doc: Fix pkg-config recommendation.

[gnutls.git] / doc / gnutls.texi

\input texinfo   @c -*-texinfo-*-
@comment %**start of header
@setfilename gnutls.info
@include version.texi
@settitle GNU TLS @value{VERSION}

@c don't indent the paragraphs.
@paragraphindent 0

@c Unify some of the indices.
@syncodeindex tp fn
@syncodeindex pg cp

@comment %**end of header
@finalout
@copying
This manual is last updated @value{UPDATED} for version
@value{VERSION} of GNU TLS.

Copyright @copyright{} 2001, 2002, 2003, 2004, 2005, 2006, 2007, 2008, 2009, 2010 Free Software Foundation, Inc.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 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''.
@end quotation
@end copying

@dircategory Software libraries
@direntry
* GnuTLS: (gnutls).             GNU Transport Layer Security Library.
@end direntry

@dircategory System Administration
@direntry
* certtool: (gnutls)Invoking certtool.  Manipulate certificates and keys.
* gnutls-serv: (gnutls)Invoking gnutls-serv.    GNU TLS test server.
* gnutls-cli: (gnutls)Invoking gnutls-cli.      GNU TLS test client.
* gnutls-cli-debug: (gnutls)Invoking gnutls-cli-debug.  GNU TLS debug client.
* psktool: (gnutls)Invoking psktool.    Simple TLS-Pre-Shared-Keys manager.
* srptool: (gnutls)Invoking srptool.    Simple SRP password tool.
@end direntry

@titlepage
@title GNU TLS
@subtitle Transport Layer Security Library for the GNU system
@subtitle for version @value{VERSION}, @value{UPDATED}
@sp 7
@image{gnutls-logo,6cm,6cm}
@author Nikos Mavrogiannopoulos
@author Simon Josefsson (@email{bug-gnutls@@gnu.org})
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@macro xcite{ref}
[\ref\] (@pxref{Bibliography})
@end macro

@contents

@ifnottex
@node Top
@top GNU TLS

@insertcopying
@end ifnottex

@menu
* Preface::
* The Library::
* Introduction to TLS::
* Authentication methods::
* More on certificate authentication::
* How to use TLS in application protocols::
* How to use GnuTLS in applications::
* Included programs::
* Function reference::
* All the supported ciphersuites in GnuTLS::
* Guile Bindings::
* Internal architecture of GnuTLS::
* Copying Information::
* Concept Index::
* Function and Data Index::
@c * @mybibnode{}::
* Bibliography::
@end menu


@node Preface
@chapter Preface

This document tries to demonstrate and explain the @acronym{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 @acronym{GnuTLS} purpose and actual offerings.
Even if @acronym{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 @acronym{GnuTLS}; there are several ways to compromise
a program or a communication line and @acronym{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 @xcite{STEVENS} and for
Public Key Infrastructure in @xcite{GUTPKI}.

@anchor{Availability}

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

@menu
* Getting help::
* Commercial Support::
* Downloading and Installing::
* Bug Reports::
* Contributing::
@end menu

@node Getting help
@section Getting Help

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

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

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

@node Commercial Support
@section Commercial Support

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

@itemize

@item Implement new features.
Such as a new TLS extension.

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

@item Integrating TLS as a security environment in your existing project.

@item System design of components related to TLS.

@end itemize

If you are interested, please write to:

@verbatim
Simon Josefsson Datakonsult
Hagagatan 24
113 47 Stockholm
Sweden

E-mail: simon@josefsson.org
@end verbatim

If your company provides support related to GnuTLS and would like to
be mentioned here, contact the author (@pxref{Bug Reports}).

@node Downloading and Installing
@section Downloading and Installing
@cindex Installation
@cindex Download

GnuTLS is available for download from the following URL:

@url{http://www.gnutls.org/download.html}

The latest version is stored in a file, e.g.,
@samp{gnutls-@value{VERSION}.tar.gz} where the @samp{@value{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
@url{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
@url{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 @file{INSTALL} file that is part of the
distribution archive.  Typically you invoke @code{./configure} and
then @code{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 @url{http://www.zlib.net/}.  You can get lzo
from @url{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
@url{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
@code{--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 @xref{Guile Preparations}.

A few @code{configure} options may be relevant, summarized in the
table.

@table @code

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

@end table

For the complete list, refer to the output from @code{configure
--help}.

@node Bug Reports
@section Bug Reports
@cindex Reporting Bugs

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

@itemize @bullet

@item 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.

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

@item 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.

@end itemize

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:

@center @samp{bug-gnutls@@gnu.org}

@node Contributing
@section Contributing
@cindex Contributing
@cindex Hacking

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 (@pxref{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:

@itemize @bullet

@item Coding Style.
Follow the GNU Standards document (@pxref{top, GNU Coding Standards,,
standards}).

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

@item Use the unified diff format @samp{diff -u}.

@item 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.

@item 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.

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

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

@item Supply a ChangeLog and NEWS entries, where appropriate.

@end itemize
@node The Library
@chapter The Library

In brief @acronym{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 @acronym{GnuTLS} is a portable ANSI C based library which
implements the TLS 1.1 and SSL 3.0 protocols (@xref{Introduction to
TLS}, for a more detailed description of the protocols), accompanied
with the required framework for authentication and public key
infrastructure.  Important features of the @acronym{GnuTLS} library
include:

@itemize

@item Support for TLS 1.0, TLS 1.1, and SSL 3.0 protocols.

@item Support for both @acronym{X.509} and @acronym{OpenPGP} certificates.

@item Support for handling and verification of certificates.

@item Support for @acronym{SRP} for TLS authentication.

@item Support for @acronym{PSK} for TLS authentication.

@item Support for TLS Extension mechanism.

@item Support for TLS Compression Methods.

@end itemize

Additionally @acronym{GnuTLS} provides a limited emulation API for the
widely used OpenSSL@footnote{@url{http://www.openssl.org/}} library,
to ease integration with existing applications.

@acronym{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
@acronym{GnuTLS} library.  The `Certificate part' consists of the
certificate parsing, and verification functions which is partially
implemented in the @acronym{GnuTLS} library.  The
@acronym{Libtasn1}@footnote{@url{ftp://ftp.gnupg.org/gcrypt/alpha/gnutls/libtasn1/}},
a library which offers @acronym{ASN.1} parsing capabilities, is used
for the @acronym{X.509} certificate parsing functions.  A smaller
version of
@acronym{OpenCDK}@footnote{@url{ftp://ftp.gnupg.org/gcrypt/alpha/gnutls/opencdk/}}
is used for the @acronym{OpenPGP} key support in @acronym{GnuTLS}.
The ``Cryptographic backend'' is provided by the
@acronym{Libgcrypt}@footnote{@url{ftp://ftp.gnupg.org/gcrypt/alpha/libgcrypt/}}
library@footnote{On current versions of GnuTLS it is possible to
override the default crypto backend. Check @pxref{Cryptographic
Backend} for details}.

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

@menu
* General Idea::
* Error handling::
* Memory handling::
* Callback functions::
@end menu

@node General Idea
@section General Idea

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

@image{gnutls-internals,12cm,8cm}

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 @acronym{ASN.1} parser.  This
structure is never modified by any @acronym{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 @acronym{GnuTLS}.

The credentials structure is used by some authentication methods, such
as certificate authentication (@pxref{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 @acronym{TLS} session.

A @acronym{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
@acronym{GnuTLS} session after a successful handshake calls the
appropriate backend function (@xref{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@footnote{The first message in a @acronym{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.

@node Error handling
@section Error Handling

In @acronym{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
@code{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
@code{GNUTLS_E_REHANDSHAKE} returned by @ref{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
@ref{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.  @xref{Error Codes}, for all the error codes.

@node Memory handling
@section Memory Handling

@acronym{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 @ref{gnutls_global_set_mem_functions} function is
available which can be used to set other memory handlers than the
defaults.

The @acronym{Libgcrypt} library on which @acronym{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 @acronym{Libgcrypt} for more
information.

@node Callback functions
@section Callback Functions
@cindex Callback functions

There are several cases where @acronym{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.

@itemize

@item @ref{gnutls_transport_set_push_function}

@item @ref{gnutls_transport_set_pull_function}

@end itemize

Other callback functions such as the one set by
@ref{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.

