Classic Cryptography: Transpositions

Transpositions
Transposition is not really a full-fledged method of cipher in its own, but it can form and usually is used in a stage of more complex cryptosystems (such as in applying key-based encryption). Transposition is simply moving the relative positions of letters within a message. We will discuss a columnar transposition below, so called because the text is arranged into columns and the columns are transposed.

When performing a columnar transposition, a keyword is first needed. The message is then written into rows beneath the keyword. The example message which we'll use will be "SECRET MESSAGES".

	e n c r y p t
	2 3 1 5 7 4 6
	S E C R E T M
	E S S A G E S

Adding some numbers beneath the keyword, the numbers refer to the relative positions of the keyword letters in the alphabet. Having formed the table we can read back the message in the order of the keyword letters. This message becomes CSETRME SESEASG. The spaces have been included for clarity, although in practice this will not happen.

Transpositions are often used as part of a more complex system. If a transposition is used in conjunction with a monoalphabetic substitution then the transposition as above may be solved, after having first worked out the plaintext letters of the substitution by looking at letter frequencies. Imagine the possibilities.

As with many cryptographic systems the greater the amount of ciphertext that is available, the easier the cipher is to solve. Transposition adds some security, however as with all cryptography, sometimes the solution is easier than one might be inclined to believe.