path: root/doc/cha-gtls-app.texi

@node How to use GnuTLS in applications
@chapter How to use @acronym{GnuTLS} in applications

@menu
* Introduction to the library::
* Preparation::
* Session initialization::
* Associating the credentials::
* Setting up the transport layer::
* TLS handshake::
* Data transfer and termination::
* Buffered data transfer::
* Handling alerts::
* Priority Strings::
* Selecting cryptographic key sizes::
* Advanced topics::
@end menu

@node Introduction to the library
@section Introduction

This chapter tries to explain the basic functionality of the current GnuTLS
library. Note that there may be additional functionality not discussed here
but included in the library. Checking the header files in @file{/usr/include/gnutls/}
and the manpages is recommended.

@menu
* General idea::
* Error handling::
* Common types::
* Debugging and auditing::
* Thread safety::
* Running in a sandbox::
* Sessions and fork::
* Callback functions::
@end menu

@node General idea
@subsection General idea

A brief description of how @acronym{GnuTLS} sessions operate is shown
at @ref{fig-gnutls-design}. This section will become more clear when it
is completely read.
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, structures needed for the @acronym{ASN.1} parser and depending
on the system's CPU, pointers to hardware accelerated encryption functions.  This
structure is never modified by any @acronym{GnuTLS} function, except
for the deinitialization function which frees all allocated memory
and must be 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 identity 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 state and 
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
@subsection Error handling

There two types of @acronym{GnuTLS} functions. The first type returns
a boolean value, true (non-zero) or false (zero) value; these functions
are defined to return an unsigned integer type. The other type returns a
signed integer type with zero (or a positive number) indicating
success and a negative value indicating failure. For the latter
type it is recommended to check for errors as following.
@example
    ret = gnutls_function();
    if (ret < 0) @{
        return -1;
    @}
@end example
The above example checks for a failure condition rather than
for explicit success (e.g., equality to zero). That has the advantage
that future extensions of the API can be extended to provide
additional information via positive returned values (see for example
@funcref{gnutls_certificate_set_x509_key_file}).

For certain operations such as TLS handshake and TLS packet receive
there is the notion of fatal and non-fatal error codes.
Fatal errors terminate the TLS session 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}.
All errors can be converted to a descriptive string using @funcref{gnutls_strerror}.

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. For example the error codes @code{GNUTLS_@-E_@-WARNING_@-ALERT_@-RECEIVED} and @code{GNUTLS_@-E_@-FATAL_@-ALERT_@-RECEIVED}
that may returned when receiving data, should be handled by notifying the
user of the alert (as explained in @ref{Handling alerts}).
See @ref{Error codes}, for a description of the available error codes.

@node Common types
@subsection Common types
@cindex gnutls_datum_t
@cindex giovec_t

All strings that are to provided as input to @acronym{GnuTLS} functions
should be in UTF-8 unless otherwise specified. Output strings are also
in UTF-8 format unless otherwise specified. When functions take as input
passwords, they will normalize them using @xcite{RFC7613} rules (since
GnuTLS 3.5.7).

When data of a fixed size are provided to @acronym{GnuTLS} functions then
the helper structure @code{gnutls_datum_t} is often used. Its definition is
shown below.
@verbatim
  typedef struct
  {
    unsigned char *data;
    unsigned int size;
  } gnutls_datum_t;
@end verbatim

In functions where this structure is a returned type, if the function succeeds,
it is expected from the caller to use @code{gnutls_free()} to deinitialize the
data element after use, unless otherwise specified. If the function fails, the
contents of the @code{gnutls_datum_t} should be considered undefined and must
not be deinitialized.

Other functions that require data for scattered read use a structure similar
to @code{struct iovec} typically used by @funcintref{readv}. It is shown
below.
@verbatim
  typedef struct
  {
    void *iov_base;             /* Starting address */
    size_t iov_len;             /* Number of bytes to transfer */
  } giovec_t;
@end verbatim


@node Debugging and auditing
@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}

Alternatively the environment variable @code{GNUTLS_DEBUG_LEVEL} can be
set to a logging level and GnuTLS will output debugging output to standard
error. Other available environment variables are shown in @ref{tab:environment}.

@float Table,tab:environment
@multitable @columnfractions .30 .70

@headitem Variable @tab Purpose

@item @code{GNUTLS_DEBUG_LEVEL}
@tab When set to a numeric value, it sets the default debugging level for GnuTLS applications.

@item @code{SSLKEYLOGFILE}
@tab When set to a filename, GnuTLS will append to it the session keys in the NSS Key Log
format. That format can be read by wireshark and will allow decryption of the session for debugging.

@item @code{GNUTLS_CPUID_OVERRIDE}
@tab That environment variable can be used to
explicitly enable/disable the use of certain CPU capabilities. Note that CPU
detection cannot be overridden, i.e., VIA options cannot be enabled on an Intel
CPU. The currently available options are:
@itemize
@item 0x1: Disable all run-time detected optimizations
@item 0x2: Enable AES-NI
@item 0x4: Enable SSSE3
@item 0x8: Enable PCLMUL
@item 0x10: Enable AVX
@item 0x100000: Enable VIA padlock
@item 0x200000: Enable VIA PHE
@item 0x400000: Enable VIA PHE SHA512
@end itemize

@item @code{GNUTLS_FORCE_FIPS_MODE}
@tab In setups where GnuTLS is compiled with support for FIPS140-2 (see @ref{FIPS140-2 mode}).

@end multitable
@caption{Environment variables used by the library.}
@end float


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
@subsection Thread safety
@cindex 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.
Read-only access to objects, for example the credentials holding structures,
is also thread-safe. 

A @code{gnutls_session_t} object could also be shared by two threads, one sending,
the other receiving. In that case however, care must be taken during key
updates and re-handshakes to be handled only by a single thread. The termination of a session 
should be handled, either by a single thread being active, or by the sender thread 
using @funcref{gnutls_bye} with @code{GNUTLS_SHUT_WR} and the receiving thread 
waiting for a return value of zero.

For several aspects of the library (e.g., the random generator, PKCS#11
operations), the library may utilize mutex locks (e.g., pthreads on GNU/Linux and CriticalSection on Windows)
which are transparently setup on library initialization. Prior to version 3.3.0
these were setup by explicitly calling @funcref{gnutls_global_init}.@footnote{On special systems
you could manually specify the locking system using
the function @funcref{gnutls_global_set_mutex} before calling any other
GnuTLS function. Setting mutexes manually is not recommended.}

Note that, on Glibc systems, unless the application is explicitly linked
with the libpthread library, no mutex locks are used and setup by GnuTLS. It 
will use the Glibc mutex stubs.

@node Running in a sandbox
@subsection Running in a sandbox
@cindex seccomp
@cindex isolated mode

Given that TLS protocol handling as well as X.509 certificate
parsing are complicated processes involving several thousands lines of code,
it is often desirable (and recommended) to run the TLS session handling in
a sandbox like seccomp. That has to be allowed by the overall software design,
but if available, it adds an additional layer of protection by
preventing parsing errors from becoming vessels for further security issues such
as code execution.

GnuTLS requires the following system calls to be available for its proper
operation.

@itemize
@item nanosleep
@item time
@item gettimeofday
@item clock_gettime
@item getrusage
@item getpid
@item send
@item recv
@item sendmsg
@item read (to read from /dev/urandom)
@item getrandom (this is Linux-kernel specific)
@item poll
@end itemize

As well as any calls needed for memory allocation to work. Note however, that GnuTLS
depends on libc for the system calls, and there is no guarantee that libc will
call the expected system call. For that it is recommended to test your
program in all the targetted platforms when filters like seccomp are in place.

An example with a seccomp filter from GnuTLS' test suite is at:
@url{http://gitlab.com/gnutls/gnutls/blob/master/tests/seccomp.c}.

@node Sessions and fork
@subsection Sessions and fork
@cindex fork

A @code{gnutls_session_t} object can be shared by two processes after a fork,
one sending, the other receiving. In that case rehandshakes, 
cannot and must not be performed. As with threads, the termination of a session should be
handled by the sender process using @funcref{gnutls_bye} with @code{GNUTLS_SHUT_WR}
and the receiving process waiting for a return value of zero.


@node Callback functions
@subsection 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 
@funcintref{gnutls_malloc} and @funcintref{gnutls_free}.


@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::
* 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.

@node Initialization
@subsection Initialization

The GnuTLS library is initialized on load; prior to 3.3.0 was initialized by calling @funcref{gnutls_global_init}@footnote{
The original behavior of requiring explicit initialization can obtained by setting the
GNUTLS_NO_EXPLICIT_INIT environment variable to 1, or by using the macro GNUTLS_SKIP_GLOBAL_INIT
in a global section of your program --the latter works in systems with
support for weak symbols only.}. 
The initialization typically enables CPU-specific acceleration, performs any required
precalculations needed, opens any required system devices (e.g., /dev/urandom on Linux)
and initializes subsystems that could be used later.

The resources allocated by the initialization process will be released 
on library deinitialization.

