     [file version.texi not included]

GNU TLS
*******

This manual is last updated UPDATED for version VERSION of GNU TLS.

Copyright (C) 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008 Free Software Foundation, Inc.

     Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.2 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.  A copy of the license is included in the section entitled ``GNU Free Documentation License''.

Preface
*******

This document tries to demonstrate and explain the GnuTLS library API.  A brief introduction to the protocols and the technology involved, is also included so that an application programmer can better understand the GnuTLS purpose and actual offerings. Even if GnuTLS is a typical library software, it operates over several security and cryptographic protocols, which require the programmer to make careful and correct usage of them, otherwise he risks to offer just a false sense of security. Security and the network security terms are very general terms even for computer software thus cannot be easily restricted to a single cryptographic library.  For that reason, do not consider a program secure just because it uses GnuTLS; there are several ways to compromise a program or a communication line and GnuTLS only helps with some of them.

Although this document tries to be self contained, basic network programming and PKI knowlegde is assumed in most of it. A good introduction to networking can be found in [STEVENS] (see Bibliography) and for Public Key Infrastructure in [GUTPKI] (see Bibliography).

Updated versions of the GnuTLS software and this document will be available from http://www.gnutls.org/ and http://www.gnu.org/software/gnutls/.

Getting Help
============

A mailing list where users may help each other exists, and you can reach it by sending e-mail to help-gnutls@gnu.org.  Archives of the mailing list discussions, and an interface to manage subscriptions, is available through the World Wide Web at http://lists.gnu.org/mailman/listinfo/help-gnutls.

A mailing list for developers are also available, see http://www.gnu.org/software/gnutls/lists.html.

Bug reports should be sent to bug-gnutls@gnu.org, see Bug Reports.

Commercial Support
==================

Commercial support is available for users of GnuTLS.  The kind of support that can be purchased may include:

  * Implement new features. Such as a new TLS extension.

  * Port GnuTLS to new platforms. This could include porting to an embedded platforms that may need memory or size optimization.

  * Integrating TLS as a security environment in your existing project.

  * System design of components related to TLS.

If you are interested, please write to:

Simon Josefsson Datakonsult
Hagagatan 24
113 47 Stockholm
Sweden

E-mail: simon@josefsson.org

If your company provide support related to GnuTLS and would like to be mentioned here, contact the author (see Bug Reports).

Downloading and Installing
==========================

GnuTLS is available for download from the following URL:

http://www.gnutls.org/download.html

The latest version is stored in a file, e.g., gnutls-VERSION.tar.gz where the VERSION value is the highest version number in the directory.

GnuTLS uses a Linux-like development cycle: even minor version numbers indicate a stable release and a odd minor version number indicates a development release.  For example, GnuTLS 1.6.3 denote a stable release since 6 is even, and GnuTLS 1.7.11 denote a development release since 7 is odd.

GnuTLS depends on Libgcrypt, and you will need to install Libgcrypt before installing GnuTLS.  Libgcrypt is available from ftp://ftp.gnupg.org/gcrypt/libgcrypt.  Libgcrypt needs another library, libgpg-error, and you need to install libgpg-error before installing Libgcrypt.  Libgpg-error is available from ftp://ftp.gnupg.org/gcrypt/libgpg-error.

Don't forget to verify the cryptographic signature after downloading source code packages.

The package is then extracted, configured and built like many other packages that use Autoconf.  For detailed information on configuring and building it, refer to the INSTALL file that is part of the distribution archive.  Typically you invoke ./configure and then make check install.  There are a number of compile-time parameters, as discussed below.

The compression libraries (libz and lzo) are optional dependencies. You can get libz from http://www.zlib.net/.  You can get lzo from http://www.oberhumer.com/opensource/lzo/.

The X.509 part of GnuTLS needs ASN.1 functionality, from a library called libtasn1.  A copy of libtasn1 is included in GnuTLS.  If you want to install it separately (e.g., to make it possibly to use libtasn1 in other programs), you can get it from http://www.gnu.org/software/gnutls/download.html.

The OpenPGP part of GnuTLS uses a stripped down version of OpenCDK for parsing OpenPGP packets.  It is included GnuTLS.  Use parameter --disable-openpgp-authentication to disable the OpenPGP functionality in GnuTLS.  Unfortunately, we didn't have resources to maintain the code in a separate library.

Regarding the Guile bindings, there are additional installation considerations, see Guile Preparations.

A few configure options may be relevant, summarized in the table.

--disable-srp-authentication
--disable-psk-authentication
--disable-anon-authentication
--disable-extra-pki
--disable-openpgp-authentication
--disable-openssl-compatibility
     Disable or enable particular features.  Generally not recommended.

For the complete list, refer to the output from configure --help.

Bug Reports
===========

If you think you have found a bug in GnuTLS, please investigate it and report it.

  * Please make sure that the bug is really in GnuTLS, and preferably also check that it hasn't already been fixed in the latest version.

  * You have to send us a test case that makes it possible for us to reproduce the bug.

  * You also have to explain what is wrong; if you get a crash, or if the results printed are not good and in that case, in what way. Make sure that the bug report includes all information you would need to fix this kind of bug for someone else.

Please make an effort to produce a self-contained report, with something definite that can be tested or debugged.  Vague queries or piecemeal messages are difficult to act on and don't help the development effort.

If your bug report is good, we will do our best to help you to get a corrected version of the software; if the bug report is poor, we won't do anything about it (apart from asking you to send better bug reports).

If you think something in this manual is unclear, or downright incorrect, or if the language needs to be improved, please also send a note.

Send your bug report to:

bug-gnutls@gnu.org

Contributing
============

If you want to submit a patch for inclusion -- from solve a typo you discovered, up to adding support for a new feature -- you should submit it as a bug report (see Bug Reports).  There are some things that you can do to increase the chances for it to be included in the official package.

Unless your patch is very small (say, under 10 lines) we require that you assign the copyright of your work to the Free Software Foundation. This is to protect the freedom of the project.  If you have not already signed papers, we will send you the necessary information when you submit your contribution.

For contributions that doesn't consist of actual programming code, the only guidelines are common sense.  Use it.

For code contributions, a number of style guides will help you:

  * Coding Style. Follow the GNU Standards document (see GNU Coding Standards (standards)).

     If you normally code using another coding standard, there is no problem, but you should use indent to reformat the code (see GNU Indent (indent)) before submitting your work.

  * Use the unified diff format diff -u.

  * Return errors. No reason whatsoever should abort the execution of the library.  Even memory allocation errors, e.g. when malloc return NULL, should work although result in an error code.

  * Design with thread safety in mind. Don't use global variables.  Don't even write to per-handle global variables unless the documented behaviour of the function you write is to write to the per-handle global variable.

  * Avoid using the C math library. It causes problems for embedded implementations, and in most situations it is very easy to avoid using it.

  * Document your functions. Use comments before each function headers, that, if properly formatted, are extracted into Texinfo manuals and GTK-DOC web pages.

  * Supply a ChangeLog and NEWS entries, where appropriate.

The Library
***********

In brief GnuTLS can be described as a library which offers an API to access secure communication protocols. These protocols provide privacy over insecure lines, and were designed to prevent eavesdropping, tampering, or message forgery.

Technically GnuTLS is a portable ANSI C based library which implements the TLS 1.1 and SSL 3.0 protocols (see Introduction to TLS, for a more detailed description of the protocols), accompanied with the required framework for authentication and public key infrastructure.  The library is available under the GNU Lesser GPL license (A copy of the license is included in the distribution).  Important features of the GnuTLS library include:

  * Support for TLS 1.0, TLS 1.1, and SSL 3.0 protocols.

  * Support for both X.509 and OpenPGP certificates.

  * Support for handling and verification of certificates.

  * Support for SRP for TLS authentication.

  * Support for PSK for TLS authentication.

  * Support for TLS Extension mechanism.

  * Support for TLS Compression Methods.

Additionally GnuTLS provides a limited emulation API for the widely used OpenSSL (http://www.openssl.org/) library, to ease integration with existing applications.

GnuTLS consists of three independent parts, namely the ``TLS protocol part'', the ``Certificate part'', and the ``Cryptographic backend'' part.  The `TLS protocol part' is the actual protocol implementation, and is entirely implemented within the GnuTLS library.  The `Certificate part' consists of the certificate parsing, and verification functions which is partially implemented in the GnuTLS library.  The Libtasn1 (ftp://ftp.gnupg.org/gcrypt/alpha/gnutls/libtasn1/), a library which offers ASN.1 parsing capabilities, is used for the X.509 certificate parsing functions.  A smaller version of OpenCDK (ftp://ftp.gnupg.org/gcrypt/alpha/gnutls/opencdk/) is used for the OpenPGP key support in GnuTLS. The ``Cryptographic backend'' is provided by the Libgcrypt (ftp://ftp.gnupg.org/gcrypt/alpha/libgcrypt/) library (On current versions of GnuTLS it is possible to override the default crypto backend. Check see Cryptographic Backend for details).

In order to ease integration in embedded systems, parts of the GnuTLS library can be disabled at compile time. That way a small library, with the required features, can be generated.

General Idea
============

A brief description of how GnuTLS works internally is shown at the figure below. This section may be easier to understand after having seen the examples (see examples).

As shown in the figure, there is a read-only global state that is initialized once by the global initialization function.  This global structure, among others, contains the memory allocation functions used, and some structures needed for the ASN.1 parser.  This structure is never modified by any GnuTLS function, except for the deinitialization function which frees all memory allocated in the global structure and is called after the program has permanently finished using GnuTLS.

The credentials structure is used by some authentication methods, such as certificate authentication (see Certificate Authentication).  A credentials structure may contain certificates, private keys, temporary parameters for diffie hellman or RSA key exchange, and other stuff that may be shared between several TLS sessions.

This structure should be initialized using the appropriate initialization functions. For example an application which uses certificate authentication would probably initialize the credentials, using the appropriate functions, and put its trusted certificates in this structure. The next step is to associate the credentials structure with each TLS session.

A GnuTLS session contains all the required stuff for a session to handle one secure connection. This session calls directly to the transport layer functions, in order to communicate with the peer.  Every session has a unique session ID shared with the peer.

Since TLS sessions can be resumed, servers would probably need a database backend to hold the session's parameters.  Every GnuTLS session after a successful handshake calls the appropriate backend function (see resume, for information on initialization) to store the newly negotiated session. The session database is examined by the server just after having received the client hello (The first message in a TLS handshake), and if the session ID sent by the client, matches a stored session, the stored session will be retrieved, and the new session will be a resumed one, and will share the same session ID with the previous one.

Error Handling
==============

In GnuTLS most functions return an integer type as a result.  In almost all cases a zero or a positive number means success, and a negative number indicates failure, or a situation that some action has to be taken. Thus negative error codes may be fatal or not.

Fatal errors terminate the connection immediately and further sends and receives will be disallowed. An example of a fatal error code is GNUTLS_E_DECRYPTION_FAILED. Non-fatal errors may warn about something, i.e., a warning alert was received, or indicate the some action has to be taken. This is the case with the error code GNUTLS_E_REHANDSHAKE returned by gnutls_record_recv. This error code indicates that the server requests a re-handshake. The client may ignore this request, or may reply with an alert.  You can test if an error code is a fatal one by using the gnutls_error_is_fatal.

If any non fatal errors, that require an action, are to be returned by a function, these error codes will be documented in the function's reference.  See Error Codes, for all the error codes.

Memory Handling
===============

GnuTLS internally handles heap allocated objects differently, depending on the sensitivity of the data they contain. However for performance reasons, the default memory functions do not overwrite sensitive data from memory, nor protect such objects from being written to the swap.  In order to change the default behavior the gnutls_global_set_mem_functions function is available which can be used to set other memory handlers than the defaults.

The Libgcrypt library on which GnuTLS depends, has such secure memory allocation functions available. These should be used in cases where even the system's swap memory is not considered secure. See the documentation of Libgcrypt for more information.

Callback Functions
==================

There are several cases where GnuTLS may need some out of band input from your program. This is now implemented using some callback functions, which your program is expected to register.

An example of this type of functions are the push and pull callbacks which are used to specify the functions that will retrieve and send data to the transport layer.

  * gnutls_transport_set_push_function

  * gnutls_transport_set_pull_function

Other callback functions such as the one set by gnutls_srp_set_server_credentials_function, may require more complicated input, including data to be allocated.  These callbacks should allocate and free memory using the functions shown below.

  * gnutls_malloc

  * gnutls_free

Introduction to TLS
*******************

TLS stands for ``Transport Layer Security'' and is the successor of SSL, the Secure Sockets Layer protocol [SSL3] (see Bibliography) designed by Netscape.  TLS is an Internet protocol, defined by IETF (IETF, or Internet Engineering Task Force, is a large open international community of network designers, operators, vendors, and researchers concerned with the evolution of the Internet architecture and the smooth operation of the Internet. It is open to any interested individual.), described in RFC 4346 and also in [RESCORLA] (see Bibliography).  The protocol provides confidentiality, and authentication layers over any reliable transport layer.  The description, below, refers to TLS 1.0 but also applies to TLS 1.1 [RFC4346] (see Bibliography) and SSL 3.0, since the differences of these protocols are minor.  Older protocols such as SSL 2.0 are not discussed nor implemented in GnuTLS since they are not considered secure today.  GnuTLS also supports X.509 and OpenPGP [RFC4880] (see Bibliography).

