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authorSimon Josefsson <simon@josefsson.org>2005-06-24 11:09:33 +0000
committerSimon Josefsson <simon@josefsson.org>2005-06-24 11:09:33 +0000
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+
+
+
+
+
+
+ Tim Dierks
+ Independent
+ Eric Rescorla
+INTERNET-DRAFT RTFM, Inc.
+<draft-ietf-tls-rfc2246-bis-13.txt> June 2005 (Expires December 2005)
+
+ The TLS Protocol
+ Version 1.1
+
+Status of this Memo
+
+By submitting this Internet-Draft, each author represents that
+any applicable patent or other IPR claims of which he or she is
+aware have been or will be disclosed, and any of which he or she
+becomes aware will be disclosed, in accordance with Section 6 of
+BCP 79.
+
+Internet-Drafts are working documents of the Internet Engineering
+Task Force (IETF), its areas, and its working groups. Note that other
+groups may also distribute working documents as Internet-Drafts.
+
+Internet-Drafts are draft documents valid for a maximum of six months
+and may be updated, replaced, or obsoleted by other documents at any
+time. It is inappropriate to use Internet-Drafts as reference
+material or to cite them other than a "work in progress."
+
+The list of current Internet-Drafts can be accessed at
+http://www.ietf.org/1id-abstracts.html
+
+The list of Internet-Draft Shadow Directories can be accessed at
+http://www.ietf.org/shadow.html
+
+Copyright Notice
+
+ Copyright (C) The Internet Society (2005). All Rights Reserved.
+
+Abstract
+
+ This document specifies Version 1.1 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
+ 5 1.1 Differences from TLS 1.0
+
+
+
+Dierks & Rescorla Standards Track [Page 1] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 6 1.1 Requirements Terminology
+ 7 2. Goals
+ 7 3. Goals of this document
+ 7 4. Presentation language
+ 8 4.1. Basic block size
+ 9 4.2. Miscellaneous
+ 9 4.3. Vectors
+ 9 4.4. Numbers
+ 10 4.5. Enumerateds
+ 10 4.6. Constructed types
+ 11 4.6.1. Variants
+ 12 4.7. Cryptographic attributes
+ 13 4.8. Constants
+ 14 5. HMAC and the pseudorandom function
+ 14 6. The TLS Record Protocol
+ 17 6.1. Connection states
+ 18 6.2. Record layer
+ 20 6.2.1. Fragmentation
+ 20 6.2.2. Record compression and decompression
+ 21 6.2.3. Record payload protection
+ 22 6.2.3.1. Null or standard stream cipher
+ 23 6.2.3.2. CBC block cipher
+ 23 6.3. Key calculation
+ 26 7. The TLS Handshaking Protocols
+ 27 7.1. Change cipher spec protocol
+ 28 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
+ 41 7.4.2. Server certificate
+ 42 7.4.3. Server key exchange message
+ 44 7.4.4. Certificate request
+ 46 7.4.5. Server hello done
+ 48 7.4.6. Client certificate
+ 48 7.4.7. Client key exchange message
+ 48 7.4.7.1. RSA encrypted premaster secret message
+ 49 7.4.7.2. Client Diffie-Hellman public value
+ 51 7.4.8. Certificate verify
+ 52 7.4.9. Finished
+ 52 8. Cryptographic computations
+ 53 8.1. Computing the master secret
+ 54 8.1.1. RSA
+ 55 8.1.2. Diffie-Hellman
+
+
+
+Dierks & Rescorla Standards Track [Page 2] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 55 9. Mandatory Cipher Suites
+ 55 A. Protocol constant values
+ 57 A.1. Record layer
+ 57 A.2. Change cipher specs message
+ 58 A.3. Alert messages
+ 58 A.4. Handshake protocol
+ 59 A.4.1. Hello messages
+ 59 A.4.2. Server authentication and key exchange messages
+ 60 A.4.3. Client authentication and key exchange messages
+ 61 A.4.4. Handshake finalization message
+ 62 A.5. The CipherSuite
+ 62 A.6. The Security Parameters
+ 65 B. Glossary
+ 67 C. CipherSuite definitions
+ 71 D. Implementation Notes
+ 73 D.1 Random Number Generation and Seeding
+ 73 D.2 Certificates and authentication
+ 73 D.3 CipherSuites
+ 73 E. Backward Compatibility With SSL
+ 74 E.1. Version 2 client hello
+ 75 E.2. Avoiding man-in-the-middle version rollback
+ 76 F. Security analysis
+ 78 F.1. Handshake protocol
+ 78 F.1.1. Authentication and key exchange
+ 78 F.1.1.1. Anonymous key exchange
+ 78 F.1.1.2. RSA key exchange and authentication
+ 79 F.1.1.3. Diffie-Hellman key exchange with authentication
+ 80 F.1.2. Version rollback attacks
+ 80 F.1.3. Detecting attacks against the handshake protocol
+ 81 F.1.4. Resuming sessions
+ 81 F.1.5. MD5 and SHA
+ 82 F.2. Protecting application data
+ 82 F.3. Explicit IVs
+ 82 F.4 Security of Composite Cipher Modes
+ 83 F.5 Denial of Service
+ 84 F.6. Final notes
+ 84
+
+
+Change history
+
+ 22-Jun-05 ekr@rtfm.com
+ * IESG comments
+ * IANA comments
+ * Cleaned up some references
+
+ 31-May-05 ekr@rtfm.com
+ * IETF Last Call comments (minor cleanups)
+
+
+
+Dierks & Rescorla Standards Track [Page 3] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 03-Dec-04 ekr@rtfm.com
+ * Removed export cipher suites
+
+ 26-Oct-04 ekr@rtfm.com
+ * Numerous cleanups from Last Call comments
+
+ 10-Aug-04 ekr@rtfm.com
+ * Added clarifying material about interleaved application data.
+
+ 27-Jul-04 ekr@rtfm.com
+ * Premature closes no longer cause a session to be nonresumable.
+ Response to WG consensus.
+
+ * Added IANA considerations and registry for cipher suites
+ and ClientCertificateTypes
+
+ 26-Jun-03 ekr@rtfm.com
+ * Incorporated Last Call comments from Franke Marcus, Jack Lloyd,
+ Brad Wetmore, and others.
+
+ 22-Apr-03 ekr@rtfm.com
+ * coverage of the Vaudenay, Boneh-Brumley, and KPR attacks
+ * cleaned up IV text a bit.
+ * Added discussion of Denial of Service attacks.
+
+ 11-Feb-02 ekr@rtfm.com
+ * Clarified the behavior of empty certificate lists [Nelson Bolyard]
+ * Added text explaining the security implications of authenticate
+ then encrypt.
+ * Cleaned up the explicit IV text.
+ * Added some more acknowledgement names
+
+ 02-Nov-02 ekr@rtfm.com
+ * Changed this to be TLS 1.1.
+ * Added fixes for the Rogaway and Vaudenay CBC attacks
+ * Separated references into normative and informative
+
+ 01-Mar-02 ekr@rtfm.com
+ * Tightened up the language in F.1.1.2 [Peter Watkins]
+ * Fixed smart quotes [Bodo Moeller]
+ * Changed handling of padding errors to prevent CBC-based attack
+ [Bodo Moeller]
+ * Fixed certificate_list spec in the appendix [Aman Sawrup]
+ * Fixed a bug in the V2 definitions [Aman Sawrup]
+ * Fixed S 7.2.1 to point out that you don't need a close notify
+ if you just sent some other fatal alert [Andreas Sterbenz]
+ * Marked alert 41 reserved [Andreas Sterbenz]
+ * Changed S 7.4.2 to point out that 512-bit keys cannot be used for
+
+
+
+Dierks & Rescorla Standards Track [Page 4] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ signing [Andreas Sterbenz]
+ * Added reserved client key types from SSLv3 [Andreas Sterbenz]
+ * Changed EXPORT40 to "40-bit EXPORT" in S 9 [Andreas Sterbenz]
+ * Removed RSA patent statement [Andreas Sterbenz]
+ * Removed references to BSAFE and RSAREF [Andreas Sterbenz]
+
+ 14-Feb-02 ekr@rtfm.com
+ * Re-converted to I-D from RFC
+ * Made RSA/3DES the mandatory cipher suite.
+ * Added discussion of the EncryptedPMS encoding and PMS version number
+ issues to 7.4.7.1
+ * Removed the requirement in 7.4.1.3 that the Server random must be
+ different from the Client random, since these are randomly generated
+ and we don't expect servers to reject Server random values which
+ coincidentally are the same as the Client random.
+ * Replaced may/should/must with MAY/SHOULD/MUST where appropriate.
+ In many cases, shoulds became MUSTs, where I believed that was the
+ actual sense of the text. Added an RFC 2119 bulletin.
+ * Clarified the meaning of "empty certificate" message. [Peter Gutmann]
+ * Redid the CertificateRequest grammar to allow no distinguished names.
+ [Peter Gutmann]
+ * Removed the reference to requiring the master secret to generate
+ the CertificateVerify in F.1.1 [Bodo Moeller]
+ * Deprecated EXPORT40.
+ * Fixed a bunch of errors in the SSLv2 backward compatible client hello.
+
+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., DES [DES], 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, MD5, 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
+
+
+
+Dierks & Rescorla Standards Track [Page 5] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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], DSS [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.
+
+ 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 up to the judgment of the designers and
+ implementors of protocols which run on top of TLS.
+
+1.1 Differences from TLS 1.0
+ This document is a revision of the TLS 1.0 [TLS1.0] protocol which
+ contains some small security improvements, clarifications, and
+ editorial improvements. The major changes are:
+
+ - The implicit Initialization Vector (IV) is replaced with an
+ explicit
+ IV to protect against CBC attacks [CBCATT].
+
+ - Handling of padding errors is changed to use the bad_record_mac
+ alert rather than the decryption_failed alert to protect against
+ CBC attacks.
+
+ - IANA registries are defined for protocol parameters.
+
+ - Premature closes no longer cause a session to be nonresumable.
+
+
+
+Dierks & Rescorla Standards Track [Page 6] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ - Additional informational notes were added for various new attacks
+ on TLS.
+
+ In addition, a number of minor clarifications and editorial
+ improvements were made.
+
+
+
+1.1 Requirements Terminology
+
+ Keywords "MUST", "MUST NOT", "REQUIRED", "SHOULD", "SHOULD NOT" and
+ "MAY" that appear in this document are to be interpreted as described
+ in RFC 2119 [REQ].
