Randy is mounting a known-ciphertext attack: the hardest kind. He has the ciphertext (the Arethusa intercepts) and nothing else. He doesn’t even know the algorithm that was used to encrypt them. In modern cryptanalysis, this is unusual; normally the algorithms are public knowledge. That is because algorithms that have been openly discussed and attacked within the academic community tend to be much stronger than ones that have been kept secret. People who rely on keeping their algorithms secret are ruined as soon as that secret gets out. But Arethusa dates from World War II, when people were much less canny about such things.