\input texinfo   @c -*- mode: texinfo; coding: us-ascii; -*-
@setfilename gsasl.info
@include version.texi
@settitle GNU Simple Authentication and Security Layer @value{VERSION}
@finalout

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

@copying
This manual was last updated @value{UPDATED} for version
@value{VERSION} of GNU SASL.

Copyright @copyright{} 2002--2026 Simon Josefsson.

@quotation
Permission is granted to copy, distribute and/or modify this document
under the terms of the GNU Free Documentation License, Version 1.3 or
any later version published by the Free Software Foundation; with no
Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts.  A
copy of the license is included in the section entitled ``GNU Free
Documentation License''.
@end quotation
@end copying

@dircategory Software libraries
@direntry
* GNU SASL: (gsasl).	GNU Simple Authentication and Security Layer Library.
@end direntry

@dircategory Security
@direntry
* gsasl: (gsasl)Invoking gsasl.		Command line interface to GNU SASL.
@end direntry

@titlepage
@title GNU SASL
@subtitle Simple Authentication and Security Layer for the GNU system
@subtitle for version @value{VERSION}, @value{UPDATED}
@author Simon Josefsson
@page
@vskip 0pt plus 1filll
@insertcopying
@end titlepage

@contents

@ifnottex
@node Top
@top GNU Simple Authentication and Security Layer
@insertcopying
@end ifnottex

@menu
* Introduction::                Information about protocol and implementation.
* Preparation::                 What you should do before using the library.
* Using the Library::           High level overview of how to use the library.
* Properties::                  How to specify username, password, etc.
* Mechanisms::                  Mechanism specific information.
* Global Functions::            Functions that can be used at all times.
* Callback Functions::          Set and use callbacks.
* Property Functions::          Specify username, password etc.
* Session Functions::           Perform an authentication.
* Utilities::                   Functions for various odd things.
* Memory Handling::             Memory de-allocation.
* Error Handling::              Error codes and such.
* Examples::                    Demonstrate how to use the library.
* Acknowledgements::            Whom to blame.

* Invoking gsasl::              Command line interface to the library.

Appendices

* Protocol Clarifications::     Our notes on the SASL protocol.
* Copying Information::		How you can copy and share GNU SASL.

Indices

* Function and Data Index::     Index of functions, variables and data types.
* Concept Index::               Index of concepts and programs.
@end menu

@c **********************************************************
@c *******************  Introduction  ***********************
@c **********************************************************
@node Introduction
@chapter Introduction

GNU SASL is an implementation of the Simple Authentication and Security
Layer (SASL) framework and a few common SASL mechanisms.  SASL is used
by network servers (e.g., IMAP, SMTP, XMPP) to request authentication
from clients, and in clients to authenticate against servers.

GNU SASL consists of a C library (libgsasl), a command-line application
(gsasl), and a manual.  The library supports the ANONYMOUS, CRAM-MD5,
DIGEST-MD5, EXTERNAL, GS2-KRB5, GSSAPI, LOGIN, NTLM, OPENID20, PLAIN,
SCRAM-SHA-1, SCRAM-SHA-1-PLUS, SCRAM-SHA-256, SCRAM-SHA-256-PLUS,
SAML20, and SECURID mechanisms.

This manual can be used in several ways.  If read from the beginning to
the end, it gives the reader an understanding of the SASL framework and
the GNU SASL implementation, and how the GNU SASL library is used in an
application.  Forward references are included where necessary.  Later
on, the manual can be used as a reference manual to get just the
information needed about any particular interface of the library.
Experienced programmers might want to start looking at the examples at
the end of the manual, and then only read up those parts of the
interface which are unclear.

@menu
* SASL Overview::
* Implementation::
* Features::
* Requirements::
* Supported Platforms::
* Getting help::
* Commercial Support::
* Downloading and Installing::
* Bug Reports::
* Contributing::
@end menu

@node SASL Overview
@section SASL Overview

SASL is a framework for application protocols, such as SMTP or IMAP,
to add authentication support.  For example, SASL is used to prove to
the server who you are when you access an IMAP server to read your
e-mail.

The SASL framework does not specify the technology used to perform the
authentication, that is the responsibility for each SASL mechanism.
Popular SASL mechanisms include CRAM-MD5 and GSSAPI (for Kerberos V5).

Typically a SASL negotiation works as follows.  First the client
requests authentication (possibly implicitly by connecting to the
server).  The server responds with a list of supported mechanisms.
The client chose one of the mechanisms.  The client and server then
exchange data, one round-trip at a time, until authentication either
succeeds or fails.  After that, the client and server knows more about
who is on the other end of the channel.

For example, in SMTP communication happens like this:

@smallexample
250-mail.example.com Hello pc.example.org [192.168.1.42], pleased to meet you
250-AUTH DIGEST-MD5 CRAM-MD5 LOGIN PLAIN
250 HELP
AUTH CRAM-MD5
334 PDk5MDgwNDEzMDUwNTUyMTE1NDQ5LjBAbG9jYWxob3N0Pg==
amFzIDBkZDRkODZkMDVjNjI4ODRkYzc3OTcwODE4ZGI5MGY3
235 2.0.0 OK Authenticated
@end smallexample

Here the first three lines are sent by the server and contains the
list of supported mechanisms (DIGEST-MD5, CRAM-MD5, etc).  The next
line is sent by the client to select the CRAM-MD5 mechanism.  The
server replies with a challenge, which is a message that can be
generated by calling GNU SASL functions.  The client replies with a
response, which also is a message that can be generated by GNU SASL
functions.  Depending on the mechanism, there can be more than one
round trip, so do not assume all authentication exchanges consists of
one message from the server and one from the client.  The server
accepts the authentication.  At that point it knows it is talking to a
authenticated client, and the application protocol can continue.

Essentially, your application is responsible for implementing the
framing protocol (e.g., SMTP or XMPP) according to the particular
specifications.  Your application uses GNU SASL to generate the
authentication messages.

@node Implementation
@section Implementation

The library is easily ported because it does not do network
communication by itself, but rather leaves it up to the calling
application.  The library is flexible with regards to the
authorization infrastructure used, as it utilizes a callback into the
application to decide whether a user is authorized or not.

GNU SASL is developed for the GNU/Linux system, but runs on over 20
platforms including most major Unix platforms and Windows, and many
kind of devices including iPAQ handhelds and S/390 mainframes.

GNU SASL is written in pure ANSI C89 to be portable to embedded and
otherwise limited platforms.  The entire library, with full support
for ANONYMOUS, EXTERNAL, PLAIN, LOGIN and CRAM-MD5, and the front-end
that supports client and server mode, and the IMAP and SMTP protocols,
fits in under 80kb on an Intel x86 platform, without any modifications
to the code.  (This figure was accurate as of version 1.1.)

The design of the library and the intended interaction between
applications and the library through the official API is illustrated
below.

@float Illustration,fig:abstraction
@image{gsasl-abstraction,10cm,5cm}

@caption{Logical overview showing how applications use authentication
mechanisms through an abstract interface.}

@end float

@node Features
@section Features

GNU SASL might have a couple of advantages over other libraries doing
a similar job.

@table @asis
@item It's Free Software
Anybody can use, modify, and redistribute it under the terms of the
GNU General Public License version 3.0 or later.  The library uses the
GNU Lesser General Public License version 2.1 or later.

@item It's thread-safe
No global variables are used and multiple library handles and session
handles may be used in parallel.

@item It's internationalized
It handles non-ASCII usernames and passwords and user visible strings
used in the library (error messages) can be translated into the users'
language.

@item It's portable
It should work on all Unix like operating systems, including Windows.
The library itself should be portable to any C89 system, not even
POSIX is required.

@item It's small
The library has been rewritten with embedded platforms in mind.  For
example, no API consumes more than around 250 bytes of stack space.

@end table

Note that the library does not implement any policy to decide whether
a certain user is ``authenticated'' or ``authorized'' or not.  Rather,
it uses a callback into the application to answer these questions.

@node Requirements
@section Requirements

The GNU SASL library does not have any required external dependencies,
but some optional features are enabled if you have a specific external
library.

@table @asis
@item LibNTLM
The NTLM mechanism requires the library LibNTLM,
@url{https://www.nongnu.org/libntlm/}.

@item GSS-API
The GSSAPI and GS2-KRB5 mechanisms requires a GSS-API library, see GNU
GSS (@url{https://www.gnu.org/software/gss/}).  Libgssglue, MIT
Kerberos, and Heimdal are also supported.  You are encouraged to try
Libgssglue
(@url{https://blog.josefsson.org/2022/07/14/towards-pluggable-gss-api-modules/}).

@item LibIDN
Processing of non-ASCII usernames and passwords requires the SASLprep
implementation in LibIDN (@url{https://www.gnu.org/software/libidn/}).
This is needed for full conformance with the latest SASL protocol
drafts, but is optional in the library for improved portability.

@item Libgcrypt
The GNU SASL library ships with its own cryptographic implementation,
but it can use the one in libgcrypt (@url{https://www.gnupg.org/})
instead, if it is available.  This is typically useful for desktop
machines which have libgcrypt installed.
@end table

The command-line interface to GNU SASL requires a POSIX or Windows
platform for network connectivity.  The command-line tool can make use
of GnuTLS (@url{https://www.gnutls.org/}) to support the STARTTLS modes
of IMAP and SMTP, but GnuTLS is not required.

Note that the library does not need a POSIX platform or network
connectivity.

@node Supported Platforms
@section Supported Platforms

GNU SASL has at some point in time been tested on the following
platforms.

@enumerate

@item Debian GNU/Linux
@cindex Debian

GCC and GNU Make.  This is the main development platform.
@code{x86_64-linux-gnu}, @code{alphaev67-unknown-linux-gnu},
@code{alphaev6-unknown-linux-gnu}, @code{arm-unknown-linux-gnu},
@code{hppa-unknown-linux-gnu}, @code{hppa64-unknown-linux-gnu},
@code{i686-pc-linux-gnu}, @code{ia64-unknown-linux-gnu},
@code{m68k-unknown-linux-gnu}, @code{mips-unknown-linux-gnu},
@code{mipsel-unknown-linux-gnu}, @code{powerpc-unknown-linux-gnu},
@code{s390-ibm-linux-gnu}, @code{sparc-unknown-linux-gnu},
@code{armv4l-unknown-linux-gnu}.

@item Tru64 UNIX
@cindex Tru64

Tru64 UNIX C compiler and Tru64 Make. @code{alphaev67-dec-osf5.1},
@code{alphaev68-dec-osf5.1}.

@item SuSE Linux 7.1
@cindex SuSE

GCC 2.96 and GNU Make. @code{alphaev6-unknown-linux-gnu},
@code{alphaev67-unknown-linux-gnu}.

@item SuSE Linux 7.2a
@cindex SuSE Linux

GCC 3.0 and GNU Make. @code{ia64-unknown-linux-gnu}.

@item RedHat Linux 7.2
@cindex RedHat

GCC 2.96 and GNU Make. @code{alphaev6-unknown-linux-gnu},
@code{alphaev67-unknown-linux-gnu}, @code{ia64-unknown-linux-gnu}.

@item RedHat Linux 8.0
@cindex RedHat

GCC 3.2 and GNU Make. @code{i686-pc-linux-gnu}.

@item RedHat Advanced Server 2.1
@cindex RedHat Advanced Server

GCC 2.96 and GNU Make. @code{i686-pc-linux-gnu}.

@item Slackware Linux 8.0.01
@cindex RedHat

GCC 2.95.3 and GNU Make. @code{i686-pc-linux-gnu}.

