Codes and Ciphers

transposition cipher

transposition cipher

In cryptography, a transposition cipher is a method of encryption by which the positions held by units of plaintext are shifted according to a regular system, so that the ciphertext constitutes a permutation of the plaintext. 

Transposition Ciphers are a bit different to Substitution Ciphers. Whereas Substitution ciphers replace each letter with a different letter or symbol to produce the ciphertext, in a Transposition cipher, the letters are just moved around.

The letters or words of the plaintext are reordered in some way, fixed by a given rule (the key).

One example of a transposition cipher, is to reverse the order of the letters in a plaintext. So "a simple example" becomes "ELPMAXE ELPMIS A". Another, similar, way to encrypt a message would be to reverse the letters of each word, but not the order in which the words are written. In this case "a simple example" becomes "A ELPMIS ELPMAXE". Both of these are available in the activity at the bottom of the page.

Another type of transposition cipher is the Scytale, which was an encryption device used by the Ancient Greeks and Spartans. It consisted of a polygonal rod or cylinder, around which was wrapped a piece of parchment. The sender would write the message along the faces of the rod as seen in the image below. When the parchment is removed from the Scytale, it leaves a nonsensical message going down the strip (in the case below it would read "STSF...").

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Arielilie~