+
+2. Goals
+
+ The goals of TLS Protocol, in order of their priority, are:
+
+ 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 will then be able to
+ 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: to prevent
+ the need to create a new protocol (and risking the introduction
+ of possible new weaknesses) and to avoid 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.
+
+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 TLS 1.1, TLS 1.0, and SSL 3.0 do not
+ interoperate (although each protocol incorporates a mechanism by
+ which an implementation can back down prior versions. This document
+
+
+
+Dierks & Rescorla Standards Track [Page 7] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ is intended primarily for readers who will be implementing the
+ protocol and 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 nor 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, not to
+ have general application beyond that particular goal.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 8] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+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 bytestream 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.
+
+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 */
+
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 9] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ Variable length vectors are defined by specifying a subrange of legal
+ lengths, inclusively, using the notation <floor..ceiling>. When
+ 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, 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 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" or "big-endian" order; the uint32 represented by the hex
+ bytes 01 02 03 04 is equivalent to the decimal value 16909060.
+
+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
+
+
+
+
+Dierks & Rescorla Standards Track [Page 10] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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;
+
+ Enumerateds occupy 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;
+
+ 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]];
+
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 11] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ The fields within a structure may be qualified using the type's name
+ using 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. 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 en: Ten;
+ } [[fv]];
+ } [[Tv]];
+
+ For example:
+
+ enum { apple, orange } 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: V2; /* VariantBody, tag = orange */
+ } variant_body; /* optional label on variant */
+ } VariantRecord;
+
+ Variant structures may be qualified (narrowed) by specifying a value
+ for the selector prior to the type. For example, a
+
+
+
+Dierks & Rescorla Standards Track [Page 12] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ orange VariantRecord
+
+ is a narrowed type of a VariantRecord containing a variant_body of
+ type V2.
+
+4.7. Cryptographic attributes
+
+ The four cryptographic operations digital signing, stream cipher
+ encryption, block cipher encryption, and public key encryption are
+ designated digitally-signed, stream-ciphered, block-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).
+
+ In digital signing, one-way hash functions are used as input for a
+ signing algorithm. A digitally-signed element is encoded as an opaque
+ vector <0..2^16-1>, where the length is specified by the signing
+ algorithm and key.
+
+ In RSA signing, a 36-byte structure of two hashes (one SHA and one
+ MD5) is signed (encrypted with the private key). It is encoded with
+ PKCS #1 block type 0 or type 1 as described in [PKCS1A].
+
+ Note: the standard reference for PKCS#1 is now RFC 3447 [PKCS1B].
+ However, to minimize differences with TLS 1.0 text, we are using the
+ terminology of RFC 2313 [PKCS1A].
+
+ In DSS, the 20 bytes of the SHA hash are run directly through the
+ Digital Signing Algorithm with no additional hashing. This produces
+ two values, r and s. The DSS signature is an opaque vector, as above,
+ the contents of which are the DER encoding of:
+
+ Dss-Sig-Value ::= SEQUENCE {
+ r INTEGER,
+ s INTEGER
+ }
+
+ 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 which are block-ciphered
+ will be an exact multiple of the cipher block length.
+
+ In public key encryption, a public key algorithm is used to encrypt
+
+
+
+Dierks & Rescorla Standards Track [Page 13] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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 signing
+ algorithm and key.
+
+ An RSA encrypted value is encoded with PKCS #1 block type 2 as
+ described in [PKCS1A].
+
+ In the following example:
+
+ stream-ciphered struct {
+ uint8 field1;
+ uint8 field2;
+ digitally-signed opaque hash[20];
+ } UserType;
+
+ The contents of hash are used as input for the signing algorithm,
+ then the entire structure is encrypted with a stream cipher. The
+ length of this structure, in bytes would be equal to 2 bytes for
+ field1 and field2, plus two bytes for the length of the signature,
+ plus the length of the output of the signing algorithm. This is known
+ due to the fact that 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
+
+ A number of operations in the TLS record and handshake layer required
+ a keyed MAC; this is a secure digest of some data protected by a
+ secret. Forging the MAC is infeasible without knowledge of the MAC
+ secret. The construction we use for this operation is known as HMAC,
+ described in [HMAC].
+
+
+
+Dierks & Rescorla Standards Track [Page 14] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ HMAC can be used with a variety of different hash algorithms. TLS
+ uses it in the handshake with two different algorithms: MD5 and
+ SHA-1, denoting these as HMAC_MD5(secret, data) and HMAC_SHA(secret,
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 15] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ data). Additional hash algorithms can be defined by cipher suites and
+ used to protect record data, but MD5 and SHA-1 are hard coded into
+ the description of the handshaking for this version of the protocol.
+
+ In addition, a construction is required to do expansion of secrets
+ into blocks of data for the purposes of key generation or validation.
+ This pseudo-random function (PRF) takes as input a secret, a seed,
+ and an identifying label and produces an output of arbitrary length.
+
+ In order to make the PRF as secure as possible, it uses two hash
+ algorithms in a way which should guarantee its security if either
+ algorithm remains secure.
+
+ First, we define a data expansion function, P_hash(secret, data)
+ which uses a single hash function to expand a secret and seed into an
+ arbitrary quantity of output:
+
+ 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 is necessary to produce the
+ required quantity of data. For example, if P_SHA-1 was being used to
+ create 64 bytes of data, it would have to be iterated 4 times
+ (through A(4)), creating 80 bytes of output data; the last 16 bytes
+ of the final iteration would then be discarded, leaving 64 bytes of
+ output data.
+
+ TLS's PRF is created by splitting the secret into two halves and
+ using one half to generate data with P_MD5 and the other half to
+ generate data with P_SHA-1, then exclusive-or'ing the outputs of
+ these two expansion functions together.
+
+ S1 and S2 are the two halves of the secret and each is the same
+ length. S1 is taken from the first half of the secret, S2 from the
+ second half. Their length is created by rounding up the length of the
+ overall secret divided by two; thus, if the original secret is an odd
+ number of bytes long, the last byte of S1 will be the same as the
+ first byte of S2.
+
+ L_S = length in bytes of secret;
+ L_S1 = L_S2 = ceil(L_S / 2);
+
+
+
+Dierks & Rescorla Standards Track [Page 16] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ The secret is partitioned into two halves (with the possibility of
+ one shared byte) as described above, S1 taking the first L_S1 bytes
+ and S2 the last L_S2 bytes.
+
+ The PRF is then defined as the result of mixing the two pseudorandom
+ streams by exclusive-or'ing them together.
+
+ PRF(secret, label, seed) = P_MD5(S1, label + seed) XOR
+ P_SHA-1(S2, 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
+
+ Note that because MD5 produces 16 byte outputs and SHA-1 produces 20
+ byte outputs, the boundaries of their internal iterations will not be
+ aligned; to generate a 80 byte output will involve P_MD5 being
+ iterated through A(5), while P_SHA-1 will only iterate through A(4).
+
+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, and reassembled, then delivered to
+ higher level clients.
+
+ Four record protocol clients 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 types can be
+ supported by the record protocol. Any new record types SHOULD
+ allocate type values immediately beyond the ContentType values for
+ the four record types described here (see Appendix A.1). All such
+ values must be defined by RFC 2434 Standards Action. See section 11
+ for IANA Considerations for ContentType values.
+
+ If a TLS implementation receives a record type it does not
+ understand, it SHOULD just ignore it. Any protocol designed for use
+ over TLS MUST be carefully designed to deal with all possible attacks
+ against it. Note that because the type and length of a record are
+ not protected by encryption, care SHOULD be taken to minimize the
+ value of traffic analysis of these values.
+
+
+
+Dierks & Rescorla Standards Track [Page 17] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+6.1. Connection states
+
+ A TLS connection state is the operating environment of the TLS Record
+ Protocol. It specifies a compression algorithm, encryption algorithm,
+ and MAC algorithm. In addition, the parameters for these algorithms
+ are known: the MAC secret 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 Change Cipher Spec 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 which 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.
+
+ bulk encryption algorithm
+ An algorithm to be used for bulk encryption. This specification
+ includes the key size of this algorithm, how much of that key is
+ secret, whether it is a block or stream cipher, the block size of
+ the cipher (if appropriate).
+
+ MAC algorithm
+ An algorithm to be used for message authentication. This
+ specification includes the size of the hash which is 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.
+
+
+
+
+Dierks & Rescorla Standards Track [Page 18] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ server random
+ A 32 byte value provided by the server.
+
+ These parameters are defined in the presentation language as:
+
+ enum { server, client } ConnectionEnd;
+
+ enum { null, rc4, rc2, des, 3des, des40, idea, aes } BulkCipherAlgorithm;
+
+ enum { stream, block } CipherType;
+
+ enum { null, md5, sha } MACAlgorithm;
+
+ enum { null(0), (255) } CompressionMethod;
+
+ /* The algorithms specified in CompressionMethod,
+ BulkCipherAlgorithm, and MACAlgorithm may be added to. */
+
+ struct {
+ ConnectionEnd entity;
+ BulkCipherAlgorithm bulk_cipher_algorithm;
+ CipherType cipher_type;
+ uint8 key_size;
+ uint8 key_material_length;
+ MACAlgorithm mac_algorithm;
+ uint8 hash_size;
+ 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 four items:
+
+ client write MAC secret
+ server write MAC secret
+ client write key
+ server write key
+
+ 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
+
+
+
+Dierks & Rescorla Standards Track [Page 19] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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 the necessary state information
+ is to allow the stream to continue to encrypt or decrypt data.
+
+ MAC secret
+ The MAC secret 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 which is 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, minor;
+ } ProtocolVersion;
+
+ enum {
+ change_cipher_spec(20), alert(21), handshake(22),
+ application_data(23), (255)
+ } ContentType;
+
+
+
+Dierks & Rescorla Standards Track [Page 20] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ struct {
+ ContentType type;
+ ProtocolVersion version;
+ uint16 length;
+ opaque fragment[TLSPlaintext.length];
+ } TLSPlaintext;
+
+ type
+ The higher level protocol used to process the enclosed fragment.
+
+ version
+ The version of the protocol being employed. This document
+ describes TLS Version 1.1, which uses the version { 3, 2 }. The
+ version value 3.2 is historical: TLS version 1.1 is a minor
+ modification to the TLS 1.0 protocol, which was itself a minor
+ modification to the SSL 3.0 protocol, which bears the version
+ value 3.0. (See Appendix A.1).
+
+ length
+ The length (in bytes) of the following TLSPlaintext.fragment.
+ The length should 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.
