatenation of + plaintext and MAC and uses a new, independent, and unpredictable IV + for each new pair of plaintext and MAC. In versions of TLS prior to + 1.1, CBC mode was used properly EXCEPT that it used a predictable IV + in the form of the last block of the previous ciphertext. This made + TLS open to chosen plaintext attacks. This version of the protocol + is immune to those attacks. For exact details in the encryption + modes proven secure, see [ENCAUTH]. + +F.5. Denial of Service + + TLS is susceptible to a number of denial-of-service (DoS) attacks. + In particular, an attacker who initiates a large number of TCP + connections can cause a server to consume large amounts of CPU for + doing RSA decryption. However, because TLS is generally used over + TCP, it is difficult for the attacker to hide his point of origin if + proper TCP SYN randomization is used [SEQNUM] by the TCP stack. + + Because TLS runs over TCP, it is also susceptible to a number of DoS + attacks on individual connections. In particular, attackers can + forge RSTs, thereby terminating connections, or forge partial TLS + records, thereby causing the connection to stall. These attacks + cannot in general be defended against by a TCP-using protocol. + Implementors or users who are concerned with this class of attack + should use IPsec AH [AH] or ESP [ESP]. + +F.6. Final Notes + + For TLS to be able to provide a secure connection, both the client + and server systems, keys, and applications must be secure. In + addition, the implementation must be free of security errors. + + + + + +Dierks & Rescorla Standards Track [Page 96] + +RFC 5246 TLS August 2008 + + + The system is only as strong as the weakest key exchange and + authentication algorithm supported, and only trustworthy + cryptographic functions should be used. Short public keys and + anonymous servers should be used with great caution. Implementations + and users must be careful when deciding which certificates and + certificate authorities are acceptable; a dishonest certificate + authority can do tremendous damage. + +Normative References + + [AES] National Institute of Standards and Technology, + "Specification for the Advanced Encryption Standard (AES)" + FIPS 197. November 26, 2001. + + [3DES] National Institute of Standards and Technology, + "Recommendation for the Triple Data Encryption Algorithm + (TDEA) Block Cipher", NIST Special Publication 800-67, May + 2004. + + [DSS] NIST FIPS PUB 186-2, "Digital Signature Standard", + National Institute of Standards and Technology, U.S. + Department of Commerce, 2000. + + [HMAC] Krawczyk, H., Bellare, M., and R. Canetti, "HMAC: Keyed- + Hashing for Message Authentication", RFC 2104, February + 1997. + + [MD5] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, + April 1992. + + [PKCS1] Jonsson, J. and B. Kaliski, "Public-Key Cryptography + Standards (PKCS) #1: RSA Cryptography Specifications + Version 2.1", RFC 3447, February 2003. + + [PKIX] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet + X.509 Public Key Infrastructure Certificate and + Certificate Revocation List (CRL) Profile", RFC 3280, + April 2002. + + [SCH] B. Schneier. "Applied Cryptography: Protocols, Algorithms, + and Source Code in C, 2nd ed.", Published by John Wiley & + Sons, Inc. 1996. + + [SHS] NIST FIPS PUB 180-2, "Secure Hash Standard", National + Institute of Standards and Technology, U.S. Department of + Commerce, August 2002. + + + + + +Dierks & Rescorla Standards Track [Page 97] + +RFC 5246 TLS August 2008 + + + [REQ] Bradner, S., "Key words for use in RFCs to Indicate + Requirement Levels", BCP 14, RFC 2119, March 1997. + + [RFC2434] Narten, T. and H. Alvestrand, "Guidelines for Writing an + IANA Considerations Section in RFCs", BCP 26, RFC 2434, + October 1998. + + [X680] ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002, + Information technology - Abstract Syntax Notation One + (ASN.1): Specification of basic notation. + + [X690] ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002, + Information technology - ASN.1 encoding Rules: + Specification of Basic Encoding Rules (BER), Canonical + Encoding Rules (CER) and Distinguished Encoding Rules + (DER). + +Informative References + + [AEAD] McGrew, D., "An Interface and Algorithms for Authenticated + Encryption", RFC 5116, January 2008. + + [AH] Kent, S., "IP Authentication Header", RFC 4302, December + 2005. + + [BLEI] Bleichenbacher D., "Chosen Ciphertext Attacks against + Protocols Based on RSA Encryption Standard PKCS #1" in + Advances in Cryptology -- CRYPTO'98, LNCS vol. 