Cryptography Reference
In-Depth Information
encrypted as D, B as E, and so forth. As many a school-
boy has discovered to his embarrassment, cyclical-shift
substitution ciphers are not secure. And as is pointed
out in the section "Cryptanalysis," neither is any other
monoalphabetic substitution cipher in which a given
plaintext symbol is always encrypted into the same
ciphertext symbol. Because of the redundancy of the
English language, only about 25 symbols of ciphertext
are required to permit the cryptanalysis of monoalpha-
betic substitution ciphers, which makes them a popular
source for recreational cryptograms. The explanation
for this weakness is that the frequency distributions of
symbols in the plaintext and in the ciphertext are iden-
tical, only the symbols having been relabeled. In fact,
any structure or pattern in the plaintext is preserved
intact in the ciphertext, so that the cryptanalyst's task
is an easy one.
There are two main approaches that have been
employed with substitution ciphers to lessen the extent to
which structure in the plaintext—primarily single-letter
frequencies—survives in the ciphertext. One approach is
to encrypt elements of plaintext consisting of two or more
symbols; e.g., digraphs and trigraphs. The other is to use
several cipher alphabets. When this approach of polyal-
phabetic substitution is carried to its limit, it results in
onetime keys, or pads.
P
layfair
C
iPhers
In cryptosystems for manually encrypting units of
plaintext made up of more than a single letter, only
digraphs were ever used. By treating digraphs in the
plaintext as units rather than as single letters, the extent
to which the raw frequency distribution survives the