TLS Layers
==========

TLS is a layered protocol, and consists of the Record Protocol, the Handshake Protocol and the Alert Protocol. The Record Protocol is to serve all other protocols and is above the transport layer.  The Record protocol offers symmetric encryption, data authenticity, and optionally compression.

The Alert protocol offers some signaling to the other protocols. It can help informing the peer for the cause of failures and other error conditions.  See The Alert Protocol, for more information.  The alert protocol is above the record protocol.

The Handshake protocol is responsible for the security parameters' negotiation, the initial key exchange and authentication.  See The Handshake Protocol, for more information about the handshake protocol.  The protocol layering in TLS is shown in the figure below.

The Transport Layer
===================

TLS is not limited to one transport layer, it can be used above any transport layer, as long as it is a reliable one.  A set of functions is provided and their purpose is to load to GnuTLS the required callbacks to access the transport layer.

  * gnutls_transport_set_push_function

  * gnutls_transport_set_pull_function

  * gnutls_transport_set_ptr

  * gnutls_transport_set_lowat

  * gnutls_transport_set_errno

These functions accept a callback function as a parameter.  The callback functions should return the number of bytes written, or -1 on error and should set errno appropriately.

In some environments, setting errno is unreliable, for example Windows have several errno variables in different CRTs, or it may be that errno is not a thread-local variable.  If this is a concern to you, call gnutls_transport_set_errno with the intended errno value instead of setting errno directly.

GnuTLS currently only interprets the EINTR and EAGAIN errno values and returns the corresponding GnuTLS error codes GNUTLS_E_INTERRUPTED and GNUTLS_E_AGAIN.  These values are usually returned by interrupted system calls, or when non blocking IO is used.  All GnuTLS functions can be resumed (called again), if any of these error codes is returned.  The error codes above refer to the system call, not the GnuTLS function, since signals do not interrupt GnuTLS' functions.

For non blocking sockets or other custom made pull/push functions the gnutls_transport_set_lowat must be called, with a zero low water mark value.

By default, if the transport functions are not set, GnuTLS will use the Berkeley Sockets functions.  In this case GnuTLS will use some hacks in order for select to work, thus making it easy to add TLS support to existing TCP/IP servers.

The TLS Record Protocol
=======================

The Record protocol is the secure communications provider. Its purpose is to encrypt, authenticate and ---optionally--- compress packets. The following functions are available:

gnutls_record_send:
     To send a record packet (with application data).

gnutls_record_recv:
     To receive a record packet (with application data).

gnutls_record_get_direction:
     To get the direction of the last interrupted function call.

As you may have already noticed, the functions which access the Record protocol, are quite limited, given the importance of this protocol in TLS.  This is because the Record protocol's parameters are all set by the Handshake protocol.

The Record protocol initially starts with NULL parameters, which means no encryption, and no MAC is used. Encryption and authentication begin just after the handshake protocol has finished.

Encryption Algorithms Used in the Record Layer
----------------------------------------------

Confidentiality in the record layer is achieved by using symmetric block encryption algorithms like 3DES, AES (AES, or Advanced Encryption Standard, is actually the RIJNDAEL algorithm. This is the algorithm that replaced DES.), or stream algorithms like ARCFOUR_128 (ARCFOUR_128 is a compatible algorithm with RSA's RC4 algorithm, which is considered to be a trade secret.). Ciphers are encryption algorithms that use a single, secret, key to encrypt and decrypt data. Block algorithms in TLS also provide protection against statistical analysis of the data.  Thus, if you're using the TLS protocol, a random number of blocks will be appended to data, to prevent eavesdroppers from guessing the actual data size.

Supported cipher algorithms:

3DES_CBC
     3DES_CBC is the DES block cipher algorithm used with triple encryption (EDE). Has 64 bits block size and is used in CBC mode.

ARCFOUR_128
     ARCFOUR is a fast stream cipher.

ARCFOUR_40
     This is the ARCFOUR cipher that is fed with a 40 bit key, which is considered weak.

AES_CBC
     AES or RIJNDAEL is the block cipher algorithm that replaces the old DES algorithm.  Has 128 bits block size and is used in CBC mode. This is not officially supported in TLS.

Supported MAC algorithms:

MAC_MD5
     MD5 is a cryptographic hash algorithm designed by Ron Rivest. Outputs 128 bits of data.

MAC_SHA
     SHA is a cryptographic hash algorithm designed by NSA. Outputs 160 bits of data.

Compression Algorithms Used in the Record Layer
-----------------------------------------------

The TLS record layer also supports compression.  The algorithms implemented in GnuTLS can be found in the table below. All the algorithms except for DEFLATE which is referenced in [RFC3749] (see Bibliography), should be considered as GnuTLS' extensions (You should use gnutls_handshake_set_private_extensions to enable private extensions.), and should be advertised only when the peer is known to have a compliant client, to avoid interoperability problems.

The included algorithms perform really good when text, or other compressible data are to be transfered, but offer nothing on already compressed data, such as compressed images, zipped archives etc. These compression algorithms, may be useful in high bandwidth TLS tunnels, and in cases where network usage has to be minimized. As a drawback, compression increases latency.

The record layer compression in GnuTLS is implemented based on the proposal [RFC3749] (see Bibliography). The supported compression algorithms are:

DEFLATE
     Zlib compression, using the deflate algorithm.

LZO
     LZO is a very fast compression algorithm.  This algorithm is only available if the GnuTLS-extra library has been initialized and the private extensions are enabled, and if GnuTLS was built with LZO support.

Weaknesses and Countermeasures
------------------------------

Some weaknesses that may affect the security of the Record layer have been found in TLS 1.0 protocol. These weaknesses can be exploited by active attackers, and exploit the facts that

 1. TLS has separate alerts for ``decryption_failed'' and ``bad_record_mac''

 2. The decryption failure reason can be detected by timing the response time.

 3. The IV for CBC encrypted packets is the last block of the previous encrypted packet.

Those weaknesses were solved in TLS 1.1 [RFC4346] (see Bibliography) which is implemented in GnuTLS. For a detailed discussion see the archives of the TLS Working Group mailing list and the paper [CBCATT] (see Bibliography).

The TLS Alert Protocol
======================

The Alert protocol is there to allow signals to be sent between peers. These signals are mostly used to inform the peer about the cause of a protocol failure. Some of these signals are used internally by the protocol and the application protocol does not have to cope with them (see GNUTLS_A_CLOSE_NOTIFY), and others refer to the application protocol solely (see GNUTLS_A_USER_CANCELLED).  An alert signal includes a level indication which may be either fatal or warning. Fatal alerts always terminate the current connection, and prevent future renegotiations using the current session ID.

The alert messages are protected by the record protocol, thus the information that is included does not leak. You must take extreme care for the alert information not to leak to a possible attacker, via public log files etc.

gnutls_alert_send:
     To send an alert signal.

gnutls_error_to_alert:
     To map a gnutls error number to an alert signal.

gnutls_alert_get:
     Returns the last received alert.

gnutls_alert_get_name:
     Returns the name, in a character array, of the given alert.

The TLS Handshake Protocol
==========================

The Handshake protocol is responsible for the ciphersuite negotiation, the initial key exchange, and the authentication of the two peers. This is fully controlled by the application layer, thus your program has to set up the required parameters. Available functions to control the handshake protocol include:

gnutls_priority_init:
     To initialize a priority set of ciphers.

gnutls_priority_deinit:
     To deinitialize a priority set of ciphers.

gnutls_priority_set:
     To associate a priority set with a TLS session.

gnutls_priority_set_direct:
     To directly associate a session with a given priority string.

gnutls_credentials_set:
     To set the appropriate credentials structures.

gnutls_certificate_server_set_request:
     To set whether client certificate is required or not.

gnutls_handshake:
     To initiate the handshake.

TLS Cipher Suites
-----------------

The Handshake Protocol of TLS negotiates cipher suites of the form TLS_DHE_RSA_WITH_3DES_CBC_SHA.  The usual cipher suites contain these parameters:

  * The key exchange algorithm. DHE_RSA in the example.

  * The Symmetric encryption algorithm and mode 3DES_CBC in this example.

  * The MAC (MAC stands for Message Authentication Code. It can be described as a keyed hash algorithm. See RFC2104.) algorithm used for authentication. MAC_SHA is used in the above example.

The cipher suite negotiated in the handshake protocol will affect the Record Protocol, by enabling encryption and data authentication.  Note that you should not over rely on TLS to negotiate the strongest available cipher suite. Do not enable ciphers and algorithms that you consider weak.

The priority functions, dicussed above, allow the application layer to enable and set priorities on the individual ciphers. It may imply that all combinations of ciphersuites are allowed, but this is not true. For several reasons, not discussed here, some combinations were not defined in the TLS protocol.  The supported ciphersuites are shown in ciphersuites.

Client Authentication
---------------------

In the case of ciphersuites that use certificate authentication, the authentication of the client is optional in TLS.  A server may request a certificate from the client --- using the gnutls_certificate_server_set_request function. If a certificate is to be requested from the client during the handshake, the server will send a certificate request message that contains a list of acceptable certificate signers. In GnuTLS the certificate signers list is constructed using the trusted Certificate Authorities by the server. That is the ones set using

  * gnutls_certificate_set_x509_trust_file

  * gnutls_certificate_set_x509_trust_mem

Sending of the names of the CAs can be controlled using gnutls_certificate_send_x509_rdn_sequence. The client, then, may send a certificate, signed by one of the server's acceptable signers.

Resuming Sessions
-----------------

The gnutls_handshake function, is expensive since a lot of calculations are performed. In order to support many fast connections to the same server a client may use session resuming. Session resuming is a feature of the TLS protocol which allows a client to connect to a server, after a successful handshake, without the expensive calculations.  This is achieved by using the previously established keys. GnuTLS supports this feature, and the example (see ex:resume-client) illustrates a typical use of it.

Keep in mind that sessions are expired after some time, for security reasons, thus it may be normal for a server not to resume a session even if you requested that.  Also note that you must enable, using the priority functions, at least the algorithms used in the last session.

Resuming Internals
------------------

The resuming capability, mostly in the server side, is one of the problems of a thread-safe TLS implementations. The problem is that all threads must share information in order to be able to resume sessions. The gnutls approach is, in case of a client, to leave all the burden of resuming to the client. I.e., copy and keep the necessary parameters. See the functions:

  * gnutls_session_get_data

  * gnutls_session_get_id

  * gnutls_session_set_data

The server side is different. A server has to specify some callback functions which store, retrieve and delete session data. These can be registered with:

  * gnutls_db_set_remove_function

  * gnutls_db_set_store_function

  * gnutls_db_set_retrieve_function

  * gnutls_db_set_ptr

It might also be useful to be able to check for expired sessions in order to remove them, and save space. The function gnutls_db_check_entry is provided for that reason.

TLS Extensions
==============

A number of extensions to the TLS protocol have been proposed mainly in [TLSEXT] (see Bibliography). The extensions supported in GnuTLS are:

  * Maximum fragment length negotiation

  * Server name indication

and they will be discussed in the subsections that follow.

Maximum Fragment Length Negotiation
-----------------------------------

This extension allows a TLS implementation to negotiate a smaller value for record packet maximum length. This extension may be useful to clients with constrained capabilities. See the gnutls_record_set_max_size and the gnutls_record_get_max_size functions.

Server Name Indication
----------------------

A common problem in HTTPS servers is the fact that the TLS protocol is not aware of the hostname that a client connects to, when the handshake procedure begins. For that reason the TLS server has no way to know which certificate to send.

This extension solves that problem within the TLS protocol, and allows a client to send the HTTP hostname before the handshake begins within the first handshake packet.  The functions gnutls_server_name_set and gnutls_server_name_get can be used to enable this extension, or to retrieve the name sent by a client.

Selecting Cryptographic Key Sizes
=================================

In TLS, since a lot of algorithms are involved, it is not easy to set a consistent security level. For this reason this section will present some correspondance between key sizes of symmetric algorithms and public key algorithms based on the most conservative values of [SELKEY] (see Bibliography). Those can be used to generate certificates with appropriate key sizes as well as parameters for Diffie Hellman and SRP authentication.

Year
Symmetric key size
RSA key size, DH and SRP prime size
ECC key size

1982
56
417
105

1988
61
566
114

2002
72
1028
139

2015
82
1613
173

2028
92
2362
210

2040
101
3214
244

2050
109
4047
272

The first column provides an estimation of the year until these parameters are considered safe and the rest of the columns list the parameters for the various algorithms.