Note that on certain systems file descriptors may be kept open by
GnuTLS (e.g. /dev/urandom) on library load. Applications closing all unknown file
descriptors must immediately call @funcref{gnutls_global_init}, after that, to
ensure they don't disrupt GnuTLS' operation.

@c In order to take advantage of the internationalization features in
@c GnuTLS, such as translated error messages, the application must set
@c 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 start-up.
See the function @funcref{gnutls_check_version}.

On the other hand, it is often desirable to support more than one
versions of the library. In that case you could utilize compile-time 
feature checks using the @code{GNUTLS_VERSION_NUMBER} macro. 
For example, to conditionally add code for GnuTLS 3.2.1 or later, you may use:
@example
#if GNUTLS_VERSION_NUMBER >= 0x030201
 ...
#endif
@end example

@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 library (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

When a program uses the GNU autoconf system, then the following
line or similar can be used to detect the presence of GnuTLS.

@example
PKG_CHECK_MODULES([LIBGNUTLS], [gnutls >= 3.3.0])

AC_SUBST([LIBGNUTLS_CFLAGS])
AC_SUBST([LIBGNUTLS_LIBS])
@end example

@node Session initialization
@section Session initialization

In the previous sections we have discussed the global initialization
required for GnuTLS as well as the initialization required for each
authentication method's credentials (see @ref{Authentication}).
In this section we elaborate on the TLS or DTLS session initiation.
Each session is initialized using @funcref{gnutls_init} which among
others is used to specify the type of the connection (server or client), 
and the underlying protocol type, i.e., datagram (UDP) or reliable (TCP).

@showfuncdesc{gnutls_init}

@showenumdesc{gnutls_init_flags_t,The @code{gnutls_init_@-flags_t} enumeration.}

After the session initialization details on the allowed ciphersuites
and protocol versions should be set using the priority functions
such as @funcref{gnutls_priority_set} and @funcref{gnutls_priority_set_direct}.
We elaborate on them in @ref{Priority Strings}.
The credentials used for the key exchange method, such as certificates 
or usernames and passwords should also be associated with the session
current session using @funcref{gnutls_credentials_set}. 

@showfuncdesc{gnutls_credentials_set}

@node Associating the credentials
@section Associating the credentials

@menu
* Certificate credentials::
* SRP credentials::
* PSK credentials::
* Anonymous credentials::
@end menu

Each authentication method is associated with a key exchange method, and a credentials type. 
The contents of the credentials is method-dependent, e.g. certificates
for certificate authentication and should be initialized and associated
with a session (see @funcref{gnutls_credentials_set}).  A mapping of the key exchange methods
with the credential types is shown in @ref{tab:key-exchange-cred}.

@float Table,tab:key-exchange-cred
@multitable @columnfractions .25 .25 .2 .2

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

@item Certificate
@tab @code{KX_RSA},
@code{KX_DHE_RSA},
@code{KX_DHE_DSS},
@code{KX_ECDHE_RSA},
@code{KX_ECDHE_ECDSA}
@tab @code{CRD_CERTIFICATE}
@tab @code{CRD_CERTIFICATE}

@item Password and certificate
@tab @code{KX_SRP_RSA}, @code{KX_SRP_DSS}
@tab @code{CRD_SRP}
@tab @code{CRD_CERTIFICATE}, @code{CRD_SRP}

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

@item Anonymous
@tab @code{KX_ANON_DH},
@code{KX_ANON_ECDH}
@tab @code{CRD_ANON}
@tab @code{CRD_ANON}

@item Pre-shared key
@tab @code{KX_PSK},
@code{KX_DHE_PSK}, @code{KX_ECDHE_PSK}
@tab @code{CRD_PSK}
@tab @code{CRD_PSK}

@end multitable
@caption{Key exchange algorithms and the corresponding credential types.}
@end float

@node Certificate credentials
@subsection Certificates
@subsubheading Server certificate authentication

When using certificates the server is required to have at least one
certificate and private key pair. Clients may not hold such
a pair, but a server could require it. In this section we discuss
general issues applying to both client and server certificates. The next
section will elaborate on issues arising from client authentication only.

@showfuncB{gnutls_certificate_allocate_credentials,gnutls_certificate_free_credentials}

After the credentials structures are initialized, the certificate 
and key pair must be loaded. This occurs before any @acronym{TLS} 
session is initialized, and the same structures are reused for multiple sessions.
Depending on the certificate type different loading functions
are available, as shown below.
For @acronym{X.509} certificates, the functions will
accept and use a certificate chain that leads to a trusted
authority. The certificate chain 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.

@showfuncC{gnutls_certificate_set_x509_key_file2,gnutls_certificate_set_x509_key_mem2,gnutls_certificate_set_x509_key}

It is recommended to use the higher level functions such as @funcref{gnutls_certificate_set_x509_key_file2}
which accept not only file names but URLs that specify objects stored in token,
or system certificates and keys (see @ref{Application-specific keys}). For these cases, another important 
function is @funcref{gnutls_certificate_set_pin_function}, that
allows setting a callback function to retrieve a PIN if the input keys are
protected by PIN.

@showfuncdesc{gnutls_certificate_set_pin_function}

If the imported keys and certificates need to be accessed before any TLS session
is established, it is convenient to use @funcref{gnutls_certificate_set_key}
in combination with @funcref{gnutls_pcert_import_x509_raw} and @funcref{gnutls_privkey_import_x509_raw}.

@showfuncdesc{gnutls_certificate_set_key}

If multiple certificates are used with the functions above each
client's request will be served with the certificate that matches the
requested name (see @ref{Server name indication}).

As an alternative to loading from files or buffers, a callback may be used for the 
server or the client to specify the certificate and the key at the handshake time.
In that case a certificate should be selected according the peer's signature
algorithm preferences. To get those preferences use
@funcref{gnutls_sign_algorithm_get_requested}. Both functions are shown below.

@showfuncC{gnutls_certificate_set_retrieve_function,gnutls_certificate_set_retrieve_function2,gnutls_sign_algorithm_get_requested}

The functions above do not handle the requested server name automatically.
A server would need to check the name requested by the client
using @funcref{gnutls_server_name_get}, and serve the appropriate
certificate. Note that some of these functions require the @code{gnutls_pcert_st} structure to be
filled in. Helper functions to fill in the structure are listed below.

@verbatim
typedef struct gnutls_pcert_st
{
  gnutls_pubkey_t pubkey;
  gnutls_datum_t cert;
  gnutls_certificate_type_t type;
} gnutls_pcert_st;
@end verbatim

@showfuncC{gnutls_pcert_import_x509,gnutls_pcert_import_x509_raw,gnutls_pcert_deinit}

In a handshake, the negotiated cipher suite depends on the
certificate's parameters, so some key exchange methods might not be
available with all 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, that require
decryption. It is not recommended to use RSA keys for both
signing and encryption. If possible use a different key for the
@code{DHE-RSA} which uses signing and @code{RSA} that requires decryption.
All the key exchange methods shown in @ref{tab:key-exchange} are
available in certificate authentication.


@subsubheading Client certificate authentication

If a certificate is to be requested from the client during the handshake, the server
will send a certificate request message. This behavior is controlled @funcref{gnutls_certificate_server_set_request}.
The request contains a list of the acceptable by the server certificate signers. This list
is constructed using the trusted certificate authorities of the server.
In cases where the server supports a large number of certificate authorities
it makes sense not to advertise all of the names to save bandwidth. That can
be controlled using the function @funcref{gnutls_certificate_send_x509_rdn_sequence}. 
This however will have the side-effect of not restricting the client to certificates
signed by server's acceptable signers.

@showfuncdesc{gnutls_certificate_server_set_request}

@showfuncdesc{gnutls_certificate_send_x509_rdn_sequence}

On the client side, it needs to set its certificates on the credentials
structure, similarly to server side from a file, or via a callback. Once the
certificates are available in the credentials structure, the client will
send them if during the handshake the server requests a certificate signed
by the issuer of its CA.

In the case a single certificate is available and the server does not
specify a signer's list, then that certificate is always sent. It is,
however possible, to send a certificate even when the advertised CA
list by the server contains CAs other than its signer. That can be achieved
using the @code{GNUTLS_FORCE_CLIENT_CERT} flag in @funcref{gnutls_init}.

@showfuncC{gnutls_certificate_set_x509_key_file,gnutls_certificate_set_x509_simple_pkcs12_file,gnutls_certificate_set_retrieve_function2}

@subsubheading Client or server certificate verification

Certificate verification is possible by loading the trusted
authorities into the credentials structure by using
the following functions, applicable to X.509 certificates.
In modern systems it is recommended to utilize @funcref{gnutls_certificate_set_x509_system_trust}
which will load the trusted authorities from the system store.

@showfuncdesc{gnutls_certificate_set_x509_system_trust}
@showfuncB{gnutls_certificate_set_x509_trust_file,gnutls_certificate_set_x509_trust_dir}

The peer's certificate will be automatically verified if
@funcref{gnutls_session_set_verify_cert} is called prior to handshake.