@item Mandrake Linux 9.0
@cindex Mandrake

GCC 3.2 and GNU Make. @code{i686-pc-linux-gnu}.

@item IRIX 6.5
@cindex IRIX

MIPS C compiler, IRIX Make. @code{mips-sgi-irix6.5}.

@item AIX 4.3.2
@cindex AIX

IBM C for AIX compiler, AIX Make.  @code{rs6000-ibm-aix4.3.2.0}.

@item Microsoft Windows 2000 (Cygwin)
@cindex Windows

GCC 3.2, GNU make. @code{i686-pc-cygwin}.

@item HP-UX 11
@cindex HP-UX

HP-UX C compiler and HP Make. @code{ia64-hp-hpux11.22},
@code{hppa2.0w-hp-hpux11.11}.

@item SUN Solaris 2.8
@cindex Solaris

Sun WorkShop Compiler C 6.0 and SUN Make. @code{sparc-sun-solaris2.8}.

@item SUN Solaris 2.9
@cindex Solaris

Sun Forte Developer 7 C compiler and GNU
Make. @code{sparc-sun-solaris2.9}.

@item NetBSD 1.6
@cindex NetBSD

GCC 2.95.3 and GNU Make. @code{alpha-unknown-netbsd1.6},
@code{i386-unknown-netbsdelf1.6}.

@item OpenBSD 3.1 and 3.2
@cindex OpenBSD

GCC 2.95.3 and GNU Make. @code{alpha-unknown-openbsd3.1},
@code{i386-unknown-openbsd3.1}.

@item FreeBSD 4.7
@cindex FreeBSD

GCC 2.95.4 and GNU Make. @code{alpha-unknown-freebsd4.7},
@code{i386-unknown-freebsd4.7}.

@item Cross compiled to uClinux/uClibc on Motorola Coldfire.
@cindex Motorola Coldfire
@cindex uClinux
@cindex uClibc

GCC 3.4 and GNU Make @code{m68k-uclinux-elf}.

@end enumerate

If you port GNU SASL to a new platform, please report it to the author
so this list can be updated.

@node Getting help
@section Getting help

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

@node Commercial Support
@section Commercial Support

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

@itemize

@item Implement new features.
Such as a new SASL mechanism.

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

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

@item System design of components related to SASL.

@end itemize

The following companies have expressed an interest in providing this
support:

@itemize

@item Simon Josefsson Datakonsult AB

Contact through @url{https://josefsson.org/} and email
@email{simon@@josefsson.org}.

@end itemize

If your company provides support related to GNU SASL and would like to
be mentioned here, please let us know (@pxref{Bug Reports}).

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

The package can be downloaded from several places, including:

@url{https://ftp.gnu.org/gnu/gsasl/}

The latest version is stored in a file, e.g.,
@samp{gsasl-@value{VERSION}.tar.gz} where the @samp{@value{VERSION}}
value is the highest version number in the directory.

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.

Here is an example terminal session that downloads, configures, builds
and install the package.  You will need a few basic tools, such as
@samp{sh}, @samp{make} and @samp{cc}.

@example
$ wget -q https://ftp.gnu.org/gnu/gsasl/gsasl-@value{VERSION}.tar.gz
$ tar xfz gsasl-@value{VERSION}.tar.gz
$ cd gsasl-@value{VERSION}/
$ ./configure
...
$ make
...
$ make install
...
@end example

After that gsasl should be properly installed and ready for use.

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

@table @code

@item --disable-client
@itemx --disable-server
If your target system require a minimal implementation, you may wish
to disable the client or the server part of the code.  This does not
remove symbols from the library, so if you attempt to call an
application that uses server functions in a library built with
@code{--disable-server}, the function will return an error code.

@item --disable-anonymous
@itemx --disable-external
@itemx --disable-plain
@itemx --disable-login
@itemx --disable-securid
@itemx --disable-ntlm
@itemx --disable-cram-md5
@itemx --disable-digest-md5
@itemx --disable-gssapi
@itemx --disable-gs2
@itemx --disable-scram-sha1
@itemx --disable-scram-sha256
@itemx --disable-saml20
@itemx --disable-openid20
Disable or enable individual mechanisms (@pxref{Mechanisms}).

@item --without-stringprep
Disable internationalized string processing.  Note that this will result
in a SASL library that is compatible with RFC 2222 but not RFC 4422.

@end table

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

@menu
* Installing under Windows::    Windows specific build instructions.
* Kerberos on Windows::         Building with Kerberos via GSS-API on Windows.
@end menu

@node Installing under Windows
@subsection Installing under Windows

There are two ways to build GNU SASL on Windows: via MinGW or via
Microsoft Visual Studio.  Note that a binary release for Windows is
available from @url{http://josefsson.org/gnutls4win/}.

With MinGW, you can build a GNU SASL DLL and use it from other
applications.  After installing MinGW (@url{http://mingw.org/}) follow
the generic installation instructions (@pxref{Downloading and
Installing}).  The DLL is installed by default.

For information on how to use the DLL in other applications, see:
@url{http://www.mingw.org/mingwfaq.shtml#faq-msvcdll}.

You can build GNU SASL as a native Visual Studio C++ project.  This
allows you to build the code for other platforms that VS supports,
such as Windows Mobile.  You need Visual Studio 2005 or later.

First download and unpack the archive as described in the generic
installation instructions (@pxref{Downloading and Installing}).  Don't
run @code{./configure}.  Instead, start Visual Studio and open the
project file @file{lib/win32/libgsasl.sln} inside the GNU SASL
directory.  You should be able to build the project using Build
Project.

Output libraries will be written into the @code{lib/win32/lib} (or
@code{lib/win32/lib/debug} for Debug versions) folder.

Warning!  Unless you build GNU SASL linked with libgcrypt, GNU SASL
uses the Windows function @code{CryptGenRandom} for generating
cryptographic random data.  The function is known to have some
security weaknesses.  See @url{http://eprint.iacr.org/2007/419} for
more information.  The code will attempt to use the Intel RND crypto
provider if it is installed, see @file{lib/gl/gc-gnulib.c}.

@node Kerberos on Windows
@subsection Kerberos on Windows

Building GNU SASL with support for Kerberos via GSS-API on Windows is
straight forward if you use GNU GSS and GNU Shishi as the Kerberos
implementation.

If you are using MIT Kerberos for Windows (KfW), getting GNU SASL to
build with Kerberos support is not straightforward because KfW does
not follow the GNU coding style and it has bugs that needs to be
worked around.  We provide instructions for this environment as well,
in the hope that it will be useful for GNU SASL users.

Our instructions assumes you are building the software on a dpkg-based
GNU/Linux systems (e.g., gNewSense) using the MinGW cross-compiler
suite.  These instructions were compiled for KfW version 3.2.2 which
were the latest as of 2010-09-25.

We assume that you have installed a normal build environment including
the MinGW cross-compiler.  Download and unpack the KfW SDK like this:

@example
$ mkdir ~/kfw
$ cd ~/kfw
$ wget -q http://web.mit.edu/kerberos/dist/kfw/3.2/kfw-3.2.2/kfw-3-2-2-sdk.zip
$ unzip kfw-3-2-2-sdk.zip
@end example

Fix a bug in the "win-mac.h" header inside KfW by replacing
@code{#include <sys\foo.h>} with @code{#include <sys/foo.h>}:

@example
perl -pi -e 's,sys\\,sys/,' ~/kfw/kfw-3-2-2-final/inc/krb5/win-mac.h
@end example

Unpack your copy of GNU SASL:

@example
$ wget -q ftp://alpha.gnu.org/gnu/gsasl/gsasl-@value{VERSION}.tar.gz
$ tar xfz gsasl-@value{VERSION}.tar.gz
$ cd gsasl-@value{VERSION}
@end example

Configure GNU SASL like this:

@example
$ lt_cv_deplibs_check_method=pass_all ./configure --host=i586-mingw32msvc --build=i686-pc-linux-gnu --with-gssapi-impl=kfw LDFLAGS="-L$HOME/kfw/kfw-3-2-2-final/lib/i386" CPPFLAGS="-I$HOME/kfw/kfw-3-2-2-final/inc/krb5 -DSSIZE_T_DEFINED"
@end example

The 'lt_cv_deplibs_check_method=pass_all' setting is required because
the KfW SDK does not ship with Libtool @code{*.la} files and is using
non-standard DLL names.  The @code{-DSSIZE_T_DEFINED} is necessary
because the @code{win-mac.h} file would provide an incorrect duplicate
definitions of @code{ssize_t} otherwise.  By passing
@code{--with-gssapi-impl=kfw} you activate other bug workarounds, such
as providing a @code{GSS_C_NT_HOSTBASED_SERVICE} symbol.

Build the software using:

@example
$ make
@end example

If you have Wine installed and your kernel is able to invoke it
automatically for Windows programs, you can run the self tests.  This
is recommended to make sure the build is sane.

@example
$ make check
@end example

You may get error messages about missing DLLs, like this error:

@example
err:module:import_dll Library gssapi32.dll (which is needed by L"Z:\\home\\jas\\src\\gsasl-1.5.2\\lib\\src\\.libs\\libgsasl-7.dll") not found
@end example

If that happens, you need to make sure that Wine can find the
appropriate DLL.  The simplest solution is to copy the necessary DLLs
to @code{~/.wine/drive_c/windows/system32/}.

You may now copy the following files onto the Windows machine (e.g.,
through a USB memory device):

@example
lib/src/.libs/libgsasl-7.dll
src/.libs/gsasl.exe
@end example

The remaining steps are done on the Windows XP machine.  Install KfW
and configure it for your realm.  To make sure KfW is working
properly, acquire a user ticket and then remove it.  For testing
purposes, you may use the realm 'interop.josefsson.org' with KDC
'interop.josefsson.org' and username 'user' and password 'pass'.

Change to the directory where you placed the files above, and invoke a
command like this:

@example
gsasl.exe -d interop.josefsson.org
@end example

KfW should query you for a password, and the tool should negotiate
authentication against the server using GS2-KRB5.

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

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

@itemize @bullet

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

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

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

@end itemize

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

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

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

Send your bug report to:

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

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

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

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

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

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

@itemize @bullet

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

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

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

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

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

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

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

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

@end itemize

@c **********************************************************
@c *******************  Preparation  ************************
@c **********************************************************
@node Preparation
@chapter Preparation

To use GNU SASL, you have to perform some changes to your sources and
the build system.  The necessary changes are small and explained in
the following sections.  At the end of this chapter, it is described
how the library is initialized, and how the requirements of the
library are verified.

A faster way to find out how to adapt your application for use with
GNU SASL may be to look at the examples at the end of this manual
(@pxref{Examples}).

@menu
* Header::
* Initialization::
* Version Check::
* Building the source::
* Autoconf tests::
@end menu

@node Header
@section Header

All interfaces (data types and functions) of the library are defined
in the header file @code{gsasl.h}.  You must include this in all
programs using the library, either directly or through some other
header file, like this:

@example
#include <gsasl.h>
@end example

The name space is @code{gsasl_*} for function names, @code{Gsasl*} for
data types and @code{GSASL_*} for other symbols.  In addition the same
name prefixes with one prepended underscore are reserved for internal
use and should never be used by an application.