Note however that the values suggested here are nothing more than an educated guess that is valid today. There are no guarrantees that an algorithm will remain unbreakable or that these values will remain constant in time. There could be scientific breakthroughs that cannot be predicted or total failure of the current public key systems by quantum computers. On the other hand though the cryptosystems used in TLS are selected in a conservative way and such catastrophic breakthroughs or failures are believed to be unlikely.

On SSL 2 and Older Protocols
============================

One of the initial decisions in the GnuTLS development was to implement the known security protocols for the transport layer. Initially TLS 1.0 was implemented since it was the latest at that time, and was considered to be the most advanced in security properties.  Later the SSL 3.0 protocol was implemented since it is still the only protocol supported by several servers and there are no serious security vulnerabilities known.

One question that may arise is why we didn't implement SSL 2.0 in the library.  There are several reasons, most important being that it has serious security flaws, unacceptable for a modern security library.  Other than that, this protocol is barely used by anyone these days since it has been deprecated since 1996.  The security problems in SSL 2.0 include:

  * Message integrity compromised. The SSLv2 message authentication uses the MD5 function, and is insecure.

  * Man-in-the-middle attack. There is no protection of the handshake in SSLv2, which permits a man-in-the-middle attack.

  * Truncation attack. SSLv2 relies on TCP FIN to close the session, so the attacker can forge a TCP FIN, and the peer cannot tell if it was a legitimate end of data or not.

  * Weak message integrity for export ciphers. The cryptographic keys in SSLv2 are used for both message authentication and encryption, so if weak encryption schemes are negotiated (say 40-bit keys) the message authentication code use the same weak key, which isn't necessary.

Other protocols such as Microsoft's PCT 1 and PCT 2 were not implemented because they were also abandoned and deprecated by SSL 3.0 and later TLS 1.0.

On Record Padding
=================

The TLS protocol allows for random padding of records, to make it more difficult to perform analysis on the length of exchanged messages. (In RFC 4346 this is specified in section 6.2.3.2.)  GnuTLS appears to be one of few implementation that take advantage of this text, and pad records by a random length.

The TLS implementation in the Symbian operating system, frequently used by Nokia and Sony-Ericsson mobile phones, cannot handle non-minimal record padding.  What happens when one of these clients handshake with a GnuTLS server is that the client will fail to compute the correct MAC for the record.  The client sends a TLS alert (bad_record_mac) and disconnects.  Typically this will result in error messages such as 'A TLS fatal alert has been received', 'Bad record MAC', or both, on the GnuTLS server side.

GnuTLS implements a work around for this problem.  However, it has to be enabled specifically.  It can be enabled by using gnutls_record_disable_padding, or gnutls_priority_set with the %COMPAT priority string.

If you implement an application that have a configuration file, we recommend that you make it possible for users or administrators to specify a GnuTLS protocol priority string, which is used by your application via gnutls_priority_set.  To allow the best flexibility, make it possible to have a different priority string for different incoming IP addresses.

To enable the workaround in the gnutls-cli client or the gnutls-serv server, for testing of other implementations, use the following parameter: --priority "%COMPAT".

This problem has been discussed on mailing lists and in bug reports. This section tries to collect all pieces of information that we know about the problem.  If you wish to go back to the old discussions, here are some links:

http://bugs.debian.org/390712

http://bugs.debian.org/402861

http://bugs.debian.org/438137

http://thread.gmane.org/gmane.ietf.tls/3079

Authentication Methods
**********************

The TLS protocol provides confidentiality and encryption, but also offers authentication, which is a prerequisite for a secure connection. The available authentication methods in GnuTLS are:

  * Certificate authentication

  * Anonymous authentication

  * SRP authentication

  * PSK authentication

Certificate Authentication
==========================

Authentication Using X.509 Certificates
---------------------------------------

X.509 certificates contain the public parameters, of a public key algorithm, and an authority's signature, which proves the authenticity of the parameters.  See The X.509 trust model, for more information on X.509 protocols.

Authentication Using OpenPGP Keys
---------------------------------

OpenPGP keys also contain public parameters of a public key algorithm, and signatures from several other parties. Depending on whether a signer is trusted the key is considered trusted or not. GnuTLS's OpenPGP authentication implementation is based on the [TLSPGP] (see Bibliography) proposal.

See The OpenPGP trust model, for more information about the OpenPGP trust model.  For a more detailed introduction to OpenPGP and GnuPG see [GPGH] (see Bibliography).

Using Certificate Authentication
--------------------------------

In GnuTLS both the OpenPGP and X.509 certificates are part of the certificate authentication and thus are handled using a common API.

When using certificates the server is required to have at least one certificate and private key pair. A client may or may not have such a pair. The certificate and key pair should be loaded, before any TLS session is initialized, in a certificate credentials structure. This should be done by using gnutls_certificate_set_x509_key_file or gnutls_certificate_set_openpgp_key_file depending on the certificate type.  In the X.509 case, the functions will also accept and use a certificate list that leads to a trusted authority. The certificate list must be ordered in such way that every certificate certifies the one before it. The trusted authority's certificate need not to be included, since the peer should possess it already.

As an alternative, a callback may be used so the server or the client specify the certificate and the key at the handshake time.  That callback can be set using the functions:

  * gnutls_certificate_server_set_retrieve_function

  * gnutls_certificate_client_set_retrieve_function

Certificate verification is possible by loading the trusted authorities into the credentials structure by using gnutls_certificate_set_x509_trust_file or gnutls_certificate_set_openpgp_keyring_file for openpgp keys. Note however that the peer's certificate is not automatically verified, you should call gnutls_certificate_verify_peers2, after a successful handshake, to verify the signatures of the certificate.  An alternative way, which reports a more detailed verification output, is to use gnutls_certificate_get_peers to obtain the raw certificate of the peer and verify it using the functions discussed in The X.509 trust model.

In a handshake, the negotiated cipher suite depends on the certificate's parameters, so not all key exchange methods will be available with some certificates. GnuTLS will disable ciphersuites that are not compatible with the key, or the enabled authentication methods.  For example keys marked as sign-only, will not be able to access the plain RSA ciphersuites, but only the DHE_RSA ones. It is recommended not to use RSA keys for both signing and encryption. If possible use the same key for the DHE_RSA and RSA_EXPORT ciphersuites, which use signing, and a different key for the plain RSA ciphersuites, which use encryption.  All the key exchange methods shown below are available in certificate authentication.

Note that the DHE key exchange methods are generally slower (It really depends on the group used.  Primes with lesser bits are always faster, but also easier to break.  Values less than 768 should not be used today) than plain RSA and require Diffie Hellman parameters to be generated and associated with a credentials structure, by the server.  The RSA-EXPORT method also requires 512 bit RSA parameters, that should also be generated and associated with the credentials structure.  See the functions:

  * gnutls_dh_params_generate2

  * gnutls_certificate_set_dh_params

  * gnutls_rsa_params_generate2

  * gnutls_certificate_set_rsa_export_params

Sometimes in order to avoid bottlenecks in programs it is usefull to store and read parameters from formats that can be generated by external programs such as certtool. This is possible with GnuTLS by using the following functions:

  * gnutls_dh_params_import_pkcs3

  * gnutls_rsa_params_import_pkcs1

  * gnutls_dh_params_export_pkcs3

  * gnutls_rsa_params_export_pkcs1

Key exchange algorithms for OpenPGP and X.509 certificates:

RSA:
     The RSA algorithm is used to encrypt a key and send it to the peer. The certificate must allow the key to be used for encryption.

RSA_EXPORT:
     The RSA algorithm is used to encrypt a key and send it to the peer. In the EXPORT algorithm, the server signs temporary RSA parameters of 512 bits --- which are considered weak --- and sends them to the client.

DHE_RSA:
     The RSA algorithm is used to sign Ephemeral Diffie Hellman parameters which are sent to the peer. The key in the certificate must allow the key to be used for signing. Note that key exchange algorithms which use Ephemeral Diffie Hellman parameters, offer perfect forward secrecy. That means that even if the private key used for signing is compromised, it cannot be used to reveal past session data.

DHE_DSS:
     The DSS algorithm is used to sign Ephemeral Diffie Hellman parameters which are sent to the peer. The certificate must contain DSA parameters to use this key exchange algorithm. DSS stands for Digital Signature Standard.

Anonymous Authentication
========================

The anonymous key exchange performs encryption but there is no indication of the identity of the peer.  This kind of authentication is vulnerable to a man in the middle attack, but this protocol can be used even if there is no prior communication and trusted parties with the peer, or when full anonymity is required.  Unless really required, do not use anonymous authentication.  Available key exchange methods are shown below.

Note that the key exchange methods for anonymous authentication require Diffie Hellman parameters to be generated by the server and associated with an anonymous credentials structure.

Supported anonymous key exchange algorithms:

ANON_DH:
     This algorithm exchanges Diffie Hellman parameters.

Authentication using SRP
========================

Authentication via the Secure Remote Password protocol, SRP (SRP is described in [RFC2945] (see Bibliography)), is supported.  The SRP key exchange is an extension to the TLS protocol, and it is a password based authentication (unlike X.509 or OpenPGP that use certificates). The two peers can be identified using a single password, or there can be combinations where the client is authenticated using SRP and the server using a certificate.

The advantage of SRP authentication, over other proposed secure password authentication schemes, is that SRP does not require the server to hold the user's password.  This kind of protection is similar to the one used traditionally in the UNIX /etc/passwd file, where the contents of this file did not cause harm to the system security if they were revealed.  The SRP needs instead of the plain password something called a verifier, which is calculated using the user's password, and if stolen cannot be used to impersonate the user. Check [TOMSRP] (see Bibliography) for a detailed description of the SRP protocol and the Stanford SRP libraries, which includes a PAM module that synchronizes the system's users passwords with the SRP password files. That way SRP authentication could be used for all the system's users.

The implementation in GnuTLS is based on paper [TLSSRP] (see Bibliography).  The supported SRP key exchange methods are:

SRP:
     Authentication using the SRP protocol.

SRP_DSS:
     Client authentication using the SRP protocol. Server is authenticated using a certificate with DSA parameters.

SRP_RSA:
     Client authentication using the SRP protocol. Server is authenticated using a certificate with RSA parameters.

If clients supporting SRP know the username and password before the connection, should initialize the client credentials and call the function gnutls_srp_set_client_credentials. Alternatively they could specify a callback function by using the function gnutls_srp_set_client_credentials_function.  This has the advantage that allows probing the server for SRP support.  In that case the callback function will be called twice per handshake.  The first time is before the ciphersuite is negotiated, and if the callback returns a negative error code, the callback will be called again if SRP has been negotiated.  This uses a special TLS-SRP handshake idiom in order to avoid, in interactive applications, to ask the user for SRP password and username if the server does not negotiate an SRP ciphersuite.

In server side the default behaviour of GnuTLS is to read the usernames and SRP verifiers from password files. These password files are the ones used by the Stanford srp libraries and can be specified using the gnutls_srp_set_server_credentials_file.  If a different password file format is to be used, then the function gnutls_srp_set_server_credentials_function, should be called, in order to set an appropriate callback.

Some helper functions such as

  * gnutls_srp_verifier

  * gnutls_srp_base64_encode

  * gnutls_srp_base64_decode

are included in GnuTLS, and can be used to generate and maintain SRP verifiers and password files.  A program to manipulate the required parameters for SRP authentication is also included.  See srptool, for more information.

Authentication using PSK
========================

Authentication using Pre-shared keys is a method to authenticate using usernames and binary keys. This protocol avoids making use of public key infrastructure and expensive calculations, thus it is suitable for constraint clients.

The implementation in GnuTLS is based on paper [TLSPSK] (see Bibliography).  The supported PSK key exchange methods are:

PSK:
     Authentication using the PSK protocol.

DHE-PSK:
     Authentication using the PSK protocol and Diffie Hellman key exchange. This method offers perfect forward secrecy.

Clients supporting PSK should supply the username and key before the connection to the client credentials by calling the function gnutls_psk_set_client_credentials. Alternatively they could specify a callback function by using the function gnutls_psk_set_client_credentials_function. This has the advantage that the callback will be called only if PSK has been negotiated.

In server side the default behaviour of GnuTLS is to read the usernames and PSK keys from a password file. The password file should contain usernames and keys in hexadecimal format. The name of the password file can be stored to the credentials structure by calling gnutls_psk_set_server_credentials_file.  If a different password file format is to be used, then the function gnutls_psk_set_server_credentials_function, should be used instead.

The server can help the client chose a suitable username and password, by sending a hint.  In the server, specify the hint by calling gnutls_psk_set_server_credentials_hint.  The client can retrieve the hint, for example in the callback function, using gnutls_psk_client_get_hint.

There is no standard mechanism to derive a PSK key from a password specified by the TLS PSK document.  However, GnuTLS provides gnutls_psk_netconf_derive_key which follows the algorithm specified in draft-ietf-netconf-tls-02.txt.