+
+ Note: Data of different TLS Record layer content types MAY be
+ interleaved. Application data is generally of higher precedence
+ for transmission than other content types and therefore handshake
+ records may be held if application data is pending. 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.
+
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 21] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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 should report a fatal decompression failure error.
+
+ 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 should 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 (CipherSpec.cipher_type) {
+ case stream: GenericStreamCipher;
+ case block: GenericBlockCipher;
+ } fragment;
+ } TLSCiphertext;
+
+ type
+ The type field is identical to TLSCompressed.type.
+
+ version
+ The version field is identical to TLSCompressed.version.
+
+
+
+Dierks & Rescorla Standards Track [Page 22] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ length
+ The length (in bytes) of the following TLSCiphertext.fragment.
+ The length may not exceed 2^14 + 2048.
+
+ 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[CipherSpec.hash_size];
+ } GenericStreamCipher;
+
+ The MAC is generated as:
+
+ HMAC_hash(MAC_write_secret, seq_num + TLSCompressed.type +
+ TLSCompressed.version + TLSCompressed.length +
+ TLSCompressed.fragment));
+
+ where "+" denotes concatenation.
+
+ seq_num
+ The sequence number for this record.
+
+ hash
+ The hashing 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 CipherSuite 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).
+ TLSCiphertext.length is TLSCompressed.length plus
+ CipherSpec.hash_size.
+
+6.2.3.2. CBC block cipher
+
+ For block ciphers (such as RC2, DES, or AES), the encryption and MAC
+ functions convert TLSCompressed.fragment structures to and from block
+ TLSCiphertext.fragment structures.
+
+
+
+Dierks & Rescorla Standards Track [Page 23] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ block-ciphered struct {
+ opaque IV[CipherSpec.block_length];
+ opaque content[TLSCompressed.length];
+ opaque MAC[CipherSpec.hash_size];
+ uint8 padding[GenericBlockCipher.padding_length];
+ uint8 padding_length;
+ } GenericBlockCipher;
+
+ The MAC is generated as described in Section 6.2.3.1.
+
+ IV
+ Unlike previous versions of SSL and TLS, TLS 1.1 uses an explicit
+ IV in order to prevent the attacks described by [CBCATT].
+ We recommend the following equivalently strong procedures.
+ For clarity we use the following notation.
+
+ IV -- the transmitted value of the IV field in the
+ GenericBlockCipher structure.
+ CBC residue -- the last ciphertext block of the previous record
+ mask -- the actual value which the cipher XORs with the
+ plaintext prior to encryption of the first cipher block
+ of the record.
+
+ In prior versions of TLS, there was no IV field and the CBC residue
+ and mask were one and the same. See Sections 6.1, 6.2.3.2 and 6.3,
+ of [TLS1.0] for details of TLS 1.0 IV handling.
+
+ One of the following two algorithms SHOULD be used to generate the
+ per-record IV:
+
+ (1) Generate a cryptographically strong random string R of
+ length CipherSpec.block_length. Place R
+ in the IV field. Set the mask to R. Thus, the first
+ cipher block will be encrypted as E(R XOR Data).
+
+ (2) Generate a cryptographically strong random number R of
+ length CipherSpec.block_length and prepend it to the plaintext
+ prior to encryption. In
+ this case either:
+
+ (a) The cipher may use a fixed mask such as zero.
+ (b) The CBC residue from the previous record may be used
+ as the mask. This preserves maximum code compatibility
+ with TLS 1.0 and SSL 3. It also has the advantage that
+ it does not require the ability to quickly reset the IV,
+ which is known to be a problem on some systems.
+
+ In either 2(a) or 2(b) the data (R || data) is fed into the
+
+
+
+Dierks & Rescorla Standards Track [Page 24] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ encryption process. The first cipher block (containing
+ E(mask XOR R) is placed in the IV field. The first
+ block of content contains E(IV XOR data)
+
+ The following alternative procedure MAY be used: However, it has
+ not been demonstrated to be equivalently cryptographically strong
+ to the above procedures. The sender prepends a fixed block F to
+ the plaintext (or alternatively a block generated with a weak
+ PRNG). He then encrypts as in (2) above, using the CBC residue
+ from the previous block as the mask for the prepended block. Note
+ that in this case the mask for the first record transmitted by
+ the application (the Finished) MUST be generated using a
+ cryptographically strong PRNG.
+
+ The decryption operation for all three alternatives is the same.
+ The receiver decrypts the entire GenericBlockCipher structure and
+ then discards the first cipher block, corresponding to the IV
+ component.
+
+ 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 long, 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 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 SHOULD 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 TLSCompressed.length, CipherSpec.hash_size, 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, the length before padding is 82 bytes (this does not
+ include the IV, which may or may not be encrypted, as
+ discussed above). Thus, the padding length modulo 8 must be
+ equal to 6 in order to make the total length an even multiple
+
+
+
+Dierks & Rescorla Standards Track [Page 25] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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 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.
+
+6.3. Key calculation
+
+ The Record Protocol requires an algorithm to generate keys, and MAC
+ secrets from the security parameters provided by the handshake
+ protocol.
+
+ The master secret is hashed into a sequence of secure bytes, which
+ are assigned to the MAC secrets and keys required by the current
+ connection state (see Appendix A.6). CipherSpecs require a client
+ write MAC secret, a server write MAC secret, a client write key, and
+ a server write key, which are generated from the master secret in
+ that order. Unused values are empty.
+
+ When generating keys and MAC secrets, 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 +
+
+
+
+Dierks & Rescorla Standards Track [Page 26] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ SecurityParameters.client_random);
+
+ until enough output has been generated. Then the key_block is
+ partitioned as follows:
+
+ client_write_MAC_secret[SecurityParameters.hash_size]
+ server_write_MAC_secret[SecurityParameters.hash_size]
+ client_write_key[SecurityParameters.key_material_length]
+ server_write_key[SecurityParameters.key_material_length]
+
+
+ Implementation note:
+ The currently defined which requires the most material is
+ AES_256_CBC_SHA, defined in [TLSAES]. It requires 2 x 32 byte
+ keys and 2 x 20 byte MAC secrets, for a total 104 bytes of key
+ material.
+
+7. The TLS Handshaking Protocols
+
+ TLS has three subprotocols which are used to allow peers to agree
+ upon security parameters for the record layer, authenticate
+ themselves, instantiate negotiated security parameters, and
+ report error conditions to each other.
+
+ The Handshake Protocol is responsible for negotiating a session,
+ which consists of the following items:
+
+ session identifier
+ An arbitrary byte sequence chosen by the server to identify an
+ active or resumable session state.
+
+ peer certificate
+ X509v3 [X509] 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 bulk data encryption algorithm (such as null,
+ DES, etc.) and a MAC algorithm (such as MD5 or SHA). It also
+ defines cryptographic attributes such as the hash_size. (See
+ Appendix A.6 for formal definition)
+
+ master secret
+ 48-byte secret shared between the client and server.
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 27] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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 change cipher spec message is sent by both the client and 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.
+ (See section 6.1.) The change cipher spec message is sent during the
+ handshake after the security parameters have been agreed upon, but
+ before the verifying finished message is sent (see section 7.4.9).
+
+ 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 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
+
+
+
+Dierks & Rescorla Standards Track [Page 28] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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(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),
+ insufficient_security(71),
+ internal_error(80),
+ user_canceled(90),
+ no_renegotiation(100),
+ (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
+
+
+
+Dierks & Rescorla Standards Track [Page 29] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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 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 an 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. 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.
+
+ bad_record_mac
+ This alert is returned if a record is received with an incorrect
+
+
+
+Dierks & Rescorla Standards Track [Page 30] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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.
+
+ decryption_failed
+ This alert MAY be returned if 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.
+
+ Note: Differentiating between bad_record_mac and
+ decryption_failed alerts may permit certain attacks against CBC
+ mode as used in TLS [CBCATT]. It is preferable to uniformly use
+ the bad_record_mac alert to hide the specific type of the error.
+
+
+ record_overflow
+ A TLSCiphertext record was received which 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.
+
+ decompression_failure
+ The decompression function received improper input (e.g. data
+ that would expand to excessive length). This message is always
+ fatal.
+
+ 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 in TLS. It should 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.
+
+ certificate_expired
+ A certificate has expired or is not currently valid.
+
+
+
+Dierks & Rescorla Standards Track [Page 31] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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 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.
+
+ decrypt_error
+ A handshake cryptographic operation failed, including being
+ unable to correctly verify a signature, decrypt a key exchange,
+ or validate a finished message.
+
+ export_restriction_RESERVED
+ This alert was used in TLS 1.0 but not TLS 1.1.
+
+ 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.
+
+ 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 makes it impossible to continue (such as a memory
+ allocation failure). This message is always fatal.
+
+
+
+Dierks & Rescorla Standards Track [Page 32] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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 would be 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.
+
+ For all errors where an alert level is not explicitly specified, the
+ sending party MAY determine at its discretion whether this is a fatal
+ error or not; if an alert with a level of warning is received, the
+ receiving party MAY decide at its discretion whether to treat this as
+ a fatal error or not. However, all messages which are transmitted
+ with a level of fatal MUST be treated as fatal messages.
+
+ New alerts values MUST be defined by RFC 2434 Standards Action. See
+ Section 11 for IANA Considerations for alert values.
+
+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.
+
+ 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
+
+
+
+Dierks & Rescorla Standards Track [Page 33] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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 TLS always
+ negotiating the strongest possible connection between two peers:
+ there are a number of ways 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.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 34] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ However, you SHOULD never send data over a link encrypted with 40 bit
+ security unless you feel that data is worth no more than the effort
+ required to break that encryption.
+
+ These goals are achieved by the handshake protocol, which can be
+ summarized as follows: The client sends a client hello message to
+ which the server must respond with a server hello message, or else a
+ fatal error will occur and the connection will fail. The client hello
+ and server hello are used to establish security enhancement
+ capabilities between client and server. The client hello and server
+ hello 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.
+
+ The actual key exchange uses up to four messages: the server
+ certificate, the server key exchange, the client certificate, and the
+ client key exchange. New key exchange methods can be created by
+ specifying a format for these messages and 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 which range from 48 to 128 bytes in
+ length.
+
+ Following the hello messages, the server will send its certificate,
+ if it is to be authenticated. Additionally, a server key exchange
+ message may be sent, if it is required (e.g. if their 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. Now the
+ server will send the server hello done 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 certificate
+ request message, the client must send the certificate message. The
+ client key exchange message is now sent, and the content of that
+ message will depend on the public key algorithm selected between the
+ client hello and the server hello. If the client has sent a
+ certificate with signing ability, a digitally-signed certificate
+ verify message is sent to explicitly verify the certificate.