@itemize

@item @ref{gnutls_malloc}

@item @ref{gnutls_free}

@end itemize

@node Introduction to TLS
@chapter Introduction to @acronym{TLS}

@acronym{TLS} stands for ``Transport Layer Security'' and is the
successor of SSL, the Secure Sockets Layer protocol @xcite{SSL3}
designed by Netscape.  @acronym{TLS} is an Internet protocol, defined
by @acronym{IETF}@footnote{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 @acronym{RFC}
4346 and also in @xcite{RESCORLA}.  The protocol provides
confidentiality, and authentication layers over any reliable transport
layer.  The description, below, refers to @acronym{TLS} 1.0 but also
applies to @acronym{TLS} 1.1 @xcite{RFC4346} and @acronym{SSL} 3.0,
since the differences of these protocols are minor.  Older protocols
such as @acronym{SSL} 2.0 are not discussed nor implemented in
@acronym{GnuTLS} since they are not considered secure today.  GnuTLS
also supports @acronym{X.509} and @acronym{OpenPGP} @xcite{RFC4880}.

@menu
* TLS layers::
* The transport layer::
* The TLS record protocol::
* The TLS Alert Protocol::
* The TLS Handshake Protocol::
* TLS Extensions::
* Selecting cryptographic key sizes::
* On SSL 2 and older protocols::
* On Record Padding::
@end menu

@node TLS layers
@section TLS Layers
@cindex TLS Layers

@acronym{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.  @xref{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.  @xref{The
Handshake Protocol}, for more information about the handshake
protocol.  The protocol layering in TLS is shown in the figure below.

@image{gnutls-layers,12cm,8cm}

@node The transport layer
@section The Transport Layer
@cindex Transport protocol

@acronym{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 @acronym{GnuTLS} the
required callbacks to access the transport layer.

@itemize
@item @ref{gnutls_transport_set_push_function}
@item @ref{gnutls_transport_set_pull_function}
@item @ref{gnutls_transport_set_ptr}
@item @ref{gnutls_transport_set_lowat}
@item @ref{gnutls_transport_set_errno}
@end itemize

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 @code{errno} appropriately.

In some environments, setting @code{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 @code{gnutls_transport_set_errno} with the intended errno
value instead of setting @code{errno} directly.

@acronym{GnuTLS} currently only interprets the EINTR and EAGAIN errno
values and returns the corresponding @acronym{GnuTLS} error codes
@code{GNUTLS_E_INTERRUPTED} and @code{GNUTLS_E_AGAIN}.  These values
are usually returned by interrupted system calls, or when non blocking
IO is used.  All @acronym{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 @acronym{GnuTLS} function,
since signals do not interrupt @acronym{GnuTLS}' functions.

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

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

@node The TLS record protocol
@section The TLS Record Protocol
@cindex 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:

@table @asis

@item @ref{gnutls_record_send}:
To send a record packet (with application data).

@item @ref{gnutls_record_recv}:
To receive a record packet (with application data).

@item @ref{gnutls_record_get_direction}:
To get the direction of the last interrupted function call.
@end table

As you may have already noticed, the functions which access the Record
protocol, are quite limited, given the importance of this protocol in
@acronym{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.

@menu
* Encryption algorithms used in the record layer::
* Compression algorithms used in the record layer::
* Weaknesses and countermeasures::
@end menu

@node Encryption algorithms used in the record layer
@subsection Encryption Algorithms Used in the Record Layer
@cindex Symmetric encryption algorithms

Confidentiality in the record layer is achieved by using symmetric
block encryption algorithms like @code{3DES}, @code{AES}@footnote{AES,
or Advanced Encryption Standard, is actually the RIJNDAEL algorithm.
This is the algorithm that replaced DES.}, or stream algorithms like
@code{ARCFOUR_128}@footnote{@code{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 @acronym{TLS} protocol, a random number of blocks will be
appended to data, to prevent eavesdroppers from guessing the actual
data size.

Supported cipher algorithms:

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

@item ARCFOUR_128
ARCFOUR is a fast stream cipher.

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

@item 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.
@end table

Supported MAC algorithms:

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

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

@end table

@node Compression algorithms used in the record layer
@subsection Compression Algorithms Used in the Record Layer
@cindex Compression algorithms

The TLS record layer also supports compression.  The algorithms
implemented in @acronym{GnuTLS} can be found in the table below.
All the algorithms except for DEFLATE which is
referenced in @xcite{RFC3749}, should be considered as
@acronym{GnuTLS}' extensions@footnote{You should use
@ref{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 @acronym{GnuTLS} is implemented based
on the proposal @xcite{RFC3749}.
The supported compression algorithms are:

@table @code
@item DEFLATE
Zlib compression, using the deflate algorithm.

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

@end table

@node Weaknesses and countermeasures
@subsection Weaknesses and Countermeasures

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

@enumerate

@item
@acronym{TLS} has separate alerts for ``decryption_failed'' and
``bad_record_mac''

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

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

@end enumerate

Those weaknesses were solved in @acronym{TLS} 1.1 @xcite{RFC4346}
which is implemented in @acronym{GnuTLS}. For a detailed discussion
see the archives of the TLS Working Group mailing list and the paper
@xcite{CBCATT}.

@node The TLS Alert Protocol
@section The TLS Alert Protocol
@anchor{The Alert Protocol}
@cindex 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 @code{GNUTLS_A_CLOSE_NOTIFY}), and others refer to the
application protocol solely (see @code{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.

@table @asis
@item @ref{gnutls_alert_send}:
To send an alert signal.

@item @ref{gnutls_error_to_alert}:
To map a gnutls error number to an alert signal.

@item @ref{gnutls_alert_get}:
Returns the last received alert.

@item @ref{gnutls_alert_get_name}:
Returns the name, in a character array, of the given alert.

@end table

@node The TLS Handshake Protocol
@section The TLS Handshake Protocol
@anchor{The Handshake Protocol}
@cindex 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:

@table @asis
@item @ref{gnutls_priority_init}:
To initialize a priority set of ciphers.

@item @ref{gnutls_priority_deinit}:
To deinitialize a priority set of ciphers.

@item @ref{gnutls_priority_set}:
To associate a priority set with a @acronym{TLS} session.

@item @ref{gnutls_priority_set_direct}:
To directly associate a session with a given priority string.

@item @ref{gnutls_credentials_set}:
To set the appropriate credentials structures.

@item @ref{gnutls_certificate_server_set_request}:
To set whether client certificate is required or not.

@item @ref{gnutls_handshake}:
To initiate the handshake.
@end table

@subsection TLS Cipher Suites

The Handshake Protocol of @acronym{TLS} negotiates cipher suites of
the form @code{TLS_DHE_RSA_WITH_3DES_CBC_SHA}.  The usual cipher
suites contain these parameters:

@itemize

@item The key exchange algorithm.
@code{DHE_RSA} in the example.

@item The Symmetric encryption algorithm and mode
@code{3DES_CBC} in this example.

@item The MAC@footnote{MAC stands for Message Authentication Code. It can be described as a keyed hash algorithm. See RFC2104.} algorithm used for authentication.
@code{MAC_SHA} is used in the above example.

@end itemize

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 @acronym{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 @acronym{TLS} protocol.  The supported ciphersuites
are shown in @ref{ciphersuites}.

@subsection Client Authentication
@cindex Client Certificate authentication

In the case of ciphersuites that use certificate authentication, the
authentication of the client is optional in @acronym{TLS}.  A server
may request a certificate from the client --- using the
@ref{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 @acronym{GnuTLS} the certificate
signers list is constructed using the trusted Certificate Authorities
by the server. That is the ones set using
@itemize
@item @ref{gnutls_certificate_set_x509_trust_file}
@item @ref{gnutls_certificate_set_x509_trust_mem}
@end itemize

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

@subsection Resuming Sessions
@anchor{resume}
@cindex Resuming sessions

The @ref{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. @strong{Session
resuming} is a feature of the @acronym{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. @acronym{GnuTLS} supports this feature, and the
example (@pxref{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.