Some helper functions such as:

  * gnutls_hex_encode

  * gnutls_hex_decode

are included in GnuTLS, and may be used to generate and maintain PSK keys.

Authentication and Credentials
==============================

In GnuTLS every key exchange method is associated with a credentials type. So in order to enable to enable a specific method, the corresponding credentials type should be initialized and set using gnutls_credentials_set.  A mapping is shown below.

Key exchange algorithms and the corresponding credential types:

Key exchange   Client credentials   Server credentials

KX_RSA
KX_DHE_RSA
KX_DHE_DSS
KX_RSA_EXPORT
CRD_CERTIFICATE
CRD_CERTIFICATE

KX_SRP_RSA
CRD_SRP
CRD_SRP
KX_SRP_DSS

CRD_CERTIFICATE

KX_SRP
CRD_SRP
CRD_SRP

KX_ANON_DH
CRD_ANON
CRD_ANON

KX_PSK
CRD_PSK
CRD_PSK

Parameters Stored in Credentials
================================

Several parameters such as the ones used for Diffie-Hellman authentication are stored within the credentials structures, so all sessions can access them. Those parameters are stored in structures such as gnutls_dh_params_t and gnutls_rsa_params_t, and functions like gnutls_certificate_set_dh_params and gnutls_certificate_set_rsa_export_params can be used to associate those parameters with the given credentials structure.

Since those parameters need to be renewed from time to time and a global structure such as the credentials, may not be easy to modify since it is accessible by all sessions, an alternative interface is available using a callback function.  This can be set using the gnutls_certificate_set_params_function.  An example is shown below.

     #include <gnutls.h>

     gnutls_rsa_params_t rsa_params;
     gnutls_dh_params_t dh_params;

     /* This function will be called once a session requests DH
      * or RSA parameters. The parameters returned (if any) will
      * be used for the first handshake only.
      */
     static int get_params( gnutls_session_t session,
             gnutls_params_type_t type,
             gnutls_params_st *st)
     {
        if (type == GNUTLS_PARAMS_RSA_EXPORT)
           st->params.rsa_export = rsa_params;
        else if (type == GNUTLS_PARAMS_DH)
           st->params.dh = dh_params;
        else return -1;

        st->type = type;
        /* do not deinitialize those parameters.
         */
        st->deinit = 0;

        return 0;
     }

     int main()
     {
        gnutls_certificate_credentials_t cert_cred;

        initialize_params();

        /* ...
         */

        gnutls_certificate_set_params_function( cert_cred, get_params);
     }

More on Certificate Authentication
**********************************

The X.509 Trust Model
=====================

The X.509 protocols rely on a hierarchical trust model. In this trust model Certification Authorities (CAs) are used to certify entities.  Usually more than one certification authorities exist, and certification authorities may certify other authorities to issue certificates as well, following a hierarchical model.

One needs to trust one or more CAs for his secure communications. In that case only the certificates issued by the trusted authorities are acceptable.  See the figure above for a typical example.  The API for handling X.509 certificates is described at section sec:x509api.  Some examples are listed below.

X.509 Certificates
------------------

An X.509 certificate usually contains information about the certificate holder, the signer, a unique serial number, expiration dates and some other fields [RFC3280] (see Bibliography) as shown in the table below.

version:
     The field that indicates the version of the certificate.

serialNumber:
     This field holds a unique serial number per certificate.

issuer:
     Holds the issuer's distinguished name.

validity:
     The activation and expiration dates.

subject:
     The subject's distinguished name of the certificate.

extensions:
     The extensions are fields only present in version 3 certificates.

The certificate's subject or issuer name is not just a single string.  It is a Distinguished name and in the ASN.1 notation is a sequence of several object IDs with their corresponding values. Some of available OIDs to be used in an X.509 distinguished name are defined in gnutls/x509.h.

The Version field in a certificate has values either 1 or 3 for version 3 certificates.  Version 1 certificates do not support the extensions field so it is not possible to distinguish a CA from a person, thus their usage should be avoided.

The validity dates are there to indicate the date that the specific certificate was activated and the date the certificate's key would be considered invalid.

Certificate extensions are there to include information about the certificate's subject that did not fit in the typical certificate fields. Those may be e-mail addresses, flags that indicate whether the belongs to a CA etc.  All the supported X.509 version 3 extensions are shown in the table below.

subject key id (2.5.29.14):
     An identifier of the key of the subject.

authority key id (2.5.29.35):
     An identifier of the authority's key used to sign the certificate.

subject alternative name (2.5.29.17):
     Alternative names to subject's distinguished name.

key usage (2.5.29.15):
     Constraints the key's usage of the certificate.

extended key usage (2.5.29.37):
     Constraints the purpose of the certificate.

basic constraints (2.5.29.19):
     Indicates whether this is a CA certificate or not, and specify the maximum path lengths of certificate chains.

CRL distribution points (2.5.29.31):
     This extension is set by the CA, in order to inform about the issued CRLs.

Proxy Certification Information (1.3.6.1.5.5.7.1.14):
     Proxy Certificates includes this extension that contains the OID of the proxy policy language used, and can specify limits on the maximum lengths of proxy chains.  Proxy Certificates are specified in [RFC3820] (see Bibliography).

In GnuTLS the X.509 certificate structures are handled using the gnutls_x509_crt_t type and the corresponding private keys with the gnutls_x509_privkey_t type.  All the available functions for X.509 certificate handling have their prototypes in gnutls/x509.h. An example program to demonstrate the X.509 parsing capabilities can be found at section ex:x509-info.

Verifying X.509 Certificate Paths
---------------------------------

Verifying certificate paths is important in X.509 authentication. For this purpose the function gnutls_x509_crt_verify is provided. The output of this function is the bitwise OR of the elements of the gnutls_certificate_status_t enumeration.  A detailed description of these elements can be found in figure below. The function gnutls_certificate_verify_peers2 is equivalent to the previous one, and will verify the peer's certificate in a TLS session.

CERT_INVALID:
     The certificate is not signed by one of the known authorities, or the signature is invalid.

CERT_REVOKED:
     The certificate has been revoked by its CA.

CERT_SIGNER_NOT_FOUND:
     The certificate's issuer is not known. This is the case when the issuer is not in the trusted certificates list.

GNUTLS_CERT_SIGNER_NOT_CA:
     The certificate's signer was not a CA. This may happen if this was a version 1 certificate, which is common with some CAs, or a version 3 certificate without the basic constrains extension.

GNUTLS_CERT_INSECURE_ALGORITHM:
     The certificate was signed using an insecure algorithm such as MD2 or MD5.  These algorithms have been broken and should not be trusted.

There is also to possibility to pass some input to the verification functions in the form of flags. For gnutls_x509_crt_verify the flags are passed straightforward, but gnutls_certificate_verify_peers2 depends on the flags set by calling gnutls_certificate_set_verify_flags.  All the available flags are part of the enumeration gnutls_certificate_verify_flags and are explained in the table below.

GNUTLS_VERIFY_DISABLE_CA_SIGN:
     If set a signer does not have to be a certificate authority. This flag should normaly be disabled, unless you know what this means.

GNUTLS_VERIFY_ALLOW_X509_V1_CA_CRT:
     Allow only trusted CA certificates that have version 1.  This is safer than GNUTLS_VERIFY_ALLOW_ANY_X509_V1_CA_CRT, and should be used instead. That way only signers in your trusted list will be allowed to have certificates of version 1.

GNUTLS_VERIFY_ALLOW_ANY_X509_V1_CA_CRT:
     Allow CA certificates that have version 1 (both root and intermediate). This is dangerous since those haven't the basicConstraints extension. Must be used in combination with GNUTLS_VERIFY_ALLOW_X509_V1_CA_CRT.

GNUTLS_VERIFY_DO_NOT_ALLOW_SAME:
     If a certificate is not signed by anyone trusted but exists in the trusted CA list do not treat it as trusted.

GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD2:
     Allow certificates to be signed using the old MD2 algorithm.

GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5:
     Allow certificates to be signed using the broken MD5 algorithm.

Although the verification of a certificate path indicates that the certificate is signed by trusted authority, does not reveal anything about the peer's identity. It is required to verify if the certificate's owner is the one you expect. For more information consult [RFC2818] (see Bibliography) and section ex:verify for an example.

PKCS #10 Certificate Requests
-----------------------------

A certificate request is a structure, which contain information about an applicant of a certificate service.  It usually contains a private key, a distinguished name and secondary data such as a challenge password. GnuTLS supports the requests defined in PKCS #10 [RFC2986] (see Bibliography). Other certificate request's format such as PKIX's [RFC4211] (see Bibliography) are not currently supported.

In GnuTLS the PKCS #10 structures are handled using the gnutls_x509_crq_t type.  An example of a certificate request generation can be found at section ex:crq.

PKCS #12 Structures
-------------------

A PKCS #12 structure [PKCS12] (see Bibliography) usually contains a user's private keys and certificates. It is commonly used in browsers to export and import the user's identities.

In GnuTLS the PKCS #12 structures are handled using the gnutls_pkcs12_t type. This is an abstract type that may hold several gnutls_pkcs12_bag_t types.  The Bag types are the holders of the actual data, which may be certificates, private keys or encrypted data.  An Bag of type encrypted should be decrypted in order for its data to be accessed.

An example of a PKCS #12 structure generation can be found at section ex:pkcs12.

The OpenPGP Trust Model
=======================

The OpenPGP key authentication relies on a distributed trust model, called the ``web of trust''. The ``web of trust'' uses a decentralized system of trusted introducers, which are the same as a CA. OpenPGP allows anyone to sign anyone's else public key. When Alice signs Bob's key, she is introducing Bob's key to anyone who trusts Alice. If someone trusts Alice to introduce keys, then Alice is a trusted introducer in the mind of that observer.

For example: If David trusts Alice to be an introducer, and Alice signed Bob's key, Dave also trusts Bob's key to be the real one.

There are some key points that are important in that model. In the example Alice has to sign Bob's key, only if she is sure that the key belongs to Bob. Otherwise she may also make Dave falsely believe that this is Bob's key. Dave has also the responsibility to know who to trust.  This model is similar to real life relations.

Just see how Charlie behaves in the previous example. Although he has signed Bob's key - because he knows, somehow, that it belongs to Bob - he does not trust Bob to be an introducer. Charlie decided to trust only Kevin, for some reason. A reason could be that Bob is lazy enough, and signs other people's keys without being sure that they belong to the actual owner.

OpenPGP Keys
------------

In GnuTLS the OpenPGP key structures [RFC2440] (see Bibliography) are handled using the gnutls_openpgp_crt_t type and the corresponding private keys with the gnutls_openpgp_privkey_t type. All the prototypes for the key handling functions can be found at gnutls/openpgp.h.

Verifying an OpenPGP Key
------------------------

The verification functions of OpenPGP keys, included in GnuTLS, are simple ones, and do not use the features of the ``web of trust''.  For that reason, if the verification needs are complex, the assistance of external tools like GnuPG and GPGME (http://www.gnupg.org/related_software/gpgme/) is recommended.

There is one verification function in GnuTLS, the gnutls_openpgp_crt_verify_ring. This checks an OpenPGP key against a given set of public keys (keyring) and returns the key status. The key verification status is the same as in X.509 certificates, although the meaning and interpretation are different. For example an OpenPGP key may be valid, if the self signature is ok, even if no signers were found.  The meaning of verification status is shown in the figure below.

CERT_INVALID:
     A signature on the key is invalid. That means that the key was modified by somebody, or corrupted during transport.

CERT_REVOKED:
     The key has been revoked by its owner.

CERT_SIGNER_NOT_FOUND:
     The key was not signed by a known signer.

GNUTLS_CERT_INSECURE_ALGORITHM:
     The certificate was signed using an insecure algorithm such as MD2 or MD5. These algorithms have been broken and should not be trusted.

Digital Signatures
==================

     [file signatures.texi not included]

How To Use TLS in Application Protocols
***************************************

This chapter is intended to provide some hints on how to use the TLS over simple custom made application protocols.  The discussion below mainly refers to the TCP/IP transport layer but may be extended to other ones too.

Separate Ports
==============

Traditionally SSL was used in application protocols by assigning a new port number for the secure services. That way two separate ports were assigned, one for the non secure sessions, and one for the secured ones. This has the benefit that if a user requests a secure session then the client will try to connect to the secure port and fail otherwise. The only possible attack with this method is a denial of service one. The most famous example of this method is the famous ``HTTP over TLS'' or HTTPS protocol [RFC2818] (see Bibliography).

Despite its wide use, this method is not as good as it seems.  This approach starts the TLS Handshake procedure just after the client connects on the ---so called--- secure port.  That way the TLS protocol does not know anything about the client, and popular methods like the host advertising in HTTP do not work (See also the Server Name Indication extension on serverind.).  There is no way for the client to say ``I connected to YYY server'' before the Handshake starts, so the server cannot possibly know which certificate to use.

