This document discusses rotor machines, which are electro-mechanical stream cipher devices used to encrypt and decrypt secret messages. Rotor machines were widely used for cryptography from the 1920s to 1970s. The most famous example is the German Enigma machine, whose messages were deciphered by the Allies during World War II to produce intelligence code-named Ultra. The document also briefly mentions that substitution ciphers encrypt by replacing plaintext units like single letters or pairs of letters with ciphertext units.