@subsection 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:

@itemize

@item @ref{gnutls_session_get_data}

@item @ref{gnutls_session_get_id}

@item @ref{gnutls_session_set_data}

@end itemize

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:

@itemize

@item @ref{gnutls_db_set_remove_function}

@item @ref{gnutls_db_set_store_function}

@item @ref{gnutls_db_set_retrieve_function}

@item @ref{gnutls_db_set_ptr}

@end itemize

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

@node TLS Extensions
@section TLS Extensions
@cindex TLS Extensions

A number of extensions to the @acronym{TLS} protocol have been
proposed mainly in @xcite{TLSEXT}. The extensions supported
in @acronym{GnuTLS} are:

@itemize
@item Maximum fragment length negotiation
@item Server name indication
@item Session tickets
@end itemize

and they will be discussed in the subsections that follow.

@subsection Maximum Fragment Length Negotiation
@cindex TLS Extensions
@cindex Maximum fragment length

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

@subsection Server Name Indication
@anchor{serverind}
@cindex TLS Extensions
@cindex Server name indication

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

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

@subsection Session Tickets
@cindex TLS Extensions
@cindex Session Tickets
@cindex Ticket

To resume a TLS session the server normally store some state.  This
complicates deployment, and typical situations the client can cache
information and send it to the server instead.  The Session Ticket
extension implements this idea, and it is documented in
RFC 5077 @xcite{TLSTKT}.

Clients can enable support for TLS tickets with
@ref{gnutls_session_ticket_enable_client} and servers use
@ref{gnutls_session_ticket_key_generate} to generate a key and
@ref{gnutls_session_ticket_enable_server} to enable the extension.
Clients resume sessions using the ticket using the normal session
resume functions, @ref{resume}.

@node Selecting cryptographic key sizes
@section Selecting Cryptographic Key Sizes
@cindex 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
@xcite{SELKEY}.  Those can be used to generate certificates with
appropriate key sizes as well as parameters for Diffie-Hellman and SRP
authentication.

@multitable @columnfractions .15 .20 .20 .20

@item Year
@tab Symmetric key size
@tab RSA key size, DH and SRP prime size
@tab ECC key size

@item 1982
@tab 56
@tab 417
@tab 105

@item 1988
@tab 61
@tab 566
@tab 114

@item 2002
@tab 72
@tab 1028
@tab 139

@item 2015
@tab 82
@tab 1613
@tab 173

@item 2028
@tab 92
@tab 2362
@tab 210

@item 2040
@tab 101
@tab 3214
@tab 244

@item 2050
@tab 109
@tab 4047
@tab 272

@end multitable

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.

NIST publication SP 800-57 @xcite{NISTSP80057} contains a similar
table that extends beyond the key sizes given above.

@multitable @columnfractions .15 .20 .20 .20

@item Bits of security
@tab Symmetric key algorithms
@tab RSA key size, DSA, DH and SRP prime size
@tab ECC key size

@item 80
@tab 2TDEA
@tab 1024
@tab 160-223

@item 112
@tab 3DES
@tab 2048
@tab 224-255

@item 128
@tab AES-128
@tab 3072
@tab 256-383

@item 192
@tab AES-192
@tab 7680
@tab 384-511

@item 256
@tab AES-256
@tab 15360
@tab 512+

@end multitable

The recommendations are fairly consistent.

@node On SSL 2 and older protocols
@section On SSL 2 and Older Protocols
@cindex SSL 2

One of the initial decisions in the @acronym{GnuTLS} development was
to implement the known security protocols for the transport layer.
Initially @acronym{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 @acronym{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 @acronym{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 @acronym{SSL} 2.0 include:

@itemize

@item Message integrity compromised.
The @acronym{SSLv2} message authentication uses the MD5 function, and
is insecure.

@item Man-in-the-middle attack.
There is no protection of the handshake in @acronym{SSLv2}, which
permits a man-in-the-middle attack.

@item Truncation attack.
@acronym{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.

@item Weak message integrity for export ciphers.
The cryptographic keys in @acronym{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.

@end itemize

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

@node On Record Padding
@section On Record Padding
@cindex Record padding
@cindex Bad record MAC

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
(@code{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
@ref{gnutls_record_disable_padding}, or @ref{gnutls_priority_set} with
the @code{%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 @ref{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 @code{gnutls-cli} client or the
@code{gnutls-serv} server, for testing of other implementations, use
the following parameter: @code{--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:

@url{http://bugs.debian.org/390712}

@url{http://bugs.debian.org/402861}

@url{http://bugs.debian.org/438137}

@url{http://thread.gmane.org/gmane.ietf.tls/3079}

@node Authentication methods
@chapter Authentication Methods

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

@itemize

@item Certificate authentication

@item Anonymous authentication

@item @acronym{SRP} authentication

@item @acronym{PSK} authentication

@end itemize

@menu
* Certificate authentication::
* Anonymous authentication::
* Authentication using SRP::
* Authentication using PSK::
* Authentication and credentials::
* Parameters stored in credentials::
@end menu

@node Certificate authentication
@section Certificate Authentication

@subsection Authentication Using @acronym{X.509} Certificates
@cindex @acronym{X.509} certificates

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

@subsection Authentication Using @acronym{OpenPGP} Keys
@cindex @acronym{OpenPGP} Keys

@acronym{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.
@acronym{GnuTLS}'s @acronym{OpenPGP} authentication implementation is
based on the @xcite{TLSPGP} proposal.

@xref{The OpenPGP trust model}, for more information about the
@acronym{OpenPGP} trust model.  For a more detailed introduction to
@acronym{OpenPGP} and @acronym{GnuPG} see @xcite{GPGH}.

@subsection Using Certificate Authentication

In @acronym{GnuTLS} both the @acronym{OpenPGP} and @acronym{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
@acronym{TLS} session is initialized, in a certificate credentials
structure. This should be done by using
@ref{gnutls_certificate_set_x509_key_file} or
@ref{gnutls_certificate_set_openpgp_key_file} depending on the
certificate type.  In the @acronym{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:

@itemize

@item @ref{gnutls_certificate_server_set_retrieve_function}

@item @ref{gnutls_certificate_client_set_retrieve_function}

@end itemize

Clients and servers that will select certificates using callback
functions should select a certificate according the peer's signature
algorithm preferences. To get those preferences use
@ref{gnutls_sign_algorithm_get_requested}.

Certificate verification is possible by loading the trusted
authorities into the credentials structure by using
@ref{gnutls_certificate_set_x509_trust_file} or
@ref{gnutls_certificate_set_openpgp_keyring_file} for openpgp
keys. Note however that the peer's certificate is not automatically
verified, you should call @ref{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 @ref{gnutls_certificate_get_peers} to
obtain the raw certificate of the peer and verify it using the
functions discussed in @ref{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. @acronym{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
@code{DHE_RSA} ones. It is recommended not to use RSA keys for both
signing and encryption. If possible use the same key for the
@code{DHE_RSA} and @code{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@footnote{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 @code{RSA-EXPORT} method also requires
512 bit RSA parameters, that should also be generated and associated
with the credentials structure.  See the functions:

@itemize

@item @ref{gnutls_dh_params_generate2}

@item @ref{gnutls_certificate_set_dh_params}

@item @ref{gnutls_rsa_params_generate2}

@item @ref{gnutls_certificate_set_rsa_export_params}

@end itemize

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 @code{certtool}. This is possible with
@acronym{GnuTLS} by using the following functions:

@itemize

@item @ref{gnutls_dh_params_import_pkcs3}

@item @ref{gnutls_rsa_params_import_pkcs1}

@item @ref{gnutls_dh_params_export_pkcs3}

@item @ref{gnutls_rsa_params_export_pkcs1}

@end itemize

Key exchange algorithms for @acronym{OpenPGP} and @acronym{X.509}
certificates:

@table @code

@item 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.

@item 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.

@item 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.

@item 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.

@end table

@node Anonymous authentication
@section Anonymous Authentication
@cindex 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:

@table @code

@item ANON_DH:
This algorithm exchanges Diffie-Hellman parameters.

@end table

@node Authentication using SRP
@section Authentication using @acronym{SRP}
@cindex @acronym{SRP} authentication

Authentication via the Secure Remote Password protocol,
@acronym{SRP}@footnote{@acronym{SRP} is described in @xcite{RFC2945}},
is supported.  The @acronym{SRP} key exchange is an extension to the
@acronym{TLS} protocol, and it is a password based authentication
(unlike @acronym{X.509} or @acronym{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 @acronym{SRP}
and the server using a certificate.