Other than that it requires two separate ports to run a single service, which is unnecessary complication. Due to the fact that there is a limitation on the available privileged ports, this approach was soon obsoleted.

Upward Negotiation
==================

Other application protocols (See LDAP, IMAP etc.)  use a different approach to enable the secure layer.  They use something called the ``TLS upgrade'' method. This method is quite tricky but it is more flexible. The idea is to extend the application protocol to have a ``STARTTLS'' request, whose purpose it to start the TLS protocols just after the client requests it.  This is a really neat idea and does not require an extra port.

This method is used by almost all modern protocols and there is even the [RFC2817] (see Bibliography) paper which proposes extensions to HTTP to support it.

The tricky part, in this method, is that the ``STARTTLS'' request is sent in the clear, thus is vulnerable to modifications.  A typical attack is to modify the messages in a way that the client is fooled and thinks that the server does not have the ``STARTTLS'' capability. See a typical conversation of a hypothetical protocol:

     (client connects to the server)

     CLIENT: HELLO I'M MR. XXX

     SERVER: NICE TO MEET YOU XXX

     CLIENT: PLEASE START TLS

     SERVER: OK

     *** TLS STARTS

     CLIENT: HERE ARE SOME CONFIDENTIAL DATA

And see an example of a conversation where someone is acting in between:

     (client connects to the server)

     CLIENT: HELLO I'M MR. XXX

     SERVER: NICE TO MEET YOU XXX

     CLIENT: PLEASE START TLS

     (here someone inserts this message)

     SERVER: SORRY I DON'T HAVE THIS CAPABILITY

     CLIENT: HERE ARE SOME CONFIDENTIAL DATA

As you can see above the client was fooled, and was dummy enough to send the confidential data in the clear.

How to avoid the above attack? As you may have already thought this one is easy to avoid. The client has to ask the user before it connects whether the user requests TLS or not. If the user answered that he certainly wants the secure layer the last conversation should be:

     (client connects to the server)

     CLIENT: HELLO I'M MR. XXX

     SERVER: NICE TO MEET YOU XXX

     CLIENT: PLEASE START TLS

     (here someone inserts this message)

     SERVER: SORRY I DON'T HAVE THIS CAPABILITY

     CLIENT: BYE

     (the client notifies the user that the secure connection was not possible)

This method, if implemented properly, is far better than the traditional method, and the security properties remain the same, since only denial of service is possible. The benefit is that the server may request additional data before the TLS Handshake protocol starts, in order to send the correct certificate, use the correct password file (in SRP authentication), or anything else!

How To Use GnuTLS in Applications
*********************************

Preparation
===========

To use GnuTLS, you have to perform some changes to your sources and your build system. The necessary changes are explained in the following subsections.

Headers
-------

All the data types and functions of the GnuTLS library are defined in the header file gnutls/gnutls.h.  This must be included in all programs that make use of the GnuTLS library.

The extra functionality of the GnuTLS-extra library is available by including the header file gnutls/extra.h in your programs.

Version Check
-------------

It is often desirable to check that the version of `gnutls' used is indeed one which fits all requirements.  Even with binary compatibility new features may have been introduced but due to problem with the dynamic linker an old version is actually used.  So you may want to check that the version is okay right after program startup. See the function gnutls_check_version.

Debugging
---------

In many cases things may not go as expected and further information, to assist debugging, from GnuTLS is desired. Those are the case where the gnutls_global_set_log_level and gnutls_global_set_log_function are to be used. Those will print verbose information on the GnuTLS functions internal flow.

Building the Source
-------------------

If you want to compile a source file including the `gnutls/gnutls.h' header file, you must make sure that the compiler can find it in the directory hierarchy.  This is accomplished by adding the path to the directory in which the header file is located to the compilers include file search path (via the -I option).

However, the path to the include file is determined at the time the source is configured.  To solve this problem, GnuTLS ships with two small helper programs libgnutls-config and libgnutls-extra-config that knows about the path to the include file and other configuration options.  The options that need to be added to the compiler invocation at compile time are output by the --cflags option to libgnutls-config.  The following example shows how it can be used at the command line:

     gcc -c foo.c `libgnutls-config --cflags`

Adding the output of libgnutls-config --cflags to the compilers command line will ensure that the compiler can find the GnuTLS header file.

A similar problem occurs when linking the program with the library. Again, the compiler has to find the library files.  For this to work, the path to the library files has to be added to the library search path (via the -L option).  For this, the option --libs to libgnutls-config can be used.  For convenience, this option also outputs all other options that are required to link the program with the GnuTLS libararies.  The example shows how to link `foo.o' with the GnuTLS libraries to a program foo.

     gcc -o foo foo.o `libgnutls-config --libs`

Of course you can also combine both examples to a single command by specifying both options to `libgnutls-config':

     gcc -o foo foo.c `libgnutls-config --cflags --libs`

Multi-Threaded Applications
===========================

Although the GnuTLS library is thread safe by design, some parts of the crypto backend, such as the random generator, are not. Since libgcrypt 1.1.92 there was an automatic detection of the thread library used by the application, so most applications wouldn't need to do any changes to ensure thread-safety. Due to the unportability of the automatic thread detection, this was removed from later releases of libgcrypt, so applications have now to register callback functions to ensure proper locking in sensitive parts of libgcrypt.

There are helper macros to help you properly initialize the libraries. Examples are shown below.

  * POSIX threads

          #include <gnutls.h>
          #include <gcrypt.h>
          #include <errno.h>
          #include <pthread.h>
          GCRY_THREAD_OPTION_PTHREAD_IMPL;

          int main()
          {
             /* The order matters.
              */
             gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_pthread);
             gnutls_global_init();
          }

  * GNU PTH threads

          #include <gnutls.h>
          #include <gcrypt.h>
          #include <errno.h>
          #include <pth.h>
          GCRY_THREAD_OPTION_PTH_IMPL;

          int main()
          {
             gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_pth);
             gnutls_global_init();
          }

  * Other thread packages

          /* The gcry_thread_cbs structure must have been
           * initialized.
           */
          static struct gcry_thread_cbs gcry_threads_other = { ... };

          int main()
          {
             gcry_control (GCRYCTL_SET_THREAD_CBS, &gcry_threads_other);
          }

Client Examples
===============

This section contains examples of TLS and SSL clients, using GnuTLS.  Note that these examples contain little or no error checking.  Some of the examples require functions implemented by another example.

Simple Client Example with Anonymous Authentication
---------------------------------------------------

The simplest client using TLS is the one that doesn't do any authentication.  This means no external certificates or passwords are needed to set up the connection.  As could be expected, the connection is vulnerable to man-in-the-middle (active or redirection) attacks. However, the data is integrity and privacy protected.

     [file ex-client1.c not included]

Simple Client Example with X.509 Certificate Support
----------------------------------------------------

Let's assume now that we want to create a TCP client which communicates with servers that use X.509 or OpenPGP certificate authentication. The following client is a very simple TLS client, it does not support session resuming, not even certificate verification. The TCP functions defined in this example are used in most of the other examples below, without redefining them.

     [file ex-client2.c not included]

Obtaining Session Information
-----------------------------

Most of the times it is desirable to know the security properties of the current established session.  This includes the underlying ciphers and the protocols involved.  That is the purpose of the following function.  Note that this function will print meaningful values only if called after a successful gnutls_handshake.

     [file ex-session-info.c not included]

Verifying Peer's Certificate
----------------------------

A TLS session is not secure just after the handshake procedure has finished.  It must be considered secure, only after the peer's certificate and identity have been verified. That is, you have to verify the signature in peer's certificate, the hostname in the certificate, and expiration dates.  Just after this step you should treat the connection as being a secure one.

     [file ex-rfc2818.c not included]

An other example is listed below which provides a more detailed verification output.

     [file ex-verify.c not included]

Using a Callback to Select the Certificate to Use
-------------------------------------------------

There are cases where a client holds several certificate and key pairs, and may not want to load all of them in the credentials structure.  The following example demonstrates the use of the certificate selection callback.

     [file ex-cert-select.c not included]

Client with Resume Capability Example
-------------------------------------

This is a modification of the simple client example. Here we demonstrate the use of session resumption. The client tries to connect once using TLS, close the connection and then try to establish a new connection using the previously negotiated data.

     [file ex-client-resume.c not included]

Simple Client Example with SRP Authentication
---------------------------------------------

The following client is a very simple SRP TLS client which connects to a server and authenticates using a username and a password. The server may authenticate itself using a certificate, and in that case it has to be verified.

     [file ex-client-srp.c not included]

Simple Client Example with TLS/IA Support
-----------------------------------------

The following client is a simple client which uses the TLS/IA extension to authenticate with the server.

     [file ex-client-tlsia.c not included]

Simple Client Example using the C++ API
---------------------------------------

The following client is a simple example of a client client utilizing the GnuTLS C++ API.

     [file ex-cxx.cpp not included]

Helper Function for TCP Connections
-----------------------------------

This helper function abstracts away TCP connection handling from the other examples.  It is required to build some examples.

     [file tcp.c not included]

Server Examples
===============

This section contains examples of TLS and SSL servers, using GnuTLS.

Echo Server with X.509 Authentication
-------------------------------------

This example is a very simple echo server which supports X.509 authentication, using the RSA ciphersuites.

     [file ex-serv1.c not included]

Echo Server with X.509 Authentication II
----------------------------------------

The following example is a server which supports X.509 authentication.  This server supports the export-grade cipher suites, the DHE ciphersuites and session resuming.

     [file ex-serv-export.c not included]

Echo Server with OpenPGP Authentication
---------------------------------------

The following example is an echo server which supports OpenPGP key authentication. You can easily combine this functionality ---that is have a server that supports both X.509 and OpenPGP certificates--- but we separated them to keep these examples as simple as possible.

     [file ex-serv-pgp.c not included]

Echo Server with SRP Authentication
-----------------------------------

This is a server which supports SRP authentication. It is also possible to combine this functionality with a certificate server. Here it is separate for simplicity.

     [file ex-serv-srp.c not included]

Echo Server with Anonymous Authentication
-----------------------------------------

This example server support anonymous authentication, and could be used to serve the example client for anonymous authentication.

     [file ex-serv-anon.c not included]

Miscellaneous Examples
======================

Checking for an Alert
---------------------

This is a function that checks if an alert has been received in the current session.

     [file ex-alert.c not included]

X.509 Certificate Parsing Example
---------------------------------

To demonstrate the X.509 parsing capabilities an example program is listed below.  That program reads the peer's certificate, and prints information about it.

     [file ex-x509-info.c not included]

Certificate Request Generation
------------------------------

The following example is about generating a certificate request, and a private key. A certificate request can be later be processed by a CA, which should return a signed certificate.

     [file ex-crq.c not included]

PKCS #12 Structure Generation
-----------------------------

The following example is about generating a PKCS #12 structure.

     [file ex-pkcs12.c not included]

Compatibility with the OpenSSL Library
======================================

To ease GnuTLS' integration with existing applications, a compatibility layer with the widely used OpenSSL library is included in the gnutls-openssl library. This compatibility layer is not complete and it is not intended to completely reimplement the OpenSSL API with GnuTLS.  It only provides source-level compatibility. There is currently no attempt to make it binary-compatible with OpenSSL.

The prototypes for the compatibility functions are in the gnutls/openssl.h header file.

Current limitations imposed by the compatibility layer include:

  * Error handling is not thread safe.

Opaque PRF Input TLS Extension
==============================

GnuTLS supports the Opaque PRF Input TLS extension (draft-rescorla-tls-opaque-prf-input-00.txt).  The API consists of one API for use in the client, gnutls_oprfi_enable_client, and one API for use in the server, gnutls_oprfi_enable_server. You must invoke both functions before calling gnutls_handshake. The server utilizes a callback function into the application.  The callback can look at the random string provided by the client, and also set the server string.  The string lengths must be equal according to the protocol.

Included Programs
*****************

Included with GnuTLS are also a few command line tools that let you use the library for common tasks without writing an application.  The applications are discussed in this chapter.

Invoking certtool
=================

This is a program to generate X.509 certificates, certificate requests, CRLs and private keys.