@node Initialization
@section Initialization

The library must be initialized before it can be used.  The library is
initialized by calling @code{gsasl_init} (@pxref{Global Functions}).
The resources allocated by the initialization process can be released
if the application no longer has a need to call `Libgsasl' functions,
this is done by calling @code{gsasl_done}.  For example:

@example
int
main (int argc, char *argv[])
@{
  Gsasl *ctx = NULL;
  int rc;
...
  rc = gsasl_init (&ctx);
  if (rc != GSASL_OK)
    @{
      printf ("SASL initialization failure (%d): %s\n",
              rc, gsasl_strerror (rc));
      return 1;
    @}
...
@end example

In order to make error messages from @code{gsasl_strerror} be
translated (@pxref{Top,,,gettext,GNU Gettext}) the application must
set the current locale using @code{setlocale} before calling
@code{gsasl_init}.  For example:

@example
int
main (int argc, char *argv[])
@{
  Gsasl *ctx = NULL;
  int rc;
...
  setlocale (LC_ALL, "");
...
  rc = gsasl_init (&ctx);
  if (rc != GSASL_OK)
    @{
      printf (gettext ("SASL initialization failure (%d): %s\n"),
              rc, gsasl_strerror (rc));
      return 1;
    @}
...
@end example

In order to take advantage of the secure memory features in
Libgcrypt@footnote{Note that GNU SASL normally use its own internal
implementation of the cryptographic functions.  Take care to verify
that GNU SASL really use Libgcrypt, if this is what you want.}, you
need to initialize secure memory in your application, and for some
platforms even make your application setuid root.  See the Libgcrypt
documentation for more information.  Here is example code to
initialize secure memory in your code:

@example
#include <gcrypt.h>
...
int
main (int argc, char *argv[])
@{
  Gsasl *ctx = NULL;
  int rc;
...
  /* Check version of libgcrypt. */
  if (!gcry_check_version (GCRYPT_VERSION))
    die ("version mismatch\n");

  /* Allocate a pool of 16k secure memory.  This also drops privileges
     on some systems. */
  gcry_control (GCRYCTL_INIT_SECMEM, 16384, 0);

  /* Tell Libgcrypt that initialization has completed. */
  gcry_control (GCRYCTL_INITIALIZATION_FINISHED, 0);
...
  rc = gsasl_init (&ctx);
  if (rc != GSASL_OK)
    @{
      printf ("SASL initialization failure (%d): %s\n",
              rc, gsasl_strerror (rc));
      return 1;
    @}
...
@end example

If you do not do this, keying material will not be allocated in secure
memory (which, for most applications, is not the biggest secure
problem anyway).  Note that the GNU SASL Library has not been audited
to make sure it stores passwords or keys in secure memory.

@node Version Check
@section Version Check

It is often desirable to check that the version of the library 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 may actually be used.
So you may want to check that the version is okay right after program
startup.

@include texi/gsasl_check_version.texi

The normal way to use the function is to put something similar to the
following early in your @code{main}:

@example
  if (!gsasl_check_version (GSASL_VERSION))
    @{
      printf ("gsasl_check_version failed:\n"
              "Header file incompatible with shared library.\n");
      exit(1);
    @}
@end example

@node Building the source
@section Building the source
@cindex Compiling your application

If you want to compile a source file including the @file{gsasl.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 libgsasl}.  The
following example shows how it can be used at the command line:

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

Adding the output of @samp{pkg-config libgsasl --cflags} to the
compiler command line will ensure that the compiler can find the
@file{gsasl.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 libgsasl} 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{-lidn} 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 libgsasl --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 libgsasl --cflags --libs`
@end example

@node Autoconf tests
@section Autoconf tests
@cindex Autoconf tests
@cindex Configure tests

If you work on a project that uses Autoconf (@pxref{top, GNU
Autoconf,, autoconf}) to help find installed libraries, the
suggestions in the previous section are not the entire story.  There
are a few methods to detect and incorporate the GNU SASL Library into
your Autoconf based package.  The preferred approach, is to use
Libtool in your project, and use the normal Autoconf header file and
library tests.

@subsection Autoconf test via @samp{pkg-config}

If your audience is a typical GNU/Linux desktop, you can often assume
they have the @samp{pkg-config} tool installed, in which you can use
its Autoconf M4 macro to find and set up your package for use with
Libgsasl.  The following example illustrates this scenario.

@example
AC_ARG_ENABLE(gsasl,
  AC_HELP_STRING([--disable-gsasl], [don't use GNU SASL]),
  gsasl=$enableval)
if test "$gsasl" != "no" ; then
  PKG_CHECK_MODULES(GSASL, libgsasl >= @value{VERSION},
    [gsasl=yes],
    [gsasl=no])
  if test "$gsasl" != "yes" ; then
    gsasl=no
    AC_MSG_WARN([Cannot find GNU SASL, disabling])
  else
    gsasl=yes
    AC_DEFINE(USE_GSASL, 1, [Define to 1 if you want GNU SASL.])
  fi
fi
AC_MSG_CHECKING([if GNU SASL should be used])
AC_MSG_RESULT($gsasl)
@end example

@subsection Standalone Autoconf test using Libtool

If your package uses Libtool (@pxref{top, GNU Libtool,, libtool}), you
can use the normal Autoconf tests to find Libgsasl and rely on the
Libtool dependency tracking to include the proper dependency libraries
(e.g., Libidn).  The following example illustrates this scenario.

@example
AC_CHECK_HEADER(gsasl.h,
  AC_CHECK_LIB(gsasl, gsasl_check_version,
    [gsasl=yes AC_SUBST(GSASL_LIBS, -lgsasl)],
    gsasl=no),
  gsasl=no)
AC_ARG_ENABLE(gsasl,
  AC_HELP_STRING([--disable-gsasl], [don't use GNU SASL]),
  gsasl=$enableval)
if test "$gsasl" != "no" ; then
  AC_DEFINE(USE_SASL, 1, [Define to 1 if you want GNU SASL.])
else
  AC_MSG_WARN([Cannot find GNU SASL, disabling])
fi
AC_MSG_CHECKING([if GNU SASL should be used])
AC_MSG_RESULT($gsasl)
@end example

@c **********************************************************
@c *****************  Using the Library  ********************
@c **********************************************************
@node Using the Library
@chapter Using the Library
@cindex Overview
@cindex Library Overview

Your application's use of the library can be roughly modeled into the
following steps: initialize the library, optionally specify the
callback, perform the authentication, and finally clean up.  The
following image illustrates this.

@image{gsasl-controlflow,15cm,5cm}

The third step may look complex, but for a simple client it will
actually not involve any code.  If your application needs to handle
several concurrent clients, or if it is a server that needs to serve
many clients simultaneous, things do get a bit more complicated.

For illustration, we will write a simple client.  Writing a server
would be similar, the only difference is that, later on, instead of
supplying a username and password, you need to decide whether someone
should be allowed to log in or not.  The code for what we have
discussed so far make up the @code{main} function in our client
(@pxref{Example 1}):

@example
int main (int argc, char *argv[])
@{
  Gsasl *ctx = NULL;
  int rc;

  if ((rc = gsasl_init (&ctx)) != GSASL_OK)
    @{
      printf ("Cannot initialize libgsasl (%d): %s",
              rc, gsasl_strerror (rc));
      return 1;
    @}

  client (ctx);

  gsasl_done (ctx);

  return 0;
@}
@end example

Here, the call to the function @code{client} correspond to the third
step in the image above.

For a more complicated application, having several clients running
simultaneous, instead of a simple call to @code{client}, it may have
created new threads for each session, and call @code{client} within
each thread.  The library is thread safe.

An actual authentication session is more complicated than what we have
seen so far.  These are the steps: decide which mechanism to use,
start the session, optionally specify the callback, optionally set any
properties, perform the authentication loop, and clean up.  Naturally,
your application will start to talk its own protocol (e.g., SMTP or
IMAP) after these steps have concluded.

The authentication loop is based on sending tokens (typically short
messages encoded in base 64) back and forth between the client and
server.  It continues until authentication succeeds or an error
occurs.  The format of the data to be transferred, the number of
iterations in the loop, and other details are specified by each
mechanism.  The goal of the library is to isolate your application
from the details of all different mechanisms.

Note that the library does not send data to the server itself, but
returns it in an buffer.  You must send it to the server, following an
application protocol profile.  For example, the SASL application
protocol profile for SMTP is described in RFC 2554.

The following image illustrates the steps we have been talking about.

@image{gsasl-controlflow2,16cm,12cm}

We will now show the implementation of the @code{client} function used
before.

@example
void client (Gsasl *ctx)
@{
  Gsasl_session *session;
  const char *mech = "PLAIN";
  int rc;

  /* Create new authentication session. */
  if ((rc = gsasl_client_start (ctx, mech, &session)) != GSASL_OK)
    @{
      printf ("Cannot initialize client (%d): %s\n",
              rc, gsasl_strerror (rc));
      return;
    @}

  /* Set username and password in session handle.  This info will be
     lost when this session is deallocated below.  */
  rc = gsasl_property_set (session, GSASL_AUTHID, "jas");
  if (rc != GSASL_OK)
    @{
      printf ("Cannot set property (%d): %s\n", rc, gsasl_strerror (rc));
      return;
    @}
  rc = gsasl_property_set (session, GSASL_PASSWORD, "secret");
  if (rc != GSASL_OK)
    @{
      printf ("Cannot set property (%d): %s\n", rc, gsasl_strerror (rc));
      return;
    @}

  /* Do it. */
  client_authenticate (session);

  /* Cleanup. */
  gsasl_finish (session);
@}
@end example

This function is responsible for deciding which mechanism to use.  In
this case, the @samp{PLAIN} mechanism is hard coded, but you will see
later how this can be made more flexible.  The function creates a new
session, then it stores the username and password in the session
handle, then it calls another function @code{client_authenticate} to
handle the authentication loop, and finally it cleans up up.  Let's
continue with the implementation of @code{client_authenticate}.

@example
void client_authenticate (Gsasl_session * session)
@{
  char buf[BUFSIZ] = "";
  char *p;
  int rc;

  /* This loop mimics a protocol where the server sends data
     first. */

  do
    @{
      printf ("Input base64 encoded data from server:\n");
      fgets (buf, sizeof (buf) - 1, stdin);
      if (buf[strlen (buf) - 1] == '\n')
        buf[strlen (buf) - 1] = '\0';

      rc = gsasl_step64 (session, buf, &p);

      if (rc == GSASL_NEEDS_MORE || rc == GSASL_OK)
        @{
          printf ("Output:\n%s\n", p);
          free (p);
        @}
    @}
  while (rc == GSASL_NEEDS_MORE);

  printf ("\n");

  if (rc != GSASL_OK)
    @{
      printf ("Authentication error (%d): %s\n",
              rc, gsasl_strerror (rc));
      return;
    @}

  /* The client is done.  Here you would typically check if the
     server let the client in.  If not, you could try again. */

  printf ("If server accepted us, we're done.\n");
@}
@end example

This last function needs to be discussed in some detail.  First, you
should be aware that there are two versions of this function, that
differ in a subtle way.  The version above (@pxref{Example 2}) is used
for application profiles where the server sends data first.  For some
mechanisms, this may waste a roundtrip, because the server needs input
from the client to proceed.  Therefore, today the recommended approach
is to permit client to send data first (@pxref{Example 1}).  Which
version you should use depends on which application protocol you are
implementing.

Further, you should realize that it is bad programming style to use a
fixed size buffer.  On GNU systems, you may use the @code{getline}
functions instead of @code{fgets}.  However, in practice, there are
few mechanisms that use very large tokens.  In typical configurations,
the mechanism with the largest tokens (GSSAPI) can use at least 500
bytes.  A fixed buffer size of 8192 bytes may thus be sufficient for
now.  But don't say I didn't warn you, when a future mechanism doesn't
work in your application, because of a fixed size buffer.

