This document describes the client protocol for SASL authentication, as implemented in charybdis and atheme.
SASL authentication relies on the
CAP client capability framework .
Support for SASL authentication is indicated with the "sasl" capability.
The client MUST enable the sasl capability before using the AUTHENTICATE
command defined by this specification.
The AUTHENTICATE command
The AUTHENTICATE command MUST be used before registration is complete and with the sasl capability enabled. To enforce the former, it is RECOMMENDED to only send CAP END when the SASL exchange is completed or needs to be aborted. Clients SHOULD be prepared for timeouts at all times during the SASL authentication.
There are two forms of the AUTHENTICATE command: initial client message and later messages.
The initial client message specifies the SASL mechanism to be used. (When this is received, the IRCD will attempt to establish an association with a SASL agent.) If this fails, a 904 numeric will be sent and the session state remains unchanged; the client MAY try another mechanism. Otherwise, the server sends a set of regular AUTHENTICATE messages with the initial server response.
initial-authenticate = "AUTHENTICATE" SP mechanism CRLF
A set of regular AUTHENTICATE messages transmits a response from client to server or vice versa. The server MAY intersperse other IRC protocol messages between the AUTHENTICATE messages of a set. The "+" form is used for an empty response. The server MAY place a limit on the total length of a response.
The client can abort an authentication by sending an asterisk as the data. The server will send a 904 numeric.
authenticate-abort = "AUTHENTICATE" SP "*" CRLF
If authentication fails, a 904 or 905 numeric will be sent and the
client MAY retry from the
AUTHENTICATE <mechanism> command.
If authentication is successful, a 900 and 903 numeric will be sent.
If the client attempts to issue the AUTHENTICATE command after already authenticating successfully, the server MUST reject it with a 907 numeric.
If the client completes registration (with CAP END, NICK, USER and any other necessary messages) while the SASL authentication is still in progress, the server SHOULD abort it and send a 906 numeric, then register the client without authentication.
This document does not specify use of the AUTHENTICATE command in registered (person) state.
Example protocol exchange
C: indicates lines sent by the client, S: indicates lines sent by the server.
The client is using the PLAIN SASL mechanism with authentication identity jilles, authorization identity jilles and password sesame.
Note that the CAP command sent by a server includes the user's nick or *, differently from what  specifies.
Alternatively the client could request the list of capabilities and enable an additional capability.
 K. Mitchell, P. Lorier (Undernet IRC Network), L. Hardy (ircd-ratbox), P. Kucharski (IRCnet), IRC Client Capabilities Extension. March 2005. This internet-draft has expired; it can still be found on http://www.leeh.co.uk/draft-mitchell-irc-capabilities-02.html
See also http://sasl.charybdis.be/ and http://wiki.atheme.net/index.php/PR:SASL_Authentication (these links are currently dead but may be resurrected in the future).