SASL Authentication

This document describes the client protocol for SASL authentication, as implemented in charybdis and atheme.

SASL authentication relies on the CAP client capability framework [1]. 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.

regular-authenticate-set = *("AUTHENTICATE" SP 400BASE64 CRLF)
    "AUTHENTICATE" SP (1*399BASE64 / "+") CRLF

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.

C: CAP REQ :sasl
C: NICK jilles
C: USER jilles cheetah.stack.nl 1 :Jilles Tjoelker
S: NOTICE AUTH :*** Processing connection to jaguar.test
S: NOTICE AUTH :*** Looking up your hostname...
S: NOTICE AUTH :*** Checking Ident
S: NOTICE AUTH :*** No Ident response
S: NOTICE AUTH :*** Found your hostname
S: :jaguar.test CAP jilles ACK :sasl
C: AUTHENTICATE PLAIN
S: AUTHENTICATE +
C: AUTHENTICATE amlsbGVzAGppbGxlcwBzZXNhbWU=
S: :jaguar.test 900 jilles jilles!jilles@localhost.stack.nl jilles :You are now logged in as jilles.
S: :jaguar.test 903 jilles :SASL authentication successful
C: CAP END
S: :jaguar.test 001 jilles :Welcome to the jillestest Internet Relay Chat Network jilles
<usual welcome messages>

Note that the CAP command sent by a server includes the user's nick or *, differently from what [1] specifies.

Alternatively the client could request the list of capabilities and enable an additional capability.

C: CAP LS
C: NICK jilles
C: USER jilles cheetah.stack.nl 1 :Jilles Tjoelker
S: NOTICE AUTH :*** Processing connection to jaguar.test
S: NOTICE AUTH :*** Looking up your hostname...
S: NOTICE AUTH :*** Checking Ident
S: NOTICE AUTH :*** No Ident response
S: NOTICE AUTH :*** Found your hostname
S: :jaguar.test CAP * LS :multi-prefix sasl
C: CAP REQ :multi-prefix sasl
S: :jaguar.test CAP jilles ACK :multi-prefix sasl
C: AUTHENTICATE PLAIN
S: AUTHENTICATE +
C: AUTHENTICATE amlsbGVzAGppbGxlcwBzZXNhbWU=
S: :jaguar.test 900 jilles jilles!jilles@localhost.stack.nl jilles :You are now logged in as jilles.
S: :jaguar.test 903 jilles :SASL authentication successful
C: CAP END
S: :jaguar.test 001 jilles :Welcome to the jillestest Internet Relay Chat Network jilles
<usual welcome messages>

[1] 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).

The AUTHENTICATE command​

The AUTHENTICATE command