Unlocking Classic Ciphers: Column Transposition, Hill Cipher & More 🔐

Tanja discussed the column transposition cipher. Tanja discussed a bit about rotor machines and cryptanalysis. Visit the exhibition of rotor machines from the Cryptomuseum that is currently on show in the MetaForum and take a look at their website (and museum if you get a chance).

Tanja discussed how to break the Hill cipher given some plaintext-ciphertext pairs and in particular repeated the extended Euclidean algorithm.

One-time pad with bits, and problems with reuse.

Finally, Tanja discussed stream ciphers. These are much more practical than the OTP in that the key is much shorter. To encrypt a message, expand the key into a stream of pseudo-random bits and xor those to the message, i.e., treat the stream-cipher output as the one-time pad. To encrypt multiple message it becomes necessary to remember how many bits have been used and either stay in that state or forward by that many positions the next time one uses the cipher. This is impractical. Initialization Vectors (IVs) deal with that problem in that they move the beginning of the stream to a random position. The IV is then sent in clear along with the ciphertext, so that the receiving end can compute the same starting position.