GNU SASL is an implementation of the Simple Authentication and Security Layer framework and a few common SASL mechanisms. SASL is used by network servers (e.g., IMAP, SMTP) to request authentication from clients, and in clients to authenticate against servers. GNU SASL consists of a library (`libgsasl'), a command line utility (`gsasl') to access the library from the shell, and a manual. The library includes support for the framework (with authentication functions and application data privacy and integrity functions) and at least partial support for the CRAM-MD5, EXTERNAL, GSSAPI, ANONYMOUS, PLAIN, SECURID, DIGEST-MD5, LOGIN, and NTLM mechanisms. 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 utilize 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 support client and server mode, and the IMAP and SMTP protocols, fits in under 60kb on an Intel x86 platform, without any modifications to the code. (This figure was accurate as of version 0.0.13.) The library is licensed under the GNU Lesser General Public License version 2.1. The command-line application (src/), examples (examples/), self-test suite (tests/) are licensed under the GNU General Public License license version 3.0. The documentation (doc/) is licensed under the GNU Free Documentation License version 1.2. A conceptual view of how your application, the library, and each mechanism interact is shown in .

The operation of an application using the library can best be understood in terms of a flow chart diagram, as shown in . The details on how the actual negotiation are carried out are illustrated in .