IRCv3 SASL DH-BLOWFISH Authentication Mechanism

This mechanism has been deemed unsafe and has been deprecated. Please refer to the SASL Mechanisms page for mechanisms that the IRCv3 WG now recommends.

This specification has been deprecated. We do not recommend implementing this specification or the features described here.

Copyright © 2013-2015 Mantas Mikulėnas <grawity@gmail.com>

Unlimited redistribution and modification of this document is allowed provided that the above copyright notice and this permission notice remains intact.


This specification documents the DH-BLOWFISH SASL mechanism currently implemented by various IRC clients and services.

The mechanism is non-standard, and specific to IRC software. It uses Diffie-Hellman key exchange to choose a shared encryption key, which is then used to encrypt the password sent to the server.

The mechanism does not authenticate the server, and is considered insecure against active MITM attacks.

The mechanism does not allow an authorization identity to be specified.

The mechanism is considered deprecated and should be avoided, as it gives a false sense of security to many users.

Step 1

The server starts by sending a challenge containing the DH parameters p, g, and the server’s public key, each serialized as two bytes containing the parameter’s length (network byte order), followed by the parameter itself as an OpenSSL bignum.

message = bignum(p) . bignum(g) . bignum(pub_key)

Step 2

The client computes the shared key from the received DH parameters, and uses it to encrypt the password using Blowfish in ECB mode. The password must be encoded as UTF-8 and null-terminated.

The client then sends the response consisting of its own DH public key, the authentication identity as a null-terminated string, and the encrypted password. Again, the public key is serialized as two bytes containing the length, followed by the key itself as a bignum.

raw_credentials = pad(8, cstring(password))

enc_credentials = blowfish_ecb[shared_key](raw_credentials)

message = bignum(pub_key) . cstring(authn_id) . enc_credentials

Step 3

The server computes the same shared key from the DH parameters and client’s public key, and uses it to decrypt the password.