The function @code{gsasl_step64} (and of course also @code{gasl_step})
returns two non-error return codes.  @code{GSASL_OK} is used for
success, indicating that the library considers the authentication
finished.  That may include a successful server authentication,
depending on the mechanism.  You must not let the client continue to
the application protocol part unless you receive @code{GSASL_OK} from
these functions.  In particular, don't be fooled into believing
authentication were successful if the server replies ``OK'' but these
functions have failed with an error.  The server may have been hacked,
and could be tricking you into sending confidential data, without
having successfully authenticated the server.

The non-error return code @code{GSASL_NEEDS_MORE} is used to signal to
your application that you should send the output token to the peer,
and wait for a new token, and do another iteration.  If the server
concludes the authentication process, with no data, you should call
@code{gsasl_step64} (or @code{gsasl_step}) specifying a zero-length
token.

If the functions (@code{gsasl_step} and @code{gsasl_step64}) return
any non-error code, the content of the output buffer is undefined.
Otherwise, it is the callers responsibility to deallocate the buffer,
by calling @code{free}.  Note that in some situations, where the
buffer is empty, @code{NULL} is returned as the buffer value.  You
should treat this as an empty buffer.

@section Choosing a mechanism

Our earlier code was hard coded to use a specific mechanism.  This is
rarely a good idea.  Instead, it is recommended to select the best
mechanism available from the list of mechanisms supported by the
server.  Note that without TLS or similar, the list may have been
maliciously altered, by an attacker.  This means that you should abort
if you cannot find any mechanism that exceeds your minimum security
level.  There is a function @code{gsasl_client_suggest_mechanism}
(@pxref{Global Functions}) that will try to pick the ``best''
available mechanism from a list of mechanisms.  Our simple interactive
example client (@pxref{Example 3}) includes the following function to
decide which mechanism to use.  Note that the code doesn't blindly use
what is returned from @code{gsasl_client_suggest_mechanism}, rather it
lets some logic (in this case the user, through an interactive query)
decide which mechanism is acceptable.

@example
const char *client_mechanism (Gsasl *ctx)
@{
  static char mech[GSASL_MAX_MECHANISM_SIZE + 1] = "";
  char mechlist[BUFSIZ] = "";
  const char *suggestion;

  printf ("Enter list of server supported mechanisms, separate by SPC:\n");
  fgets (mechlist, sizeof (mechlist) - 1, stdin);

  suggestion = gsasl_client_suggest_mechanism (ctx, mechlist);
  if (suggestion)
    printf ("Library suggests use of `%s'.\n", suggestion);

  printf ("Enter mechanism to use:\n");
  fgets (mech, sizeof (mech) - 1, stdin);
  mech[strlen (mech) - 1] = '\0';

  return mech;
@}
@end example

When running this example code, it might look like in the following
output.

@example
Enter list server supported mechanisms, separate by SPC:
CRAM-MD5 DIGEST-MD5 GSSAPI FOO BAR
Library suggests use of `GSSAPI'.
Enter mechanism to use:
CRAM-MD5
Input base64 encoded data from server:
Zm5vcmQ=
Output:
amFzIDkyY2U1NWE5MTM2ZTY4NzEyMTUyZTFjYmFmNjVkZjgx

If server accepted us, we're done.
@end example

@section Using a callback

Our earlier code specified the username and password before the
authentication loop, as in:

@example
gsasl_property_set (ctx, GSASL_AUTHID, "jas");
gsasl_property_set (ctx, GSASL_PASSWORD, "secret");
@end example

This may work for simple mechanisms, that need only a username and a
password.  But some mechanism requires more information, such as an
authorization identity, a special PIN or passcode, a realm, a
hostname, a service name, or an anonymous identifier.  Querying the
user for all that information, without knowing exactly which of it is
really needed will result in a poor user interface.  The user should
not have to input private information, if it isn't required.

The approach is a bad idea for another reason.  What if the server
aborts the authentication process?  Then your application has already
queried the user for a username and password.  It would be better if
you only asked the user for this information, annoying to input, when
it is known to be needed.

A better approach to this problem is to use a callback.  Then the
mechanism may query your application whenever it needs some
information, like the username and password.  It will only do this at
the precise step in the authentication when the information is
actually needed.  Further, if the user aborts, e.g., a password
prompt, the mechanism is directly informed of this (because it invoked
the callback), and could recover somehow.

Our final example (@pxref{Example 4}) specifies a callback function,
inside @code{main} as below.

@example
/* Set the callback handler for the library. */
gsasl_callback_set (ctx, callback);
@end example

The function itself is implemented as follows.

@example
int callback (Gsasl * ctx, Gsasl_session * sctx, Gsasl_property prop)
@{
  char buf[BUFSIZ] = "";
  int rc = GSASL_NO_CALLBACK;

  /* Get user info from user. */

  printf ("Callback invoked, for property %d.\n", prop);

  switch (prop)
    @{
    case GSASL_PASSCODE:
      printf ("Enter passcode:\n");
      fgets (buf, sizeof (buf) - 1, stdin);
      buf[strlen (buf) - 1] = '\0';

      rc = gsasl_property_set (sctx, GSASL_PASSCODE, buf);
      break;

    case GSASL_AUTHID:
      printf ("Enter username:\n");
      fgets (buf, sizeof (buf) - 1, stdin);
      buf[strlen (buf) - 1] = '\0';

      rc = gsasl_property_set (sctx, GSASL_AUTHID, buf);
      break;

    default:
      printf ("Unknown property!  Don't worry.\n");
      break;
    @}

  return rc;
@}
@end example

Again, it is bad style to use a fixed size buffer.  Mmm'kay.

Which properties you should handle is up to you.  If you don't know how
to respond to a certain property, simply return
@code{GSASL_NO_CALLBACK}.  The basic properties to support are
authentication identity (@code{GSASL_AUTHID}), authorization identity
(@code{GSASL_AUTHZID}), and password (@code{GSASL_PASSWORD}).
@xref{Properties}, for the list of all properties, and what your callback
should (ideally) do for them, and which properties each mechanism
require in order to work.

@c **********************************************************
@c *******************  Properties  *************************
@c **********************************************************
@node Properties
@chapter Properties

The library uses a concept called ``properties'' to request and pass
data between the application and the individual authentication
mechanisms.  The application can set property values using the
@code{gsasl_property_set} function.  If a mechanism needs a property
value the application has not yet provided, this is handled through a
callback.  The application provides a callback, using
@code{gsasl_callback_set}, which will be invoked with a property
parameter.  The callback should set the property before returning, or
fail.  @xref{Callback Functions}, for more information.

There are two kind of properties.  The first, a ``data property'' is
the simplest to understand because it normally refers to short
strings.  For example, the property called @code{GSASL_AUTHID}
correspond to the username string, e.g., @code{simon}.

The latter properties, called ``logical properties'', are used by the
server to make a authentication decision, and is used as a way to get
the application callback invoked.  For example, the property
@code{GSASL_VALIDATE_SIMPLE} is used by the server-side part of
mechanisms like @code{PLAIN}.  The purpose is to ask the server
application to decide whether the user should be authenticated
successfully or not.  The callback typically look at other property
fields, such as @code{GSASL_AUTHID} and @code{GSASL_PASSWORD}, and
compare those values with external information (for example data
stored in a database or on a LDAP server) and then return OK or not.

@quotation Warning
Don't expect that all mechanisms invoke one of the ``logical
properties'' in the server mode.  For example, the CRAM-MD5 and
SCRAM-SHA-1 mechanisms will use the data properties (i.e., username and
password) provided by the application to internally decide whether to
successfully authenticate the user.  User authorization decisions needs
to be made by the application outside of the SASL mechanism negotiation.
@end quotation

The logical properties are currently only used by servers, but data
properties are used by both client and servers.  It makes sense to
think about the latter category as @samp{server properties} but the
reverse is not valid nor useful.

The semantics associated with a data property is different when it is
used in client context and in the server context.  For example, in the
client context, the application is expected to set the property
@code{GSASL_AUTHID} to signal to the mechanism the username to use,
but in the server context, the @code{GSASL_AUTHID} property is set by
the mechanism and can be used by the application (in the callback) to
find out what username the client provided.

Below is a list of all properties and an explanation for each.  First
is the list of data properties:

@itemize

@item @code{GSASL_AUTHID}

The authentication identity.

@item @code{GSASL_AUTHZID}

The authorization identity.

@item @code{GSASL_PASSWORD}

The password of the authentication identity.

@item @code{GSASL_ANONYMOUS_TOKEN}

The anonymous token.  This is typically the email address of the user.

@item @code{GSASL_SERVICE}

The registered GSSAPI service name of the application service,
e.g. ``imap''.  While the names are registered for GSSAPI, other
mechanisms such as DIGEST-MD5 may also use this.

@item @code{GSASL_HOSTNAME}

Should be the local host name of the machine.

@item @code{GSASL_GSSAPI_DISPLAY_NAME}

Contain the GSSAPI ``display name'', set by the server GSSAPI
mechanism.  Typically you retrieve this property in your callback,
when invoked for @code{GSASL_VALIDATE_GSSAPI}.

@item @code{GSASL_REALM}

The name of the authentication domain.  This is used by several
mechanisms, including DIGEST-MD5, GSS-API, and NTLM.

@item @code{GSASL_PASSCODE}

The SecurID passcode.

@item @code{GSASL_PIN}

The SecurID personal identification number (PIN).

@item @code{GSASL_SUGGESTED_PIN}

A SecurID personal identification number (PIN) suggested by the server.

@item @code{GSASL_DIGEST_MD5_HASHED_PASSWORD}

For the DIGEST-MD5 mechanism, this is a hashed password.  It is used
in servers to avoid storing clear-text credentials.

@item @code{GSASL_QOPS}

The DIGEST-MD5 server query for this property to get the set of
quality of protection (QOP) values to advertise.  The property holds
strings with comma separated keywords denoting the set of qops to use,
for example @code{qop-auth, qop-int}.  Valid keywords are
@code{qop-auth}, @code{qop-int}, and @code{qop-conf}.

@item @code{GSASL_QOP}

The DIGEST-MD5 client query for this property to get the quality of
protection (QOP) values to request.  The property value is one of the
keywords for @code{GSASL_QOPS}.  The client must chose one of the QOP
values offered by the server (which may be inspected through the
@code{GSASL_QOPS} property).

@item @code{GSASL_SCRAM_SALTED_PASSWORD}

In a client, the SCRAM mechanism (@pxref{SCRAM}) will request this
property from the application.  The value should be hex-encoded string
(40 characters for SCRAM-SHA-1 and 64 characters for SCRAM-SHA-256) with
the user's PBKDF2-prepared password.  Note that the value is different
for the same password for each value of the @code{GSASL_SCRAM_ITER} and
@code{GSASL_SCRAM_SALT} properties.  The property can be used to avoid
storing a clear-text credential in the client, however note that an
attacker who steal it may impersonate both a SCRAM client and SCRAM
server.  If the property is not available, the mechanism will ask for
the @code{GSASL_PASSWORD} property instead.

The @code{GSASL_SCRAM_SALTED_PASSWORD} property is set by the SCRAM
mechanism if it derived the value from a @code{GSASL_PASSWORD} value
supplied during authentication.  Thus, the application may cache this
value for future authentication attempts.

@item @code{GSASL_SCRAM_ITER}
@item @code{GSASL_SCRAM_SALT}

@cindex iteration count
@cindex salt
In the server, the application can set these properties to influence the
hash iteration count and hash salt to use when deriving the password in
the SCRAM mechanism (@pxref{SCRAM}).  The default hash iteration count
is 4096 and often you should use a higher value.  The salt should be a
base64-encoded string with random data, typical length 4 to 16 bytes.

