Lecture 17: Introduction to Encryption

Transposition Ciphers

In this system, the order of the plaintext characters is changed, but the characters themselves are not -- that is, the encrypted message contains all the same characters as the plaintext. For example, a simple columnar cipher could be constructed thus:

Hence, the ciphertext is:

EATITNIHMEXNETMGMEDT

Multi-stage transposition was the basis of the famous Enigma encryption machine used by German armed forces and famously cracked by the British intelligence service at Bletchely Park in the second world war.

Neither substitution nor transposition ciphers alone are regarded as secure for serious modern use. The current approach is to combine both techniques, on binary data and with much more complex algorithms (see DES, later).

Lecture 17: Introduction to Encryption

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