The advantage of @acronym{SRP} authentication, over other proposed
secure password authentication schemes, is that @acronym{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 @emph{UNIX}
@file{/etc/passwd} file, where the contents of this file did not cause
harm to the system security if they were revealed.  The @acronym{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 @xcite{TOMSRP} for a detailed
description of the @acronym{SRP} protocol and the Stanford
@acronym{SRP} libraries, which includes a PAM module that synchronizes
the system's users passwords with the @acronym{SRP} password
files. That way @acronym{SRP} authentication could be used for all the
system's users.

The implementation in @acronym{GnuTLS} is based on paper
@xcite{TLSSRP}.  The supported @acronym{SRP} key exchange methods are:

@table @code

@item SRP:
Authentication using the @acronym{SRP} protocol.

@item SRP_DSS:
Client authentication using the @acronym{SRP} protocol. Server is
authenticated using a certificate with DSA parameters.

@item SRP_RSA:
Client authentication using the @acronym{SRP} protocol. Server is
authenticated using a certificate with RSA parameters.

@end table

If clients supporting @acronym{SRP} know the username and password
before the connection, should initialize the client credentials and
call the function @ref{gnutls_srp_set_client_credentials}.
Alternatively they could specify a callback function by using the
function @ref{gnutls_srp_set_client_credentials_function}.  This has
the advantage that allows probing the server for @acronym{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 @acronym{SRP} has been negotiated.  This uses a
special @acronym{TLS}-@acronym{SRP} handshake idiom in order to avoid,
in interactive applications, to ask the user for @acronym{SRP}
password and username if the server does not negotiate an
@acronym{SRP} ciphersuite.

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

Some helper functions such as

@itemize

@item @ref{gnutls_srp_verifier}

@item @ref{gnutls_srp_base64_encode}

@item @ref{gnutls_srp_base64_decode}

@end itemize

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


@node Authentication using PSK
@section Authentication using @acronym{PSK}
@cindex @acronym{PSK} authentication

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 @acronym{GnuTLS} is based on paper
@xcite{TLSPSK}.  The supported @acronym{PSK} key exchange methods are:

@table @code

@item PSK:
Authentication using the @acronym{PSK} protocol.

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

@end table

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

In server side the default behaviour of @acronym{GnuTLS} is to read
the usernames and @acronym{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 @ref{gnutls_psk_set_server_credentials_file}.  If
a different password file format is to be used, then the function
@ref{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
@ref{gnutls_psk_set_server_credentials_hint}.  The client can retrieve
the hint, for example in the callback function, using
@ref{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
@ref{gnutls_psk_netconf_derive_key} which follows the algorithm
specified in @file{draft-ietf-netconf-tls-02.txt}.

Some helper functions such as:

@itemize

@item @ref{gnutls_hex_encode}

@item @ref{gnutls_hex_decode}

@end itemize

are included in @acronym{GnuTLS}, and may be used to generate and
maintain @acronym{PSK} keys.


@node Authentication and credentials
@section Authentication and Credentials

In @acronym{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
@ref{gnutls_credentials_set}.  A mapping is shown below.

Key exchange algorithms and the corresponding credential types:

@multitable @columnfractions .3 .3 .3

@headitem Key exchange @tab Client credentials @tab Server credentials

@item @code{KX_RSA}
@item @code{KX_DHE_RSA}
@item @code{KX_DHE_DSS}
@item @code{KX_RSA_EXPORT}
@tab @code{CRD_CERTIFICATE}
@tab @code{CRD_CERTIFICATE}

@item @code{KX_SRP_RSA}
@tab @code{CRD_SRP}
@tab @code{CRD_SRP}
@item @code{KX_SRP_DSS}
@tab
@tab @code{CRD_CERTIFICATE}

@item @code{KX_SRP}
@tab @code{CRD_SRP}
@tab @code{CRD_SRP}

@item @code{KX_ANON_DH}
@tab @code{CRD_ANON}
@tab @code{CRD_ANON}

@item @code{KX_PSK}
@tab @code{CRD_PSK}
@tab @code{CRD_PSK}

@end multitable

@node Parameters stored in credentials
@section 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 @code{gnutls_dh_params_t} and @code{gnutls_rsa_params_t}, and
functions like @ref{gnutls_certificate_set_dh_params} and
@ref{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
@ref{gnutls_certificate_set_params_function}.  An example is shown
below.

@example
#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);
@}
@end example

@node More on certificate authentication
@chapter More on Certificate Authentication
@anchor{Certificate Authentication}
@cindex Certificate authentication

@menu
* The X.509 trust model::
* The OpenPGP trust model::
* Digital signatures::
@end menu

@node The X.509 trust model
@section The @acronym{X.509} Trust Model
@cindex @acronym{X.509} certificates

The @acronym{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.

@image{gnutls-x509,7cm,9.5cm}

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 @acronym{X.509} certificates is described at section
@ref{sec:x509api}.  Some examples are listed below.

@menu
* X.509 certificates::
* Verifying X.509 certificate paths::
* PKCS #10 certificate requests::
* PKCS #12 structures::
@end menu

@node X.509 certificates
@subsection @acronym{X.509} Certificates

An @acronym{X.509} certificate usually contains information about the
certificate holder, the signer, a unique serial number, expiration
dates and some other fields @xcite{PKIX} as shown in the table below.

@table @code

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

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

@item issuer:
Holds the issuer's distinguished name.

@item validity:
The activation and expiration dates.

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

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

@end table

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

The @emph{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 @emph{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 @emph{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 @acronym{X.509} version 3
extensions are shown in the table below.

@table @code

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

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

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

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

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

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

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

@item 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
@xcite{RFC3820}.

@end table

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

@node Verifying X.509 certificate paths
@subsection Verifying @acronym{X.509} Certificate Paths
@cindex Verifying certificate paths

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

@table @code

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

@item GNUTLS_CERT_REVOKED:
The certificate has been revoked by its CA.

@item GNUTLS_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.

@item 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.

@anchor{GNUTLS_CERT_INSECURE_ALGORITHM}
@item 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.

@end table

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

@anchor{gnutls_certificate_verify_flags}
@tindex gnutls_certificate_verify_flags
@table @code
@item 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.

@item 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.

@item 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.

@item 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.

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

@item GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5:
Allow certificates to be signed using the broken MD5 algorithm.
@end table

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 @xcite{RFC2818} and section @ref{ex:verify} for an example.

@node PKCS #10 certificate requests
@subsection @acronym{PKCS} #10 Certificate Requests
@cindex Certificate requests
@cindex @acronym{PKCS} #10

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. @acronym{GnuTLS} supports the requests defined in
@acronym{PKCS} #10 @xcite{RFC2986}. Other certificate request's format
such as PKIX's @xcite{RFC4211} are not currently supported.

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

@node PKCS #12 structures
@subsection @acronym{PKCS} #12 Structures
@cindex @acronym{PKCS} #12

A @acronym{PKCS} #12 structure @xcite{PKCS12} usually contains a user's
private keys and certificates. It is commonly used in browsers to
export and import the user's identities.

In @acronym{GnuTLS} the @acronym{PKCS} #12 structures are handled
using the @code{gnutls_pkcs12_t} type. This is an abstract type that
may hold several @code{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 @acronym{PKCS} #12 structure generation can be found
at section @ref{ex:pkcs12}.

@node The OpenPGP trust model
@section The @acronym{OpenPGP} Trust Model
@cindex @acronym{OpenPGP} Keys

The @acronym{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. @acronym{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.

@image{gnutls-pgp,11cm,9cm}

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.

@subsection @acronym{OpenPGP} Keys

In @acronym{GnuTLS} the @acronym{OpenPGP} key structures
@xcite{RFC2440} are handled using the @code{gnutls_openpgp_crt_t} type
and the corresponding private keys with the
@code{gnutls_openpgp_privkey_t} type. All the prototypes for the key
handling functions can be found at @file{gnutls/openpgp.h}.

@subsection Verifying an @acronym{OpenPGP} Key

The verification functions of @acronym{OpenPGP} keys, included in
@acronym{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 @acronym{GnuPG} and
GPGME (@url{http://www.gnupg.org/related_software/gpgme/}) is
recommended.