+
+ At this point, a change cipher spec 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 change cipher spec message, transfer the pending to the
+ current Cipher Spec, and send its finished message under the new
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 35] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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
+ TLS_NULL_WITH_NULL_NULL is established).
+ Client Server
+
+ ClientHello -------->
+ ServerHello
+ Certificate*
+ ServerKeyExchange*
+ CertificateRequest*
+ <-------- ServerHelloDone
+ Certificate*
+ ClientKeyExchange
+ CertificateVerify*
+ [ChangeCipherSpec]
+ Finished -------->
+ [ChangeCipherSpec]
+ <-------- Finished
+ Application Data <-------> Application Data
+
+ Fig. 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 change cipher spec 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] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ Client Server
+
+ ClientHello -------->
+ ServerHello
+ [ChangeCipherSpec]
+ <-------- Finished
+ [ChangeCipherSpec]
+ Finished -------->
+ Application Data <-------> Application Data
+
+ Fig. 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] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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 which is not bound by these ordering rules is the Hello
+ Request 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 type values MUST be defined via RFC 2434
+ Standards Action. See Section 11 for IANA Considerations for these
+ values.
+
+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 hello request message MAY be sent by the server at any time.
+
+ Meaning of this message:
+ Hello request is a simple notification that the client should
+ begin the negotiation process anew by sending a client hello
+ message when convenient. 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 hello request but does not receive a client hello in
+ response, it may close the connection with a fatal alert.
+
+ After sending a hello request, servers SHOULD not repeat the request
+ until the subsequent handshake negotiation is complete.
+
+ Structure of this message:
+ struct { } HelloRequest;
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 38] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ Note: This message MUST NOT be included in the message hashes which 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 client hello as its first message. The client can also send a
+ client hello in response to a hello request or on its own
+ initiative in order to renegotiate the security parameters in an
+ existing connection.
+
+ Structure of this message:
+ The client hello 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, GMT, 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.
+
+ random_bytes
+ 28 bytes generated by a secure random number generator.
+
+ The client hello 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, this
+ connection, or 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, while 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 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.
+
+
+
+Dierks & Rescorla Standards Track [Page 39] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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 CipherSuite list, passed from the client to the server in the
+ client hello message, contains the combinations of cryptographic
+ algorithms supported by the client in order of the client's
+ preference (favorite choice first). Each CipherSuite defines a key
+ exchange algorithm, a bulk encryption algorithm (including secret key
+ length) and a MAC algorithm. The server will select a cipher suite
+ or, if no acceptable choices are presented, return a handshake
+ failure alert and close the connection.
+
+ uint8 CipherSuite[2]; /* Cryptographic suite selector */
+
+ The client hello includes a list of compression algorithms supported
+ by the client, ordered according to the client's preference.
+
+ enum { null(0), (255) } CompressionMethod;
+
+ struct {
+ ProtocolVersion client_version;
+ Random random;
+ SessionID session_id;
+ CipherSuite cipher_suites<2..2^16-1>;
+ CompressionMethod compression_methods<1..2^8-1>;
+ } ClientHello;
+
+ 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 3.2 (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 should be empty if no session_id is available or the
+
+
+
+Dierks & Rescorla Standards Track [Page 40] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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 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.
+
+ After sending the client hello message, the client waits for a server
+ hello message. Any other handshake message returned by the server
+ except for a hello request is treated as a fatal error.
+
+ Forward compatibility note:
+ In the interests of forward compatibility, it is permitted for a
+ client hello message to include extra data after the compression
+ methods. This data MUST be included in the handshake hashes, but
+ must otherwise be ignored. This is the only handshake message for
+ which this is legal; for all other messages, the amount of data
+ in the message MUST match the description of the message
+ precisely.
+
+Note: For the intended use of trailing data in the ClientHello, see RFC
+ 3546 [TLSEXT].
+
+7.4.1.3. Server hello
+
+ When this message will be sent:
+ The server will send this message in response to a client hello
+ 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;
+
+
+
+Dierks & Rescorla Standards Track [Page 41] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ CompressionMethod compression_method;
+ } 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 3.2 (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.
+
+ 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.
+
+7.4.2. Server certificate
+
+ When this message will be sent:
+ The server MUST send a certificate whenever the agreed-upon key
+ exchange method is not an anonymous one. This message will always
+ immediately follow the server hello message.
+
+ Meaning of this message:
+ The certificate type MUST be appropriate for the selected cipher
+ suite's key exchange algorithm, and is generally an X.509v3
+
+
+
+Dierks & Rescorla Standards Track [Page 42] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ certificate. It MUST contain a key which matches the key exchange
+ method, as follows. Unless otherwise specified, the signing
+ algorithm for the certificate MUST be the same as the algorithm
+ for the certificate key. Unless otherwise specified, the public
+ key MAY be of any length.
+
+ Key Exchange Algorithm Certificate Key Type
+
+ RSA RSA public key; the certificate MUST
+ allow the key to be used for encryption.
+
+ DHE_DSS DSS public key.
+
+ DHE_RSA RSA public key which can be used for
+ signing.
+
+ DH_DSS Diffie-Hellman key. The algorithm used
+ to sign the certificate MUST be DSS.
+
+ DH_RSA Diffie-Hellman key. The algorithm used
+ to sign the certificate MUST be RSA.
+
+ All certificate profiles, key and cryptographic formats are defined
+ by the IETF PKIX working group [PKIX]. When a key usage extension is
+ present, the digitalSignature bit MUST be set for the key to be
+ eligible for signing, as described above, and the keyEncipherment bit
+ MUST be present to allow encryption, as described above. The
+ keyAgreement bit must be set on Diffie-Hellman certificates.
+
+ As CipherSuites which specify new key exchange methods are specified
+ for the TLS Protocol, they will imply certificate format and the
+ required encoded keying information.
+
+ 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 X.509v3 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 which specifies the
+ root certificate authority may optionally be omitted from the
+ chain, under the assumption that the remote end must already
+
+
+
+Dierks & Rescorla Standards Track [Page 43] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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.
+
+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 server hello message, if this is an
+ anonymous negotiation).
+
+ The server key exchange 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 server key exchange message for the
+ following key exchange methods:
+
+ RSA
+ DH_DSS
+ DH_RSA
+
+ Meaning of this message:
+ This message conveys cryptographic information to allow the
+ client to communicate the premaster secret: either an RSA public
+ key to encrypt the premaster secret with, or a Diffie-Hellman
+ public key with which the client can complete a key exchange
+ (with the result being the premaster secret.)
+
+ As additional CipherSuites are defined for TLS which 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.
+
+
+
+
+Dierks & Rescorla Standards Track [Page 44] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ Structure of this message:
+ enum { rsa, diffie_hellman } KeyExchangeAlgorithm;
+
+ struct {
+ opaque rsa_modulus<1..2^16-1>;
+ opaque rsa_exponent<1..2^16-1>;
+ } ServerRSAParams;
+
+ rsa_modulus
+ The modulus of the server's temporary RSA key.
+
+ rsa_exponent
+ The public exponent of the server's temporary RSA key.
+
+ 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).
+
+ struct {
+ select (KeyExchangeAlgorithm) {
+ case diffie_hellman:
+ ServerDHParams params;
+ Signature signed_params;
+ case rsa:
+ ServerRSAParams params;
+ Signature signed_params;
+ };
+ } ServerKeyExchange;
+
+ struct {
+ select (KeyExchangeAlgorithm) {
+ case diffie_hellman:
+ ServerDHParams params;
+ case rsa:
+ ServerRSAParams params;
+ };
+ } ServerParams;
+
+
+
+Dierks & Rescorla Standards Track [Page 45] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ params
+ The server's key exchange parameters.
+
+ signed_params
+ For non-anonymous key exchanges, a hash of the corresponding
+ params value, with the signature appropriate to that hash
+ applied.
+
+ md5_hash
+ MD5(ClientHello.random + ServerHello.random + ServerParams);
+
+ sha_hash
+ SHA(ClientHello.random + ServerHello.random + ServerParams);
+
+ enum { anonymous, rsa, dsa } SignatureAlgorithm;
+
+
+ struct {
+ select (SignatureAlgorithm) {
+ case anonymous: struct { };
+ case rsa:
+ digitally-signed struct {
+ opaque md5_hash[16];
+ opaque sha_hash[20];
+ };
+ case dsa:
+ digitally-signed struct {
+ opaque sha_hash[20];
+ };
+ };
+ };
+ } Signature;
+
+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 Server Key Exchange
+ message (if it is sent; otherwise, the Server 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)
+
+
+
+Dierks & Rescorla Standards Track [Page 46] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ } ClientCertificateType;
+
+ opaque DistinguishedName<1..2^16-1>;
+
+ struct {
+ ClientCertificateType certificate_types<1..2^8-1>;
+ DistinguishedName certificate_authorities<0..2^16-1>;
+ } CertificateRequest;
+
+ certificate_types
+ This field is a list of the types of certificates requested,
+ sorted in order of the server's preference.
+
+ certificate_authorities
+ A list of the distinguished names of acceptable certificate
+ authorities. These distinguished names may specify a desired
+ distinguished name for a root CA or for a subordinate CA;
+ thus, this message can be used both to describe known roots
+ and 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.
+
+
+ ClientCertificateType values are divided into three groups:
+
+ 1. Values from 0 (zero) through 63 decimal (0x3F) inclusive are
+ reserved for IETF Standards Track protocols.
+
+ 2. Values from 64 decimal (0x40) through 223 decimal (0xDF) inclusive
+ are reserved for assignment for non-Standards Track methods.
+
+ 3. Values from 224 decimal (0xE0) through 255 decimal (0xFF)
+ inclusive are reserved for private use.
+
+ Additional information describing the role of IANA in the
+ allocation of ClientCertificateType code points is described
+ in Section 11.
+
+ Note: Values listed as RESERVED may not be used. They were used in SSLv3.
+
+ Note: DistinguishedName is derived from [X501]. DistinguishedNames are
+ represented in DER-encoded format.
+
+ Note: It is a fatal handshake_failure alert for an anonymous server to
+ request client authentication.
+
+
+
+
+Dierks & Rescorla Standards Track [Page 47] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+7.4.5. Server hello done
+
+ When this message will be sent:
+ The server hello done message is sent by the server to indicate
+ the end of the server hello 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 server hello done 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
+ server hello done message. This message is only sent if the
+ server requests a certificate. If no suitable certificate is
+ available, the client SHOULD send a certificate message
+ containing no certificates. That is, the certificate_list
+ structure has a length of zero. If client authentication is
+ required by the server for the handshake to continue, it may
+ respond with a fatal handshake failure alert. Client certificates
+ are sent using the Certificate structure defined in Section
+ 7.4.2.