Certtool help
Usage: certtool [options]
     -s, --generate-self-signed
                              Generate a self-signed certificate.
     -c, --generate-certificate
                              Generate a signed certificate.
     --generate-proxy         Generate a proxy certificate.
     --generate-crl           Generate a CRL.
     -u, --update-certificate
                              Update a signed certificate.
     -p, --generate-privkey   Generate a private key.
     -q, --generate-request   Generate a PKCS #10 certificate
                              request.
     -e, --verify-chain       Verify a PEM encoded certificate chain.
                              The last certificate in the chain must
                              be a self signed one.
     --verify-crl             Verify a CRL.
     --generate-dh-params     Generate PKCS #3 encoded Diffie Hellman
                              parameters.
     --get-dh-params          Get the included PKCS #3 encoded Diffie
                              Hellman parameters.
     --load-privkey FILE      Private key file to use.
     --load-request FILE      Certificate request file to use.
     --load-certificate FILE
                              Certificate file to use.
     --load-ca-privkey FILE   Certificate authority's private key
                              file to use.
     --load-ca-certificate FILE
                              Certificate authority's certificate
                              file to use.
     --password PASSWORD      Password to use.
     -i, --certificate-info   Print information on a certificate.
     -l, --crl-info           Print information on a CRL.
     --p12-info               Print information on a PKCS #12
                              structure.
     --p7-info                Print information on a PKCS #7
                              structure.
     --smime-to-p7            Convert S/MIME to PKCS #7 structure.
     -k, --key-info           Print information on a private key.
     --fix-key                Regenerate the parameters in a private
                              key.
     --to-p12                 Generate a PKCS #12 structure.
     -8, --pkcs8              Use PKCS #8 format for private keys.
     --dsa                    Use DSA keys.
     --hash STR               Hash algorithm to use for signing
                              (MD5,SHA1,RMD160).
     --export-ciphers         Use weak encryption algorithms.
     --inder                  Use DER format for input certificates
                              and private keys.
     --outder                 Use DER format for output certificates
                              and private keys.
     --bits BITS              specify the number of bits for key
                              generation.
     --outfile FILE           Output file.
     --infile FILE            Input file.
     --template FILE          Template file to use for non
                              interactive operation.
     -d, --debug LEVEL        specify the debug level. Default is 1.
     -h, --help               shows this help text
     -v, --version            shows the program's version
     --copyright              shows the program's license

The program can be used interactively or non interactively by specifying the --template command line option. See below for an example of a template file.

How to use certtool interactively:

  * To generate parameters for Diffie Hellman key exchange, use the command:

          $ certtool --generate-dh-params --outfile dh.pem

  * To generate parameters for the RSA-EXPORT key exchange, use the command:

          $ certtool --generate-privkey --bits 512 --outfile rsa.pem

  * To create a self signed certificate, use the command:

          $ certtool --generate-privkey --outfile ca-key.pem
          $ certtool --generate-self-signed --load-privkey ca-key.pem \
             --outfile ca-cert.pem

     Note that a self-signed certificate usually belongs to a certificate authority, that signs other certificates.

  * To create a private key, run:

          $ certtool --generate-privkey --outfile key.pem

  * To generate a certificate using the private key, use the command:

          $ certtool --generate-certificate --load-privkey key.pem \
             --outfile cert.pem --load-ca-certificate ca-cert.pem \
             --load-ca-privkey ca-key.pem

  * To create a certificate request (needed when the certificate is issued by another party), run:

          $ certtool --generate-request --load-privkey key.pem \
            --outfile request.pem

  * To generate a certificate using the previous request, use the command:

          $ certtool --generate-certificate --load-request request.pem \
             --outfile cert.pem \
             --load-ca-certificate ca-cert.pem --load-ca-privkey ca-key.pem

  * To view the certificate information, use:

          $ certtool --certificate-info --infile cert.pem

  * To generate a PKCS #12 structure using the previous key and certificate, use the command:

          $ certtool --load-certificate cert.pem --load-privkey key.pem \
            --to-p12 --outder --outfile key.p12

  * Proxy certificate can be used to delegate your credential to a temporary, typically short-lived, certificate.  To create one from the previously created certificate, first create a temporary key and then generate a proxy certificate for it, using the commands:

          $ certtool --generate-privkey > proxy-key.pem
          $ certtool --generate-proxy --load-ca-privkey key.pem \
            --load-privkey proxy-key.pem --load-certificate cert.pem \
            --outfile proxy-cert.pem

  * To create an empty Certificate Revocation List (CRL) do:

          $ certtool --generate-crl --load-ca-privkey x509-ca-key.pem --load-ca-certificate x509-ca.pem

     To create a CRL that contains some revoked certificates, place the certificates in a file and use --load-certificate as follows:

          $ certtool --generate-crl --load-ca-privkey x509-ca-key.pem --load-ca-certificate x509-ca.pem --load-certificate revoked-certs.pem

  * To verify a Certificate Revocation List (CRL) do:

          $ certtool --verify-crl --load-ca-certificate x509-ca.pem < crl.pem

Certtool's template file format:

  * Firstly create a file named 'cert.cfg' that contains the information about the certificate. An example file is listed below.

  * Then execute:

          $ certtool --generate-certificate cert.pem --load-privkey key.pem  \
             --template cert.cfg \
             --load-ca-certificate ca-cert.pem --load-ca-privkey ca-key.pem

An example certtool template file:

     # X.509 Certificate options
     #
     # DN options

     # The organization of the subject.
     organization = "Koko inc."

     # The organizational unit of the subject.
     unit = "sleeping dept."

     # The locality of the subject.
     # locality =

     # The state of the certificate owner.
     state = "Attiki"

     # The country of the subject. Two letter code.
     country = GR

     # The common name of the certificate owner.
     cn = "Cindy Lauper"

     # A user id of the certificate owner.
     #uid = "clauper"

     # If the supported DN OIDs are not adequate you can set
     # any OID here.
     # For example set the X.520 Title and the X.520 Pseudonym
     # by using OID and string pairs.
     #dn_oid = "2.5.4.12" "Dr." "2.5.4.65" "jackal"

     # This is deprecated and should not be used in new
     # certificates.
     # pkcs9_email = "none@none.org"

     # The serial number of the certificate
     serial = 007

     # In how many days, counting from today, this certificate will expire.
     expiration_days = 700

     # X.509 v3 extensions

     # A dnsname in case of a WWW server.
     #dns_name = "www.none.org"

     # An IP address in case of a server.
     #ip_address = "192.168.1.1"

     # An email in case of a person
     email = "none@none.org"

     # An URL that has CRLs (certificate revocation lists)
     # available. Needed in CA certificates.
     #crl_dist_points = "http://www.getcrl.crl/getcrl/"

     # Whether this is a CA certificate or not
     #ca

     # Whether this certificate will be used for a TLS client
     #tls_www_client

     # Whether this certificate will be used for a TLS server
     #tls_www_server

     # Whether this certificate will be used to sign data (needed
     # in TLS DHE ciphersuites).
     signing_key

     # Whether this certificate will be used to encrypt data (needed
     # in TLS RSA ciphersuites). Note that it is prefered to use different
     # keys for encryption and signing.
     #encryption_key

     # Whether this key will be used to sign other certificates.
     #cert_signing_key

     # Whether this key will be used to sign CRLs.
     #crl_signing_key

     # Whether this key will be used to sign code.
     #code_signing_key

     # Whether this key will be used to sign OCSP data.
     #ocsp_signing_key

     # Whether this key will be used for time stamping.
     #time_stamping_key

Invoking gnutls-cli
===================

Simple client program to set up a TLS connection to some other computer.  It sets up a TLS connection and forwards data from the standard input to the secured socket and vice versa.

GNU TLS test client
Usage:  gnutls-cli [options] hostname

     -d, --debug integer      Enable debugging
     -r, --resume             Connect, establish a session. Connect
                              again and resume this session.
     -s, --starttls           Connect, establish a plain session and
                              start TLS when EOF or a SIGALRM is
                              received.
     --crlf                   Send CR LF instead of LF.
     --x509fmtder             Use DER format for certificates to read
                              from.
     -f, --fingerprint        Send the openpgp fingerprint, instead
                              of the key.
     --disable-extensions     Disable all the TLS extensions.
     --print-cert             Print the certificate in PEM format.
     -p, --port integer       The port to connect to.
     --recordsize integer     The maximum record size to advertize.
     -V, --verbose            More verbose output.
     --ciphers cipher1 cipher2...
                              Ciphers to enable.
     --protocols protocol1 protocol2...
                              Protocols to enable.
     --comp comp1 comp2...    Compression methods to enable.
     --macs mac1 mac2...      MACs to enable.
     --kx kx1 kx2...          Key exchange methods to enable.
     --ctypes certType1 certType2...
                              Certificate types to enable.
     --x509cafile FILE        Certificate file to use.
     --x509crlfile FILE       CRL file to use.
     --pgpkeyfile FILE        PGP Key file to use.
     --pgpkeyring FILE        PGP Key ring file to use.
     --pgptrustdb FILE        PGP trustdb file to use.
     --pgpcertfile FILE       PGP Public Key (certificate) file to
                              use.
     --x509keyfile FILE       X.509 key file to use.
     --x509certfile FILE      X.509 Certificate file to use.
     --srpusername NAME       SRP username to use.
     --srppasswd PASSWD       SRP password to use.
     --insecure               Don't abort program if server
                              certificate can't be validated.
     -l, --list               Print a list of the supported
                              algorithms  and modes.
     -h, --help               prints this help
     -v, --version            prints the program's version number
     --copyright              prints the program's license

To connect to a server using PSK authentication, you may use something like:

     $ gnutls-cli -p 5556 test.gnutls.org --pskusername jas --pskkey 9e32cf7786321a828ef7668f09fb35db --priority NORMAL:+PSK:-RSA:-DHE-RSA -d 4711

Example client PSK connection
-----------------------------

If your server only supports the PSK ciphersuite, connecting to it should be as simple as connecting to the server:

     $ ./gnutls-cli -p 5556 localhost
     Resolving 'localhost'...
     Connecting to '127.0.0.1:5556'...
     - PSK client callback. PSK hint 'psk_identity_hint'
     Enter PSK identity: psk_identity
     Enter password:
     - PSK authentication. PSK hint 'psk_identity_hint'
     - Version: TLS1.1
     - Key Exchange: PSK
     - Cipher: AES-128-CBC
     - MAC: SHA1
     - Compression: NULL
     - Handshake was completed

     - Simple Client Mode:

If the server supports several cipher suites, you may need to force it to chose PSK by using a cipher priority parameter such as --priority NORMAL:+PSK:-RSA:-DHE-RSA:-DHE-PSK.

Instead of using the Netconf-way to derive the PSK key from a password, you can also give the PSK username and key directly on the command line:

     $ ./gnutls-cli -p 5556 localhost --pskusername psk_identity --pskkey 88f3824b3e5659f52d00e959bacab954b6540344
     Resolving 'localhost'...
     Connecting to '127.0.0.1:5556'...
     - PSK authentication. PSK hint 'psk_identity_hint'
     - Version: TLS1.1
     - Key Exchange: PSK
     - Cipher: AES-128-CBC
     - MAC: SHA1
     - Compression: NULL
     - Handshake was completed

     - Simple Client Mode:

By keeping the --pskusername parameter and removing the --pskkey parameter, it will query only for the password during the handshake.

Invoking gnutls-cli-debug
=========================

This program was created to assist in debugging GnuTLS, but it might be useful to extract a TLS server's capabilities. It's purpose is to connect onto a TLS server, perform some tests and print the server's capabilities. If called with the `-v' parameter a more checks will be performed. An example output is:

     crystal:/cvs/gnutls/src$ ./gnutls-cli-debug localhost -p 5556
     Resolving 'localhost'...
     Connecting to '127.0.0.1:5556'...
     Checking for TLS 1.1 support... yes
     Checking fallback from TLS 1.1 to... N/A
     Checking for TLS 1.0 support... yes
     Checking for SSL 3.0 support... yes
     Checking for version rollback bug in RSA PMS... no
     Checking for version rollback bug in Client Hello... no
     Checking whether we need to disable TLS 1.0... N/A
     Checking whether the server ignores the RSA PMS version... no
     Checking whether the server can accept Hello Extensions... yes
     Checking whether the server can accept cipher suites not in SSL 3.0 spec... yes
     Checking whether the server can accept a bogus TLS record version in the client hello... yes
     Checking for certificate information... N/A
     Checking for trusted CAs... N/A
     Checking whether the server understands TLS closure alerts... yes
     Checking whether the server supports session resumption... yes
     Checking for export-grade ciphersuite support... no
     Checking RSA-export ciphersuite info... N/A
     Checking for anonymous authentication support... no
     Checking anonymous Diffie Hellman group info... N/A
     Checking for ephemeral Diffie Hellman support... no
     Checking ephemeral Diffie Hellman group info... N/A
     Checking for AES cipher support (TLS extension)... yes
     Checking for 3DES cipher support... yes
     Checking for ARCFOUR 128 cipher support... yes
     Checking for ARCFOUR 40 cipher support... no
     Checking for MD5 MAC support... yes
     Checking for SHA1 MAC support... yes
     Checking for ZLIB compression support (TLS extension)... yes
     Checking for LZO compression support (GnuTLS extension)... yes
     Checking for max record size (TLS extension)... yes
     Checking for SRP authentication support (TLS extension)... yes
     Checking for OpenPGP authentication support (TLS extension)... no

Invoking gnutls-serv
====================

Simple server program that listens to incoming TLS connections.