Alternatively, one must set a callback function during the handshake
using @funcref{gnutls_certificate_set_verify_function}, which
will verify the peer's certificate once received. The verification
should happen using @funcref{gnutls_certificate_verify_peers3} within
the callback. It will verify the certificate's signature and the owner 
of the certificate. That will provide a brief verification output. If a
detailed output is required one should call @funcref{gnutls_certificate_get_peers}
to obtain the raw certificate of the peer and verify it using the
functions discussed in @ref{X.509 certificates}.

In both the automatic and the manual cases, the verification status returned
can be printed using @funcref{gnutls_certificate_verification_status_print}.

@showfuncdesc{gnutls_session_set_verify_cert}

@showfuncB{gnutls_certificate_verify_peers3,gnutls_certificate_set_verify_function}


@node SRP credentials
@subsection SRP

The initialization functions in SRP credentials differ between
client and server.
Clients supporting @acronym{SRP} should set the username and password
prior to connection, to the credentials structure.
Alternatively @funcref{gnutls_srp_set_client_credentials_function}
may be used instead, to specify a callback function that should return the
SRP username and password.
The callback is called once during the @acronym{TLS} handshake.

@showfuncE{gnutls_srp_allocate_server_credentials,gnutls_srp_allocate_client_credentials,gnutls_srp_free_server_credentials,gnutls_srp_free_client_credentials,gnutls_srp_set_client_credentials}

@showfuncdesc{gnutls_srp_set_client_credentials_function}

In server side the default behavior of @acronym{GnuTLS} is to read
the usernames and @acronym{SRP} verifiers from password files. These
password file format is compatible the with the @emph{Stanford srp libraries}
format.  If a different password file format is to be used, then 
@funcref{gnutls_srp_set_server_credentials_function} should be called,
to set an appropriate callback. 

@showfuncdesc{gnutls_srp_set_server_credentials_file}

@showfuncdesc{gnutls_srp_set_server_credentials_function}


@node PSK credentials
@subsection PSK
The initialization functions in PSK credentials differ between
client and server.

@showfuncD{gnutls_psk_allocate_server_credentials,gnutls_psk_allocate_client_credentials,gnutls_psk_free_server_credentials,gnutls_psk_free_client_credentials}

Clients supporting @acronym{PSK} should supply the username and key
before a TLS session is established.  Alternatively 
@funcref{gnutls_psk_set_client_credentials_function} can be used to
specify a callback function. This has the
advantage that the callback will be called only if @acronym{PSK} has
been negotiated.

@showfuncA{gnutls_psk_set_client_credentials}

@showfuncdesc{gnutls_psk_set_client_credentials_function}

In server side the default behavior 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 @funcref{gnutls_psk_set_server_credentials_file}.  If
a different password file format is to be used, then
a callback should be set instead by @funcref{gnutls_psk_set_server_credentials_function}.

The server can help the client chose a suitable username and password,
by sending a hint. Note that there is no common profile for the PSK hint and applications
are discouraged to use it.
A server, may specify the hint by calling
@funcref{gnutls_psk_set_server_credentials_hint}.  The client can retrieve
the hint, for example in the callback function, using
@funcref{gnutls_psk_client_get_hint}.

@showfuncdesc{gnutls_psk_set_server_credentials_file}

@showfuncC{gnutls_psk_set_server_credentials_function,gnutls_psk_set_server_credentials_hint,gnutls_psk_client_get_hint}

@node Anonymous credentials
@subsection Anonymous
The key exchange methods for anonymous authentication
since GnuTLS 3.6.0 will utilize the RFC7919 parameters, unless
explicit parameters have been provided and associated with an
anonymous credentials structure. Check @ref{Parameter generation} for more information.
The initialization functions for the credentials are shown below.

@showfuncD{gnutls_anon_allocate_server_credentials,gnutls_anon_allocate_client_credentials,gnutls_anon_free_server_credentials,gnutls_anon_free_client_credentials}



@node Setting up the transport layer
@section Setting up the transport layer

The next step is to setup the underlying transport layer details. The
Berkeley sockets are implicitly used by GnuTLS, thus a
call to @funcref{gnutls_transport_set_int} would be sufficient to
specify the socket descriptor. 

@showfuncB{gnutls_transport_set_int,gnutls_transport_set_int2}

If however another transport layer than TCP is selected, then
a pointer should be used instead to express the parameter to be
passed to custom functions. In that case the following functions should
be used instead.

@showfuncB{gnutls_transport_set_ptr,gnutls_transport_set_ptr2}

Moreover all of the following push and pull callbacks should be set.

@showfuncdesc{gnutls_transport_set_push_function}
@showfuncdesc{gnutls_transport_set_vec_push_function}
@showfuncdesc{gnutls_transport_set_pull_function}
@showfuncdesc{gnutls_transport_set_pull_timeout_function}


The functions above accept a callback function which
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 in other
systems it may be a non thread-local variable.  If this is a concern to
you, call @funcref{gnutls_transport_set_errno} with the intended errno
value instead of setting @code{errno} directly.

@showfuncdesc{gnutls_transport_set_errno}

@acronym{GnuTLS} currently only interprets the EINTR, EAGAIN and EMSGSIZE errno
values and returns the corresponding @acronym{GnuTLS} error codes:
@itemize
@item @code{GNUTLS_E_INTERRUPTED} 
@item @code{GNUTLS_E_AGAIN}
@item @code{GNUTLS_E_LARGE_PACKET}
@end itemize
The EINTR and EAGAIN values are returned by interrupted system calls, 
or when non blocking IO is used.  All @acronym{GnuTLS} functions can be 
resumed (called again), if any of the above error codes is returned. The
EMSGSIZE value is returned when attempting to send a large datagram.

In the case of DTLS it is also desirable to override the generic 
transport functions with functions that emulate the operation
of @code{recvfrom} and @code{sendto}. In addition
@acronym{DTLS} requires timers during the receive of a handshake
message, set using the @funcref{gnutls_transport_set_pull_timeout_function} 
function. To check the retransmission timers the function
@funcref{gnutls_dtls_get_timeout} is provided, which returns the time
remaining until the next retransmission, or better the time until 
@funcref{gnutls_handshake} should be called again.

@showfuncdesc{gnutls_transport_set_pull_timeout_function}
@showfuncdesc{gnutls_dtls_get_timeout}

@menu
* Asynchronous operation::
* Reducing round-trips::
* DTLS sessions::
* DTLS and SCTP::
@end menu

@node Asynchronous operation
@subsection Asynchronous operation

@acronym{GnuTLS} can be used with asynchronous socket or event-driven programming.
The approach is similar to using Berkeley sockets under such an environment.
The blocking, due to network interaction, calls such as
@funcref{gnutls_handshake}, @funcref{gnutls_record_recv},
can be set to non-blocking by setting the underlying sockets to non-blocking.
If other push and pull functions are setup, then they should behave the same
way as @funcintref{recv} and @funcintref{send} when used in a non-blocking
way, i.e., return -1 and set errno to @code{EAGAIN}. Since, during a TLS protocol session 
@acronym{GnuTLS} does not block except for network interaction, the non blocking
@code{EAGAIN} errno will be propagated and @acronym{GnuTLS} functions 
will return the @code{GNUTLS_E_AGAIN} error code. Such calls can be resumed the 
same way as a system call would. 
The only exception is @funcref{gnutls_record_send},
which if interrupted subsequent calls need not to include the data to be
sent (can be called with NULL argument).

When using the @funcintref{poll} or @funcintref{select} system calls though, one should remember
that they only apply to the kernel sockets API. To check for any
available buffered data in a @acronym{GnuTLS} session, 
utilize @funcref{gnutls_record_check_pending},
either before the @funcintref{poll} system call, or after a call to
@funcref{gnutls_record_recv}. Data queued by @funcref{gnutls_record_send}
(when interrupted) can be discarded using @funcref{gnutls_record_discard_queued}.

An example of GnuTLS' usage with asynchronous operation can be found
in @code{doc/examples/tlsproxy}.

The following paragraphs describe the detailed requirements for non-blocking
operation when using the TLS or DTLS protocols.

@subsubsection TLS protocol
There are no special requirements for the TLS protocol operation in non-blocking
mode if a non-blocking socket is used.

It is recommended, however, for future compatibility, when in non-blocking mode, to
call the @funcref{gnutls_init} function with the
@code{GNUTLS_NONBLOCK} flag set (see @ref{Session initialization}).

@subsubsection Datagram TLS protocol
When in non-blocking mode the function, the @funcref{gnutls_init} function 
must be called with the @code{GNUTLS_NONBLOCK} flag set (see @ref{Session initialization}). 

In contrast with the TLS protocol, the pull timeout function is required,
but will only be called with a timeout of zero. In that case it should indicate
whether there are data to be received or not. When not using the default pull function,
then @funcref{gnutls_transport_set_pull_timeout_function} should be called.