There is one verification function in @acronym{GnuTLS}, the
@ref{gnutls_openpgp_crt_verify_ring}.  This checks an
@acronym{OpenPGP} key against a given set of public keys (keyring) and
returns the key status. The key verification status is the same as in
@acronym{X.509} certificates, although the meaning and interpretation
are different. For example an @acronym{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.

@table @code

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

@item CERT_REVOKED:
The key has been revoked by its owner.

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

@item 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.

@end table

@node Digital signatures
@section Digital Signatures
@cindex Digital signatures

In this section we will provide some information about digital
signatures, how they work, and give the rationale for disabling some
of the algorithms used.

Digital signatures work by using somebody's secret key to sign some
arbitrary data.  Then anybody else could use the public key of that
person to verify the signature.  Since the data may be arbitrary it is
not suitable input to a cryptographic digital signature algorithm. For
this reason and also for performance cryptographic hash algorithms are
used to preprocess the input to the signature algorithm. This works as
long as it is difficult enough to generate two different messages with
the same hash algorithm output. In that case the same signature could
be used as a proof for both messages. Nobody wants to sign an innocent
message of donating 1 @euro{} to Greenpeace and find out that he
donated 1.000.000 @euro{} to Bad Inc.

For a hash algorithm to be called cryptographic the following three
requirements must hold:

@enumerate
@item Preimage resistance.
That means the algorithm must be one way and given the output of the
hash function @math{H(x)}, it is impossible to calculate @math{x}.

@item 2nd preimage resistance.
That means that given a pair @math{x,y} with @math{y=H(x)} it is
impossible to calculate an @math{x'} such that @math{y=H(x')}.

@item Collision resistance.
That means that it is impossible to calculate random @math{x} and
@math{x'} such @math{H(x')=H(x)}.
@end enumerate

The last two requirements in the list are the most important in
digital signatures. These protect against somebody who would like to
generate two messages with the same hash output. When an algorithm is
considered broken usually it means that the Collision resistance of
the algorithm is less than brute force. Using the birthday paradox the
brute force attack takes
@iftex
@math{2^{(\rm{hash\ size}) / 2}}
@end iftex
@ifnottex
@math{2^{((hash size) / 2)}}
@end ifnottex
operations. Today colliding certificates using the MD5 hash algorithm
have been generated as shown in @xcite{WEGER}.

There has been cryptographic results for the SHA-1 hash algorithms as
well, although they are not yet critical.  Before 2004, MD5 had a
presumed collision strength of @math{2^{64}}, but it has been showed
to have a collision strength well under @math{2^{50}}.  As of November
2005, it is believed that SHA-1's collision strength is around
@math{2^{63}}.  We consider this sufficiently hard so that we still
support SHA-1.  We anticipate that SHA-256/386/512 will be used in
publicly-distributed certificates in the future.  When @math{2^{63}}
can be considered too weak compared to the computer power available
sometime in the future, SHA-1 will be disabled as well.  The collision
attacks on SHA-1 may also get better, given the new interest in tools
for creating them.

@subsection Trading Security for Interoperability

If you connect to a server and use GnuTLS' functions to verify the
certificate chain, and get a @ref{GNUTLS_CERT_INSECURE_ALGORITHM}
validation error (@pxref{Verifying X.509 certificate paths}), it means
that somewhere in the certificate chain there is a certificate signed
using @code{RSA-MD2} or @code{RSA-MD5}.  These two digital signature
algorithms are considered broken, so GnuTLS fail when attempting to
verify the certificate.  In some situations, it may be useful to be
able to verify the certificate chain anyway, assuming an attacker did
not utilize the fact that these signatures algorithms are broken.
This section will give help on how to achieve that.

First, it is important to know that you do not have to enable any of
the flags discussed here to be able to use trusted root CA
certificates signed using @code{RSA-MD2} or @code{RSA-MD5}.  The only
attack today is that it is possible to generate certificates with
colliding signatures (collision resistance); you cannot generate a
certificate that has the same signature as an already existing
signature (2nd preimage resistance).

If you are using @ref{gnutls_certificate_verify_peers2} to verify the
certificate chain, you can call
@ref{gnutls_certificate_set_verify_flags} with the
@code{GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD2} or
@code{GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5} flag, as in:

@example
  gnutls_certificate_set_verify_flags (x509cred,
                                       GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5);
@end example

This will tell the verifier algorithm to enable @code{RSA-MD5} when
verifying the certificates.

If you are using @ref{gnutls_x509_crt_verify} or
@ref{gnutls_x509_crt_list_verify}, you can pass the
@code{GNUTLS_VERIFY_ALLOW_SIGN_RSA_MD5} parameter directly in the
@code{flags} parameter.

If you are using these flags, it may also be a good idea to warn the
user when verification failure occur for this reason.  The simplest is
to not use the flags by default, and only fall back to using them
after warning the user.  If you wish to inspect the certificate chain
yourself, you can use @ref{gnutls_certificate_get_peers} to extract
the raw server's certificate chain, then use
@ref{gnutls_x509_crt_import} to parse each of the certificates, and
then use @ref{gnutls_x509_crt_get_signature_algorithm} to find out the
signing algorithm used for each certificate.  If any of the
intermediary certificates are using @code{GNUTLS_SIGN_RSA_MD2} or
@code{GNUTLS_SIGN_RSA_MD5}, you could present a warning.



@node How to use TLS in application protocols
@chapter How To Use @acronym{TLS} in Application Protocols

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

@menu
* Separate ports::
* Upward negotiation::
@end menu

@node Separate ports
@section Separate Ports

Traditionally @acronym{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 @acronym{HTTPS} protocol @xcite{RFC2818}.

Despite its wide use, this method is not as good as it seems.  This
approach starts the @acronym{TLS} Handshake procedure just after the
client connects on the ---so called--- secure port.  That way the
@acronym{TLS} protocol does not know anything about the client, and
popular methods like the host advertising in HTTP do not
work@footnote{See also the Server Name Indication extension on
@ref{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.

@node Upward negotiation
@section Upward Negotiation

Other application protocols@footnote{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 @xcite{RFC2817} 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:

@quotation
(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
@end quotation

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

@quotation
(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
@end quotation

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 @acronym{TLS} or not. If the user
answered that he certainly wants the secure layer the last
conversation should be:

@quotation
(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)
@end quotation

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 @acronym{TLS} Handshake protocol
starts, in order to send the correct certificate, use the correct
password file@footnote{in @acronym{SRP} authentication}, or anything
else!

@node How to use GnuTLS in applications
@chapter How To Use @acronym{GnuTLS} in Applications
@anchor{examples}
@cindex Example programs

@menu
* Preparation::
* Multi-threaded applications::
* Client examples::
* Server examples::
* Miscellaneous examples::
* Compatibility with the OpenSSL library::
* Opaque PRF Input TLS Extension::
* Keying Material Exporters::
@end menu

@node Preparation
@section Preparation

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

@menu
* Headers::
* Initialization::
* Version check::
* Debugging::
* Building the source::
@end menu

@node Headers
@subsection Headers

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

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

@node Initialization
@subsection Initialization

GnuTLS must be initialized before it can be used.  The library is
initialized by calling @ref{gnutls_global_init}.  The resources
allocated by the initialization process can be released if the
application no longer has a need to call GnuTLS functions, this is
done by calling @ref{gnutls_global_deinit}.

The extra functionality of the @acronym{GnuTLS-extra} library is
available after calling @ref{gnutls_global_init_extra}.

In order to take advantage of the internationalisation features in
GnuTLS, such as translated error messages, the application must set
the current locale using @code{setlocale} before initializing GnuTLS.

@node Version check
@subsection 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 @ref{gnutls_check_version}.

@node Debugging
@subsection Debugging

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

@node Building the source
@subsection Building the Source

If you want to compile a source file including the
@file{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 @option{-I}
option).

