cryptokwot (substitution cipher game)

This is a game which the player must decipher a quotation that's been
enciphered with a randomly generated simple substitution cipher. If
you've ever done a newspaper Cryptoquote or Cryptoquip, this will be
familiar.

cryptokwot doesn't solve the puzzle for you; it basically replaces
pencil and paper, allowing you to assign plaintext letters to
ciphertext ones interactively.