+
+
+ Note: When using a static Diffie-Hellman based key exchange method
+ (DH_DSS or DH_RSA), if client authentication is requested, the
+ Diffie-Hellman group and generator encoded in the client's
+ certificate MUST match the server specified Diffie-Hellman
+ parameters if the client's parameters are to be used for the key
+ exchange.
+
+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 server hello done message.
+
+
+
+Dierks & Rescorla Standards Track [Page 48] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ Meaning of this message:
+ With this message, the premaster secret is set, either though
+ direct transmission of the RSA-encrypted secret, or by the
+ transmission of Diffie-Hellman parameters which will allow each
+ side to agree upon the same premaster secret. When the key
+ exchange method is DH_RSA or DH_DSS, client certification has
+ been requested, and the client was able to respond with a
+ certificate which contained a Diffie-Hellman public key whose
+ parameters (group and generator) matched those specified by the
+ server in its certificate, this message MUST not contain any
+ data.
+
+ 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.
+
+ struct {
+ select (KeyExchangeAlgorithm) {
+ case rsa: EncryptedPreMasterSecret;
+ case diffie_hellman: 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 or the temporary RSA
+ key provided in a server key exchange message, and sends the
+ result in an encrypted premaster secret message. This structure
+ is a variant of the client key exchange message, 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 roll-back attacks. Upon receiving the
+ premaster secret, the server SHOULD check that this value
+ matches the value transmitted by the client in the client
+ hello message.
+
+
+
+
+Dierks & Rescorla Standards Track [Page 49] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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.
+
+ Note: An attack discovered by Daniel Bleichenbacher [BLEI] can be used
+ to attack a TLS server which is using PKCS#1 v 1.5 encoded RSA.
+ The attack takes advantage of the fact that by failing in
+ different ways, a TLS server can be coerced into revealing
+ whether a particular message, when decrypted, is properly PKCS#1
+ v1.5 formatted or not.
+
+ The best way to avoid vulnerability to this attack is to treat
+ incorrectly formatted messages in a manner indistinguishable from
+ correctly formatted RSA blocks. Thus, when it receives an
+ incorrectly formatted RSA block, a server should generate a
+ random 48-byte value and proceed using it as the premaster
+ secret. Thus, the server will act identically whether the
+ received RSA block is correctly encoded or not.
+
+ [PKCS1B] defines a newer version of PKCS#1 encoding that is more
+ secure against the Bleichenbacher attack. However, for maximal
+ compatibility with TLS 1.0, TLS 1.1 retains the original
+ encoding. 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 the length bytes, encoding 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 upgrading from SSLv3 must modify their
+ implementations to generate and accept the correct encoding.
+
+
+
+Dierks & Rescorla Standards Track [Page 50] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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 SSL are possible, at least when the client and server are on
+ the same LAN. Accordingly, implementations which use static RSA
+ keys SHOULD use RSA blinding or some other anti-timing technique,
+ as described in [TIMING].
+
+ Note: The version number in the PreMasterSecret MUST be the version
+ offered by the client in the ClientHello, not the version
+ negotiated for the connection. This feature is designed to
+ prevent rollback attacks. Unfortunately, many 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 and Server
+ implementations MAY check the version number. In practice, since
+ the TLS handshake MACs prevent downgrade and no good attacks are
+ known on those MACs, ambiguity is not considered a serious
+ security risk. Note that if servers choose to to check the
+ version number, they should randomize the PreMasterSecret in case
+ of error, rather than generate an alert, in order to avoid
+ variants on the Bleichenbacher attack. [KPR03]
+
+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, not a message in itself.
+
+ Structure of this message:
+ enum { implicit, explicit } PublicValueEncoding;
+
+ implicit
+ If the client certificate already contains a suitable Diffie-
+ Hellman key, then Yc is implicit and does not need to be sent
+ again. In this case, the client key exchange message will be
+ sent, but MUST be empty.
+
+ explicit
+ Yc needs to be sent.
+
+ struct {
+ select (PublicValueEncoding) {
+
+
+
+Dierks & Rescorla Standards Track [Page 51] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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).
+
+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 {
+ Signature signature;
+ } CertificateVerify;
+
+ The Signature type is defined in 7.4.3.
+
+ CertificateVerify.signature.md5_hash
+ MD5(handshake_messages);
+
+ CertificateVerify.signature.sha_hash
+ SHA(handshake_messages);
+
+ Here handshake_messages refers to all handshake messages sent or
+ received starting at client hello 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 7.4 exchanged thus far.
+
+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 protected with the just-
+ negotiated algorithms, keys, and secrets. Recipients of finished
+
+
+
+Dierks & Rescorla Standards Track [Page 52] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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.
+
+ struct {
+ opaque verify_data[12];
+ } Finished;
+
+ verify_data
+ PRF(master_secret, finished_label, MD5(handshake_messages) +
+ SHA-1(handshake_messages)) [0..11];
+
+ finished_label
+ For Finished messages sent by the client, the string "client
+ finished". For Finished messages sent by the server, the
+ string "server finished".
+
+ 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
+ 7.4 exchanged thus far.
+
+ It is a fatal error if a finished message is not preceded by a change
+ cipher spec message at the appropriate point in the handshake.
+
+ The value handshake_messages includes all handshake messages starting
+ at client hello 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 certificate verify 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 which is sent second will include the prior one.
+
+ Note: Change cipher spec messages, alerts and any other record types
+ are not handshake messages and are not included in the hash
+ computations. Also, Hello Request 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
+
+
+
+Dierks & Rescorla Standards Track [Page 53] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ server and revealed in the server hello 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 54] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+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.
+
+ RSA digital signatures are performed using PKCS #1 [PKCS1] block type
+ 1. RSA public key encryption is performed using PKCS #1 block type 2.
+
+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_3DES_EDE_CBC_SHA.
+
+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. IANA Considerations
+
+ This document describes a number of new registries to be created by
+ IANA. We recommend that they be placed as individual registries items
+ under a common TLS category.
+
+ Section 7.4.3 describes a TLS ClientCertificateType Registry to be
+ maintained by the IANA, as defining a number of such code point
+ identifiers. ClientCertificateType identifiers with values in the
+ range 0-63 (decimal) inclusive are assigned via RFC 2434 Standards
+ Action. Values from the range 64-223 (decimal) inclusive are assigned
+ via [RFC 2434] Specification Required. Identifier values from
+
+
+
+Dierks & Rescorla Standards Track [Page 55] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 224-255 (decimal) inclusive are reserved for RFC 2434 Private Use.
+ The registry will be initially populated with the values in this
+ document, Section 7.4.4.
+
+ Section A.5 describes a TLS Cipher Suite Registry to be maintained by
+ the IANA, as well as defining a number of such cipher suite
+ identifiers. Cipher suite values with the first byte in the range
+ 0-191 (decimal) inclusive are assigned via RFC 2434 Standards Action.
+ Values with the first byte in the range 192-254 (decimal) are
+ assigned via RFC 2434 Specification Required. Values with the first
+ byte 255 (decimal) are reserved for RFC 2434 Private Use. The
+ registry will be initially populated with the values from Section A.5
+ of this document, [TLSAES], and Section 3 of [TLSKRB].
+
+ Section 6 requires that all ContentType values be defined by RFC 2434
+ Standards Action. IANA SHOULD create a TLS ContentType registry,
+ initially populated with values from Section 6.2.1 of this document.
+ Future values MUST be allocated via Standards Action as described in
+ [RFC 2434].
+
+ Section 7.2.2 requires that all Alert values be defined by RFC 2434
+ Standards Action. IANA SHOULD create a TLS Alert registry, initially
+ populated with values from Section 7.2 of this document and Section 4
+ of [TLSEXT]. Future values MUST be allocated via Standards Action as
+ described in [RFC 2434].
+
+ Section 7.4 requires that all HandshakeType values be defined by RFC
+ 2434 Standards Action. IANA SHOULD create a TLS HandshakeType
+ registry, initially populated with values from Section 7.4 of this
+ document and Section 2.4 of [TLSEXT]. Future values MUST be
+ allocated via Standards Action as described in [RFC2434].
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 56] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+A. Protocol constant values
+
+ This section describes protocol types and constants.
+
+A.1. Record layer
+
+ struct {
+ uint8 major, minor;
+ } ProtocolVersion;
+
+ ProtocolVersion version = { 3, 2 }; /* TLS v1.1 */
+
+ 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 (CipherSpec.cipher_type) {
+ case stream: GenericStreamCipher;
+ case block: GenericBlockCipher;
+ } fragment;
+ } TLSCiphertext;
+
+ stream-ciphered struct {
+ opaque content[TLSCompressed.length];
+ opaque MAC[CipherSpec.hash_size];
+ } GenericStreamCipher;
+
+ block-ciphered struct {
+ opaque IV[CipherSpec.block_length];
+
+
+
+Dierks & Rescorla Standards Track [Page 57] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ opaque content[TLSCompressed.length];
+ opaque MAC[CipherSpec.hash_size];
+ uint8 padding[GenericBlockCipher.padding_length];
+ uint8 padding_length;
+ } GenericBlockCipher;
+
+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(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),
+ insufficient_security(71),
+ internal_error(80),
+ user_canceled(90),
+ no_renegotiation(100),
+ (255)
+ } AlertDescription;
+
+ struct {
+ AlertLevel level;
+ AlertDescription description;
+ } Alert;
+
+
+
+Dierks & Rescorla Standards Track [Page 58] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+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;
+
+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..2^16-1>;
+ CompressionMethod compression_methods<1..2^8-1>;
+
+
+
+Dierks & Rescorla Standards Track [Page 59] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ } ClientHello;
+
+ struct {
+ ProtocolVersion server_version;
+ Random random;
+ SessionID session_id;
+ CipherSuite cipher_suite;
+ CompressionMethod compression_method;
+ } ServerHello;
+
+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 { rsa, diffie_hellman } KeyExchangeAlgorithm;
+
+ struct {
+ opaque rsa_modulus<1..2^16-1>;
+ opaque rsa_exponent<1..2^16-1>;
+ } ServerRSAParams;
+
+ struct {
+ opaque dh_p<1..2^16-1>;
+ opaque dh_g<1..2^16-1>;
+ opaque dh_Ys<1..2^16-1>;
+ } ServerDHParams;
+
+ struct {
+ select (KeyExchangeAlgorithm) {
+ case diffie_hellman:
+ ServerDHParams params;
+ Signature signed_params;
+ case rsa:
+ ServerRSAParams params;
+ Signature signed_params;
+ };
+ } ServerKeyExchange;
+
+ enum { anonymous, rsa, dsa } SignatureAlgorithm;
+
+ struct {
+ select (KeyExchangeAlgorithm) {
+ case diffie_hellman:
+ ServerDHParams params;
+
+
+
+Dierks & Rescorla Standards Track [Page 60] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ case rsa:
+ ServerRSAParams params;
+ };
+ } ServerParams;
+
+ struct {
+ select (SignatureAlgorithm) {
+ case anonymous: struct { };
+ case rsa:
+ digitally-signed struct {
+ opaque md5_hash[16];
+ opaque sha_hash[20];
+ };
+ case dsa:
+ digitally-signed struct {
+ opaque sha_hash[20];
+ };
+ };
+ };
+ } Signature;
+
+ 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;
+
+A.4.3. Client authentication and key exchange messages
+
+ struct {
+ select (KeyExchangeAlgorithm) {
+ case rsa: EncryptedPreMasterSecret;
+ case diffie_hellman: ClientDiffieHellmanPublic;
+ } exchange_keys;
+ } ClientKeyExchange;
+
+ struct {
+ ProtocolVersion client_version;
+
+
+
+Dierks & Rescorla Standards Track [Page 61] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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 {
+ Signature signature;
+ } CertificateVerify;
+
+A.4.4. Handshake finalization message
+
+ struct {
+ opaque verify_data[12];
+ } Finished;
+
+A.5. The CipherSuite
+
+ The following values define the CipherSuite codes used in the client
+ hello and server hello messages.