In the client, the SCRAM mechanism set these properties (using values
received from the server) before asking the application to provide a
@code{GSASL_SCRAM_SALTED_PASSWORD} value.

After the final authentication step, the properties are set by the
mechanism, to allow the application to retrieve the values used
(required when storing the @code{GSASL_SCRAM_SALTED_PASSWORD} value, for
example).

@item @code{GSASL_SCRAM_SERVERKEY}
@item @code{GSASL_SCRAM_STOREDKEY}

These properties are requested by the SCRAM server mechanism
(@pxref{SCRAM}), and if they are not available it will ask for
@code{GSASL_PASSWORD} or @code{GSASL_SCRAM_SALTED_PASSWORD} to complete
authentication.  The values are base64-encoded strings; 28 characters
for SCRAM-SHA-1 and 44 characters for SCRAM-SHA-256.  The properties are
set after completing the final authentication step; so if
@code{GSASL_PASSWORD} or @code{GSASL_SCRAM_SALTED_PASSWORD} was used for
authentication, the application may extract @code{GSASL_SCRAM_SERVERKEY}
and @code{GSASL_SCRAM_STOREDKEY} to use these values in a future
authentication instead of the password.  The values can be calculated
using @code{gsasl_scram_secrets_from_password},
@code{gsasl_scram_secrets_from_salted_password} (@pxref{Utilities}) or
using the @code{--mkpasswd} parameter for the @code{gsasl} utility
(@pxref{Invoking gsasl}).

@item @code{GSASL_CB_TLS_UNIQUE}

@cindex channel binding
@cindex tls-unique
This property holds base64 encoded @code{tls-unique} channel binding
data.  As a hint, if you use GnuTLS, the API
@code{gnutls_session_channel_binding} can be used to extract channel
bindings for a session.  To be secure, a TLS channel MUST have the
session hash extension (RFC 7627) negotiated, or session resumption MUST
NOT have been used.  The library cannot enforce this, so it is up to the
application to only provide the @code{GSASL_CB_TLS_UNIQUE} property when
the condition holds.  Note that TLS version 1.3 and later do not support
this channel binding.

@item @code{GSASL_CB_TLS_EXPORTER}

@cindex channel binding
@cindex tls-exporter
This property holds base64 encoded @code{tls-exporter} channel binding
data.  As a hint, if you use GnuTLS, the API
@code{gnutls_session_channel_binding} can be used to extract channel
bindings for a session.  This fixes some of the security problems with
the @code{tls-unique} channel binding and supports modern TLS versions.

@item @code{GSASL_SAML20_IDP_IDENTIFIER}

@cindex SAML IdP Identifier
@cindex Identity Provider Identifier
This property holds the SAML identifier of the user.  The SAML20
mechanism in client mode will send it to the other end for
identification purposes, and in server mode it will be accessible in
the @code{GSASL_SAML20_REDIRECT_URL} callback.

@item @code{GSASL_SAML20_REDIRECT_URL}
This property holds the SAML redirect URL that the server wants the
client to access.  It will be available in the
@code{GSASL_SAML20_AUTHENTICATE_IN_BROWSER} callback for the client.

@item @code{GSASL_OPENID20_REDIRECT_URL}
This property holds the SAML redirect URL that the server wants the
client to access.  It will be available in the
@code{GSASL_OPENID20_AUTHENTICATE_IN_BROWSER} callback for the client.

@item @code{GSASL_OPENID20_OUTCOME_DATA}
OpenID 2.0 authentication outcome data.  This is either the OpenID
SREG values or a value list starting with @code{"openid.error="} to
signal error.

@end itemize

Next follows a list of data properties used to trigger the callback,
typically used in servers to validate client credentials:

@itemize

@item @code{GSASL_VALIDATE_SIMPLE}

Used by multiple mechanisms in server mode.  The callback may retrieve
the @code{GSASL_AUTHID}, @code{GSASL_AUTHZID} and
@code{GSASL_PASSWORD} property values and use them to make an
authentication and authorization decision.

@item @code{GSASL_VALIDATE_EXTERNAL}

Used by EXTERNAL mechanism on the server side to validate the client.
The GSASL_AUTHID will contain the authorization identity of the
client.

@item @code{GSASL_VALIDATE_ANONYMOUS}

Used by ANONYMOUS mechanism on the server side to validate the client.
The GSASL_ANONYMOUS_TOKEN will contain token that identity the client.

@item @code{GSASL_VALIDATE_GSSAPI}

Used by the GSSAPI and GS2-KRB5 mechanisms on the server side, to
validate the client.  You may retrieve the authorization identity from
GSASL_AUTHZID and the GSS-API display name from
GSASL_GSSAPI_DISPLAY_NAME.

@item @code{GSASL_VALIDATE_SECURID}

Used by SECURID mechanism on the server side to validate client.  The
GSASL_AUTHID, GSASL_AUTHZID, GSASL_PASSCODE, and GSASL_PIN will be
set.  It can return GSASL_SECURID_SERVER_NEED_ADDITIONAL_PASSCODE to
ask the client to supply another passcode, and
GSASL_SECURID_SERVER_NEED_NEW_PIN to require the client to supply a
new PIN code.

@item @code{GSASL_VALIDATE_SAML20}

Used by the SAML20 mechanism on the server side to request that the
application perform authentication.  The callback should return
@code{GSASL_OK} if the user should be permitted access, and
@code{GSASL_AUTHENTICATION_ERROR} (or another error code) otherwise.

@item @code{GSASL_VALIDATE_OPENID20}

Used by the OPENID20 mechanism on the server side to request that the
application perform authentication.  The callback should return
@code{GSASL_OK} if the user should be permitted access, and
@code{GSASL_AUTHENTICATION_ERROR} (or another error code) otherwise.

@item @code{GSASL_SAML20_AUTHENTICATE_IN_BROWSER}
Used by the SAML20 mechanism in the client side to request that the
client should launch the SAML redirect URL (the
@code{GSASL_SAML20_REDIRECT_URL} property) in a browser to continue
with authentication.

@item @code{GSASL_OPENID20_AUTHENTICATE_IN_BROWSER}
Used by the OPENID20 mechanism in the client side to request that the
client should launch the OpenID redirect URL (the
@code{GSASL_OPENID20_REDIRECT_URL} property) in a browser to continue
with authentication.

@end itemize


@c **********************************************************
@c *******************  Mechanisms  *************************
@c **********************************************************
@node Mechanisms
@chapter Mechanisms

Different SASL mechanisms have different requirements on the
application using it.  To handle these differences the library can use
a callback function into your application in several different ways.
Some mechanisms, such as @samp{PLAIN}, are simple to explain and use.
The client callback queries the user for a username and password.  The
server callback hands the username and password into any local policy
deciding authentication system (such as @file{/etc/passwd} via PAM).

Mechanism such as @samp{CRAM-MD5} and @samp{SCRAM-SHA-256} uses hashed
passwords.  The client callback behaviour is the same as for PLAIN.
However, the server does not receive the plain text password over the
network but rather a hash of it.  Existing policy deciding systems like
PAM cannot handle this, so the server callback for these mechanisms are
more complicated.

Further, mechanisms like GSSAPI/GS2-KRB5 (Kerberos 5) assume a
specific authentication system.  In theory this means that the SASL
library would not need to interact with the application, but rather
call this specific authentication system directly.  However, some
callbacks are supported anyway, to modify the behaviour of how the
specific authentication system is used (i.e., to handle ``super-user''
login as some other user).

Some mechanisms, like @samp{EXTERNAL} and @samp{ANONYMOUS} are
entirely dependent on callbacks.

@menu
* EXTERNAL::                Authentication via out of band information.
* ANONYMOUS::               Mechanism for anonymous access to resources.
* PLAIN::                   Clear text username and password.
* LOGIN::                   Non-standard clear text username and password.
* CRAM-MD5::                Challenge-Response Authentication Mechanism.
* DIGEST-MD5::              Digest Authentication.
* SCRAM::                   Salted Challenge Response Authentication.
* NTLM::                    Microsoft NTLM authentication.
* SECURID::                 Authentication using tokens.
* GSSAPI::                  GSSAPI (Kerberos 5) authentication.
* GS2-KRB5::                Improved GSSAPI (Kerberos 5) authentication.
* SAML20::                  Authenticate using SAML 2.0 via a browser.
* OPENID20::                Authenticate using OpenID 2.0 via a browser.
@end menu

@node EXTERNAL
@section The EXTERNAL mechanism

The EXTERNAL mechanism is used to authenticate a user to a server based
on out-of-band authentication.  EXTERNAL is typically used over TLS
authenticated channels.  Note that in the server, you need to make sure
that TLS actually authenticated the client successfully and that the
negotiated ciphersuite and other parameters are acceptable.  It is
generally not sufficient that TLS is used, since TLS supports anonymous
and other variants that generally provide less assurance than you
normally want.

In the client, this mechanism is always enabled, and it will send the
@code{GSASL_AUTHZID} property as the authorization name to the server,
if the property is set.  If the property is not set, an empty
authorization name is sent.  You need not implement a callback.

In the server, this mechanism will request the
@code{GSASL_VALIDATE_EXTERNAL} callback property to decide whether the
client is authenticated and authorized to log in.  Your callback can
retrieve the @code{GSASL_AUTHZID} property to inspect the requested
authorization name from the client.

The EXTERNAL mechanism was initially specified in the core SASL
framework RFC 2222 and later revised in RFC 4422.

@node ANONYMOUS
@section The ANONYMOUS mechanism

The ANONYMOUS mechanism is used to ``authenticate'' clients to anonymous
services; or rather, just indicate that the client wishes to use the
service anonymously.  The client sends a token, usually her email
address, which serve the purpose of some trace information suitable for
logging.  The token cannot be empty.

In the client, this mechanism is always enabled, and will send the
@code{GSASL_ANONYMOUS_TOKEN} property as the trace information to the
server.

In the server, this mechanism will invoke the
@code{GSASL_VALIDATE_ANONYMOUS} callback to decide whether the client
should be permitted to log in.  Your callback can retrieve the
@code{GSASL_ANONYMOUS_TOKEN} property to, for example, record it in a
log file.  The token is normally not used to decide whether the client
should be permitted to log in or not.

The ANONYMOUS mechanism was initially specified in RFC 2245 and later
revised in RFC 4505.

@node PLAIN
@section The PLAIN mechanism

The PLAIN mechanism uses username and password to authenticate users.
Two user names are relevant.  The first, the authentication identity,
indicates the credential holder, i.e., whom the provided password
belongs to.  The second, the authorization identity, is typically empty
to indicate that the user requests to log on to the server as herself
(i.e., the authentication identity).  If the authorization identity is
not empty, the server should decide whether the authenticated user may
log on as the authorization identity.  This is typically used for
super-user accounts like @samp{admin} to take on the role of a regular
user.

In the client, this mechanism is always enabled, and require the
@code{GSASL_AUTHID} and @code{GSASL_PASSWORD} properties.  If set,
@code{GSASL_AUTHZID} will also be used.

In the server, the mechanism is always enabled.  Two approaches to
authenticate and authorize the client are provided.

In the first approach, the server side of the mechanism will request
the @code{GSASL_VALIDATE_SIMPLE} callback property to decide whether
the client should be accepted or not.  The callback may inspect the
@code{GSASL_AUTHID}, @code{GSASL_AUTHZID}, and @code{GSASL_PASSWORD}
properties.  These property values will be normalized.

If the first approach fails (because there is no callback or your
callback returns @samp{GSASL_NO_CALLBACK} to signal that it does not
implement @code{GSASL_VALIDATE_SIMPLE}) the mechanism will continue to
query the application for a password, via the @code{GSASL_PASSWORD}
property.  Your callback may use the @code{GSASL_AUTHID} and
@code{GSASL_AUTHZID} properties to select the proper password.  The
password is then normalized and compared to the client credential.

