Ciphers and Computers

A distinction is sometimes made between polygraphic substitution ciphers and polyliteral substitution ciphers. Polygraphic ciphers, as we saw in Section 1.6, transform a block of plaintext letters into a block of ciphertext letters of the same size. Polyliteral ciphers, on the other hand, transform a single letter into a block of letters or symbols. For our first example, we go back to the ancient Greeks again. In the second century BCE, the Greek historian Polybius wrote a 40-volume history of ancient Greece and Rome with considerably many digressions, including one about cryptography and, in particular, what he refers to as