Although in the TLS protocol implementation each call to receive or send
function implies to restoring the same function that was interrupted, in
the DTLS protocol this requirement isn't true.
There are cases where a retransmission is required, which are indicated by
a received message and thus @funcref{gnutls_record_get_direction} must be called 
to decide which direction to check prior to restoring a function call.
@showfuncdesc{gnutls_record_get_direction}

When calling @funcref{gnutls_handshake} through a multi-plexer,
to be able to handle properly the DTLS handshake retransmission timers,
the function @funcref{gnutls_dtls_get_timeout}
should be used to estimate when to call @funcref{gnutls_handshake} if
no data have been received.

@node Reducing round-trips
@subsection Reducing round-trips

The full TLS 1.2 handshake requires 2 round-trips to complete, and when
combined with TCP's SYN and SYN-ACK negotiation it extends to 3 full
round-trips. While, TLS 1.3 reduces that to two round-trips when under TCP,
it still adds considerable latency, making the protocol unsuitable for
certain applications.

To optimize the handshake latency, in client side, it is possible to take
advantage of the TCP fast open @xcite{RFC7413} mechanism on operating
systems that support it. That can be done either by manually crafting the push and pull
callbacks, or by utilizing @funcref{gnutls_transport_set_fastopen}. In that
case the initial TCP handshake is eliminated, reducing the TLS 1.2 handshake round-trip
to 2, and the TLS 1.3 handshake to a single round-trip.
Note, that when this function is used, any connection failures will be reported during the
@funcref{gnutls_handshake} function call with error code @code{GNUTLS_E_PUSH_ERROR}.

@showfuncdesc{gnutls_transport_set_fastopen}

When restricted to TLS 1.2, and non-resumed sessions, it is possible to further
reduce the round-trips to a single one by taking advantage of the @ref{False Start}
TLS extension. This can be enabled by setting the @acronym{GNUTLS_ENABLE_FALSE_START}
flag on @funcref{gnutls_init}.

Under TLS 1.3, the server side can start transmitting before the handshake
is complete (i.e., while the client Finished message is still in flight),
when no client certificate authentication is requested. This, unlike false
start, is part of protocol design with no known security implications.
It can be enabled by setting the @acronym{GNUTLS_ENABLE_EARLY_START} on
@funcref{gnutls_init}, and the @funcref{gnutls_handshake} function will
return early, allowing the server to send data earlier.


@node DTLS sessions
@subsection DTLS sessions

Because datagram TLS can operate over connections where the client
cannot be reliably verified, functionality in the form of cookies, is available to prevent
denial of service attacks to servers. @acronym{GnuTLS} requires a server
to generate a secret key that is used to sign a cookie@footnote{A key of 128 bits or 16 bytes should be sufficient for this purpose.}. 
That cookie is sent to the client using @funcref{gnutls_dtls_cookie_send}, and 
the client must reply using the correct cookie. The server side
should verify the initial message sent by client using @funcref{gnutls_dtls_cookie_verify}.
If successful the session should be initialized and associated with
the cookie using @funcref{gnutls_dtls_prestate_set}, before proceeding to
the handshake.

@showfuncD{gnutls_key_generate,gnutls_dtls_cookie_send,gnutls_dtls_cookie_verify,gnutls_dtls_prestate_set}

Note that the above apply to server side only and they are not mandatory to be
used. Not using them, however, allows denial of service attacks.
The client side cookie handling is part of @funcref{gnutls_handshake}. 

Datagrams are typically restricted by a maximum transfer unit (MTU). For that
both client and server side should set the correct maximum transfer unit for
the layer underneath @acronym{GnuTLS}. This will allow proper fragmentation
of DTLS messages and prevent messages from being silently discarded by the
transport layer. The ``correct'' maximum transfer unit can be obtained through
a path MTU discovery mechanism @xcite{RFC4821}.

@showfuncC{gnutls_dtls_set_mtu,gnutls_dtls_get_mtu,gnutls_dtls_get_data_mtu}

@node DTLS and SCTP
@subsection DTLS and SCTP

Although DTLS can run under any reliable or unreliable layer, there are
special requirements for SCTP according to @xcite{RFC6083}. We summarize the
most important below, however for a full treatment we refer to @xcite{RFC6083}.

@itemize
@item The MTU set via @funcref{gnutls_dtls_set_mtu} must be 2^14.
@item Replay detection must be disabled; use the flag @code{GNUTLS_NO_REPLAY_PROTECTION} with @funcref{gnutls_init}.
@item Retransmission of messages must be disabled; use @funcref{gnutls_dtls_set_timeouts}
   with a retransmission timeout larger than the total.
@item Handshake, Alert and ChangeCipherSpec messages must be sent over stream 0 with unlimited reliability
   and with the ordered delivery feature.
@item During a rehandshake, the caching of messages with unknown epoch is
   not handled by GnuTLS; this must be implemented in a special pull function.
@end itemize

@node TLS handshake
@section TLS handshake
Once a session has been initialized and a network
connection has been set up, TLS and DTLS protocols
perform a handshake. The handshake is the actual key
exchange.

@showfuncdesc{gnutls_handshake}

@showfuncdesc{gnutls_handshake_set_timeout}

In GnuTLS 3.5.0 and later it is recommended to use @funcref{gnutls_session_set_verify_cert}
for the handshake process to ensure the verification of the peer's identity.
That will verify the peer's certificate, against the trusted CA store while
accounting for stapled OCSP responses during the handshake; any error will
be returned as a handshake error.

In older GnuTLS versions it is required to verify the peer's certificate
during the handshake by setting a callback with @funcref{gnutls_certificate_set_verify_function},
and then using @funcref{gnutls_certificate_verify_peers3} from it. See @ref{Certificate authentication}
for more information.

@showfuncB{gnutls_session_set_verify_cert,gnutls_certificate_verify_peers3}

@node Data transfer and termination
@section Data transfer and termination
Once the handshake is complete and peer's identity
has been verified data can be exchanged. The available
functions resemble the POSIX @code{recv} and @code{send}
functions. It is suggested to use @funcref{gnutls_error_is_fatal}
to check whether the error codes returned by these functions are
fatal for the protocol or can be ignored.

@showfuncdesc{gnutls_record_send}

@showfuncdesc{gnutls_record_recv}

@showfuncdesc{gnutls_error_is_fatal}

Although, in the TLS protocol the receive function can be called
at any time, when DTLS is used the GnuTLS receive functions must be
called once a message is available for reading, even if no data are
expected. This is because in DTLS various (internal) actions
may be required due to retransmission timers. Moreover,
an extended receive function is shown below, which allows the extraction
of the message's sequence number. Due to the unreliable nature of the
protocol, this field allows distinguishing out-of-order messages.

@showfuncdesc{gnutls_record_recv_seq}

The @funcref{gnutls_record_check_pending} helper function is available to 
allow checking whether data are available to be read in a @acronym{GnuTLS} session 
buffers. Note that this function complements but does not replace @funcintref{poll},
i.e., @funcref{gnutls_record_check_pending} reports no data to be read, @funcintref{poll}
should be called to check for data in the network buffers.

@showfuncdesc{gnutls_record_check_pending}
@showfuncA{gnutls_record_get_direction}

Once a TLS or DTLS session is no longer needed, it is
recommended to use @funcref{gnutls_bye} to terminate the
session. That way the peer is notified securely about the
intention of termination, which allows distinguishing it
from a malicious connection termination.
A session can be deinitialized with the @funcref{gnutls_deinit} function.

@showfuncdesc{gnutls_bye}
@showfuncdesc{gnutls_deinit}

@node Buffered data transfer
@section Buffered data transfer

Although @funcref{gnutls_record_send} is sufficient to transmit data
to the peer, when many small chunks of data are to be transmitted
it is inefficient and wastes bandwidth due to the TLS record
overhead. In that case it is preferrable to combine the small chunks
before transmission. The following functions provide that functionality.

@showfuncdesc{gnutls_record_cork}

@showfuncdesc{gnutls_record_uncork}


@node Handling alerts
@section Handling alerts
During a TLS connection alert messages may be exchanged by the
two peers. Those messages may be fatal, meaning the connection
must be terminated afterwards, or warning when something needs
to be reported to the peer, but without interrupting the session.
The error codes @code{GNUTLS_E_@-WARNING_@-ALERT_@-RECEIVED}
or @code{GNUTLS_E_@-FATAL_@-ALERT_@-RECEIVED} signal those alerts
when received, and may be returned by all GnuTLS functions that receive 
data from the peer, being @funcref{gnutls_handshake} and @funcref{gnutls_record_recv}.

If those error codes are received the alert and its level should be logged
or reported to the peer using the functions below.

@showfuncdesc{gnutls_alert_get}
@showfuncdesc{gnutls_alert_get_name}

The peer may also be warned or notified of a fatal issue
by using one of the functions below. All the available alerts
are listed in @ref{The Alert Protocol}.

@showfuncdesc{gnutls_alert_send}
@showfuncdesc{gnutls_error_to_alert}


@node Priority Strings
@section Priority strings
@cindex Priority strings

@subheading How to use Priority Strings

The GnuTLS priority strings specify the TLS session's handshake
algorithms and options in a compact, easy-to-use format. These
strings are intended as a user-specified override of the library defaults.

