Unicity distance: The length of cipher text necessary for an attacker to determine whether a guessed decryption key unlocks a uniquely coherent message. For example, if Alice encrypts the single letter "A", attacker Mallory might try various keys and wind up with possible messages "Q", "Z", "W". With this little plain text, Mallory has no way of knowing whether he has come across the right decryption key. However, he is pretty safe in assuming that "Launch rockets at 7 p.m." is a real message, while "qWsl*(dk883 slOO1234 >" is an unsuccessful decryption. The actual mathematics of unicity distance depend on key length, but for DES and English prose as plain text, unicity distance is about eight characters of text.