Which approach to use?  If your database stores hashed passwords, you
have no option, but must use the first approach.  If passwords in your
user database are stored in prepared (SASLprep) form, the first approach
will be faster.  If you do not have prepared passwords available, you
can use the second approach to make sure the password is prepared
properly before comparison.

The PLAIN mechanism was initially specified in RFC 2595 and later
revised in RFC 4616.

@node LOGIN
@section The LOGIN mechanism

The LOGIN mechanism is a non-standard mechanism, and is similar to the
PLAIN mechanism except that LOGIN lacks the support for authorization
identities.  Always use PLAIN instead of LOGIN in new applications.

The callback behaviour is the same as for PLAIN, except that
@code{GSASL_AUTHZID} is neither used nor required, and that the server
does not normalize the password using SASLprep.

@xref{Use of SASLprep in LOGIN}, for a proposed clarification of the
interpretation of a hypothetical LOGIN specification.

@node CRAM-MD5
@section The CRAM-MD5 mechanism
@cindex CRAM-MD5

CRAM-MD5 is a widely used challenge-response mechanism that transfers
hashed passwords instead of clear text passwords.  It is official
deprecated, initially in favor of first DIGEST-MD5 but today
SCRAM-SHA-1.  For insecure channels (e.g., when TLS is not used), it is
has better properties than PLAIN since the unhashed password is not
leaked.  The CRAM-MD5 mechanism does not support authorization
identities; that make the relationship between CRAM-MD5 and
DIGEST-MD5/SCRAM-SHA-* similar to the relationship between LOGIN and
PLAIN.

The disadvantage with hashed passwords is that the server cannot use
normal authentication infrastructures such as PAM, because the server
must have access to the unhashed password in order to validate every
authentication attempt.

In the client, this mechanism is always enabled, and it requires the
@code{GSASL_AUTHID} and @code{GSASL_PASSWORD} properties.

In the server, the mechanism will require the @code{GSASL_PASSWORD}
callback property, which may use the @code{GSASL_AUTHID} property to
determine which users' password should be used.  The @code{GSASL_AUTHID}
will be in normalized form.  The server will then normalize the
password, and compare the client response with the computed correct
response, and accept the user accordingly.

@xref{Use of SASLprep in CRAM-MD5}, for a clarification on the
interpretation of the CRAM-MD5 specification that this implementation
rely on.

The CRAM-MD5 mechanism was initially specified in RFC 2095 but quickly
revised in RFC 2195.  Note that both were published before the core SASL
framework, which explains its lack of authorization identity.

@node DIGEST-MD5
@section The DIGEST-MD5 mechanism
@cindex DIGEST-MD5

The DIGEST-MD5 mechanism is based on repeated hashing using MD5.  After
the MD5 break may be argued to be weaker than HMAC-MD5 that CRAM-MD5
builds on, but DIGEST-MD5 supports other features.  For example,
authorization identities and data integrity and privacy protection are
supported.  Like CRAM-MD5, only a hashed password is transferred.
Consequently, DIGEST-MD5 needs access to the correct password to verify
the client response -- however the server can store the password in
hashed form, another improvement compared to CRAM-MD5 .  Alas, this
makes it impossible to use, e.g., PAM on the server side.

In the client, this mechanism is always enabled, and it requires the
@code{GSASL_AUTHID}, @code{GSASL_PASSWORD}, @code{GSASL_SERVICE}, and
@code{GSASL_HOSTNAME} properties.  If set, @code{GSASL_AUTHZID} and
@code{GSASL_REALM} will also be used.

In the server, the mechanism will first request the
@code{GSASL_DIGEST_MD5_HASHED_PASSWORD} callback property to get the
user's hashed password.  If the callback doesn't supply a hashed
password (i.e., it returns @samp{GSASL_NO_CALLBACK}), the
@code{GSASL_PASSWORD} callback property will be requested.  Both
callbacks may use the @code{GSASL_AUTHID}, @code{GSASL_AUTHZID} and
@code{GSASL_REALM} properties to determine which users' password should
be used.  The server will then compare the client response with a
computed correct response, and accept the user accordingly.

The server uses the @code{GSASL_QOPS} callback to get the set of
quality of protection values to use.  By default, it advertises
support for authentication (@code{qop-auth}) only.  You can use the
callback, for example, to make the server advertise support for
authentication with integrity layers.

The client uses the @code{GSASL_QOP} callback to get the quality of
protection value to request.  The client must choose one of the QOP
values offered by the server (which may be inspected through the
@code{GSASL_QOPS} property).  If the client does not return a value,
@code{qop-auth} is used by default.

The security layers of DIGEST-MD5 are rarely used in practice due to
interoperability and security reasons.  You are recommended to use TLS
instead.

The DIGEST-MD5 mechanism is specified in RFC 2831.  RFC 6331 labels
DIGEST-MD5 as historic and it contains a good exposition of the
disadvantages with DIGEST-MD5.

@node SCRAM
@section The SCRAM mechanisms
@cindex SCRAM

SCRAM is a family of mechanisms and we support SCRAM-SHA-1 and
SCRAM-SHA-256.  They differ only in the use of underlying hash function,
SHA-1 and SHA-256 respectively.  Channel bindings are supported through
the SCRAM-SHA-1-PLUS and SCRAM-SHA-256-PLUS mechanisms, and will bind
the authentication to a particular TLS channel.  SCRAM provides mutual
authentication, i.e., after a successful authentication the client will
know that the server knows the password, and the server will know that
the client knows the password.  Further, this can be achieved without
storing the password in clear text on either side.

The SCRAM family is designed to provide the same capabilities that
CRAM-MD5 and DIGEST-MD5 provides but with modern cryptographic
techniques such as HMAC hashing and PKCS#5 PBKDF2 key derivation.  SCRAM
supports authorization identities.  Like CRAM-MD5 and DIGEST-MD5, only a
hashed password is transferred.  Consequently, SCRAM servers needs
access to the correct password, the salted password, or the derived
ServerKey/StoredKey values, to verify the client response.

In the client, the non-PLUS mechanism is always enabled, and it requires
the @code{GSASL_AUTHID} property, and either @code{GSASL_PASSWORD} or
@code{GSASL_SCRAM_SALTED_PASSWORD}.  When the @code{GSASL_CB_TLS_UNIQUE}
property is available, the SCRAM-SHA-1-PLUS mechanism is also available
and it will negotiate channel bindings when the server also supports it.
If set, @code{GSASL_AUTHZID} will be used by the client.  To be able to
return the proper @code{GSASL_SCRAM_SALTED_PASSWORD} value, the callback
needs to check the @code{GSASL_SCRAM_ITER} and @code{GSASL_SCRAM_SALT}
values which are available when the @code{GSASL_SCRAM_SALTED_PASSWORD}
property is queried for.  The client/server may retrieve the calculated
@code{GSASL_SCRAM_SALTED_PASSWORD} value by retrieving it after the
final authentication step.  The @code{GSASL_SCRAM_SALTED_PASSWORD} value
can also be derived by using the
@code{gsasl_scram_secrets_from_password} function (@pxref{Utilities}),
or through the @code{--mkpasswd} parameter for the @code{gsasl} utility
(@pxref{Invoking gsasl}).

In the server, the @code{GSASL_AUTHID} property (and, when provided by
the client, the @code{GSASL_AUTHZID} property) will be set in order for
the callback to retrieve the user credentials.  The server mechanism
will request the @code{GSASL_SERVERKEY} and @code{GSASL_STOREDKEY}
properties first, and will use them to complete authentication.  Using
ServerKey/StoredKey on the server make it possible for the server to
avoid storing the clear-text password.  If ServerKey/StoredKey is not
available, the @code{GSASL_SCRAM_SALTED_PASSWORD} property is request,
and used to derive the ServetKey/StoredKey secrets.  When
@code{GSASL_SCRAM_SALTED_PASSWORD} is not available, the
@code{GSASL_PASSWORD} property is requested, which will be used to
derive the ServetKey/StoredKey secrets.  The mechanism uses the
credentials to authenticate the user.  The application may set the
@code{GSASL_SCRAM_ITER} and @code{GSASL_SCRAM_SALT} properties which
allow the server to tell the clients what values to use for deriving a
key from a password.  When the application do not supply them, the SCRAM
server will default to using a fresh random salt and an iteration count
of 4096.  After the final authentication step, the application may
retrieve the @code{GSASL_SCRAM_ITER}, @code{GSASL_SCRAM_SALT},
@code{GSASL_SCRAM_SALTED_PASSWORD}, @code{GSASL_SERVERKEY}, and
@code{GSASL_STOREDKEY} properties for potential storage in a database to
avoid the need to store the cleartext password.  When the
@code{GSASL_CB_TLS_UNIQUE} property is set, the SCRAM-*-PLUS mechanism
is supported and is used to negotiate channel bindings.

It is recommended for servers to stored ServerKey/StoredKey in a
database instead of @code{GSASL_SCRAM_SALTED_PASSWORD}, when possible,
since the latter is a password-equivalent but the former cannot directly
be used to impersonate the client (although one failed authentication
exchange against the server is sufficient to recover a
plaintext-equivalent from ServerKey/StoredKey).

@cindex channel binding
The @code{GSASL_CB_TLS_UNIQUE} property signal that this side of the
authentication supports channel bindings.  Setting the property will
enable the SCRAM-SHA-1-PLUS and SCRAM-SHA-256-PLUS mechanisms.  For
clients, this also instructs the SCRAM-SHA-1 and SCRAM-SHA-256 mechanism
to tell servers that the client believes the server does not support
channel bindings if it is used (remember that clients should otherwise
have chosen the SCRAM-SHA-1-PLUS mechanism instead of the SCRAM-SHA-1
mechanism).  For servers, it means the SCRAM-SHA-1/SCRAM-SHA-256
mechanism will refuse to authenticate against a client that signals that
it believes the server does not support channel bindings.

The SCRAM-SHA-*-PLUS mechanisms will never complete authentication
successfully if channel bindings are not confirmed.

To offer the intended security, the SCRAM-SHA-*-PLUS mechanisms MUST be
used over a TLS channel that has had the session hash extension (RFC
7627) negotiated, or session resumption MUST NOT have been used.  The
library cannot enforce this, so it is up to the application to only
provide the @code{GSASL_CB_TLS_UNIQUE} property when this condition
holds.

The SCRAM-SHA-1 mechanism is specified in RFC 5802 and SCRAM-SHA-256 is
specified in RFC 7677.  How to store SCRAM credentials in LDAP on the
server side is described in RFC 5803.

@node NTLM
@section The NTLM mechanism
@cindex NTLM

The NTLM is a non-standard mechanism.  Do not use it in new
applications, and do not expect it to be secure.  Currently only the
client side is supported.

In the client, this mechanism is always enabled, and it requires the
@code{GSASL_AUTHID} and @code{GSASL_PASSWORD} properties.  It will set
the @samp{domain} field in the NTLM request to the value of
@code{GSASL_REALM}.  Some servers reportedly need non-empty but
arbitrary values in that field.

@node SECURID
@section The SECURID mechanism
@cindex SECURID

The SECURID mechanism uses authentication and authorization identity
together with a passcode from a hardware token to authenticate users.

In the client, this mechanism is always enabled, and it requires the
@code{GSASL_AUTHID} and @code{GSASL_PASSCODE} properties.  If set,
@code{GSASL_AUTHZID} will also be used.  If the server requests it,
the @code{GSASL_PIN} property is also required, and its callback may
inspect the @code{GSASL_SUGGESTED_PIN} property to discover a
server-provided PIN to use.