That is, we recommend applications using the default settings
(c.f. @funcref{gnutls_set_default_priority} or
@funcref{gnutls_set_default_priority_append}), and provide the user 
with access to priority strings for overriding the default behavior,
on configuration files, or other UI. Following such a principle,
makes the GnuTLS library as the default settings provider. That is
necessary and a good practice, because TLS protocol hardening and
phasing out of legacy algorithms, is easier to co-ordinate when happens
in a single library.

@showfuncC{gnutls_set_default_priority,gnutls_set_default_priority_append,gnutls_priority_set_direct}

The priority string translation to the internal GnuTLS form requires
processing and the generated internal form also occupies some memory.
For that, it is recommended to do that processing once in server side,
and share the generated data across sessions. The following functions
allow the generation of a "priority cache" and the sharing of it across
sessions.

@showfuncD{gnutls_priority_init2,gnutls_priority_init,gnutls_priority_set,gnutls_priority_deinit}

@subheading Using Priority Strings

A priority string string may contain a single initial keyword such as in
@ref{tab:prio-keywords} and may be followed by additional algorithm or
special keywords. Note that their description is intentionally avoiding
specific algorithm details, as the priority strings are not constant between
gnutls versions (they are periodically updated to account for cryptographic
advances while providing compatibility with old clients and servers).

@float Table,tab:prio-keywords
@multitable @columnfractions .20 .70
@headitem Keyword @tab Description
@item @@KEYWORD @tab
Means that a compile-time specified system configuration file@footnote{The default is @code{/etc/gnutls/default-priorities}.} 
will be used to expand the provided keyword. That is used to impose system-specific policies.
It may be followed by additional options that will be appended to the
system string (e.g., "@@SYSTEM:+SRP"). The system file should have the
format 'KEYWORD=VALUE', e.g., 'SYSTEM=NORMAL:+ARCFOUR-128'.

Since version 3.5.1 it is allowed to specify fallback keywords such
as @@KEYWORD1,@@KEYWORD2, and the first valid keyword will be used.

@item PERFORMANCE @tab
All the known to be secure ciphersuites are enabled,
limited to 128 bit ciphers and sorted by terms of speed
performance. The message authenticity security level is of 64 bits or more,
and the certificate verification profile is set to GNUTLS_PROFILE_LOW (80-bits).

@item NORMAL @tab
Means all the known to be secure ciphersuites. The ciphers are sorted by security
margin, although the 256-bit ciphers are included as a fallback only.
The message authenticity security level is of 64 bits or more,
and the certificate verification profile is set to GNUTLS_PROFILE_LOW (80-bits).

This priority string implicitly enables ECDHE and DHE. The ECDHE ciphersuites
are placed first in the priority order, but due to compatibility 
issues with the DHE ciphersuites they are placed last in the priority order,
after the plain RSA ciphersuites.

@item LEGACY @tab
This sets the NORMAL settings that were used for GnuTLS 3.2.x or earlier. There is
no verification profile set, and the allowed DH primes are considered
weak today (but are often used by misconfigured servers).

@item PFS @tab
Means all the known to be secure ciphersuites that support perfect forward
secrecy (ECDHE and DHE). The ciphers are sorted by security
margin, although the 256-bit ciphers are included as a fallback only.
The message authenticity security level is of 80 bits or more,
and the certificate verification profile is set to GNUTLS_PROFILE_LOW (80-bits).
This option is available since 3.2.4 or later.

@item SECURE128 @tab
Means all known to be secure ciphersuites that offer a 
security level 128-bit or more.
The message authenticity security level is of 80 bits or more,
and the certificate verification profile is set to GNUTLS_PROFILE_LOW (80-bits).

@item SECURE192 @tab
Means all the known to be secure ciphersuites that offer a 
security level 192-bit or more.
The message authenticity security level is of 128 bits or more,
and the certificate verification profile is set to GNUTLS_PROFILE_HIGH (128-bits).

@item SECURE256 @tab
Currently alias for SECURE192. This option, will enable ciphers which use a
256-bit key but, due to limitations of the TLS protocol, the overall security
level will be 192-bits (the security level depends on more factors than cipher key size).

@item SUITEB128 @tab
Means all the NSA Suite B cryptography (RFC5430) ciphersuites
with an 128 bit security level, as well as the enabling of the corresponding
verification profile.

@item SUITEB192 @tab
Means all the NSA Suite B cryptography (RFC5430) ciphersuites
with an 192 bit security level, as well as the enabling of the corresponding
verification profile.

@item NONE @tab
Means nothing is enabled.  This disables even protocol versions.
It should be followed by the algorithms to be enabled.

@end multitable
@caption{Supported initial keywords.}
@end float

Unless the initial keyword is "NONE" the defaults (in preference
order) are for TLS protocols TLS 1.2, TLS1.1, TLS1.0;
for certificate types X.509.
In key exchange algorithms when in NORMAL or SECURE levels the
perfect forward secrecy algorithms take precedence of the other
protocols.  In all cases all the supported key exchange algorithms
are enabled.

Note that the SECURE levels distinguish between overall security level and
message authenticity security level. That is because the message
authenticity security level requires the adversary to break
the algorithms at real-time during the protocol run, whilst 
the overall security level refers to off-line adversaries 
(e.g. adversaries breaking the ciphertext years after it was captured).

The NONE keyword, if used, must followed by keywords specifying 
the algorithms and protocols to be enabled. The other initial keywords 
do not require, but may be followed by such keywords. All level keywords
can be combined, and for example a level of "SECURE256:+SECURE128" is
allowed.

The order with which every algorithm or protocol
is specified is significant. Algorithms specified before others
will take precedence. The supported in the GnuTLS version corresponding
to this document algorithms and protocols are shown in @ref{tab:prio-algorithms};
to list the supported algorithms in your currently using version use
@code{gnutls-cli -l}.

To avoid collisions in order to specify a protocol version
with "VERS-", signature algorithms with "SIGN-" and certificate types with "CTYPE-". 
All other algorithms don't need a prefix. Each specified keyword (except
for @emph{special keywords}) can be prefixed with any of the following
characters.

@table @asis
@item '!' or '-' 
appended with an algorithm will remove this algorithm.
@item "+" 
appended with an algorithm will add this algorithm.
@end table

@float Table,tab:prio-algorithms
@multitable @columnfractions .20 .70
@headitem Type @tab Keywords
@item Ciphers @tab
Examples are AES-128-GCM, AES-256-GCM, AES-256-CBC; see also
@ref{tab:ciphers} for more options. Catch all name is CIPHER-ALL which will add
all the algorithms from NORMAL priority.

@item Key exchange @tab
RSA, DHE-RSA, DHE-DSS, SRP, SRP-RSA, SRP-DSS,
PSK, DHE-PSK, ECDHE-PSK, ECDHE-RSA, ECDHE-ECDSA, ANON-ECDH, ANON-DH. The
Catch all name is KX-ALL which will add all the algorithms from NORMAL
priority. Under TLS1.3, the DHE-PSK and ECDHE-PSK strings are equivalent
and instruct for a Diffie-Hellman key exchange using the enabled groups.

@item MAC @tab
MD5, SHA1, SHA256, SHA384, AEAD (used with
GCM ciphers only). All algorithms from NORMAL priority can be accessed with MAC-ALL.

@item Compression algorithms @tab
COMP-NULL, COMP-DEFLATE. Catch all is COMP-ALL.

@item TLS versions @tab
VERS-TLS1.0, VERS-TLS1.1, VERS-TLS1.2, VERS-TLS1.3,
VERS-DTLS1.0, VERS-DTLS1.2. 
Catch all are VERS-ALL, and will enable
all protocols from NORMAL priority. To distinguish between TLS and DTLS
versions you can use VERS-TLS-ALL and VERS-DTLS-ALL.

@item Signature algorithms @tab
SIGN-RSA-SHA1, SIGN-RSA-SHA224,
SIGN-RSA-SHA256, SIGN-RSA-SHA384, SIGN-RSA-SHA512, SIGN-DSA-SHA1,
SIGN-DSA-SHA224, SIGN-DSA-SHA256, SIGN-RSA-MD5, SIGN-ECDSA-SHA1,
SIGN-ECDSA-SHA224, SIGN-ECDSA-SHA256, SIGN-ECDSA-SHA384, SIGN-ECDSA-SHA512,
SIGN-RSA-PSS-SHA256, SIGN-RSA-PSS-SHA384, SIGN-RSA-PSS-SHA512.
Catch all which enables all algorithms from NORMAL priority is SIGN-ALL.
This option is only considered for TLS 1.2 and later.

@item Groups @tab
GROUP-SECP256R1, GROUP-SECP384R1, GROUP-SECP521R1, GROUP-X25519,
GROUP-FFDHE2048, GROUP-FFDHE3072, GROUP-FFDHE4096, GROUP-FFDHE6144, and
GROUP-FFDHE8192.
Groups include both elliptic curve groups, e.g., SECP256R1, as well as 
finite field groups such as FFDHE2048. Catch all which enables all groups
from NORMAL priority is GROUP-ALL. The helper keywords GROUP-DH-ALL and
GROUP-EC-ALL are also available, restricting the groups to finite fields
(DH) and elliptic curves.