+
+ A CipherSuite defines a cipher specification supported in TLS Version
+ 1.1.
+
+ 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 either an RSA or a DSS signature-capable certificate in the
+ certificate request message.
+
+ CipherSuite TLS_RSA_WITH_NULL_MD5 = { 0x00,0x01 };
+ CipherSuite TLS_RSA_WITH_NULL_SHA = { 0x00,0x02 };
+
+
+
+Dierks & Rescorla Standards Track [Page 62] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ CipherSuite TLS_RSA_WITH_RC4_128_MD5 = { 0x00,0x04 };
+ CipherSuite TLS_RSA_WITH_RC4_128_SHA = { 0x00,0x05 };
+ CipherSuite TLS_RSA_WITH_IDEA_CBC_SHA = { 0x00,0x07 };
+ CipherSuite TLS_RSA_WITH_DES_CBC_SHA = { 0x00,0x09 };
+ CipherSuite TLS_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0A };
+
+ The following CipherSuite 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 DSS or RSA certificate, which has been
+ signed by the CA. The signing algorithm used is specified after the
+ DH or DHE parameter. The server can request an RSA or DSS 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.
+
+ CipherSuite TLS_DH_DSS_WITH_DES_CBC_SHA = { 0x00,0x0C };
+ CipherSuite TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x0D };
+ CipherSuite TLS_DH_RSA_WITH_DES_CBC_SHA = { 0x00,0x0F };
+ CipherSuite TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x10 };
+ CipherSuite TLS_DHE_DSS_WITH_DES_CBC_SHA = { 0x00,0x12 };
+ CipherSuite TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA = { 0x00,0x13 };
+ CipherSuite TLS_DHE_RSA_WITH_DES_CBC_SHA = { 0x00,0x15 };
+ CipherSuite TLS_DHE_RSA_WITH_3DES_EDE_CBC_SHA = { 0x00,0x16 };
+
+ 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 and is
+ therefore deprecated.
+
+ CipherSuite TLS_DH_anon_WITH_RC4_128_MD5 = { 0x00,0x18 };
+ CipherSuite TLS_DH_anon_WITH_DES_CBC_SHA = { 0x00,0x1A };
+ CipherSuite TLS_DH_anon_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1B };
+
+ When SSLv3 and TLS 1.0 were designed, the United States restricted
+ the export of cryptographic software containing certain strong
+ encryption algorithms. A series of cipher suites were designed to
+ operate at reduced key lengths in order to comply with those
+ regulations. Due to advances in computer performance, these
+ algorithms are now unacceptably weak and export restrictions have
+ since been loosened. TLS 1.1 implementations MUST NOT negotiate these
+ cipher suites in TLS 1.1 mode. However, for backward compatibility
+ they may be offered in the ClientHello for use with TLS 1.0 or SSLv3
+ only servers. TLS 1.1 clients MUST check that the server did not
+ choose one of these cipher suites during the handshake. These
+
+
+
+Dierks & Rescorla Standards Track [Page 63] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ ciphersuites are listed below for informational purposes and to
+ reserve the numbers.
+
+ CipherSuite TLS_RSA_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x03 };
+ CipherSuite TLS_RSA_EXPORT_WITH_RC2_CBC_40_MD5 = { 0x00,0x06 };
+ CipherSuite TLS_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x08 };
+ CipherSuite TLS_DH_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0B };
+ CipherSuite TLS_DH_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x0E };
+ CipherSuite TLS_DHE_DSS_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x11 };
+ CipherSuite TLS_DHE_RSA_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x14 };
+ CipherSuite TLS_DH_anon_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x17 };
+ CipherSuite TLS_DH_anon_EXPORT_WITH_DES40_CBC_SHA = { 0x00,0x19 };
+
+ The following cipher suites were defined in [TLSKRB] and are included
+ here for completeness. See [TLSKRB] for details:
+
+ CipherSuite TLS_KRB5_WITH_DES_CBC_SHA = { 0x00,0x1E };
+ CipherSuite TLS_KRB5_WITH_3DES_EDE_CBC_SHA = { 0x00,0x1F };
+ CipherSuite TLS_KRB5_WITH_RC4_128_SHA = { 0x00,0x20 };
+ CipherSuite TLS_KRB5_WITH_IDEA_CBC_SHA = { 0x00,0x21 };
+ CipherSuite TLS_KRB5_WITH_DES_CBC_MD5 = { 0x00,0x22 };
+ CipherSuite TLS_KRB5_WITH_3DES_EDE_CBC_MD5 = { 0x00,0x23 };
+ CipherSuite TLS_KRB5_WITH_RC4_128_MD5 = { 0x00,0x24 };
+ CipherSuite TLS_KRB5_WITH_IDEA_CBC_MD5 = { 0x00,0x25 };
+
+ The following exportable cipher suites were defined in [TLSKRB] and
+ are included here for completeness. TLS 1.1 implementations MUST NOT
+ negotiate these cipher suites.
+
+ CipherSuite TLS_KRB5_EXPORT_WITH_DES_CBC_40_SHA = { 0x00,0x26
+ };
+ CipherSuite TLS_KRB5_EXPORT_WITH_RC2_CBC_40_SHA = { 0x00,0x27
+ };
+ CipherSuite TLS_KRB5_EXPORT_WITH_RC4_40_SHA = { 0x00,0x28
+ };
+ CipherSuite TLS_KRB5_EXPORT_WITH_DES_CBC_40_MD5 = { 0x00,0x29
+ };
+ CipherSuite TLS_KRB5_EXPORT_WITH_RC2_CBC_40_MD5 = { 0x00,0x2A
+ };
+ CipherSuite TLS_KRB5_EXPORT_WITH_RC4_40_MD5 = { 0x00,0x2B
+ };
+
+ The following cipher suites were defined in [TLSAES] and are included
+ here for completeness. See [TLSAES] for details:
+
+ CipherSuite TLS_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x2F };
+ CipherSuite TLS_DH_DSS_WITH_AES_128_CBC_SHA = { 0x00, 0x30 };
+ CipherSuite TLS_DH_RSA_WITH_AES_128_CBC_SHA = { 0x00, 0x31 };
+
+
+
+Dierks & Rescorla Standards Track [Page 64] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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_anon_WITH_AES_128_CBC_SHA = { 0x00, 0x34 };
+
+ CipherSuite TLS_RSA_WITH_AES_256_CBC_SHA = { 0x00, 0x35 };
+ 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_anon_WITH_AES_256_CBC_SHA = { 0x00, 0x3A };
+
+ The cipher suite space is divided into three regions:
+
+ 1. Cipher suite values with first byte 0x00 (zero)
+ through decimal 191 (0xBF) inclusive are reserved for the IETF
+ Standards Track protocols.
+
+ 2. Cipher suite values with first byte decimal 192 (0xC0)
+ through decimal 254 (0xFE) inclusive are reserved
+ for assignment for non-Standards Track methods.
+
+ 3. Cipher suite values with first byte 0xFF are
+ reserved for private use.
+ Additional information describing the role of IANA in the allocation
+ of cipher suite code points is described in Section 11.
+
+ 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 { null, rc4, rc2, des, 3des, des40, aes, idea }
+ BulkCipherAlgorithm;
+
+ enum { stream, block } CipherType;
+
+ enum { null, md5, sha } MACAlgorithm;
+
+ /* The algorithms specified in CompressionMethod,
+
+
+
+Dierks & Rescorla Standards Track [Page 65] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ BulkCipherAlgorithm, and MACAlgorithm may be added to. */
+
+ struct {
+ ConnectionEnd entity;
+ BulkCipherAlgorithm bulk_cipher_algorithm;
+ CipherType cipher_type;
+ uint8 key_size;
+ uint8 key_material_length;
+ MACAlgorithm mac_algorithm;
+ uint8 hash_size;
+ CompressionMethod compression_algorithm;
+ opaque master_secret[48];
+ opaque client_random[32];
+ opaque server_random[32];
+ } SecurityParameters;
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 66] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+B. Glossary
+
+ Advanced Encryption Standard (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. [AES] 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.
+
+ authentication
+ Authentication is the ability of one entity to determine the
+ identity of another entity.
+
+ block cipher
+ A block cipher is an algorithm that operates on plaintext in
+ groups of bits, called blocks. 64 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
+
+
+
+Dierks & Rescorla Standards Track [Page 67] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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 secret
+ 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 is a very widely used symmetric encryption algorithm. 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 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 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. [DES], [3DES]
+
+ 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, "Digital Signature
+ Standard," published May, 1994 by the U.S. Dept. of Commerce.
+ [DSS]
+
+ 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.
+
+
+
+
+Dierks & Rescorla Standards Track [Page 68] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ IDEA
+ A 64-bit block cipher designed by Xuejia Lai and James Massey.
