Ah, it was my fault then. Sorry. I had a header of an IP packet in mind. Its format is known and machine-verifiable, and even includes a simple checksum, so it'

Ah, it was my fault then. Sorry. I had a header of an IP packet in mind. Its format is known and machine-verifiable, and even includes a simple checksum, so it's quite easy to tell if the guessed encryption key has a chance of being valid.