The rules vary by country. In the US, the FCC has prohibited amateur stations from using encryption intended to 'obscure the meaning' of a transmission. It's a

The rules vary by country. In the US, the FCC has prohibited amateur stations from using encryption intended to 'obscure the meaning' of a transmission. It's a grey area whether you're permitted to encrypt something for other purposes. This permits digital signatures, since they do not obscure anything. In Canada, the Radiocommunication Regulations only permit amateur stations to 'use a code or cipher that is not secret'. That prohibits any form of encryption unless you publish all keys involved in the transmission. By my reading of it, it also seems to prohibit creating a digital signature, since such a signature could be considered a code that is based on a key that is not public (otherwise anyone could forge your signature). I'm not sure if there's any official rulings on this.