Cryptography Reference
In-Depth Information
Simple frequency counts on the ciphertext would
reveal to the cryptanalyst that letters occur with precisely
the same frequency in the cipher as in an average plaintext
and, hence, that a simple rearrangement of the letters is
probable.
The rail fence is the simplest example of a class of trans-
position ciphers, known as route ciphers, that enjoyed
considerable popularity in the early history of cryptology.
In general, the elements of the plaintext (usually single
letters) are written in a prearranged order (route) into a geo-
metric array (matrix)—typically a rectangle—agreed upon
in advance by the transmitter and receiver and then read off
by following another prescribed route through the matrix
to produce the cipher. The key in a route cipher consists of
keeping secret the geometric array, the starting point, and
the routes. Clearly both the matrix and the routes can be
much more complex than in this example; but even so, they
provide little security. One form of transposition (permuta-
tion) that was widely used depends on an easily remembered
key word for identifying the route in which the columns of
a rectangular matrix are to be read. For example, using the
key word AUTHOR and ordering the columns by the lexi-
cographic order of the letters in the key word
