The Rossignols, a family of French cryptographers and cryptanalysts, included Antoine Rossignol (1600–1682), Bonaventure Rossignol and Antoine-Bonaventure Rossignol. The family name means "nightingale" in French. As early as 1406, the word rossignol has served as the French term for "skeleton key" or for any tool which opens that which is locked.

In cryptography, a substitution cipher is a method of encrypting in which units of plaintext are replaced with the ciphertext, in a defined manner, with the help of a key; the "units" may be single letters, pairs of letters, triplets of letters, mixtures of the above, and so forth. The receiver deciphers the text by performing the inverse substitution process to extract the original message.