A polyphonic substitution cipher is one in which several different plaintext letters are enciphered into a single cipher letter or symbol. Perhaps the most simple and well-known example of a polyphonic cipher is the telephone dial, in which the letters ABC are encoded by the number 2, DEF by 3, GHI by 4, JKL by 5, MNO by 6, PRS by 7, TUV by 8, and WXY by 9. Polyphonic ciphers have tended to be shunned by cryptologists because of the inevitable ambiguity encountered in recovering a message. However, if one turns the problem around and asks how one should encode the alphabet to make it as easy as possible to recover a message, then polyphonic ciphers are deserving of study. Since the English language is highly redundant, it is possible to tolerate a considerable amount of ambiguity in decoding.
Eckler, A. Ross
"Another Polyphonic Cipher,"
Word Ways: Vol. 11
, Article 17.
Available at: http://digitalcommons.butler.edu/wordways/vol11/iss2/17