GNU TLS test server
Usage: gnutls-serv [options]

     -d, --debug integer      Enable debugging
     -g, --generate           Generate Diffie Hellman Parameters.
     -p, --port integer       The port to connect to.
     -q, --quiet              Suppress some messages.
     --nodb                   Does not use the resume database.
     --http                   Act as an HTTP Server.
     --echo                   Act as an Echo Server.
     --dhparams FILE          DH params file to use.
     --x509fmtder             Use DER format for certificates
     --x509cafile FILE        Certificate file to use.
     --x509crlfile FILE       CRL file to use.
     --pgpkeyring FILE        PGP Key ring file to use.
     --pgptrustdb FILE        PGP trustdb file to use.
     --pgpkeyfile FILE        PGP Key file to use.
     --pgpcertfile FILE       PGP Public Key (certificate) file to
                              use.
     --x509keyfile FILE       X.509 key file to use.
     --x509certfile FILE      X.509 Certificate file to use.
     --x509dsakeyfile FILE    Alternative X.509 key file to use.
     --x509dsacertfile FILE   Alternative X.509 certificate file to
                              use.
     --srppasswd FILE         SRP password file to use.
     --srppasswdconf FILE     SRP password conf file to use.
     --ciphers cipher1 cipher2...
                              Ciphers to enable.
     --protocols protocol1 protocol2...
                              Protocols to enable.
     --comp comp1 comp2...    Compression methods to enable.
     --macs mac1 mac2...      MACs to enable.
     --kx kx1 kx2...          Key exchange methods to enable.
     --ctypes certType1 certType2...
                              Certificate types to enable.
     -l, --list               Print a list of the supported
                              algorithms  and modes.
     -h, --help               prints this help
     -v, --version            prints the program's version number
     --copyright              prints the program's license

Setting Up a Test HTTPS Server
------------------------------

Running your own TLS server based on GnuTLS can be useful when debugging clients and/or GnuTLS itself.  This section describes how to use gnutls-serv as a simple HTTPS server.

The most basic server can be started as:

     gnutls-serv --http

It will only support anonymous ciphersuites, which many TLS clients refuse to use.

The next step is to add support for X.509.  First we generate a CA:

     certtool --generate-privkey > x509-ca-key.pem
     echo 'cn = GnuTLS test CA' > ca.tmpl
     echo 'ca' >> ca.tmpl
     echo 'cert_signing_key' >> ca.tmpl
     certtool --generate-self-signed --load-privkey x509-ca-key.pem \
       --template ca.tmpl --outfile x509-ca.pem
     ...

Then generate a server certificate.  Remember to change the dns_name value to the name of your server host, or skip that command to avoid the field.

     certtool --generate-privkey > x509-server-key.pem
     echo 'organization = GnuTLS test server' > server.tmpl
     echo 'cn = test.gnutls.org' >> server.tmpl
     echo 'tls_www_server' >> server.tmpl
     echo 'encryption_key' >> server.tmpl
     echo 'signing_key' >> server.tmpl
     echo 'dns_name = test.gnutls.org' >> server.tmpl
     certtool --generate-certificate --load-privkey x509-server-key.pem \
       --load-ca-certificate x509-ca.pem --load-ca-privkey x509-ca-key.pem \
       --template server.tmpl --outfile x509-server.pem
     ...

For use in the client, you may want to generate a client certificate as well.

     certtool --generate-privkey > x509-client-key.pem
     echo 'cn = GnuTLS test client' > client.tmpl
     echo 'tls_www_client' >> client.tmpl
     echo 'encryption_key' >> client.tmpl
     echo 'signing_key' >> client.tmpl
     certtool --generate-certificate --load-privkey x509-client-key.pem \
       --load-ca-certificate x509-ca.pem --load-ca-privkey x509-ca-key.pem \
       --template client.tmpl --outfile x509-client.pem
     ...

To be able to import the client key/certificate into some applications, you will need to convert them into a PKCS#12 structure. This also encrypts the security sensitive key with a password.

     certtool --to-p12 --load-privkey x509-client-key.pem --load-certificate x509-client.pem --outder --outfile x509-client.p12

For icing, we'll create a proxy certificate for the client too.

     certtool --generate-privkey > x509-proxy-key.pem
     echo 'cn = GnuTLS test client proxy' > proxy.tmpl
     certtool --generate-proxy --load-privkey x509-proxy-key.pem \
       --load-ca-certificate x509-client.pem --load-ca-privkey x509-client-key.pem \
       --load-certificate x509-client.pem --template proxy.tmpl \
       --outfile x509-proxy.pem
     ...

Then start the server again:

     gnutls-serv --http \
                 --x509cafile x509-ca.pem \
                 --x509keyfile x509-server-key.pem \
                 --x509certfile x509-server.pem

Try connecting to the server using your web browser.  Note that the server listens to port 5556 by default.

While you are at it, to allow connections using DSA, you can also create a DSA key and certificate for the server.  These credentials will be used in the final example below.

     certtool --generate-privkey --dsa > x509-server-key-dsa.pem
     certtool --generate-certificate --load-privkey x509-server-key-dsa.pem \
       --load-ca-certificate x509-ca.pem --load-ca-privkey x509-ca-key.pem \
       --template server.tmpl --outfile x509-server-dsa.pem
     ...

The next step is to create OpenPGP credentials for the server.

     gpg --gen-key
     ...enter whatever details you want, use 'test.gnutls.org' as name...

Make a note of the OpenPGP key identifier of the newly generated key, here it was 5D1D14D8.  You will need to export the key for GnuTLS to be able to use it.

     gpg -a --export 5D1D14D8 > openpgp-server.txt
     gpg --export 5D1D14D8 > openpgp-server.bin
     gpg --export-secret-keys 5D1D14D8 > openpgp-server-key.bin
     gpg -a --export-secret-keys 5D1D14D8 > openpgp-server-key.txt

Let's start the server with support for OpenPGP credentials:

     gnutls-serv --http \
                 --pgpkeyfile openpgp-server-key.txt \
                 --pgpcertfile openpgp-server.txt

The next step is to add support for SRP authentication.

     srptool --create-conf srp-tpasswd.conf
     srptool --passwd-conf srp-tpasswd.conf --username jas --passwd srp-passwd.txt
     Enter password: [TYPE "foo"]

Start the server with SRP support:

     gnutls-serv --http \
                 --srppasswdconf srp-tpasswd.conf \
                 --srppasswd srp-passwd.txt

Let's also add support for PSK.

     $ psktool --passwd psk-passwd.txt

Start the server with PSK support:

     gnutls-serv --http \
                 --pskpasswd psk-passwd.txt

Finally, we start the server with all the earlier parameters and you get this command:

     gnutls-serv --http \
                 --x509cafile x509-ca.pem \
                 --x509keyfile x509-server-key.pem \
                 --x509certfile x509-server.pem \
                 --x509dsakeyfile x509-server-key-dsa.pem \
                 --x509dsacertfile x509-server-dsa.pem \
                 --pgpkeyfile openpgp-server-key.txt \
                 --pgpcertfile openpgp-server.txt \
                 --srppasswdconf srp-tpasswd.conf \
                 --srppasswd srp-passwd.txt \
                 --pskpasswd psk-passwd.txt

Example server PSK connection
-----------------------------

To set up a PSK server with gnutls-serv you need to create PSK password file (see Invoking psktool).  In the example below, I type password at the prompt.

     $ ./psktool -u psk_identity -p psks.txt -n psk_identity_hint
     Enter password:
     Key stored to psks.txt
     $ cat psks.txt
     psk_identity:88f3824b3e5659f52d00e959bacab954b6540344
     $

After this, start the server pointing to the password file.  We disable DHE-PSK.

     $ ./gnutls-serv --pskpasswd psks.txt  --pskhint psk_identity_hint --priority NORMAL:-DHE-PSK
     Set static Diffie Hellman parameters, consider --dhparams.
     Echo Server ready. Listening to port '5556'.

You can now connect to the server using a PSK client (see Example client PSK connection).

Invoking psktool
================

This is a program to manage PSK username and keys.

PSKtool help
Usage : psktool [options]
     -u, --username username
                              specify username.
     -p, --passwd FILE        specify a password file.
     -n, --netconf-hint HINT
                              derive key from Netconf password, using
                              HINT as the psk_identity_hint.
     -s, --keysize SIZE       specify the key size in bytes.
     -v, --version            prints the program's version number
     -h, --help               shows this help text

Normally the file will generate random keys for the indicate username. You may also derive PSK keys from passwords, using the algorithm specified in draft-ietf-netconf-tls-02.txt.  The algorithm needs a PSK identity hint, which you specify using --netconf-hint.  To derive a PSK key from a password with an empty PSK identity hint, using --netconf-hint "".

Invoking srptool
================

The srptool is a very simple program that emulates the programs in the Stanford SRP libraries.  It is intended for use in places where you don't expect SRP authentication to be the used for system users.  Traditionally libsrp used two files. One called 'tpasswd' which holds usernames and verifiers, and 'tpasswd.conf' which holds generators and primes.

How to use srptool:

  * To create tpasswd.conf which holds the g and n values for SRP protocol (generator and a large prime), run:

          $ srptool --create-conf /etc/tpasswd.conf

  * This command will create /etc/tpasswd and will add user 'test' (you will also be prompted for a password). Verifiers are stored by default in the way libsrp expects.

          $ srptool --passwd /etc/tpasswd \
              --passwd-conf /etc/tpasswd.conf -u test

  * This command will check against a password. If the password matches the one in /etc/tpasswd you will get an ok.

          $ srptool --passwd /etc/tpasswd \
              --passwd-conf /etc/tpasswd.conf --verify -u test

Function Reference
******************

Core Functions
==============

The prototypes for the following functions lie in gnutls/gnutls.h.

     [file gnutls-api.texi not included]

X.509 Certificate Functions
===========================

The following functions are to be used for X.509 certificate handling. Their prototypes lie in gnutls/x509.h.

     [file x509-api.texi not included]

GnuTLS-extra Functions
======================

These functions are only available in the GPL version of the library called gnutls-extra. The prototypes for this library lie in gnutls/extra.h.

     [file gnutls-extra-api.texi not included]

OpenPGP Functions
=================

The following functions are to be used for OpenPGP certificate handling.  Their prototypes lie in gnutls/openpgp.h.  You need to link with libgnutls-extra to be able to use these functions (see GnuTLS-extra functions).

     [file pgp-api.texi not included]

TLS Inner Application (TLS/IA) Functions
========================================

The following functions are used for TLS Inner Application (TLS/IA).  Their prototypes lie in gnutls/extra.h. You need to link with libgnutls-extra to be able to use these functions (see GnuTLS-extra functions).

The typical control flow in an TLS/IA client (that would not require an Application Phase for resumed sessions) would be similar to the following:

     int client_avp (gnuls_session_t *session, void *ptr,
                     const char *last, size_t lastlen,
     		char **new, size_t *newlen)
     {
     ...
     }
     ...
     int main ()
     {
       gnutls_ia_client_credentials_t iacred;
     ...
       gnutls_init (&session, GNUTLS_CLIENT);
     ...
       /* Enable TLS/IA. */
       gnutls_ia_allocate_client_credentials(&iacred);
       gnutls_ia_set_client_avp_function(iacred, client_avp);
       gnutls_credentials_set (session, GNUTLS_CRD_IA, iacred);
     ...
       ret = gnutls_handshake (session);
       // Error handling...
     ...
       if (gnutls_ia_handshake_p (session))
         {
           ret = gnutls_ia_handshake (session);
           // Error handling...
     ...

See below for detailed descriptions of all the functions used above.

The function client_avp would have to be implemented by your application.  The function is responsible for handling the AVP data. See gnutls_ia_set_client_avp_function below for more information on how that function should be implemented.

The control flow in a typical server is similar to the above, use gnutls_ia_server_credentials_t instead of gnutls_ia_client_credentials_t, and replace the call to the client functions with the corresponding server functions.

     [file ia-api.texi not included]

Error Codes and Descriptions
============================

The error codes used throughout the library are described below.  The return code GNUTLS_E_SUCCESS indicate successful operation, and is guaranteed to have the value 0, so you can use it in logical expressions.

     [file error_codes.texi not included]

All the Supported Ciphersuites in GnuTLS
****************************************

     [file algorithms.texi not included]

     [file guile.texi not included]

Internal Architecture of GnuTLS
*******************************

This chapter is to give a brief description of the way GnuTLS works. The focus is to give an idea to potential developers and those who want to know what happens inside the black box.

The TLS Protocol
================

The main needs for the TLS protocol to be used are shown in the image below.

This is being accomplished by the following object diagram. Note that since GnuTLS is being developed in C object are just structures with attributes. The operations listed are functions that require the first parameter to be that object.

TLS Handshake Protocol
======================

The GnuTLS handshake protocol is implemented as a state machine that waits for input or returns immediately when the non-blocking transport layer functions are used. The main idea is shown in the following figure.

Also the way the input is processed varies per ciphersuite. Several implementations of the internal handlers are available and gnutls_handshake only multiplexes the input to the appropriate handler. For example a PSK ciphersuite has a different implementation of the process_client_key_exchange than a certificate ciphersuite.