However, the path to the include file is determined at the time the
source is configured.  To solve this problem, the library uses the
external package @command{pkg-config} that knows 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 @option{--cflags} option to @command{pkg-config gnutls}.  The
following example shows how it can be used at the command line:

@example
gcc -c foo.c `pkg-config gnutls --cflags`
@end example

Adding the output of @samp{pkg-config gnutls --cflags} to the
compilers command line will ensure that the compiler can find the
@file{gnutls/gnutls.h} 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 @option{-L} option).  For this, the option
@option{--libs} to @command{pkg-config gnutls} can be used.  For
convenience, this option also outputs all other options that are
required to link the program with the libarary (for instance, the
@samp{-ltasn1} option).  The example shows how to link @file{foo.o}
with the library to a program @command{foo}.

@example
gcc -o foo foo.o `pkg-config gnutls --libs`
@end example

Of course you can also combine both examples to a single command by
specifying both options to @command{pkg-config}:

@example
gcc -o foo foo.c `pkg-config gnutls --cflags --libs`
@end example

@node Multi-threaded applications
@section Multi-Threaded Applications

Although the @acronym{GnuTLS} library is thread safe by design, some
parts of Libgcrypt, such as the random generator, are not.
Applications have to register callback functions to ensure proper
locking in the sensitive parts of @emph{libgcrypt}.

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

@itemize

@item POSIX threads
@example
#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();
@}
@end example

@item GNU PTH threads
@example
#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();
@}
@end example

@item Other thread packages
@example
/* 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);
@}
@end example
@end itemize

@node Client examples
@section Client Examples

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

@menu
* Simple client example with anonymous authentication::
* Simple client example with X.509 certificate support::
* Obtaining session information::
* Verifying peer's certificate::
* Using a callback to select the certificate to use::
* Client with Resume capability example::
* Simple client example with SRP authentication::
* Simple client example with TLS/IA support::
* Simple client example in C++::
* Helper function for TCP connections::
@end menu

@node Simple client example with anonymous authentication
@subsection 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.

@verbatiminclude examples/ex-client1.c

@node Simple client example with X.509 certificate support
@subsection Simple Client Example with @acronym{X.509} Certificate Support

Let's assume now that we want to create a TCP client which
communicates with servers that use @acronym{X.509} or
@acronym{OpenPGP} certificate authentication. The following client is
a very simple @acronym{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.

@verbatiminclude examples/ex-client2.c

@node Obtaining session information
@subsection 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 @ref{gnutls_handshake}.

@verbatiminclude examples/ex-session-info.c

@node Verifying peer's certificate
@subsection Verifying Peer's Certificate
@anchor{ex:verify}

A @acronym{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.

@verbatiminclude examples/ex-rfc2818.c

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

@verbatiminclude examples/ex-verify.c

@node Using a callback to select the certificate to use
@subsection 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.

@verbatiminclude examples/ex-cert-select.c

@node Client with Resume capability example
@subsection Client with Resume Capability Example
@anchor{ex:resume-client}

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

@verbatiminclude examples/ex-client-resume.c

@node Simple client example with SRP authentication
@subsection Simple Client Example with @acronym{SRP} Authentication

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

@verbatiminclude examples/ex-client-srp.c

@node Simple client example with TLS/IA support
@subsection Simple Client Example with @acronym{TLS/IA} Support

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

@verbatiminclude examples/ex-client-tlsia.c

@node Simple client example in C++
@subsection Simple Client Example using the C++ API

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

@verbatiminclude examples/ex-cxx.cpp

@node Helper function for TCP connections
@subsection Helper Function for TCP Connections

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

@verbatiminclude examples/tcp.c

@node Server examples
@section Server Examples

This section contains examples of @acronym{TLS} and @acronym{SSL}
servers, using @acronym{GnuTLS}.

@menu
* Echo Server with X.509 authentication::
* Echo Server with X.509 authentication II::
* Echo Server with OpenPGP authentication::
* Echo Server with SRP authentication::
* Echo Server with anonymous authentication::
@end menu

@node Echo Server with X.509 authentication
@subsection Echo Server with @acronym{X.509} Authentication

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

@verbatiminclude examples/ex-serv1.c

@node Echo Server with X.509 authentication II
@subsection Echo Server with @acronym{X.509} Authentication II

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

@verbatiminclude examples/ex-serv-export.c

@node Echo Server with OpenPGP authentication
@subsection Echo Server with @acronym{OpenPGP} Authentication
@cindex @acronym{OpenPGP} Server

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

@verbatiminclude examples/ex-serv-pgp.c

@node Echo Server with SRP authentication
@subsection Echo Server with @acronym{SRP} Authentication

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

@verbatiminclude examples/ex-serv-srp.c

@node Echo Server with anonymous authentication
@subsection Echo Server with Anonymous Authentication

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

@verbatiminclude examples/ex-serv-anon.c

@node Miscellaneous examples
@section Miscellaneous Examples

@menu
* Checking for an alert::
* X.509 certificate parsing example::
* Certificate request generation::
* PKCS #12 structure generation::
@end menu

@node Checking for an alert
@subsection Checking for an Alert

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

@verbatiminclude examples/ex-alert.c

@node X.509 certificate parsing example
@subsection @acronym{X.509} Certificate Parsing Example
@anchor{ex:x509-info}

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

@verbatiminclude examples/ex-x509-info.c

@node Certificate request generation
@subsection Certificate Request Generation
@anchor{ex:crq}

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.

@verbatiminclude examples/ex-crq.c

@node PKCS #12 structure generation
@subsection @acronym{PKCS} #12 Structure Generation
@anchor{ex:pkcs12}

The following example is about generating a @acronym{PKCS} #12
structure.

@verbatiminclude examples/ex-pkcs12.c

@node Compatibility with the OpenSSL library
@section Compatibility with the OpenSSL Library
@cindex OpenSSL

To ease @acronym{GnuTLS}' integration with existing applications, a
compatibility layer with the widely used OpenSSL library is included
in the @code{gnutls-openssl} library. This compatibility layer is not
complete and it is not intended to completely reimplement the OpenSSL
API with @acronym{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
@file{gnutls/openssl.h} header file.

Current limitations imposed by the compatibility layer include:

@itemize

@item Error handling is not thread safe.

@end itemize

@node Opaque PRF Input TLS Extension
@section Opaque PRF Input TLS Extension
@cindex Opaque PRF Input

GnuTLS supports the Opaque PRF Input TLS extension
(@code{draft-rescorla-tls-opaque-prf-input-00.txt}).  The API consists
of one API for use in the client, @ref{gnutls_oprfi_enable_client},
and one API for use in the server, @ref{gnutls_oprfi_enable_server}.
You must invoke both functions before calling @ref{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.

@node Keying Material Exporters
@section Keying Material Exporters
@cindex Keying Material Exporters
@cindex Exporting Keying Material

The TLS PRF can be used by other protocols to derive data.  The API to
use is @ref{gnutls_prf}.  The function needs to be provided with the
label in the parameter @code{label}, and the extra data to mix in the
@code{extra} parameter.  Depending on whether you want to mix in the
client or server random data first, you can set the
@code{server_random_first} parameter.

For example, after establishing a TLS session using
@ref{gnutls_handshake}, you can invoke the TLS PRF with this call:

@smallexample
#define MYLABEL "EXPORTER-FOO"
#define MYCONTEXT "some context data"
char out[32];
rc = gnutls_prf (session, strlen (MYLABEL), MYLABEL, 0,
                 strlen (MYCONTEXT), MYCONTEXT, 32, out);
@end smallexample

If you don't want to mix in the client/server random, there is a more
low-level TLS PRF interface called @ref{gnutls_prf_raw}.

@node Included programs
@chapter Included Programs

Included with @acronym{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.

