Cryptography Reference
In-Depth Information
chief cryptoanalyst in the Signal Intelligence Service, notably
leading the teams that broke various Japanese codes, includ-
ing ultimately the Purple machine cipher initiated by Japan in
1939. After World War II, William Friedman worked awhile
for the National Security Agency, and Elizebeth Friedman for
the International Monetary Fund. William died on November
2, 1969, in Washington, D.C., and Elizebeth died on October
31, 1980, in Plainfield, New Jersey.
William Friedman wrote
The Index of Coincidence and Its
Applications in Cryptography
(1922), one of the standard works
in the nomenclature and classification of ciphers. Together, the
Friedmans wrote
The Shakespearean Ciphers Examined
(1957), in
which they denied Francis Bacon's purported authorship of
the William Shakespeare plays and sonnets.
producT ciphers
In the discussion of transposition ciphers it was pointed
out that by combining two or more simple transposi-
tions, a more secure encryption may result. In the days
of manual cryptography this was a useful device for the
cryptographer, and in fact double transposition or prod-
uct ciphers on key word-based rectangular matrices were
widely used. There was also some use of a class of product
ciphers known as fractionation systems, wherein a sub-
stitution was first made from symbols in the plaintext to
multiple symbols (usually pairs, in which case the cipher is
called a biliteral cipher) in the ciphertext, which was then
encrypted by a final transposition, known as superencryp-
tion. One of the most famous field ciphers of all time was
a fractionation system, the ADFGVX cipher employed
by the German army during World War I. This system
used a 6 × 6 matrix to substitution-encrypt the 26 letters
and 10 digits into pairs of the symbols A, D, F, G, V, and