+ [IDEA]
+
+ 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 is a secure hashing function that converts an arbitrarily
+ long data stream into a digest of fixed size (16 bytes). [MD5]
+
+ 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.
+
+ RC2
+ A block cipher developed by Ron Rivest at RSA Data Security, Inc.
+ [RSADSI] described in [RC2].
+
+ 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 under client.
+
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 69] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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, which 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 secret
+ The secret data used to authenticate data written by the server.
+
+ SHA
+ The Secure Hash Algorithm is defined in FIPS PUB 180-2. It
+ produces a 20-byte output. Note that all references to SHA
+ actually use the modified SHA-1 algorithm. [SHA]
+
+ 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 "Comments" at
+ the end of this document.
+
+
+
+
+
+
+
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 70] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+C. CipherSuite definitions
+
+CipherSuite Key Cipher Hash
+ 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_RC4_128_MD5 RSA RC4_128 MD5
+TLS_RSA_WITH_RC4_128_SHA RSA RC4_128 SHA
+TLS_RSA_WITH_IDEA_CBC_SHA RSA IDEA_CBC SHA
+TLS_RSA_WITH_DES_CBC_SHA RSA DES_CBC SHA
+TLS_RSA_WITH_3DES_EDE_CBC_SHA RSA 3DES_EDE_CBC SHA
+TLS_DH_DSS_WITH_DES_CBC_SHA DH_DSS DES_CBC SHA
+TLS_DH_DSS_WITH_3DES_EDE_CBC_SHA DH_DSS 3DES_EDE_CBC SHA
+TLS_DH_RSA_WITH_DES_CBC_SHA DH_RSA DES_CBC SHA
+TLS_DH_RSA_WITH_3DES_EDE_CBC_SHA DH_RSA 3DES_EDE_CBC SHA
+TLS_DHE_DSS_WITH_DES_CBC_SHA DHE_DSS DES_CBC SHA
+TLS_DHE_DSS_WITH_3DES_EDE_CBC_SHA DHE_DSS 3DES_EDE_CBC SHA
+TLS_DHE_RSA_WITH_DES_CBC_SHA DHE_RSA DES_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_DES_CBC_SHA DH_anon DES_CBC SHA
+TLS_DH_anon_WITH_3DES_EDE_CBC_SHA DH_anon 3DES_EDE_CBC SHA
+
+ Key
+ Exchange
+ Algorithm Description Key size limit
+
+ DHE_DSS Ephemeral DH with DSS signatures None
+ DHE_RSA Ephemeral DH with RSA signatures None
+ DH_anon Anonymous DH, no signatures None
+ DH_DSS DH with DSS-based certificates None
+ DH_RSA DH with RSA-based certificates None
+ RSA = none
+ NULL No key exchange N/A
+ RSA RSA key exchange None
+
+ Key Expanded IV Block
+ Cipher Type Material Key Material Size Size
+
+ NULL Stream 0 0 0 N/A
+ IDEA_CBC Block 16 16 8 8
+ RC2_CBC_40 Block 5 16 8 8
+ RC4_40 Stream 5 16 0 N/A
+ RC4_128 Stream 16 16 0 N/A
+ DES40_CBC Block 5 8 8 8
+ DES_CBC Block 8 8 8 8
+
+
+
+Dierks & Rescorla Standards Track [Page 71] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 3DES_EDE_CBC Block 24 24 8 8
+
+ 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.
+
+ Expanded Key Material
+ The number of bytes actually fed into the encryption algorithm
+
+ IV Size
+ How much data needs to be generated for the initialization
+ vector. Zero for stream ciphers; equal to the block size for
+ block ciphers.
+
+ 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.
+
+ Hash Hash Padding
+ function Size Size
+ NULL 0 0
+ MD5 16 48
+ SHA 20 40
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 72] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+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 MD5 and/or SHA, are
+ acceptable, but cannot provide more security than the size of the
+ random number generator state. (For example, MD5-based PRNGs usually
+ provide 128 bits of 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. To seed a 128-bit PRNG, one
+ 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 CipherSuites
+
+ TLS supports a range of key sizes and security levels, including some
+ which provide no or minimal security. A proper implementation will
+ probably not support many cipher suites. For example, 40-bit
+ encryption is easily broken, so implementations requiring strong
+ security should not allow 40-bit keys. Similarly, 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.
+
+
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 73] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+E. Backward Compatibility With SSL
+
+ For historical reasons and in order to avoid a profligate consumption
+ of reserved port numbers, application protocols which are secured by
+ TLS 1.1, TLS 1.0, SSL 3.0, and SSL 2.0 all frequently share the same
+ connection port: for example, the https protocol (HTTP secured by SSL
+ or TLS) uses port 443 regardless of which security protocol it is
+ using. Thus, some mechanism must be determined to distinguish and
+ negotiate among the various protocols.
+
+ TLS versions 1.1, 1.0, and SSL 3.0 are very similar; thus, supporting
+ both is easy. TLS clients who wish to negotiate with such older
+ servers SHOULD send client hello messages using the SSL 3.0 record
+ format and client hello structure, sending {3, 2} for the version
+ field to note that they support TLS 1.1. If the server supports only
+ TLS 1.0 or SSL 3.0, it will respond with a downrev 3.0 server hello;
+ if it supports TLS 1.1 it will respond with a TLS 1.1 server hello.
+ The negotiation then proceeds as appropriate for the negotiated
+ protocol.
+
+ Similarly, a TLS 1.1 server which wishes to interoperate with TLS
+ 1.0 or SSL 3.0 clients SHOULD accept SSL 3.0 client hello messages
+ and respond with a SSL 3.0 server hello if an SSL 3.0 client hello
+ with a version field of {3, 0} is received, denoting that this client
+ does not support TLS. Similarly, if a SSL 3.0 or TLS 1.0 hello with a
+ version field of {3, 1} is received, the server SHOULD respond with a
+ TLS 1.0 hello with a version field of {3, 1}.
+
+ Whenever a client already knows the highest protocol known to a
+ server (for example, when resuming a session), it SHOULD initiate the
+ connection in that native protocol.
+
+ TLS 1.1 clients that support SSL Version 2.0 servers MUST send SSL
+ Version 2.0 client hello messages [SSL2]. TLS servers SHOULD accept
+ either client hello format if they wish to support SSL 2.0 clients on
+ the same connection port. The only deviations from the Version 2.0
+ specification are the ability to specify a version with a value of
+ three and the support for more ciphering types in the CipherSpec.
+
+ Warning: The ability to send Version 2.0 client hello messages will be
+ phased out with all due haste. Implementors SHOULD make every
+ effort to move forward as quickly as possible. Version 3.0
+ provides better mechanisms for moving to newer versions.
+
+ The following cipher specifications are carryovers from SSL Version
+ 2.0. These are assumed to use RSA for key exchange and
+ authentication.
+
+
+
+
+Dierks & Rescorla Standards Track [Page 74] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ V2CipherSpec TLS_RC4_128_WITH_MD5 = { 0x01,0x00,0x80 };
+ V2CipherSpec TLS_RC4_128_EXPORT40_WITH_MD5 = { 0x02,0x00,0x80 };
+ V2CipherSpec TLS_RC2_CBC_128_CBC_WITH_MD5 = { 0x03,0x00,0x80 };
+ V2CipherSpec TLS_RC2_CBC_128_CBC_EXPORT40_WITH_MD5
+ = { 0x04,0x00,0x80 };
+ V2CipherSpec TLS_IDEA_128_CBC_WITH_MD5 = { 0x05,0x00,0x80 };
+ V2CipherSpec TLS_DES_64_CBC_WITH_MD5 = { 0x06,0x00,0x40 };
+ V2CipherSpec TLS_DES_192_EDE3_CBC_WITH_MD5 = { 0x07,0x00,0xC0 };
+
+ Cipher specifications native to TLS can be included in Version 2.0
+ client hello messages using the syntax below. Any V2CipherSpec
+ element with its first byte equal to zero will be ignored by Version
+ 2.0 servers. Clients sending any of the above V2CipherSpecs SHOULD
+ also include the TLS equivalent (see Appendix A.5):
+
+ V2CipherSpec (see TLS name) = { 0x00, CipherSuite };
+
+ Note: TLS 1.1 clients may generate the SSLv2 EXPORT cipher suites in
+ handshakes for backward compatibility but MUST NOT negotiate them in
+ TLS 1.1 mode.
+
+E.1. Version 2 client hello
+
+ The Version 2.0 client hello message is presented below using this
+ document's presentation model. The true definition is still assumed
+ to be the SSL Version 2.0 specification. Note that this message MUST
+ be sent directly on the wire, not wrapped as an SSLv3 record
+
+ 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
+ This field is the length of the following data in bytes. The high
+ bit MUST be 1 and is not part of the length.
+
+ msg_type
+ This field, in conjunction with the version field, identifies a
+
+
+
+Dierks & Rescorla Standards Track [Page 75] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ version 2 client hello message. The value SHOULD be one (1).
+
+ version
+ The highest version of the protocol supported by the client
+ (equals ProtocolVersion.version, see Appendix A.1).
+
+ 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.
+
+ challenge_length
+ The length in bytes of the client's challenge to the server to
+ authenticate itself. When using the SSLv2 backward compatible
+ handshake the client MUST use a 32-byte challenge.
+
+ cipher_specs
+ This is a list of all CipherSpecs the client is willing and able
+ to use. There MUST be at least one CipherSpec acceptable to the
+ server.
+
+ session_id
+ This field MUST be empty.
+
+ challenge
+ The client challenge to the server for the server to identify
+ itself is a (nearly) arbitrary length random. The TLS server will
+ right justify the challenge data to become the ClientHello.random
+ data (padded with leading zeroes, if necessary), as specified in
+ this protocol specification. If the length of the challenge is
+ greater than 32 bytes, only the last 32 bytes are used. It is
+ legitimate (but not necessary) for a V3 server to reject a V2
+ ClientHello that has fewer than 16 bytes of challenge data.
+
+ Note: Requests to resume a TLS session MUST use a TLS client hello.
+
+E.2. Avoiding man-in-the-middle version rollback
+
+ When TLS clients fall back to Version 2.0 compatibility mode, they
+ SHOULD use special PKCS #1 block formatting. This is done so that TLS
+ servers will reject Version 2.0 sessions with TLS-capable clients.