@menu
* Invoking certtool::
* Invoking gnutls-cli::
* Invoking gnutls-cli-debug::
* Invoking gnutls-serv::
* Invoking psktool::
* Invoking srptool::
@end menu

@node Invoking certtool
@section Invoking certtool
@cindex certtool

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

@verbatim
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
@end verbatim

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

How to use certtool interactively:

@itemize
@item
To generate parameters for Diffie-Hellman key exchange, use the command:
@example
$ certtool --generate-dh-params --outfile dh.pem
@end example

@item
To generate parameters for the RSA-EXPORT key exchange, use the command:
@example
$ certtool --generate-privkey --bits 512 --outfile rsa.pem
@end example

@end itemize

@itemize

@item
To create a self signed certificate, use the command:
@example
$ certtool --generate-privkey --outfile ca-key.pem
$ certtool --generate-self-signed --load-privkey ca-key.pem \
   --outfile ca-cert.pem
@end example

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

@item
To create a private key (RSA by default), run:

@example
$ certtool --generate-privkey --outfile key.pem
@end example

To create a DSA private key, run:

@example
$ certtool --dsa --generate-privkey --outfile key-dsa.pem
@end example

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

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

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

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

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

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

@item
To view the certificate information, use:

@example
$ certtool --certificate-info --infile cert.pem
@end example

@item
To generate a @acronym{PKCS} #12 structure using the previous key and
certificate, use the command:

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

Some tools (reportedly web browsers) have problems with that file
because it does not contain the CA certificate for the certificate.
To work around that problem in the tool, you can use the
@samp{--load-ca-certificate} parameter as follows:

@example
$ certtool --load-ca-certificate ca.pem \
  --load-certificate cert.pem --load-privkey key.pem \
  --to-p12 --outder --outfile key.p12
@end example

@item
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:

@example
$ 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
@end example

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

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

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

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

@item
To verify a Certificate Revocation List (CRL) do:

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

@end itemize

Certtool's template file format:

@itemize

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

@item
Then execute:

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

@end itemize

An example certtool template file:

@example
# 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"
#dns_name = "www.morethanone.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 preferred 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
@end example

@node Invoking gnutls-cli
@section Invoking gnutls-cli
@cindex 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.

@verbatim
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.
     --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.
     --priority PRIORITY STRING
                              Priorities string.
     --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.
     --pgpcertfile FILE       PGP Public Key (certificate) file to
                              use.
     --pgpsubkey HEX|auto     PGP subkey 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.
     --pskusername NAME       PSK username to use.
     --pskkey KEY             PSK key (in hex) to use.
     --opaque-prf-input DATA
                              Use Opaque PRF Input DATA.
     -p, --port PORT          The port to connect to.
     --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
@end verbatim

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

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

@menu
* Example client PSK connection::
@end menu

@node Example client PSK connection
@subsection Example client PSK connection
@cindex PSK client

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

@smallexample
$ ./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:
@end smallexample

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

@cindex Netconf
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:

@smallexample
$ ./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:
@end smallexample

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

@node Invoking gnutls-cli-debug
@section Invoking gnutls-cli-debug
@cindex gnutls-cli-debug

This program was created to assist in debugging @acronym{GnuTLS}, but
it might be useful to extract a @acronym{TLS} server's capabilities.
It's purpose is to connect onto a @acronym{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:

@smallexample
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
@end smallexample

@node Invoking gnutls-serv
@section Invoking gnutls-serv
@cindex gnutls-serv

Simple server program that listens to incoming TLS connections.

@verbatim
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.
     --pgpkeyfile FILE        PGP Key file to use.
     --pgpcertfile FILE       PGP Public Key (certificate) file to
                              use.
     --pgpsubkey HEX|auto     PGP subkey 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.
     -r, --require-cert       Require a valid certificate.
     -a, --disable-client-cert
                              Disable request for a client
                              certificate.
     --pskpasswd FILE         PSK password file to use.
     --pskhint HINT           PSK identity hint to use.
     --srppasswd FILE         SRP password file to use.
     --srppasswdconf FILE     SRP password conf file to use.
     --opaque-prf-input DATA
                              Use Opaque PRF Input DATA.
     --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.
     --priority PRIORITY STRING
                              Priorities string.
     -l, --list               Print a list of the supported
                              algorithms  and modes.
     -h, --help               prints this help
     -v, --version            prints the program's version number
@end verbatim

@subsection Setting Up a Test HTTPS Server
@cindex HTTPS server
@cindex debug 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 @code{gnutls-serv} as a simple HTTPS server.

The most basic server can be started as:

@example
gnutls-serv --http
@end example

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:

@example
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
...
@end example

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.

@example
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
...
@end example

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

@example
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
...
@end example

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.

@example
certtool --to-p12 --load-ca-certificate x509-ca.pem --load-privkey x509-client-key.pem --load-certificate x509-client.pem --outder --outfile x509-client.p12
@end example

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

@example
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
...
@end example

Then start the server again:

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

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.

@example
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
...
@end example

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

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

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

@example
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
@end example

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

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

The next step is to add support for SRP authentication.

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

Start the server with SRP support:

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

Let's also add support for PSK.

@example
$ psktool --passwd psk-passwd.txt
@end example

Start the server with PSK support:

@example
gnutls-serv --http \
            --pskpasswd psk-passwd.txt
@end example

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

@example
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
@end example

@menu
* Example server PSK connection::
@end menu

@node Example server PSK connection
@subsection Example server PSK connection
@cindex PSK server

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

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

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

@smallexample
$ ./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'.
@end smallexample

You can now connect to the server using a PSK client (@pxref{Example
client PSK connection}).

@node Invoking psktool
@section Invoking psktool
@cindex psktool

This is a program to manage @acronym{PSK} username and keys.

@verbatim
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
@end verbatim

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

@node Invoking srptool
@section Invoking srptool
@anchor{srptool}
@cindex srptool

The @file{srptool} is a very simple program that emulates the programs
in the @emph{Stanford SRP libraries}, see
@url{http://srp.stanford.edu/}.  It is intended for use in places
where you don't expect @acronym{SRP} authentication to be the used for
system users.

Traditionally @emph{libsrp} used two files. One called @code{tpasswd}
which holds usernames and verifiers, and @code{tpasswd.conf} which
holds generators and primes.

How to use srptool:

@itemize

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

@example
$ srptool --create-conf /etc/tpasswd.conf
@end example

@item
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.

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

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

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

@end itemize

@node Function reference
@chapter Function Reference
@cindex Function reference

@menu
* Core functions::
* X.509 certificate functions::
* GnuTLS-extra functions::
* OpenPGP functions::
* TLS Inner Application (TLS/IA) functions::
* Error codes and descriptions::
@end menu

@node Core functions
@section Core Functions

The prototypes for the following functions lie in
@file{gnutls/gnutls.h}.

@include gnutls-api.texi

@node X.509 certificate functions
@section @acronym{X.509} Certificate Functions
@anchor{sec:x509api}
@cindex @acronym{X.509} Functions

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

@include x509-api.texi

@node GnuTLS-extra functions
@section @acronym{GnuTLS-extra} Functions
@cindex @acronym{GnuTLS-extra} functions

These functions are only available in the GPLv3+ version of the
library called @code{gnutls-extra}. The prototypes for this library
lie in @file{gnutls/extra.h}.

@include extra-api.texi

@node OpenPGP functions
@section @acronym{OpenPGP} Functions
@cindex @acronym{OpenPGP} functions
@anchor{sec:openpgpapi}

The following functions are to be used for @acronym{OpenPGP}
certificate handling.  Their prototypes lie in
@file{gnutls/openpgp.h}.

@include pgp-api.texi

@node TLS Inner Application (TLS/IA) functions
@section @acronym{TLS} Inner Application (@acronym{TLS/IA}) Functions
@cindex @acronym{TLS} Inner Application (@acronym{TLS/IA}) functions
@cindex Inner Application (@acronym{TLS/IA}) functions

The following functions are used for @acronym{TLS} Inner Application
(@acronym{TLS/IA}).  Their prototypes lie in @file{gnutls/extra.h}.
You need to link with @file{libgnutls-extra} to be able to use these
functions (@pxref{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:

@example
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...
...
@end example

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

The function @code{client_avp} would have to be implemented by your
application.  The function is responsible for handling the AVP data.
See @code{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
@code{gnutls_ia_server_credentials_t} instead of
@code{gnutls_ia_client_credentials_t}, and replace the call to the
client functions with the corresponding server functions.

@include ia-api.texi

@node Error codes and descriptions
@section Error Codes and Descriptions
@anchor{Error Codes}
@cindex Error codes

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

@include error_codes.texi

@node All the supported ciphersuites in GnuTLS
@chapter All the Supported Ciphersuites in @acronym{GnuTLS}
@anchor{ciphersuites}
@cindex Ciphersuites

@include algorithms.texi

Some additional information regarding some of the algorithms:

@table @code
@item RSA
RSA is public key cryptosystem designed by Ronald Rivest, Adi Shamir
and Leonard Adleman.  It can be used with any hash functions.

