Transposition Ciphers
Instead of retaining the order of plaintext symbols as does the substitution ciphers,
transposition ciphers reorder the letters but do not perform any substitutions. The key for
the cipher is a word or phrase that does not contain any repeated letters. Without going
into minute detail, suffice it to say that the key word or phrase is used to establish the
order of the columns of a matrix and the plaintext message itself is used to establish rows
of the matrix. The column order is determined by the character’s relative order in the
alphabet. The ciphertext is then read from the matrix column by column. A simple
example using the word ROCK as the key to encrypt the message “attack now” would be:
R O C K
4 3 1 2
a t t a
c k
n
o w a b
To determine the ciphertext from the above matrix, we look at column 1, then
column 2, etc. Thus the ciphertext is t aanbtkwaco. The letters a and b that fill out the
matrix in the last row are padding. For a cryptanalyst to break this code he or she must
first determine that they are working with a transposition cipher instead of a substitution
cipher. For a message of any length, this can be determined simply by looking at the
frequency of the letters used. If the most commonly used letters e, t, a, etc. fit the normal
pattern for plaintext, then the cryptanalyst can be fairly certain it is a transposition cipher
since each letter represents itself. Next the cryptanalyst tries to determine the number of
columns in the key by looking at digrams and then once this is determined, placing the
columns in the proper order.