1462, + pages: 1-12, 1998. + + [CBCATT] Moeller, B., "Security of CBC Ciphersuites in SSL/TLS: + Problems and Countermeasures", + http://www.openssl.org/~bodo/tls-cbc.txt. + + [CBCTIME] Canvel, B., Hiltgen, A., Vaudenay, S., and M. Vuagnoux, + "Password Interception in a SSL/TLS Channel", Advances in + Cryptology -- CRYPTO 2003, LNCS vol. 2729, 2003. + + [CCM] "NIST Special Publication 800-38C: The CCM Mode for + Authentication and Confidentiality", + http://csrc.nist.gov/publications/nistpubs/800-38C/ + SP800-38C.pdf + + [DES] National Institute of Standards and Technology, "Data + Encryption Standard (DES)", FIPS PUB 46-3, October 1999. + + + + + + +Dierks & Rescorla Standards Track [Page 98] + +RFC 5246 TLS August 2008 + + + [DSS-3] NIST FIPS PUB 186-3 Draft, "Digital Signature Standard", + National Institute of Standards and Technology, U.S. + Department of Commerce, 2006. + + [ECDSA] American National Standards Institute, "Public Key + Cryptography for the Financial Services Industry: The + Elliptic Curve Digital Signature Algorithm (ECDSA)", ANS + X9.62-2005, November 2005. + + [ENCAUTH] Krawczyk, H., "The Order of Encryption and Authentication + for Protecting Communications (Or: How Secure is SSL?)", + Crypto 2001. + + [ESP] Kent, S., "IP Encapsulating Security Payload (ESP)", RFC + 4303, December 2005. + + [FI06] Hal Finney, "Bleichenbacher's RSA signature forgery based + on implementation error", ietf-openpgp@imc.org mailing + list, 27 August 2006, http://www.imc.org/ietf-openpgp/ + mail-archive/msg14307.html. + + [GCM] Dworkin, M., NIST Special Publication 800-38D, + "Recommendation for Block Cipher Modes of Operation: + Galois/Counter Mode (GCM) and GMAC", November 2007. + + [IKEALG] Schiller, J., "Cryptographic Algorithms for Use in the + Internet Key Exchange Version 2 (IKEv2)", RFC 4307, + December 2005. + + [KEYSIZ] Orman, H. and P. Hoffman, "Determining Strengths For + Public Keys Used For Exchanging Symmetric Keys", BCP 86, + RFC 3766, April 2004. + + [KPR03] Klima, V., Pokorny, O., Rosa, T., "Attacking RSA-based + Sessions in SSL/TLS", http://eprint.iacr.org/2003/052/, + March 2003. + + [MODP] Kivinen, T. and M. Kojo, "More Modular Exponential (MODP) + Diffie-Hellman groups for Internet Key Exchange (IKE)", + RFC 3526, May 2003. + + [PKCS6] RSA Laboratories, "PKCS #6: RSA Extended Certificate + Syntax Standard", version 1.5, November 1993. + + [PKCS7] RSA Laboratories, "PKCS #7: RSA Cryptographic Message + Syntax Standard", version 1.5, November 1993. + + + + + +Dierks & Rescorla Standards Track [Page 99] + +RFC 5246 TLS August 2008 + + + [RANDOM] Eastlake, D., 3rd, Schiller, J., and S. Crocker, + "Randomness Requirements for Security", BCP 106, RFC 4086, + June 2005. + + [RFC3749] Hollenbeck, S., "Transport Layer Security Protocol + Compression Methods", RFC 3749, May 2004. + + [RFC4366] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., + and T. Wright, "Transport Layer Security (TLS) + Extensions", RFC 4366, April 2006. + + [RSA] R. Rivest, A. Shamir, and L. M. Adleman, "A Method for + Obtaining Digital Signatures and Public-Key + Cryptosystems", Communications of the ACM, v. 21, n. 2, + Feb 1978, pp. 120-126. + + [SEQNUM] Bellovin, S., "Defending Against Sequence Number Attacks", + RFC 1948, May 1996. + + [SSL2] Hickman, Kipp, "The SSL Protocol", Netscape Communications + Corp., Feb 9, 1995. + + [SSL3] A. Freier, P. Karlton, and P. Kocher, "The SSL 3.0 + Protocol", Netscape Communications Corp., Nov 18, 1996. + + [SUBGROUP] Zuccherato, R., "Methods for Avoiding the "Small-Subgroup" + Attacks on the Diffie-Hellman Key Agreement Method for + S/MIME", RFC 2785, March 2000. + + [TCP] Postel, J., "Transmission Control Protocol", STD 7, RFC + 793, September 1981. + + [TIMING] Boneh, D., Brumley, D., "Remote timing attacks are + practical", USENIX Security Symposium 2003. + + [TLSAES] Chown, P., "Advanced Encryption Standard (AES) + Ciphersuites for Transport Layer Security (TLS)", RFC + 3268, June 2002. + + [TLSECC] Blake-Wilson, S., Bolyard, N., Gupta, V., Hawk, C., and B. + Moeller, "Elliptic Curve Cryptography (ECC) Cipher Suites + for Transport Layer Security (TLS)", RFC 4492, May 2006. + + [TLSEXT] Eastlake, D., 3rd, "Transport Layer Security (TLS) + Extensions: Extension Definitions", Work in Progress, + February 2008. + + + + + +Dierks & Rescorla Standards Track [Page 100] + +RFC 5246 TLS August 2008 + + + [TLSPGP] Mavrogiannopoulos, N., "Using OpenPGP Keys for Transport + Layer Security (TLS) Authentication", RFC 5081, November + 2007. + + [TLSPSK] Eronen, P., Ed., and