@item Elliptic curves (legacy) @tab
CURVE-SECP192R1, CURVE-SECP224R1, CURVE-SECP256R1, CURVE-SECP384R1,
CURVE-SECP521R1, and CURVE-X25519.
Catch all which enables all curves from NORMAL priority is CURVE-ALL. Note
that the CURVE keyword is kept for backwards compatibility only, for new
applications see the GROUP keyword above.

@end multitable
@caption{The supported algorithm keywords in priority strings.}
@end float

Note that the finite field groups (indicated by the FFDHE prefix) and DHE key
exchange methods are generally slower@footnote{It depends on the group in use.  Groups with
less bits are always faster, but the number of bits ties with the security
parameter.  See @ref{Selecting cryptographic key sizes}
for the acceptable security levels.} than their elliptic curves counterpart
(ECDHE). 

The available special keywords are shown in @ref{tab:prio-special1}
and @ref{tab:prio-special2}. 

@float Table,tab:prio-special1
@multitable @columnfractions .45 .45
@headitem Keyword @tab Description

@item %COMPAT @tab
will enable compatibility mode. It might mean that violations
of the protocols are allowed as long as maximum compatibility with
problematic clients and servers is achieved. More specifically this
string will tolerate packets over the maximum allowed TLS record,
and add a padding to TLS Client Hello packet to prevent it being in the
256-512 range which is known to be causing issues with a commonly used
firewall (see the %DUMBFW option).

@item %DUMBFW @tab
will add a private extension with bogus data that make the client
hello exceed 512 bytes. This avoids a black hole behavior in some
firewalls. This is the @xcite{RFC7685} client hello padding extension, also enabled
with %COMPAT.

@item %NO_EXTENSIONS @tab
will prevent the sending of any TLS extensions in client side. Note
that TLS 1.2 requires extensions to be used, as well as safe
renegotiation thus this option must be used with care.

@item %NO_TICKETS @tab
will prevent the advertizing of the TLS session ticket extension.
This is implied by the PFS keyword.

@item %NO_SESSION_HASH @tab
will prevent the advertizing the TLS extended master secret (session hash)
extension.

@item %SERVER_PRECEDENCE @tab
The ciphersuite will be selected according to server priorities
and not the client's.

@item %SSL3_RECORD_VERSION @tab
will use SSL3.0 record version in client hello.
By default GnuTLS will set the minimum supported version as the
client hello record version (do not confuse that version with the
proposed handshake version at the client hello).

@item %LATEST_RECORD_VERSION @tab
will use the latest TLS version record version in client hello.

@end multitable
@caption{Special priority string keywords.}
@end float

@float Table,tab:prio-special2
@multitable @columnfractions .45 .45
@headitem Keyword @tab Description

@item %STATELESS_COMPRESSION @tab
ignored; no longer used.

@item %DISABLE_WILDCARDS @tab
will disable matching wildcards when comparing hostnames
in certificates.

@item %NO_ETM @tab
will disable the encrypt-then-mac TLS extension (RFC7366). This is
implied by the %COMPAT keyword.

@item %FORCE_ETM @tab
negotiate CBC ciphersuites only when both sides of the connection support
encrypt-then-mac TLS extension (RFC7366).

@item %DISABLE_SAFE_RENEGOTIATION @tab
will completely disable safe renegotiation
completely.  Do not use unless you know what you are doing.

@item %UNSAFE_RENEGOTIATION @tab
will allow handshakes and re-handshakes
without the safe renegotiation extension.  Note that for clients
this mode is insecure (you may be under attack), and for servers it
will allow insecure clients to connect (which could be fooled by an
attacker).  Do not use unless you know what you are doing and want
maximum compatibility.

@item %PARTIAL_RENEGOTIATION @tab
will allow initial handshakes to proceed,
but not re-handshakes.  This leaves the client vulnerable to attack,
and servers will be compatible with non-upgraded clients for
initial handshakes.  This is currently the default for clients and
servers, for compatibility reasons.

@item %SAFE_RENEGOTIATION @tab
will enforce safe renegotiation.  Clients and
servers will refuse to talk to an insecure peer.  Currently this
causes interoperability problems, but is required for full protection.

@item %FALLBACK_SCSV @tab
will enable the use of the fallback signaling cipher suite value in the
client hello.  Note that this should be set only by applications that
try to reconnect with a downgraded protocol version. See RFC7507 for
details.

@item %VERIFY_ALLOW_BROKEN @tab
will allow signatures with known to be broken algorithms (such as MD5 or
SHA1) in certificate chains.

@item %VERIFY_ALLOW_SIGN_RSA_MD5 @tab
will allow RSA-MD5 signatures in certificate chains.

@item %VERIFY_ALLOW_SIGN_WITH_SHA1 @tab
will allow signatures with SHA1 hash algorithm in certificate chains.

@item %VERIFY_DISABLE_CRL_CHECKS @tab
will disable CRL or OCSP checks in the verification of the certificate chain.

@item %VERIFY_ALLOW_X509_V1_CA_CRT @tab
will allow V1 CAs in chains.

@item %PROFILE_(LOW|LEGACY|MEDIUM|HIGH|ULTRA) @tab
require a certificate verification profile the corresponds to the specified
security level, see @ref{tab:key-sizes} for the mappings to values.

@item %PROFILE_(SUITEB128|SUITEB192) @tab
require a certificate verification profile the corresponds to SUITEB. Note
that an initial keyword that enables SUITEB automatically sets the profile.


@end multitable
@caption{More priority string keywords.}
@end float

Finally the ciphersuites enabled by any priority string can be
listed using the @code{gnutls-cli} application (see @ref{gnutls-cli Invocation}), 
or by using the priority functions as in @ref{Listing the ciphersuites in a priority string}.

Example priority strings are:
@example
The system imposed security level:
    "SYSTEM"

The default priority without the HMAC-MD5:
    "NORMAL:-MD5"

Specifying RSA with AES-128-CBC:
    "NONE:+VERS-TLS-ALL:+MAC-ALL:+RSA:+AES-128-CBC:+SIGN-ALL:+COMP-NULL"

Specifying the defaults plus ARCFOUR-128:
    "NORMAL:+ARCFOUR-128"

Enabling the 128-bit secure ciphers, while disabling TLS 1.0:
    "SECURE128:-VERS-TLS1.0"

Enabling the 128-bit and 192-bit secure ciphers, while disabling all TLS versions 
except TLS 1.2:
    "SECURE128:+SECURE192:-VERS-ALL:+VERS-TLS1.2"
@end example

@node Selecting cryptographic key sizes
@section Selecting cryptographic key sizes
@cindex key sizes

Because many algorithms are involved in TLS, it is not easy to set
a consistent security level.  For this reason in @ref{tab:key-sizes} we
present some correspondence between key sizes of symmetric algorithms
and public key algorithms based on @xcite{ECRYPT}. 
Those can be used to generate certificates with
appropriate key sizes as well as select parameters for Diffie-Hellman and SRP
authentication.

@float Table,tab:key-sizes
@multitable @columnfractions .10 .12 .10 .20 .32

@headitem Security bits @tab RSA, DH and SRP parameter size @tab ECC key size @tab Security parameter (profile) @tab Description

@item <64
@tab <768
@tab <128
@tab @code{INSECURE}
@tab Considered to be insecure

@item 64
@tab 768
@tab 128
@tab @code{VERY WEAK}
@tab Short term protection against individuals

@item 72
@tab 1008
@tab 160
@tab @code{WEAK}
@tab Short term protection against small organizations

@item 80
@tab 1024
@tab 160
@tab @code{LOW}
@tab Very short term protection against agencies (corresponds to ENISA legacy level)

@item 96
@tab 1776
@tab 192
@tab @code{LEGACY}
@tab Legacy standard level

@item 112
@tab 2048
@tab 224
@tab @code{MEDIUM}
@tab Medium-term protection

@item 128
@tab 3072
@tab 256
@tab @code{HIGH}
@tab Long term protection (corresponds to ENISA future level)

@item 192
@tab 8192
@tab 384
@tab @code{ULTRA}
@tab Even longer term protection

@item 256
@tab 15424
@tab 512
@tab @code{FUTURE}
@tab Foreseeable future

@end multitable
@caption{Key sizes and security parameters.}
@end float

The first column  provides a security parameter in a number of bits. This
gives an indication of the number of combinations to be tried by an adversary
to brute force a key. For example to test all possible keys in a 112 bit security parameter
@math{2^{112}} combinations have to be tried. For today's technology this is infeasible.
The next two columns correlate the security
parameter with actual bit sizes of parameters for DH, RSA, SRP and ECC algorithms.
A mapping to @code{gnutls_sec_param_t} value is given for each security parameter, on
the next column, and finally a brief description of the level.

