path: root/doc/cha-library.texi

@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 protocols ranging from SSL 3.0 to TLS 1.2 (see @ref{Introduction to TLS}, 
for a 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 1.2, TLS 1.1, TLS 1.0 and SSL 3.0 protocols.

@item Support for Datagram TLS 1.0.

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

@item Support for handling and verification of certificates.

@item Support for password authentication using @acronym{TLS-SRP}.

@item Support for keyed authentication using @acronym{TLS-PSK}.

@item Support for @acronym{PKCS} #11 tokens and smart-cards.

@end itemize

The @acronym{GnuTLS} library consists of three independent parts, namely the ``TLS
protocol part'', the ``Certificate part'', and the ``Cryptographic
back-end'' 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 and it uses
functionality from the
libtasn1@footnote{@url{http://www.gnu.org/software/libtasn1/}} library.
The ``Cryptographic back-end'' is provided by the nettle@footnote{@url{http://www.lysator.liu.se/~nisse/nettle/}}
library. 
@menu
* Downloading and installing::
* General idea::
* Error handling::
* Thread safety::
* Callback functions::
@end menu

@node Downloading and installing
@section Downloading and installing
@cindex installation
@cindex download

GnuTLS is available for download at:
@url{http://www.gnutls.org/download.html}

GnuTLS uses a development cycle where even minor version numbers
indicate a stable release and a odd minor version number indicate 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 Libnettle, and you will need to install it
before installing GnuTLS.  Libnettle is available from
@url{http://www.lysator.liu.se/~nisse/nettle/}.  
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 library, libz, as well as p11-kit are a optional dependencies.
You can get libz from @url{http://www.zlib.net/} and p11-kit from @url{http://p11-glue.freedesktop.org/}.

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}.

A few @code{configure} options may be relevant, summarized below.
They disable or enable particular features,
to create a smaller library with only the required features.

@verbatim
--disable-srp-authentication
--disable-psk-authentication
--disable-anon-authentication
--disable-extra-pki
--disable-openpgp-authentication
--disable-openssl-compatibility
--without-p11-kit
@end verbatim

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

@node General idea
@section General idea

A brief description of how @acronym{GnuTLS} works internally is shown
at @ref{fig:gnutls-design}. This section may be easier to understand after
having seen the examples at @ref{examples}.
As shown in the figure, there is a read-only global state that is
initialized once by the global initialization function.  This global
structure, among others, contains the memory allocation functions
used, and 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 allocated memory
and is called after the program has permanently
finished using @acronym{GnuTLS}.

@float Figure,fig:gnutls-design
@image{gnutls-internals,12cm}
@caption{High level design of GnuTLS.}
@end float

The credentials structures are used by the authentication methods, such
as certificate authentication. They store certificates, privates keys,
and other information that is needed to prove the identity to the peer,
and/or verify the indentity of the peer. The information stored in
the credentials structures is initialized once and then can be 
shared by many @acronym{TLS} sessions.

A @acronym{GnuTLS} session contains all the required information
to handle one secure connection. The session communicates with the
peers using the provided functions of the transport layer.
Every session has a unique session ID shared with the peer.

Since TLS sessions can be resumed, servers need a
database back-end to hold the session's parameters.  Every
@acronym{GnuTLS} session after a successful handshake calls the
appropriate back-end function (see @ref{resume})
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
@subsection Conventions

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.  Such an example 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 @funcref{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
@funcref{gnutls_error_is_fatal}.

If any non fatal errors, that require an action, are to be returned by
a function, these error codes will be documented in the function's
reference.  See @ref{Error codes}, for a description of the available 
error codes.

@subsection Debugging and auditing

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

@showfuncB{gnutls_global_set_log_level,gnutls_global_set_log_function}

When debugging is not required, important issues, such as detected
attacks on the protocol still need to be logged. This is provided
by the logging function set by
@funcref{gnutls_global_set_audit_log_function}. The provided function
will receive an message and the corresponding
TLS session. The session information might be used to derive IP addresses
or other information about the peer involved.

@showfuncdesc{gnutls_global_set_audit_log_function}

@node Thread safety
@section Thread safety

The @acronym{GnuTLS} library is thread safe by design, meaning that
objects of the library such as TLS sessions, can be safely divided across
threads as long as a single thread accesses a single object. This is
sufficient to support a server which handles several sessions per thread.
If, however, an object needs to be shared across threads then access must be 
protected with a mutex. Read-only access to objects, for example the
credentials holding structures (see @ref{Authentication methods}), is also thread-safe. 

The random generator of the cryptographic back-end, is not thread safe and requires
mutex locks which are setup by @acronym{GnuTLS}.
Applications can either call @funcref{gnutls_global_init} which will initialize the default
operating system provided locks (i.e. @code{pthreads} on GNU/Linux and
@code{CriticalSection} on Windows), or specify manually the locking system using 
the function @funcref{gnutls_global_set_mutex} before calling @funcref{gnutls_global_init}. 
Setting manually mutexes is recommended
only to applications that have full control of the underlying libraries. If this
is not the case, the use of the operating system defaults is recommended. An example of 
non-native thread usage is shown below.

@example
#include <gnutls.h>

/* Other thread packages
 */

int main()
@{
   gnutls_global_set_mutex (mutex_init, mutex_deinit, 
                            mutex_lock, mutex_unlock);
   gnutls_global_init();
@}
@end example

@showfuncdesc{gnutls_global_set_mutex}

@node Callback functions
@section Callback functions
@cindex callback functions

There are several cases where @acronym{GnuTLS} may need 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.

@showfuncB{gnutls_transport_set_push_function,gnutls_transport_set_pull_function}

Other callback functions may require more complicated input and data
to be allocated. Such an example is 
@funcref{gnutls_srp_set_server_credentials_function}.
All callbacks should allocate and free memory using the functions shown below.

@showfuncB{gnutls_malloc,gnutls_free}