TLS Authentication Methods
==========================

In GnuTLS authentication methods can be implemented quite easily. Since the required changes to add a new authentication method affect only the handshake protocol, a simple interface is used. An authentication method needs only to implement the functions as seen in the figure below.

The functions that need to be implemented are the ones responsible for interpreting the handshake protocol messages. It is common for such functions to read data from one or more credentials_t structures (such as the gnutls_certificate_credentials_t structures) and write data, such as certificates, usernames etc. to auth_info_t structures.

Simple examples of existing authentication methods can be seen in auth_psk.c for PSK ciphersuites and auth_srp.c for SRP ciphersuites. After implementing these functions the structure holding its pointers has to be registered in gnutls_algorithms.c in the _gnutls_kx_algorithms structure.

TLS Extension Handling
======================

As with authentication methods, the TLS extensions handlers can be implemented using the following interface.

Here there are two functions, one for receiving the extension data and one for sending. These functions have to check internally whether they operate in client or server side.

A simple example of an extension handler can be seen in ext_srp.c After implementing these functions, together with the extension number they handle, they have to be registered in gnutls_extensions.c in the _gnutls_extensions structure.

Adding a New TLS Extension
--------------------------

Adding support for a new TLS extension is done from time to time, and the process to do so is not difficult.  Here are the steps you need to follow if you wish to do this yourself.  For sake of discussion, let's consider adding support for the hypothetical TLS extension foobar.

 1. Modify configure.in to add --enable-foobar or --disable-foobar.

     Which to chose depends on whether you intend to make the extension be enabled by default.  Look at existing checks (i.e., SRP, authz) for how to model the code.  For example:

          AC_MSG_CHECKING([whether to disable foobar support])
          AC_ARG_ENABLE(foobar,
          	AS_HELP_STRING([--disable-foobar],
          		[disable foobar support]),
          	ac_enable_foobar=no)
          if test x$ac_enable_foobar != xno; then
           AC_MSG_RESULT(no)
           AC_DEFINE(ENABLE_FOOBAR, 1, [enable foobar])
          else
           ac_full=0
           AC_MSG_RESULT(yes)
          fi
          AM_CONDITIONAL(ENABLE_FOOBAR, test "$ac_enable_foobar" != "no")

 2. Add IANA extension value to extensions_t in gnutls_int.h.

     A good name for the value would be GNUTLS_EXTENSION_FOOBAR.  Check with http://www.iana.org/assignments/tls-extensiontype-values for allocated values.  For experiments, you could pick a number but remember that some consider it a bad idea to deploy such modified version since it will lead to interoperability problems in the future when the IANA allocates that number to someone else, or when the foobar protocol is allocated another number.

 3. Add an entry to _gnutls_extensions in gnutls_extensions.c.

     A typical entry would be:

          #if ENABLE_FOOBAR
            GNUTLS_EXTENSION_ENTRY (GNUTLS_EXTENSION_FOOBAR,
          			  _gnutls_foobar_recv_params,
          			  _gnutls_foobar_send_params),
          #endif

     The GNUTLS_EXTENSION_FOOBAR is the integer value you added to gnutls_int.h earlier.  The two functions are new functions that you will need to implement, most likely you'll need to add an #include "ext_foobar.h" as well.

 4. Add new files ext_foobar.c and ext_foobar.h that implements the extension.

     The functions you are responsible to add are those mentioned in the previous step.  As a starter, you could add this:

          int
          _gnutls_foobar_recv_params (gnutls_session_t session,
                                      const opaque * data,
                                      size_t data_size)
          {
            return 0;
          }

          int
          _gnutls_foobar_send_params (gnutls_session_t session,
                                      opaque * data,
                                      size_t _data_size)
          {
            return 0;
          }

     The _gnutls_foobar_recv_params function is responsible for parsing incoming extension data (both in the client and server).

     The _gnutls_foobar_send_params function is responsible for sending extension data (both in the client and server).

     If you receive length fields that doesn't match, return GNUTLS_E_UNEXPECTED_PACKET_LENGTH.  If you receive invalid data, return GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER.  You can use other error codes too.  Return 0 on success.

     The function typically store some information in the session variable for later usage.  If you need to add new fields there, check tls_ext_st in gnutls_int.h and compare with existing TLS extension specific variables.

     Recall that both the client and server both send and receives parameters, and your code most likely will need to do different things depending on which mode it is in.  It may be useful to make this distinction explicit in the code.  Thus, for example, a better template than above would be:

          int
          _gnutls_foobar_recv_params (gnutls_session_t session,
                                      const opaque * data,
                                      size_t data_size)
          {
            if (session->security_parameters.entity == GNUTLS_CLIENT)
              return foobar_recv_client (session, data, data_size);
            else
              return foobar_recv_server (session, data, data_size);
          }

          int
          _gnutls_foobar_send_params (gnutls_session_t session,
                                      opaque * data,
                                      size_t data_size)
          {
            if (session->security_parameters.entity == GNUTLS_CLIENT)
              return foobar_send_client (session, data, data_size);
            else
              return foobar_send_server (session, data, data_size);
          }

     The functions used would be declared as static functions, of the appropriate prototype, in the same file.

     When adding the files, you'll need to add them to Makefile.am as well, for example:

          if ENABLE_FOOBAR
          COBJECTS += ext_foobar.c
          HFILES += ext_foobar.h
          endif

 5. Add API functions to enable/disable the extension.

     Normally the client will have one API to request use of the extension, and setting some extension specific data.  The server will have one API to let the library know that it is willing to accept the extension, often this is implemented through a callback but it doesn't have to.

     The APIs need to be added to includes/gnutls/gnutls.h or includes/gnutls/extra.h as appropriate.  It is recommended that if you don't have a requirement to use the LGPL license for your extension, that you place your work under the GPL license and thus in the libgnutls-extra library.

     You can implement the API function in the ext_foobar.c file, or if that file ends up becoming rather larger, add a gnutls_foobar.c file.

Certificate Handling
====================

What is provided by the certificate handling functions is summarized in the following diagram.

Cryptographic Backend
=====================

Several new systems provide hardware assisted cryptographic algorithm implementations that offer implementations some orders of magnitude faster than the software. For this reason in current releases of GnuTLS it is possible to override parts of the crypto backend or the whole. It is possible to override them both at runtime and compile time, however here we will discuss the runtime possibility. The API available for this functionality is in gnutls/crypto.h header file.

Override specific algorithms
----------------------------

When an optimized implementation of a single algorithm is available, say a hardware assisted version of AES-CBC then the following functions can be used to register those algorithms.

  * gnutls_crypto_single_cipher_register2 To register a cipher algorithm.

  * gnutls_crypto_single_mac_register2 To register a MAC algorithm.

     gnutls_crypto_single_digest_register2 To register a digest (hash) algorithm.

Those registration functions will only replace the specified algorithm and leave the rest of subsystem intact.

Override parts of the backend
-----------------------------

In some systems, such as embedded ones, it might be desirable to override big parts of the cryptographic backend, or even all of them. For this reason the following functions are provided.

  * gnutls_crypto_cipher_register2 To override the cryptographic algorithms backend.

  * gnutls_crypto_mac_register2 To override the MAC algorithms backend.

  * gnutls_crypto_digest_register2 To override the digest algorithms backend.

  * gnutls_crypto_rnd_register2 To override the random number generator backend.

  * gnutls_crypto_bigint_register2 To override the big number number operations backend.

  * gnutls_crypto_pk_register2 To override the public key encryption backend. This is tight to the big number operations so either both of them should be updated or care must be taken to use the same format.

If all of them are used then GnuTLS will no longer use libgcrypt.

Copying Information
*******************

GNU Free Documentation License
==============================

     [file fdl.texi not included]

GNU Lesser General Public License
=================================

     [file lgpl-2.1.texi not included]

GNU General Public License
==========================

     [file gpl-3.0.texi not included]

Bibliography
************

[CBCATT]
     Bodo Moeller, "Security of CBC Ciphersuites in SSL/TLS: Problems and Countermeasures", 2002, available from http://www.openssl.org/~bodo/tls-cbc.txt.

[GPGH]
     Mike Ashley, "The GNU Privacy Handbook", 2002, available from http://www.gnupg.org/gph/en/manual.pdf.

[GUTPKI]
     Peter Gutmann, "Everything you never wanted to know about PKI but were forced to find out", Available from http://www.cs.auckland.ac.nz/~pgut001/.

[RFC2246]
     Tim Dierks and Christopher Allen, "The TLS Protocol Version 1.0", January 1999, Available from http://www.ietf.org/rfc/rfc2246.txt.

[RFC4346]
     Tim Dierks and Eric Rescorla, "The TLS Protocol Version 1.1", Match 2006, Available from http://www.ietf.org/rfc/rfc4346.txt.

[RFC2440]
     Jon Callas, Lutz Donnerhacke, Hal Finney and Rodney Thayer, "OpenPGP Message Format", November 1998, Available from http://www.ietf.org/rfc/rfc2440.txt.

[RFC4880]
     Jon Callas, Lutz Donnerhacke, Hal Finney, David Shaw and Rodney Thayer, "OpenPGP Message Format", November 2007, Available from http://www.ietf.org/rfc/rfc4880.txt.

[RFC4211]
     J. Schaad, "Internet X.509 Public Key Infrastructure Certificate Request Message Format (CRMF)", September 2005, Available from http://www.ietf.org/rfc/rfc4211.txt.

[RFC2817]
     Rohit Khare and Scott Lawrence, "Upgrading to TLS Within HTTP/1.1", May 2000, Available from http://www.ietf.org/rfc/rfc2817.txt

[RFC2818]
     Eric Rescorla, "HTTP Over TLS", May 2000, Available from http://www.ietf/rfc/rfc2818.txt.

[RFC2945]
     Tom Wu, "The SRP Authentication and Key Exchange System", September 2000, Available from http://www.ietf.org/rfc/rfc2945.txt.

[RFC2986]
     Magnus Nystrom and Burt Kaliski, "PKCS 10 v1.7: Certification Request Syntax Specification", November 2000, Available from http://www.ietf.org/rfc/rfc2986.txt.

[RFC3280]
     Russell Housley, Tim Polk, Warwick Ford and David Solo, "Internet X.509 Public Key Infrastructure Certificate and Certificate Revocation List (CRL) Profile", April 2002, Available from http://www.ietf.org/rfc/rfc3280.txt.

[RFC3749]
     Scott Hollenbeck, "Transport Layer Security Protocol Compression Methods", May 2004, Available from http://www.ietf.org/rfc/rfc3749.txt.

[RFC3820]
     Steven Tuecke, Von Welch, Doug Engert, Laura Pearlman, and Mary Thompson, "Internet X.509 Public Key Infrastructure (PKI) Proxy Certificate Profile", June 2004, available from http://www.ietf.org/rfc3820.

[PKCS12]
     RSA Laboratories, "PKCS 12 v1.0: Personal Information Exchange Syntax", June 1999, Available from http://www.rsa.com.

[RESCORLA]
     Eric Rescorla, "SSL and TLS: Designing and Building Secure Systems", 2001

[SELKEY]
     Arjen Lenstra and Eric Verheul, "Selecting Cryptographic Key Sizes", 2003, available from http://www.win.tue.nl/~klenstra/key.pdf.

[SSL3]
     Alan Freier, Philip Karlton and Paul Kocher, "The SSL Protocol Version 3.0", November 1996, Available from http://wp.netscape.com/eng/ssl3/draft302.txt.

[STEVENS]
     Richard Stevens, "UNIX Network Programming, Volume 1", Prentice Hall PTR, January 1998

[TLSEXT]
     Simon Blake-Wilson, Magnus Nystrom, David Hopwood, Jan Mikkelsen and Tim Wright, "Transport Layer Security (TLS) Extensions", June 2003, Available from http://www.ietf.org/rfc/rfc3546.txt.

[TLSPGP]
     Nikos Mavrogiannopoulos, "Using OpenPGP keys for TLS authentication", April 2004, November 2007. Available from http://www.ietf.org/rfc/rfc5081.txt.

[TLSSRP]
     David Taylor, Trevor Perrin, Tom Wu and Nikos Mavrogiannopoulos, "Using SRP for TLS Authentication", November 2007. Available from http://www.ietf.org/rfc/rfc5054.txt.

[TLSPSK]
     Pasi Eronen and Hannes Tschofenig, "Pre-shared key Ciphersuites for TLS", December 2005, Available from http://www.ietf.org/rfc/rfc4279.txt.

[TOMSRP]
     Tom Wu, "The Stanford SRP Authentication Project", Available at http://srp.stanford.edu/.

[WEGER]
     Arjen Lenstra and Xiaoyun Wang and Benne de Weger, "Colliding X.509 Certificates", Cryptology ePrint Archive, Report 2005/067, Available at http://eprint.iacr.org/.