H. Tschofenig, Ed., "Pre-Shared Key + Ciphersuites for Transport Layer Security (TLS)", RFC + 4279, December 2005. + + [TLS1.0] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", + RFC 2246, January 1999. + + [TLS1.1] Dierks, T. and E. Rescorla, "The Transport Layer Security + (TLS) Protocol Version 1.1", RFC 4346, April 2006. + + [X501] ITU-T Recommendation X.501: Information Technology - Open + Systems Interconnection - The Directory: Models, 1993. + + [XDR] Eisler, M., Ed., "XDR: External Data Representation + Standard", STD 67, RFC 4506, May 2006. + +Working Group Information + + The discussion list for the IETF TLS working group is located at the + e-mail address <tls@ietf.org>. Information on the group and + information on how to subscribe to the list is at + <https://www1.ietf.org/mailman/listinfo/tls> + + Archives of the list can be found at: + <http://www.ietf.org/mail-archive/web/tls/current/index.html> + +Contributors + + Christopher Allen (co-editor of TLS 1.0) + Alacrity Ventures + ChristopherA@AlacrityManagement.com + + Martin Abadi + University of California, Santa Cruz + abadi@cs.ucsc.edu + + Steven M. Bellovin + Columbia University + smb@cs.columbia.edu + + Simon Blake-Wilson + BCI + sblakewilson@bcisse.com + + + + +Dierks & Rescorla Standards Track [Page 101] + +RFC 5246 TLS August 2008 + + + Ran Canetti + IBM + canetti@watson.ibm.com + + Pete Chown + Skygate Technology Ltd + pc@skygate.co.uk + + Taher Elgamal + taher@securify.com + Securify + + Pasi Eronen + pasi.eronen@nokia.com + Nokia + + Anil Gangolli + anil@busybuddha.org + + Kipp Hickman + + Alfred Hoenes + + David Hopwood + Independent Consultant + david.hopwood@blueyonder.co.uk + + Phil Karlton (co-author of SSLv3) + + Paul Kocher (co-author of SSLv3) + Cryptography Research + paul@cryptography.com + + Hugo Krawczyk + IBM + hugo@ee.technion.ac.il + + Jan Mikkelsen + Transactionware + janm@transactionware.com + + Magnus Nystrom + RSA Security + magnus@rsasecurity.com + + Robert Relyea + Netscape Communications + relyea@netscape.com + + + +Dierks & Rescorla Standards Track [Page 102] + +RFC 5246 TLS August 2008 + + + Jim Roskind + Netscape Communications + jar@netscape.com + + Michael Sabin + + Dan Simon + Microsoft, Inc. + dansimon@microsoft.com + + Tom Weinstein + + Tim Wright + Vodafone + timothy.wright@vodafone.com + +Editors' Addresses + + Tim Dierks + Independent + EMail: tim@dierks.org + + Eric Rescorla + RTFM, Inc. + EMail: ekr@rtfm.com + + + + + + + + + + + + + + + + + + + + + + + + + + +Dierks & Rescorla Standards Track [Page 103] + +RFC 5246 TLS August 2008 + + +Full Copyright Statement + + Copyright (C) The IETF Trust (2008). + + This document is subject to the rights, licenses and restrictions + contained in BCP 78, and except as set forth therein, the authors + retain all their rights. + + This document and the information contained herein are provided on an + "AS IS" basis and THE CONTRIBUTOR, THE ORGANIZATION HE/SHE REPRESENTS + OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY, THE IETF TRUST AND + THE INTERNET ENGINEERING TASK FORCE DISCLAIM ALL WARRANTIES, EXPRESS + OR IMPLIED, INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF + THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED + WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE. + +Intellectual Property + + The IETF takes no position regarding the validity or scope of any + Intellectual Property Rights or other rights that might be claimed to + pertain to the implementation or use of the technology described in + this document or the extent to which any license under such rights + might or might not be available; nor does it represent that it has + made any independent effort to identify any such rights. Information + on the procedures with respect to rights in RFC documents can be + found in BCP 78 and BCP 79. + + Copies of IPR disclosures made to the IETF Secretariat and any + assurances of licenses to be made available, or the result of an + attempt made to obtain a general license or permission for the use of + such proprietary rights by implementers or users of this + specification can be obtained from the IETF on-line IPR repository at + http://www.ietf.org/ipr. + + The IETF invites any interested party to bring to its attention any + copyrights, patents or patent applications, or other proprietary + rights that may cover technology that may be required to implement + this standard. Please address the information to the IETF at + ietf-ipr@ietf.org. + + + + + + + + + + + + +Dierks & Rescorla Standards Track [Page 104] + |