In the server, this mechanism will invoke the
@code{GSASL_VALIDATE_SECURID} callback.  The callback may inspect the
@code{GSASL_AUTHID}, @code{GSASL_AUTHZID}, and @code{GSASL_PASSCODE}
properties.  The callback can return
@code{GSASL_SECURID_SERVER_NEED_ADDITIONAL_PASSCODE} to ask for
another additional passcode from the client.  The callback can return
@code{GSASL_SECURID_SERVER_NEED_NEW_PIN} to ask for a new PIN code
from the client, in which case it may also set the
@code{GSASL_SUGGESTED_PIN} property to indicate a recommended new PIN.
If the callbacks is invoked again, after having returned
@code{GSASL_SECURID_SERVER_NEED_NEW_PIN}, it may also inspect the
@code{GSASL_PIN} property, in addition to the other properties, to
find out the client selected PIN code.

The SECURID mechanism is specified in RFC 2808.

@node GSSAPI
@section The GSSAPI mechanism
@cindex GSSAPI

The GSSAPI mechanism allows you to authenticate using Kerberos V5.  The
mechanism was originally designed to allow for any GSS-API mechanism to
be used, but problems with the protocol made it unpractical and it is
today restricted for use with Kerberos V5.  See the GS2 mechanism
(@pxref{GS2-KRB5}) for a general solution.  However, GSSAPI continues to
be widely used in Kerberos V5 environments.

In the client, the mechanism is enabled only if the user has acquired
credentials (i.e., a ticket granting ticket), and it requires the
@code{GSASL_AUTHZID}, @code{GSASL_SERVICE}, and @code{GSASL_HOSTNAME}
properties.  (Earlier versions of the library incorrectly probed for
@code{GSASL_AUTHID} and used it as the authorization identity.)

In the server, the mechanism requires the @code{GSASL_SERVICE} and
@code{GSASL_HOSTNAME} properties, and it will invoke the
@code{GSASL_VALIDATE_GSSAPI} callback property in order to validate the
user.  The callback may inspect the @code{GSASL_AUTHZID} and
@code{GSASL_GSSAPI_DISPLAY_NAME} properties to decide whether to
authorize the user.  Note that authentication is performed by the
GSS-API library and that @code{GSASL_AUTHID} is not used by the server
mechanism, its role is played by @code{GSASL_GSSAPI_DISPLAY_NAME}.

The protocol does not distinguish between an absent authorization
identity and the empty authorization identity.  Earlier versions of the
library returned the empty string but currently it returns NULL, it is
suggested to treat both the same for this mechanism.

The server-part does not support security layers.  You are recommended
to use TLS instead.

The GSSAPI mechanism was specified as part of the initial core SASL
framework, in RFC 2222, but later revised in RFC 4752 to only apply to
Kerberos V5.

@node GS2-KRB5
@section The GS2-KRB5 mechanism
@cindex GS2
@cindex GS2-KRB5

GS2 is a protocol bridge between GSS-API and SASL, and allows every
GSS-API mechanism that supports mutual authentication and channel
bindings to be used as a SASL mechanism.  Currently GS2-KRB5 is
supported, for Kerberos V5 authentication, however our GS2
implementation is flexible enough to easily support other GSS-API
mechanism if any gains popularity.

In the client, the mechanism is enabled only if the user has acquired
credentials (i.e., a ticket granting ticket), and it requires the
@code{GSASL_AUTHZID}, @code{GSASL_SERVICE}, and @code{GSASL_HOSTNAME}
properties.

In the server, the mechanism requires the @code{GSASL_SERVICE} and
@code{GSASL_HOSTNAME} properties, and it will invoke the
@code{GSASL_VALIDATE_GSSAPI} callback property in order to validate the
user.  The callback may inspect the @code{GSASL_AUTHZID} and
@code{GSASL_GSSAPI_DISPLAY_NAME} properties to decide whether to
authorize the user.  Note that authentication is performed by the
GSS-API library and that @code{GSASL_AUTHID} is not used by the server
mechanism, its role is played by @code{GSASL_GSSAPI_DISPLAY_NAME}.

@cindex GS2-KRB5-PLUS
The GS2 framework supports a variant of each mechanism, called the PLUS
variant, which can also bind the authentication to a secure channel
through channel bindings.  Currently this is not supported by GNU SASL.

The GS2 mechanism family was specified in RFC 5801.

@node SAML20
@section The SAML20 mechanism
@cindex SAML

The SAML20 mechanism makes it possible to use SAML in SASL, in a way
that offloads the authentication exchange to an external web browser.

The mechanism makes use of the following properties:
@code{GSASL_AUTHZID}, @code{GSASL_SAML20_IDP_IDENTIFIER},
@code{GSASL_SAML20_REDIRECT_URL},
@code{GSASL_SAML20_AUTHENTICATE_IN_BROWSER} and
@code{GSASL_VALIDATE_SAML20}.

In client mode, the mechanism will retrieve the @code{GSASL_AUTHZID} and
@code{GSASL_SAML20_IDP_IDENTIFIER} properties and form a request to the
server.  The server will respond with a redirect URL stored in the
@code{GSASL_SAML20_REDIRECT_URL} property, which the client can retrieve
from the @code{GSASL_SAML20_AUTHENTICATE_IN_BROWSER} callback.  The
intention is that the client launches a browser to the given URL, and
then proceeds with authentication.  The server responds whether
authentication was successful or not.

In server mode, the mechanism will invoke the
@code{GSASL_SAML20_REDIRECT_URL} callback and the application can
inspect the @code{GSASL_AUTHZID} and @code{GSASL_SAML20_IDP_IDENTIFIER}
properties when forming the redirect URL.  The URL is passed to the
client which will hopefully complete authentication in the browser.  The
server callback @code{GSASL_VALIDATE_SAML20} should check whether the
authentication attempt was successful.

Note that SAML itself is not implemented by the GNU SASL library.  On
the client side, no SAML knowledge is needed, it is only required on the
server side.  The client only needs to be able to start a web browser to
access the redirect URL.  The server side is expected to call a SAML
library to generate the AuthRequest and to implement an
AssertionConsumerService (ACS) to validate the AuthResponse.  There is a
complete proof-of-concept example of a SMTP server with SAML 2.0 support
distributed with GNU SASL in the @code{examples/saml20/} sub-directory.
The example uses the Lasso SAML implementation
(@url{http://lasso.entrouvert.org/}) and require a web server for the
ACS side.  The example may be used as inspiration for your own server
implementation.  The @code{gsasl} command line client supports SAML20 as
a client.

The SAML20 mechanism is specified in RFC 6595.

@node OPENID20
@section The OPENID20 mechanism
@cindex OpenID

The OPENID20 mechanism makes it possible to use OpenID in SASL, in a way
that offloads the authentication exchange to an external web browser.

The mechanism makes use of the following properties: @code{GSASL_AUTHID}
(for the OpenID User-Supplied Identifier), @code{GSASL_AUTHZID},
@code{GSASL_OPENID20_REDIRECT_URL}, @code{GSASL_OPENID20_OUTCOME_DATA},
@code{GSASL_OPENID20_AUTHENTICATE_IN_BROWSER}, and
@code{GSASL_VALIDATE_OPENID20}.

In the client, the mechanism is enabled by default.  The
@code{GSASL_AUTHID} property is required and should contain the
User-Supplied OpenID Identifier (for example
@code{https://josefsson.org}).  If set, @code{GSASL_AUTHZID} will be used
by the client.  The client will be invoked with the
@code{GSASL_OPENID20_AUTHENTICATE_IN_BROWSER} callback to perform the
OpenID authentication in a web browser.  The callback can retrieve the
@code{GSASL_OPENID20_REDIRECT_URL} property to find out the URL to
redirect the user to.  After authentication, the client can retrieve the
@code{GSASL_OPENID20_OUTCOME_DATA} property with the OpenID Simple
Registry (SREG) attributes sent by the server (if any).

In the server, the mechanism is enabled by default.  The server will
request the @code{GSASL_OPENID20_REDIRECT_URL} property, and your
callback may inspect the @code{GSASL_AUTHID} to find the OpenID
User-Supplied Identifier.  The server callback should perform OpenID
discovery and return the URL to redirect the user to.  After this, the
user would access the URL and proceed with authentication in the
browser.  The server is invoked with the
@code{GSASL_VALIDATE_OPENID20} callback to perform the actual
validation of the authentication.  Usually the callback will perform
some IPC communication with an OpenID consumer running in a web
server.  The callback should return @code{GSASL_OK} on successful
authentication and @code{GSASL_AUTHENTICATION_ERROR} on authentication
errors, or any other error code.  If the server received some OpenID
Simple Registry (SREG) attributes from the OpenID Identity Provider,
it may use the @code{GSASL_OPENID20_OUTCOME_DATA} property to send
these to the client.

Note that OpenID itself is not implemented by the GNU SASL library.  On
the client side, no OpenID knowledge is required, it is only required on
the server side.  The client only needs to be able to start a web
browser to access the redirect URL.  The server side is expected to use
an OpenID library to generate the redirect URL and to implement the
Service Provider (SP) to validate the response from the IdP.  There is a
complete proof-of-concept example with a SMTP server with OpenID 2.0
support distributed with GNU SASL in the @code{examples/openid20/}
sub-directory.  It uses the JanRain PHP5 OpenID implementation and
require a web server to implement the OpenID SP.  The example may be
used as inspiration for your own server implementation.  The
@code{gsasl} command line client supports OPENID20 as a client.

The OPENID20 mechanism is specified in RFC 6616.

@c **********************************************************
@c *****************  Global Functions  *********************
@c **********************************************************
@node Global Functions
@chapter Global Functions

@include texi/gsasl_init.texi
@include texi/gsasl_done.texi
@include texi/gsasl_client_mechlist.texi
@include texi/gsasl_server_mechlist.texi
@include texi/gsasl_client_support_p.texi
@include texi/gsasl_server_support_p.texi
@include texi/gsasl_client_suggest_mechanism.texi
@include texi/gsasl_register.texi
@include texi/gsasl_mechanism_name_p.texi

@c **********************************************************
@c ******************  Callback Functions  ******************
@c **********************************************************
@node Callback Functions
@chapter Callback Functions
@cindex Callbacks

The callback is used by mechanisms to retrieve information, such as
username and password, from the application.  In a server, the
callback is used to decide whether a user is permitted to log in or
not.  You tell the library of your callback function by calling
@code{gsasl_callback_set}.

Since your callback may need access to data from other parts of your
application, there are hooks to store and retrieve application
specific pointers.  This avoids the use of global variables, which
wouldn't be thread safe.  You store a pointer to some information
(opaque from the point of view of the library) by calling
@code{gsasl_callback_hook_set} and can later retrieve this data in
your callback by calling @code{gsasl_callback_hook_get}.

