def sign_ssh_data(self, rng, data):
return '<STR_LIT>'<EOL>
Sign a blob of data with this private key, and return a L{Message}
representing an SSH signature message.

@param rng: a secure random number generator.
@type rng: L{Crypto.Util.rng.RandomPool}
@param data: the data to sign.
@type data: str
@return: an SSH signature message.
@rtype: L{Message}