Cryptography is usually banned in forum mafia.
This raise a question, though, if a cryptography ban is actually enforceable in forum mafia, in games with unlimited amounts of data per post. Would the potential for information hiding in posts make it possible for two players to solicit eachother for a cryptographic exchange channel without the moderator who views the post knowing, and/or somehow being able to always have plausible deniability that they were soliciting a cryptographic channel of communication (I wasn't signalling him, my post just happened to have that pattern in it).
My knowledge of cryptography is limited enough that I'm not really for sure. It would counterintuitive/surprising if you could do so under a moderator's nose, but I find unbreakable cryptography counterintuitive/suprising also so I can't just lean on that intuition.
Of course, in practice, forum mafia is more fun without cryptography, if you break the moderator's rule you are being a Sea Hag, and it's extra hard to find a second person that also wants to be a Sea Hag with you.