The silliest part of all is that bad guys have been using slang and in-jokes for centuries. That’s cryptography, and it’s also impossible to ban.
Also, if the message is short, even an extremely simple cypher is unbreakable. If the NSA has a voice conversation, that’s a ton of data there, but if you can issue orders to your minions in 140 characters, there’s simply not enough information to begin cracking.
