Reply To: Stuff to do while you wait (with a message from Harry added)
@Harry reminds me of the forum conversation we had a few years back, about two-time pads.
On the topic of two-time pads:
A two-time pad is solvable as a vigenere with key length half that of the ciphertext length, if you concatenate the ciphertexts. Theoretically, this method gets easier every time the key is used on a same-length plaintext.
Are there any other ways to crack a two-time pad?
[The puzzle master at GCHQ gave a good talk about this at the prize giving a while ago. Cribs are your friend. If you have one key, K and two messages M1 and M2 then the cipher texts are C1=M1+K; C2=M2+K, so the difference D is C1-C2=(M1+K-(M2+K) = M1-M2. The effect is that D is the encryption of M1 using the plaintext M2 as the OTP. If you know (or strongly believe) that a particular word or phrase appears in M1 you can search for it by substituting it back in at all the possible locations. Typically this gives you a break that allows you to reconstruct a fragment of M2, and if that is partial then you can guess its completion to get more of M2, substitute it back in and get some more of M1 and so on. Harry]