transposition cipher

transposition cipher, simple data encryption scheme in which plaintext characters are shifted in some regular pattern to form ciphertext.

In manual systems transpositions are generally carried out with the aid of an easily remembered mnemonic. For example, a popular schoolboy cipher is the “rail fence,” in which letters of the plaintext are written alternating between rows and the rows are then read sequentially to give the cipher. In a depth-two rail fence (two rows) the message WE ARE DISCOVERED SAVE YOURSELF would be written

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 transposition 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 geometric 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 (permutation) 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 lexicographic order of the letters in the key word

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 transposition, 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.

In the same class also fall systems that make use of perforated cardboard matrices called grilles; descriptions of such systems can be found in most older books on cryptography. In contemporary cryptography, transpositions serve principally as one of several encryption steps in forming a compound or product cipher.