@item DSA
DSA is the USA's Digital Signature Standard.  It uses only the SHA-1
hash algorithm.

@item MD2
MD2 is a cryptographic hash algorithm designed by Ron Rivest.  It is
optimized for 8-bit processors. Outputs 128 bits of data.  There are
no known weaknesses of this algorithm but since this algorithm is
rarely used and not really studied it should not be used today.

@item MD5
MD5 is a cryptographic hash algorithm designed by Ron Rivest. Outputs
128 bits of data.  It is considered to be broken.

@item SHA-1
SHA is a cryptographic hash algorithm designed by NSA. Outputs 160
bits of data.  It is also considered to be broken, though no practical
attacks have been found.

@item RMD160
RIPEMD is a cryptographic hash algorithm developed in the framework of
the EU project RIPE.  Outputs 160 bits of data.

@end table

@c
@c Guile Bindings
@c

@include guile.texi


@node Internal architecture of GnuTLS
@chapter Internal Architecture of GnuTLS
@cindex Internal architecture

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

@menu
* The TLS Protocol::
* TLS Handshake Protocol::
* TLS Authentication Methods::
* TLS Extension Handling::
* Cryptographic Backend::
@end menu

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

@image{gnutls-client-server-use-case,9cm}

This is being accomplished by the following object diagram.
Note that since @acronym{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.
@image{gnutls-objects,15cm}

@node TLS Handshake Protocol
@section TLS Handshake Protocol
The @acronym{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.

@image{gnutls-handshake-state,9cm}

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

@image{gnutls-handshake-sequence,12cm}

@node TLS Authentication Methods
@section TLS Authentication Methods
In @acronym{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.

@image{gnutls-mod_auth_st,12cm}

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 @code{credentials_t}
structures@footnote{such as the
@code{gnutls_certificate_credentials_t} structures} and write data,
such as certificates, usernames etc. to @code{auth_info_t} structures.

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

@node TLS Extension Handling
@section TLS Extension Handling
As with authentication methods, the TLS extensions handlers can be
implemented using the following interface.

@image{gnutls-extensions_st,12cm}

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
@code{ext_srp.c} After implementing these functions, together with the
extension number they handle, they have to be registered in
@code{gnutls_extensions.c} in the @code{_gnutls_extensions} structure.

@subsection 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
@code{foobar}.

@enumerate

@item Add @code{configure} option like @code{--enable-foobar} or @code{--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:

@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")
@end example

These lines should go in @code{lib/m4/hooks.m4}.

@item Add IANA extension value to @code{extensions_t} in @code{gnutls_int.h}.

A good name for the value would be GNUTLS_EXTENSION_FOOBAR.  Check
with @url{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.

@item Add an entry to @code{_gnutls_extensions} in @code{gnutls_extensions.c}.

A typical entry would be:

@example
  int ret;

  /* ...
   */

#if ENABLE_FOOBAR
  ret = gnutls_ext_register (GNUTLS_EXTENSION_FOOBAR,
                             "FOOBAR",
                             GNUTLS_EXT_TLS,
                             _gnutls_foobar_recv_params,
                             _gnutls_foobar_send_params);
  if (ret != GNUTLS_E_SUCCESS)
    return ret;
#endif
@end example

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

@item Add new files @code{ext_foobar.c} and @code{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:

@example
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;
@}
@end example

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

The @code{_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
@code{GNUTLS_E_UNEXPECTED_PACKET_LENGTH}.  If you receive invalid
data, return @code{GNUTLS_E_RECEIVED_ILLEGAL_PARAMETER}.  You can use
other error codes too.  Return 0 on success.

The function typically store some information in the @code{session}
variable for later usage.  If you need to add new fields there, check
@code{tls_ext_st} in @code{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:

@example
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);
@}
@end example

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

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

@example
if ENABLE_FOOBAR
COBJECTS += ext_foobar.c
HFILES += ext_foobar.h
endif
@end example

@item 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 @code{includes/gnutls/gnutls.h} or
@code{includes/gnutls/extra.h} as appropriate.  It is recommended that
if you don't have a requirement to use the LGPLv2.1+ license for your
extension, that you place your work under the GPLv3+ license and thus
in the libgnutls-extra library.

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

@end enumerate

@section Certificate Handling
What is provided by the certificate handling functions
is summarized in the following diagram.

@image{gnutls-certificate-user-use-case,12cm}

@node Cryptographic Backend
@section 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 GnuTLS supports by default
the /dev/crypto device usually found in FreeBSD and OpenBSD system, to
take advantage of installed hardware. 

In addition 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 @code{gnutls/crypto.h} header
file.

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

@itemize

@item @ref{gnutls_crypto_single_cipher_register2}
To register a cipher algorithm.

@ref{gnutls_crypto_single_digest_register2}
To register a hash (digest) or MAC algorithm.

@end itemize

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

@subsection 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.

@itemize

@item @ref{gnutls_crypto_cipher_register2}
To override the cryptographic algorithms backend.

@item @ref{gnutls_crypto_digest_register2}
To override the digest algorithms backend.

@item @ref{gnutls_crypto_rnd_register2}
To override the random number generator backend.

@item @ref{gnutls_crypto_bigint_register2}
To override the big number number operations backend.

@item @ref{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.

@end itemize

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

@node Copying Information
@appendix Copying Information

@menu
* GNU Free Documentation License::   License for copying this manual.
* GNU LGPL::                     License for copying the core GnuTLS library.
* GNU GPL::                      License for copying GNUTLS extra and tools.
@end menu

@node GNU Free Documentation License
@appendixsec GNU Free Documentation License

@cindex FDL, GNU Free Documentation License

@include fdl-1.3.texi

@node GNU LGPL
@appendixsec GNU Lesser General Public License
@cindex LGPL, GNU Lesser General Public License
@cindex License, GNU LGPL

@include lgpl-2.1.texi

@node GNU GPL
@appendixsec GNU General Public License
@cindex GPL, GNU General Public License
@cindex License, GNU GPL

@include gpl-3.0.texi

@node Bibliography
@unnumbered Bibliography

@table @asis

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

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

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

@item @anchor{NISTSP80057}[NISTSP80057]
NIST Special Publication 800-57, "Recommendation for Key Management -
Part 1: General (Revised)", March 2007, available from
@url{http://csrc.nist.gov/publications/nistpubs/800-57/sp800-57-Part1-revised2_Mar08-2007.pdf}.

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

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

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

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

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

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

@item @anchor{RFC2818}[RFC2818]
Eric Rescorla, "HTTP Over TLS", May 2000, Available from
@url{http://www.ietf/rfc/rfc2818.txt}.

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

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

@item @anchor{PKIX}[PKIX]
D. Cooper, S. Santesson, S. Farrel, S. Boeyen, R. Housley, W. Polk,
"Internet X.509 Public Key Infrastructure Certificate and Certificate
Revocation List (CRL) Profile", May 2008, available from
@url{http://www.ietf.org/rfc/rfc5280.txt}.

@item @anchor{RFC3749}[RFC3749]
Scott Hollenbeck, "Transport Layer Security Protocol Compression
Methods", May 2004, available from
@url{http://www.ietf.org/rfc/rfc3749.txt}.

@item @anchor{RFC3820}[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
@url{http://www.ietf.org/rfc/rfc3820}.

@item @anchor{TLSTKT}[TLSTKT]
Joseph Salowey, Hao Zhou, Pasi Eronen, Hannes Tschofenig, "Transport
Layer Security (TLS) Session Resumption without Server-Side State",
January 2008, available from @url{http://www.ietf.org/rfc/rfc5077}.

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

@item @anchor{RESCORLA}[RESCORLA]
Eric Rescorla, "SSL and TLS: Designing and Building Secure Systems",
2001

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

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

@item @anchor{STEVENS}[STEVENS]
Richard Stevens, "UNIX Network Programming, Volume 1", Prentice Hall
PTR, January 1998

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

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

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

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

@item @anchor{TOMSRP}[TOMSRP]
Tom Wu, "The Stanford SRP Authentication Project", Available at
@url{http://srp.stanford.edu/}.

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

@end table

@node Function and Data Index
@unnumbered Function and Data Index

@printindex fn

@node Concept Index
@unnumbered Concept Index

@printindex cp

@bye