@c @showenumdesc{gnutls_sec_param_t,The @code{gnutls_sec_@-param_t} enumeration.}

Note, however, that the values suggested here are nothing more than an
educated guess that is valid today. There are no guarantees 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.
The NIST publication SP 800-57 @xcite{NISTSP80057} contains a similar
table.

When using @acronym{GnuTLS} and a decision on bit sizes for a public
key algorithm is required, use of the following functions is  
recommended:

@showfuncdesc{gnutls_sec_param_to_pk_bits}

@showfuncdesc{gnutls_pk_bits_to_sec_param}

Those functions will convert a human understandable security parameter
of @code{gnutls_sec_param_t} type, to a number of bits suitable for a public 
key algorithm.

@showfuncA{gnutls_sec_param_get_name}

The following functions will set the minimum acceptable group size for Diffie-Hellman
and SRP authentication. 
@showfuncB{gnutls_dh_set_prime_bits,gnutls_srp_set_prime_bits}


@node Advanced topics
@section Advanced topics

@menu
* Virtual hosts and credentials::
* Session resumption::
* Certificate verification::
* TLS 1.2 re-authentication::
* TLS 1.3 re-authentication and re-key::
* Parameter generation::
* Deriving keys for other applications/protocols::
* Channel Bindings::
* Interoperability::
* Compatibility with the OpenSSL library::
@end menu

@node Virtual hosts and credentials
@subsection Virtual hosts and credentials
@cindex virtual hosts
@cindex credentials

Often when operating with virtual hosts, one may not want to associate
a particular certificate set to the credentials function early, before
the virtual host is known. That can be achieved by calling
@funcref{gnutls_credentials_set} within a handshake pre-hook for client
hello. That message contains the peer's intended hostname, and if read,
and the appropriate credentials are set, gnutls will be able to
continue in the handshake process. A brief usage example is shown
below.

@example
static int ext_hook_func(void *ctx, unsigned tls_id,
                         const unsigned char *data, unsigned size)
@{
	if (tls_id == 0) @{ /* server name */
		/* figure the advertized name - the following hack
                 * relies on the fact that this extension only supports
                 * DNS names, and due to a protocol bug cannot be extended
                 * to support anything else. */
		if (name < 5) return 0;
		name = data+5;
		name_size = size-5;
	@}
@}

static int
handshake_hook_func(gnutls_session_t session, unsigned int htype,
                    unsigned when, unsigned int incoming, const gnutls_datum_t *msg)
@{
    assert(htype == GNUTLS_HANDSHAKE_CLIENT_HELLO);
    assert(when == GNUTLS_HOOK_PRE);

    ret = gnutls_ext_raw_parse(NULL, ext_hook_func, msg,
                               GNUTLS_EXT_RAW_FLAG_TLS_CLIENT_HELLO);
    assert(ret >= 0);

    gnutls_credentials_set(session, GNUTLS_CRD_CERTIFICATE, cred);
@}

int main()
@{
  ...

  gnutls_handshake_set_hook_function(server, GNUTLS_HANDSHAKE_CLIENT_HELLO,
                                     GNUTLS_HOOK_PRE, handshake_hook_func);
  ...
@}
@end example

@showfuncdesc{gnutls_handshake_set_hook_function}

@node Session resumption
@subsection Session resumption
@cindex resuming sessions
@cindex session resumption

To reduce time and network traffic spent in a handshake the client can
request session resumption from a server that previously shared a
session with the client.

Under TLS 1.2, in order to support resumption a server can either store
the session security parameters in a local database or use session
tickets (see @ref{Session tickets}) to delegate storage to the client.

Under TLS 1.3, session resumption is only available through session
tickets, and multiple tickets could be sent from server to client. That
provides the following advantages:
@itemize
@item When tickets are not re-used the subsequent client sessions cannot be associated with each other by an eavesdropper
@item On post-handshake authentication the server may send different tickets asynchronously for each identity used by client.
@end itemize

@subsubheading Client side

The client has to retrieve and store the session parameters. Before
establishing a new session to the same server the parameters must be
re-associated with the GnuTLS session using
@funcref{gnutls_session_set_data}.

@showfuncB{gnutls_session_get_data2,gnutls_session_set_data}

Keep in mind that sessions will be expired after some time, depending
on the server, and a server may choose not to resume a session
even when requested to.  The expiration is to prevent temporal session keys
from becoming long-term keys. Also note that as a client you must enable, 
using the priority functions, at least the algorithms used in the last session.

@showfuncdesc{gnutls_session_is_resumed}

@showfuncdesc{gnutls_session_get_id2}

@subsubheading Server side

A server enabling both session tickets and a storage for session data
would use session tickets when clients support it and the storage otherwise.

A storing server needs to specify callback functions to store, retrieve and delete session data. These can be
registered with the functions below. The stored sessions in the database can be checked using @funcref{gnutls_db_check_entry}
for expiration.

@showfuncD{gnutls_db_set_retrieve_function,gnutls_db_set_store_function,gnutls_db_set_ptr,gnutls_db_set_remove_function}
@showfuncA{gnutls_db_check_entry}

A server supporting session tickets must generate ticket encryption
and authentication keys using @funcref{gnutls_session_ticket_key_generate}.
Those keys should be associated with the GnuTLS session using
@funcref{gnutls_session_ticket_enable_server}, and should be rotated regularly
(e.g., every few hours), to prevent them from becoming long-term keys which
if revealed could be used to decrypt all previous sessions.

@showfuncdesc{gnutls_session_ticket_enable_server}
@showfuncdesc{gnutls_session_ticket_key_generate}
@showfuncdesc{gnutls_session_resumption_requested}

The expiration time for session resumption, either in tickets or stored data
is set using @funcref{gnutls_db_set_cache_expiration}.

Under TLS 1.3, the server can send a new session ticket at any time
using @funcref{gnutls_session_ticket_send}.

@showfuncdesc{gnutls_session_ticket_send}

@node Certificate verification
@subsection Certificate verification
@cindex DANE
@cindex DNSSEC
@cindex SSH-style authentication
@cindex Trust on first use
@cindex Key pinning
@tindex gnutls_certificate_verify_flags

In this section the functionality for additional certificate verification methods is listed. 
These methods are intended to be used in addition to normal PKI verification, in order to reduce 
the risk of a compromised CA being undetected.

@subsubsection Trust on first use

The GnuTLS library includes functionality to use an SSH-like trust on first use authentication.
The available functions to store and verify public keys are listed below.

@showfuncdesc{gnutls_verify_stored_pubkey}
@showfuncdesc{gnutls_store_pubkey}

In addition to the above the @funcref{gnutls_store_commitment} can be 
used to implement a key-pinning architecture as in @xcite{KEYPIN}. 
This provides a way for web server to commit on a public key that is
not yet active.

@showfuncdesc{gnutls_store_commitment}

The storage and verification functions may be used with the default
text file based back-end, or another back-end may be specified. That
should contain storage and retrieval functions and specified as below.

@showfuncE{gnutls_tdb_init,gnutls_tdb_deinit,gnutls_tdb_set_verify_func,gnutls_tdb_set_store_func,gnutls_tdb_set_store_commitment_func}

@subsubsection DANE verification
Since the DANE library is not included in GnuTLS it requires programs
to be linked against it. This can be achieved with the following commands.

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

When a program uses the GNU autoconf system, then the following
line or similar can be used to detect the presence of the library.

@example
PKG_CHECK_MODULES([LIBDANE], [gnutls-dane >= 3.0.0])

AC_SUBST([LIBDANE_CFLAGS])
AC_SUBST([LIBDANE_LIBS])
@end example

The high level functionality provided by the DANE library is shown below.

@showfuncdesc{dane_verify_crt}

@showfuncB{dane_verify_session_crt,dane_strerror}

Note that the @code{dane_state_t} structure that is accepted by both
verification functions is optional. It is required when many queries
are performed to optimize against multiple re-initializations of the
resolving back-end and loading of DNSSEC keys.

The following flags are returned by the verify functions to
indicate the status of the verification.

@showenumdesc{dane_verify_status_t,The DANE verification status flags.}

In order to generate a DANE TLSA entry to use in a DNS server 
you may use danetool (see @ref{danetool Invocation}).



@node TLS 1.2 re-authentication
@subsection TLS 1.2 re-authentication
@cindex re-negotiation
@cindex re-authentication

In TLS 1.2 or earlier there is no distinction between re-key, re-authentication, and re-negotiation.
All of these use cases are handled by the TLS' rehandshake process. For that reason
in GnuTLS rehandshake is not transparent to the application, and the application
must explicitly take control of that process. In addition GnuTLS since version 3.5.0 will not
allow the peer to switch identities during a rehandshake.
The threat addressed by that behavior depends on the application protocol,
but primarily it protects applications from being misled
by a rehandshake which switches the peer's identity. Applications can
disable this protection by using the @code{GNUTLS_ALLOW_ID_CHANGE} flag in
@funcref{gnutls_init}.

The following paragraphs explain how to safely use the rehandshake process.

