Blowfish Encryption Manual » History » Version 4
Blowfish can be used to encrypt messages between 2 persons in query, messages in channel and the topic. In Quassel all messages will be de-/encrypted on core. So it is highly recommend to setup SSL in case of you are not running monolithic client.
a) setting a key for a user or channel
/setkey <nick|channel> <key>
b) deleting a key for a user or channel
Blowfish support for Quassel depends on QCA (Qt Cryptographic Architecture) library. It is needed to compile your core/monolithic client with crypt compile option.
... -DWITH_CRYPT=ON ...
For detailed build instructions have at look general build instructons.