@include texi/gsasl_callback_set.texi
@include texi/gsasl_callback.texi
@include texi/gsasl_callback_hook_set.texi
@include texi/gsasl_callback_hook_get.texi
@include texi/gsasl_session_hook_set.texi
@include texi/gsasl_session_hook_get.texi

@c **********************************************************
@c ******************  Property Functions  ******************
@c **********************************************************
@node Property Functions
@chapter Property Functions
@cindex Properties

@include texi/gsasl_property_free.texi
@include texi/gsasl_property_set.texi
@include texi/gsasl_property_set_raw.texi
@include texi/gsasl_property_fast.texi
@include texi/gsasl_property_get.texi


@c **********************************************************
@c *****************  Session Functions  ********************
@c **********************************************************
@node Session Functions
@chapter Session Functions
@cindex SASL sessions

@include texi/gsasl_client_start.texi
@include texi/gsasl_server_start.texi
@include texi/gsasl_step.texi
@include texi/gsasl_step64.texi
@include texi/gsasl_finish.texi
@include texi/gsasl_encode.texi
@include texi/gsasl_decode.texi
@include texi/gsasl_mechanism_name.texi



@c **********************************************************
@c *******************  Utilities  **************************
@c **********************************************************
@node Utilities
@chapter Utilities

@include texi/gsasl_saslprep.texi
@include texi/gsasl_base64_to.texi
@include texi/gsasl_base64_from.texi
@include texi/gsasl_hex_to.texi
@include texi/gsasl_hex_from.texi
@include texi/gsasl_simple_getpass.texi
@include texi/gsasl_nonce.texi
@include texi/gsasl_random.texi
@include texi/gsasl_hash_length.texi
@include texi/gsasl_scram_secrets_from_salted_password.texi
@include texi/gsasl_scram_secrets_from_password.texi

@c **********************************************************
@c ****************  Memory Handling  ***********************
@c **********************************************************
@node Memory Handling
@chapter Memory Handling

@include texi/gsasl_free.texi


@c **********************************************************
@c *******************  Errors  ****************************
@c **********************************************************
@node Error Handling
@chapter Error Handling
@cindex Error Handling

Most functions in the GNU SASL Library return an error if they fail.
For this reason, the application should always catch the error
condition and take appropriate measures, for example by releasing the
resources and passing the error up to the caller, or by displaying a
descriptive message to the user and cancelling the operation.

Some error values do not indicate a system error or an error in the
operation, but the result of an operation that failed properly.

@menu
* Error values::                A list of all error values used.
* Error strings::               How to get a descriptive string from a value.
@end menu

@node Error values
@section Error values

Errors are returned as @code{int} values.

The value of the symbol @code{GSASL_OK} is guaranteed to always be
@code{0}, and all other error codes are guaranteed to be non-@code{0},
so you may use that information to build boolean expressions involving
return codes.  Otherwise, an application should not depend on the
particular value for error codes, and are encouraged to use the
constants even for @code{GSASL_OK} to improve readability.  Possible
values are:

@table @code

@include gsasl-api-error-labels.texi

@end table

@node Error strings
@section Error strings

@include texi/gsasl_strerror.texi
@include texi/gsasl_strerror_name.texi


@c **********************************************************
@c ***********************  Examples  ***********************
@c **********************************************************
@node Examples
@chapter Examples
@cindex Examples

This chapter contains example code which illustrates how the GNU SASL
Library can be used when writing your own application.

@menu
* Example 1::		SASL client.
* Example 2::		SASL client where server send data first.
* Example 3::		SASL client, with a choice of mechanism to use.
* Example 4::		SASL client, with callback for user info.
* Example 5::		Example SMTP server with GNU SASL authentication.
@end menu

@node Example 1
@section Example 1

@example
@verbatiminclude client.c
@end example

@node Example 2
@section Example 2

@example
@verbatiminclude client-serverfirst.c
@end example

@node Example 3
@section Example 3

@example
@verbatiminclude client-mech.c
@end example

@node Example 4
@section Example 4

@example
@verbatiminclude client-callback.c
@end example

@node Example 5
@section Example 5

@example
@verbatiminclude smtp-server.c
@end example


@c **********************************************************
@c *******************  Acknowledgements  *******************
@c **********************************************************
@node Acknowledgements
@chapter Acknowledgements

The makefiles, manuals, etc borrowed much from Libgcrypt written by
Werner Koch.

Cryptographic functions for some SASL mechanisms uses Libgcrypt by
Werner Koch et al.  The NTLM mechanism uses Libntlm by Grant Edwards et
al, using code from Samba written by Andrew Tridgell, and now maintained
by Simon Josefsson.  The GSSAPI and GS2-KRB5 mechanism uses a GSS-API
implementation, such as MIT Kerberos, Heimdal or GNU GSS.

Gnulib is used to simplify portability.

This manual borrows text from the SASL specification.

@c **********************************************************
@c ********************  Invoking gsasl  ********************
@c **********************************************************
@node Invoking gsasl
@chapter Invoking gsasl

@pindex gsasl
@cindex invoking @command{gsasl}
@cindex command line

@majorheading Name

GNU SASL (gsasl) -- Command line interface to libgsasl.

@majorheading Description

@code{gsasl} is the main program of GNU SASL.

This section only lists the commands and options available.

Mandatory or optional arguments to long options are also mandatory or
optional for any corresponding short options.

@majorheading Commands

@code{gsasl} recognizes these commands:

@verbatim
  -c, --client               Act as client (the default).
      --client-mechanisms    Write name of supported client mechanisms
                             separated by space to stdout.
  -s, --server               Act as server.
      --server-mechanisms    Write name of supported server mechanisms
                             separated by space to stdout.
  -k, --mkpasswd             Derive password. Provide --mechanism as SCRAM-SHA-1 or
                             SCRAM-SHA-256.  The required inputs are
                             password (through --password or read from
                             terminal) and optional inputs are iteration
                             count (through --iteration-count, or
                             defaulting to 65536) and salt (through
                             --salt, or generated randomly).  The output
                             is a string of the form
                             "{mech}count,salt,stored-key,server-key[,salted-password]"
                             where "mech" is the mechanism, "count" is
                             the number of times password was hashed,
                             "salt" is the provided/generated
                             base64-encoded salt, "stored-key" and
                             "server-key" are the two derived and
                             base64-encoded server-side keys.  When
                             --verbose is provided, "salted-password"
                             will be included as the hex-encoded PBKDF2-derived
                             password.  (default=off)
@end verbatim

@majorheading Network Options

Normally the SASL negotiation is performed on the terminal, with
reading from stdin and writing to stdout.  It is also possible to
perform the negotiation with a server over a TCP network connection.

@verbatim
      --connect=HOSTNAME[:SERVICE]
                             Connect to TCP server and negotiate on stream
                             instead of stdin/stdout. SERVICE is the protocol
                             service, or an integer denoting the port, and
                             defaults to 143 (imap) if not specified. Also sets
                             the --hostname default.
@end verbatim

@majorheading Miscellaneous Options:

These parameters affect overall behaviour.

@verbatim
  -d, --application-data     After authentication, read data from stdin and run
                             it through the mechanism's security layer and
                             print it base64 encoded to stdout. The default is
                             to terminate after authentication.
      --imap                 Use a IMAP-like logon procedure (client only).
                             Also sets the --service default to "imap".
  -m, --mechanism=STRING     Mechanism to use.
      --no-client-first      Disallow client to send data first (client only).
@end verbatim

@majorheading SASL Mechanism Options

These options modify the behaviour of the callbacks (@pxref{Callback
Functions}) in the library.  The default is to query the user on the
terminal.

@verbatim
  -n, --anonymous-token=STRING    Token for anonymous authentication, usually
                                  mail address (ANONYMOUS only).
  -a, --authentication-id=STRING  Identity of credential owner.
  -z, --authorization-id=STRING   Identity to request service for.
      --disable-cleartext-validate
                             Disable cleartext validate hook, forcing server to
                             prompt for password.
      --enable-cram-md5-validate  Validate CRAM-MD5 challenge and response
                                  interactively.
      --hostname=STRING      Set the name of the server with the requested
                             service.
  -p, --password=STRING      Password for authentication (insecure for
                             non-testing purposes).
      --passcode=NUMBER      Passcode for authentication (SECURID only).
      --quality-of-protection=<qop-auth | qop-int | qop-conf>
                             How application payload will be protected.
                             "qop-auth" means no protection,
                             "qop-int" means integrity protection,
                             "qop-conf" means confidentiality.
                             Currently only used by DIGEST-MD5, where the
                             default is "qop-int".
  -r, --realm=STRING         Realm. Defaults to hostname.
      --service=STRING       Set the requested service name (should be a
                             registered GSSAPI host based service name).
      --service-name=STRING  Set the generic server name in case of a
                             replicated server (DIGEST-MD5 only).
      --iteration-count=NUMBER  Indicate PBKDF2 hash iteration count (SCRAM
                                  only).  (default=`65536')
      --salt=B64DATA         Indicate PBKDF2 salt as base64-encoded string
                                  (SCRAM only).
      --scram-salted-password=STRING
                                Salted SCRAM password for authentication (SCRAM
                                  only; 40 hex characters for SCRAM-SHA-1 and
                                  64 characters for SCRAM-SHA-256).
@end verbatim

@majorheading STARTTLS options

@verbatim
      --starttls                Force use of STARTTLS.  The default is to use
                                  STARTTLS when available.  (default=off)
      --no-starttls             Unconditionally disable STARTTLS.
                                  (default=off)
      --no-cb                   Don't set any channel bindings.  (default=off)
      --x509-ca-file=FILE       File containing one or more X.509 Certificate
                                  Authorities certificates in PEM format, used
                                  to verify the certificate received from the
                                  server.  If not specified, verification uses
                                  system trust settings.  If FILE is the empty
                                  string, don't fail on X.509 server
                                  certificates verification errors.
      --x509-cert-file=FILE     File containing client X.509 certificate in PEM
                                  format.  Used together with --x509-key-file
                                  to specify the certificate/key pair.
      --x509-key-file=FILE      Private key for the client X.509 certificate in
                                  PEM format.  Used together with
                                  --x509-key-file to specify the
                                  certificate/key pair.
      --priority                Cipher priority string.
@end verbatim

@majorheading Other Options

These are some standard parameters.

@verbatim
  -q, --quiet, --silent      Don't produce any diagnostic output.
  -v, --verbose              Produce verbose output.

  -?, --help                 Give this help list
      --usage                Give a short usage message
  -V, --version              Print program version
@end verbatim

@c **********************************************************
@c *******************  Appendices  *************************
@c **********************************************************

@node Protocol Clarifications
@appendix Protocol Clarifications

This appendix contains clarifications to various SASL specification
that we felt were necessary to include, if for nothing else it may
serve as a guide for other implementers that worry about the same
issues.

@menu
* Use of SASLprep in CRAM-MD5::
* Use of SASLprep in LOGIN::
@end menu

@node Use of SASLprep in CRAM-MD5
@section Use of SASLprep in CRAM-MD5

The specification, as of @file{draft-ietf-sasl-crammd5-04.txt}, is
silent on whether a SASL server implementation applying SASLprep on a
password received from an external, non-SASL specific database (i.e.,
the passwords are not stored in SASLprep form in the database), should
set or clear the AllowUnassigned bit.  The motivation for the AU-bit
in StringPrep/SASLprep is for stored vs query strings.  It could be
argued that in this situation the server can treat the external
password either as a stored string (from a database) or as a query
(the server uses the string as a query into the fixed HMAC-MD5 hash).

The specification is also unclear on whether clients should set or
clear the AllowUnassigned flag.

In the server, GNU SASL applies SASLprep to the password with the
AllowUnassigned bit cleared.

@node Use of SASLprep in LOGIN
@section Use of SASLprep in LOGIN

The non-standard mechanism LOGIN presumably does not support
non-ASCII.  We suggest that the client should send unprepared UTF-8
and that the server apply SASLprep with the AllowUnassigned bit
cleared on the received username and password.

@node Copying Information
@appendix Copying Information

@menu
* GNU Free Documentation License::   License for copying this manual.
@end menu

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

@cindex FDL, GNU Free Documentation License

@include fdl-1.3.texi

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

@printindex fn

@node Concept Index
@unnumbered Concept Index

@printindex cp

@bye
