Cryptography Reference
In-Depth Information
In decrypting a route cipher, the receiver enters
the ciphertext symbols into the agreed-upon matrix
according to the encryption route and then reads
the plaintext according to the original order of entry.
A significant improvement in cryptosecurity can be
achieved by reencrypting the cipher obtained from one
transposition with another transposition. Because the
result (product) of two transpositions is also a transpo-
sition, the effect of multiple transpositions is to define
a complex route in the matrix, which in itself would
be difficult to describe by any simple mnemonic. (
See
"Product Ciphers," later in this chapter.)
In the same class also fall systems that make use of per-
forated cardboard matrices called grilles; descriptions of
such systems can be found in most older topics on cryp-
tography. In contemporary cryptography, transpositions
serve principally as one of several encryption steps in
forming a compound or product cipher.
subsTiTuTion ciphers
In substitution ciphers, units of the plaintext (gener-
ally single letters or pairs of letters) are replaced with
other symbols or groups of symbols, which need not be
the same as those used in the plaintext. For instance, in
Sir Arthur Conan Doyle's
Adventure of the Dancing Men
(1903), Sherlock Holmes solves a monoalphabetic substi-
tution cipher in which the ciphertext symbols are stick
figures of a human in various dancelike poses.
The simplest of all substitution ciphers are those
in which the cipher alphabet is merely a cyclical shift
of the plaintext alphabet. Of these, the best-known is
the Caesar cipher, used by Julius Caesar, in which A is