+
+ When TLS clients are in Version 2.0 compatibility mode, they set the
+ right-hand (least-significant) 8 random bytes of the PKCS padding
+ (not including the terminal null of the padding) for the RSA
+
+
+
+Dierks & Rescorla Standards Track [Page 76] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ encryption of the ENCRYPTED-KEY-DATA field of the CLIENT-MASTER-KEY
+ to 0x03 (the other padding bytes are random). After decrypting the
+ ENCRYPTED-KEY-DATA field, servers that support TLS SHOULD issue an
+ error if these eight padding bytes are 0x03. Version 2.0 servers
+ receiving blocks padded in this manner will proceed normally.
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 77] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+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 CipherSpec 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 secrets (see
+ Sections 7.4.8, 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 RSA or Diffie-
+ Hellman for key exchange. With anonymous RSA, the client encrypts a
+ pre_master_secret with the server's uncertified public key extracted
+
+
+
+Dierks & Rescorla Standards Track [Page 78] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ from the server key exchange message. The result is sent in a client
+ key exchange message. Since eavesdroppers do not know the server's
+ private key, it will be infeasible for them to decode the
+ pre_master_secret.
+
+ Note: No anonymous RSA Cipher Suites are defined in this document.
+
+ With Diffie-Hellman, the server's public parameters are contained in
+ the server key exchange message and the client's are sent in the
+ 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 may be either contained in the server's certificate or may
+ be a temporary RSA key sent in a server key exchange message. When
+ temporary RSA keys are used, they are signed by the server's RSA
+ certificate. The signature includes the current ClientHello.random,
+ so old signatures and temporary keys cannot be replayed. Servers may
+ use a single temporary RSA key for multiple negotiation sessions.
+
+ Note: The temporary RSA key option is useful if servers need large
+ certificates but must comply with government-imposed size limits
+ on keys used for key exchange.
+
+ Note that if ephemeral RSA is not used, 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
+
+
+
+Dierks & Rescorla Standards Track [Page 79] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ the certificate verify message (see Section 7.4.8). The client signs
+ a value derived from the master_secret and 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
+ can use the server key exchange message to send a set of temporary
+ Diffie-Hellman parameters signed with a DSS 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.,
+ 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 DSS 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 ciphersuites 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 so 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 ciphersuites.
+
+F.1.2. Version rollback attacks
+
+
+
+
+Dierks & Rescorla Standards Track [Page 80] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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. Parties
+ concerned about attacks of this scale should not be using 40-bit
+ encryption keys anyway. Altering the padding of the least-significant
+ 8 bytes of the PKCS 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 chooses.
+
+ 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 secrets 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 (which use both SHA and MD5).
+
+ 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
+
+
+
+Dierks & Rescorla Standards Track [Page 81] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ 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.1.5. MD5 and SHA
+
+ TLS uses hash functions very conservatively. Where possible, both MD5
+ and SHA are used in tandem to ensure that non-catastrophic flaws in
+ one algorithm will not break the overall protocol.
+
+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 secret, the sequence number, the message length, the 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 secrets. 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 secrets 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.
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 82] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+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
+ ciphersuite. 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 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 is not guaranteed to
+ be secure in general. 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 ciphersuites 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 pseudo-random generator and this pad is xor-ed
+ 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 previous versions of SSL, 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 verson of the protocol is immune to
+ those attacks. For exact details in the encryption modes proven
+ secure see [ENCAUTH].
+
+
+
+
+
+Dierks & Rescorla Standards Track [Page 83] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+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 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
+ denial of service attacks on individual connections. In
+ particular, attackers can forge RSTs, terminating connections, or
+ forge partial TLS records, 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.
+
+ 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, 40-bit
+ bulk encryption 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.
+
+
+
+
+
+
+
+
+
+
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+
+
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+Dierks & Rescorla Standards Track [Page 84] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+Security Considerations
+
+ Security issues are discussed throughout this memo, especially in
+ Appendices D, E, and F.
+
+Normative References
+ [AES] National Institute of Standards and Technology,
+ "Specification for the Advanced Encryption Standard (AES)"
+ FIPS 197. November 26, 2001.
+
+ [3DES] W. Tuchman, "Hellman Presents No Shortcut Solutions To DES,"
+ IEEE Spectrum, v. 16, n. 7, July 1979, pp40-41.
+
+ [DES] ANSI X3.106, "American National Standard for Information
+ Systems-Data Link Encryption," American National Standards
+ Institute, 1983.
+
+ [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.
+
+ [IDEA] X. Lai, "On the Design and Security of Block Ciphers," ETH
+ Series in Information Processing, v. 1, Konstanz: Hartung-
+ Gorre Verlag, 1992.
+
+ [MD5] Rivest, R., "The MD5 Message Digest Algorithm", RFC 1321,
+ April 1992.
+
+ [PKCS1A] B. Kaliski, "Public-Key Cryptography Standards (PKCS) #1:
+ RSA Cryptography Specifications Version 1.5", RFC 2313,
+ March 1998.
+
+ [PKCS1B] J. Jonsson, B. Kaliski, "Public-Key Cryptography Standards
+ (PKCS) #1: RSA Cryptography Specifications Version 2.1", RFC
+ 3447, February 2003.
+
+ [PKIX] Housley, R., Ford, W., Polk, W. and D. Solo, "Internet
+ Public Key Infrastructure: Part I: X.509 Certificate and CRL
+ Profile", RFC 3280, April 2002.
+
+ [RC2] Rivest, R., "A Description of the RC2(r) Encryption
+ Algorithm", RFC 2268, January 1998.
+
+ [SCH] B. Schneier. "Applied Cryptography: Protocols, Algorithms,
+
+
+
+Dierks & Rescorla Standards Track [Page 85] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ and Source Code in C, 2ed", Published by John Wiley & Sons,
+ Inc. 1996.
+
+ [SHA] NIST FIPS PUB 180-2, "Secure Hash Standard," National
+ Institute of Standards and Technology, U.S. Department of
+ Commerce., August 2001.
+
+ [REQ] Bradner, S., "Key words for use in RFCs to Indicate
+ Requirement Levels", BCP 14, RFC 2119, March 1997.
+
+ [RFC2434] T. Narten, H. Alvestrand, "Guidelines for Writing an IANA
+ Considerations Section in RFCs", RFC 3434, October 1998.
+
+ [TLSAES] Chown, P. "Advanced Encryption Standard (AES) Ciphersuites
+ for Transport Layer Security (TLS)", RFC 3268, June 2002.
+
+ [TLSEXT] Blake-Wilson, S., Nystrom, M, Hopwood, D., Mikkelsen, J.,
+ Wright, T., "Transport Layer Security (TLS) Extensions", RFC
+ 3546, June 2003.
+ [TLSKRB] A. Medvinsky, M. Hur, "Addition of Kerberos Cipher Suites to
+ Transport Layer Security (TLS)", RFC 2712, October 1999.
+
+
+Informative References
+
+ [AH] Kent, S., and Atkinson, R., "IP Authentication Header", RFC
+ 2402, November 1998.
+
+ [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., "Password Interception in a SSL/TLS Channel",
+ http://lasecwww.epfl.ch/memo_ssl.shtml, 2003.
+
+ [ENCAUTH] Krawczyk, H., "The Order of Encryption and Authentication
+ for Protecting Communications (Or: How Secure is SSL?)",
+ Crypto 2001.
+
+ [ESP] Kent, S., and Atkinson, R., "IP Encapsulating Security
+ Payload (ESP)", RFC 2406, November 1998.
+
+ [KPR03] Klima, V., Pokorny, O., Rosa, T., "Attacking RSA-based
+
+
+
+Dierks & Rescorla Standards Track [Page 86] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ Sessions in SSL/TLS", http://eprint.iacr.org/2003/052/,
+ March 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.
+
+ [RANDOM] D. Eastlake 3rd, S. Crocker, J. Schiller. "Randomness
+ Recommendations for Security", RFC 1750, December 1994.
+
+ [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. Frier, P. Karlton, and P. Kocher, "The SSL 3.0 Protocol",
+ Netscape Communications Corp., Nov 18, 1996.
+
+ [SUBGROUP] R. Zuccherato, "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.
+
+ [TLS1.0] Dierks, T., and Allen, C., "The TLS Protocol, Version 1.0",
+ RFC 2246, January 1999.
+
+ [X501] ITU-T Recommendation X.501: Information Technology - Open
+ Systems Interconnection - The Directory: Models, 1993.
+
+ [X509] ITU-T Recommendation X.509 (1997 E): Information Technology -
+ Open Systems Interconnection - "The Directory -
+ Authentication Framework". 1988.
+
+ [XDR] R. Srinivansan, Sun Microsystems, "XDR: External Data
+ Representation Standard", RFC 1832, August 1995.
+
+
+
+Dierks & Rescorla Standards Track [Page 87] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+Credits
+
+ Working Group Chairs
+ Win Treese
+ EMail: treese@acm.org
+
+ Eric Rescorla
+ EMail: ekr@rtfm.com
+
+
+ Editors
+
+ Tim Dierks Eric Rescorla
+ Independent RTFM, Inc.
+
+ EMail: tim@dierks.org EMail: ekr@rtfm.com
+
+
+
+ Other 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
+
+ Ran Canetti
+ IBM
+ canetti@watson.ibm.com
+
+ Taher Elgamal
+ taher@securify.com
+ Securify
+
+ Anil Gangolli
+ anil@busybuddha.org
+
+ Kipp Hickman
+
+ Phil Karlton (co-author of SSLv3)
+
+ Paul Kocher (co-author of SSLv3)
+ Cryptography Research
+ paul@cryptography.com
+
+
+
+
+Dierks & Rescorla Standards Track [Page 88] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
+
+
+ Hugo Krawczyk
+ Technion Israel Institute of Technology
+ hugo@ee.technion.ac.il
+
+ Robert Relyea
+ Netscape Communications
+ relyea@netscape.com
+
+ Jim Roskind
+ Netscape Communications
+ jar@netscape.com
+
+ Michael Sabin
+
+ Dan Simon
+ Microsoft, Inc.
+ dansimon@microsoft.com
+
+ Tom Weinstein
+
+Comments
+
+ The discussion list for the IETF TLS working group is located at the
+ e-mail address <ietf-tls@lists.consensus.com>. Information on the
+ group and information on how to subscribe to the list is at
+ <http://lists.consensus.com/>.
+
+ Archives of the list can be found at:
+ <http://www.imc.org/ietf-tls/mail-archive/>
+
+
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+Dierks & Rescorla Standards Track [Page 89] draft-ietf-tls-rfc2246-bis-13.txt TLS June 2005
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+
+Full Copyright Statement
+
+ 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
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+
+ The IETF invites any interested party to bring to its attention any
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+
+Copyright Notice
+ Copyright (C) The Internet Society (2003). This document is subject
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