@subsubsection Client side

According to the TLS specification a client may initiate a rehandshake at any
time. That can be achieved by calling @funcref{gnutls_handshake} and rely on its
return value for the outcome of the handshake (the server may deny a rehandshake).
If a server requests a re-handshake, then a call to @funcref{gnutls_record_recv} will
return GNUTLS_E_REHANDSHAKE in the client, instructing it to call @funcref{gnutls_handshake}.
To deny a rehandshake request by the server it is recommended to send a warning alert
of type GNUTLS_A_NO_RENEGOTIATION.

Due to limitations of early protocol versions, it is required to check whether
safe renegotiation is in place, i.e., using @funcref{gnutls_safe_renegotiation_status},
which ensures that the server remains the same as the initial.

@showfuncdesc{gnutls_safe_renegotiation_status}

@subsubsection Server side

A server which wants to instruct the client to re-authenticate, should call
@funcref{gnutls_rehandshake} and wait for the client to re-authenticate.
It is recommended to only request re-handshake when safe renegotiation is
enabled for that session (see @funcref{gnutls_safe_renegotiation_status} and
the discussion in @ref{Safe renegotiation}). A server could also encounter
the GNUTLS_E_REHANDSHAKE error code while receiving data. That indicates
a client-initiated re-handshake request. In that case the server could
ignore that request, perform handshake (unsafe when done generally), or
even drop the connection.

@showfuncdesc{gnutls_rehandshake}

@node TLS 1.3 re-authentication and re-key
@subsection TLS 1.3 re-authentication and re-key
@cindex re-key
@cindex re-negotiation
@cindex re-authentication
@cindex post-handshake authentication

The TLS 1.3 protocol distinguishes between re-key and re-authentication.
The re-key process ensures that fresh keys are supplied to the already
negotiated parameters, and on GnuTLS can be initiated using
@funcref{gnutls_session_key_update}. The re-key process can be one-way
(i.e., the calling party only changes its keys), or two-way where the peer
is requested to change keys as well.

The re-authentication process, allows the connected client to switch
identity by presenting a new certificate. Unlike TLS 1.2, the server
is not allowed to change identities. That client re-authentication, or
post-handshake authentication can be initiated only by the server using
@funcref{gnutls_reauth}, and only if a client has advertized support for it.
Both server and client have to explicitly enable support for post handshake
authentication using the @code{GNUTLS_POST_HANDSHAKE_AUTH} flag at @funcref{gnutls_init}.

A client receiving a re-authentication request will "see" the error code
@code{GNUTLS_E_REAUTH_REQUEST} at @funcref{gnutls_record_recv}. At this
point, it should also call @funcref{gnutls_reauth}.

@node Parameter generation
@subsection Parameter generation
@cindex parameter generation
@cindex generating parameters

Prior to GnuTLS 3.6.0 for the ephemeral or anonymous Diffie-Hellman (DH) TLS ciphersuites
the application was required to generate or provide
DH parameters. That is no longer necessary as GnuTLS utilizes DH parameters
and negotiation from @xcite{RFC7919}.

Applications can tune the used parameters by explicitly specifying them
in the priority string. In server side applications can set the
minimum acceptable level of DH parameters by calling
@funcref{gnutls_certificate_set_known_dh_params},
@funcref{gnutls_anon_set_server_known_dh_params}, or
@funcref{gnutls_psk_set_server_known_dh_params}, depending on the type
of the credentials, to set the lower acceptable parameter limits. Typical
applications should rely on the default settings.

@showfuncC{gnutls_certificate_set_known_dh_params,gnutls_anon_set_server_known_dh_params,gnutls_psk_set_server_known_dh_params}


@subsubsection Legacy parameter generation
Note that older than 3.5.6 versions of GnuTLS provided functions
to generate or import arbitrary DH parameters from a file. This
practice is still supported but discouraged in current versions.
There is no known advantage from using random parameters, while there
have been several occasions where applications were utilizing incorrect,
weak or insecure parameters. This is the main reason GnuTLS includes the
well-known parameters of @xcite{RFC7919} and recommends applications
utilizing them.

In older applications which require to specify explicit DH parameters, we recommend
using @code{certtool} (of GnuTLS 3.5.6 or later) with the @code{--get-dh-params}
option to obtain the FFDHE parameters discussed above. The output
parameters of the tool are in PKCS#3 format and can be imported by
most existing applications.

The following functions are still supported but considered obsolete.

@showfuncC{gnutls_dh_params_generate2,gnutls_dh_params_import_pkcs3,gnutls_certificate_set_dh_params}


@node Deriving keys for other applications/protocols
@subsection Deriving keys for other applications/protocols
@cindex keying material exporters
@cindex exporting keying material
@cindex deriving keys
@cindex key extraction

In several cases, after a TLS connection is established, it is desirable
to derive keys to be used in another application or protocol (e.g., in an
other TLS session using pre-shared keys). The following describe GnuTLS'
implementation of RFC5705 to extract keys based on a session's master secret.

The API to use is @funcref{gnutls_prf_rfc5705}.  The 
function needs to be provided with a label,
and additional context data to mix in the @code{context} parameter. 

@showfuncdesc{gnutls_prf_rfc5705}

For example, after establishing a TLS session using
@funcref{gnutls_handshake}, you can obtain 32-bytes to be used as key, using this call:

@example
#define MYLABEL "EXPORTER-My-protocol-name"
#define MYCONTEXT "my-protocol's-1st-session"

char out[32];
rc = gnutls_prf_rfc5705 (session, sizeof(MYLABEL)-1, MYLABEL,
                         sizeof(MYCONTEXT)-1, MYCONTEXT, 32, out);
@end example

The output key depends on TLS' master secret, and is the same on both client
and server.

For legacy applications which need to use a more flexible API, there is
@funcref{gnutls_prf}, which in addition, allows to switch the mix of the
client and server random nonces, using the @code{server_random_first} parameter.
For additional flexibility and low-level access to the TLS1.2 PRF,
there is a low-level TLS PRF interface called @funcref{gnutls_prf_raw}.
That however is not functional under newer protocol versions.

@node Channel Bindings
@subsection Channel bindings
@cindex channel bindings

In user authentication protocols (e.g., EAP or SASL mechanisms) it is
useful to have a unique string that identifies the secure channel that
is used, to bind together the user authentication with the secure
channel.  This can protect against man-in-the-middle attacks in some
situations.  That unique string is called a ``channel binding''.  For
background and discussion see @xcite{RFC5056}.

In @acronym{GnuTLS} you can extract a channel binding using the
@funcref{gnutls_session_channel_binding} function.  Currently only the
type @code{GNUTLS_CB_TLS_UNIQUE} is supported, which corresponds to
the @code{tls-unique} channel binding for TLS defined in
@xcite{RFC5929}.

The following example describes how to print the channel binding data.
Note that it must be run after a successful TLS handshake.

@example
@{
  gnutls_datum_t cb;
  int rc;

  rc = gnutls_session_channel_binding (session,
                                       GNUTLS_CB_TLS_UNIQUE,
                                       &cb);
  if (rc)
    fprintf (stderr, "Channel binding error: %s\n",
             gnutls_strerror (rc));
  else
    @{
      size_t i;
      printf ("- Channel binding 'tls-unique': ");
      for (i = 0; i < cb.size; i++)
        printf ("%02x", cb.data[i]);
      printf ("\n");
    @}
@}
@end example

@node Interoperability
@subsection Interoperability

The @acronym{TLS} protocols support many ciphersuites, extensions and version
numbers. As a result, few implementations are 
not able to properly interoperate once faced with extensions or version protocols
they do not support and understand. The @acronym{TLS} protocol allows for a
graceful downgrade to the commonly supported options, but practice shows 
it is not always implemented correctly. 

Because there is no way to achieve maximum interoperability with broken peers
without sacrificing security, @acronym{GnuTLS} ignores such peers by default. 
This might not be acceptable in cases where maximum compatibility
is required. Thus we allow enabling compatibility with broken peers using
priority strings (see @ref{Priority Strings}). A conservative priority
string that would disable certain @acronym{TLS} protocol
options that are known to cause compatibility problems, is shown below. 
@verbatim
NORMAL:%COMPAT
@end verbatim

For very old broken peers that do not tolerate TLS version numbers over TLS 1.0
another priority string is:
@verbatim
NORMAL:-VERS-ALL:+VERS-TLS1.0:+VERS-SSL3.0:%COMPAT
@end verbatim
This priority string will in addition to above, only enable SSL 3.0 and 
TLS 1.0 as protocols. 


@node Compatibility with the OpenSSL library
@subsection Compatibility with the OpenSSL library
@cindex OpenSSL

To ease @acronym{GnuTLS}' integration with existing applications, a
compatibility layer with the OpenSSL library is included
in the @code{gnutls-openssl} library. This compatibility layer is not
complete and it is not intended to completely re-implement the OpenSSL
API with @acronym{GnuTLS}.  It only provides limited source-level
compatibility. 

The prototypes for the compatibility functions are in the
@file{gnutls/openssl.h} header file. The limitations 
imposed by the compatibility layer include:

@itemize

@item Error handling is not thread safe.

@end itemize