Route Transposition Cipher - Description and Cryptanalysis

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Introduction

The route transposition cipher is not so much a single cipher as a class of transposition ciphers. Since the 'route' that is chosen can be arbitrary, just about any transposition cipher including the railfence cipher and the columnar transposition cipher (for a particular key) could all be called a route cipher for a carefully chosen route.

Most routes, however, are very simple, such as a spiral shape.

Example

The key for the route cipher is actually a route, or more accurately, instructions for jumbling text. Since the words 'Route cipher' do not mean any particular algorithm, examples of route ciphers will be provided here. An example of an unkeyed route cipher is the rail fence cipher. An example of a keyed route cipher is the columnar transposition cipher.

There are a practically infinite number of possible route ciphers, limited only by the imagination. For security, the more mixing that a cipher performs, the better.

Cryptanalysis

Route ciphers are not the easiest of transposition ciphers to break, but there are statistical properties of language that can be exploited to recover the key.To greatly increase the security, a substitution cipher could be employed as well as the permutation provided by the route cipher.

A peculiarity of transposition ciphers is that the frequency distribution of the characters will be identical to that of natural text (since no substitutions have been performed, it is just the order that has been mixed up). In other words it should look just like this:

Cracking by hand is usually performed by anagramming, or trying to reconstruct the route. The more complex the route, the more difficult to crack.

For a method that works well on computers, we need a way of determining how 'english like' a piece of text is, check out the Classical Cryptanalysis section 'Text Characterisation'. The key that results in a decryption with the highest likelyhood of being english text is most probably the correct key. Of course, the more ciphertext you have, the more likely this is to be true (this is the case for all statistical measures, including the frequency approaches above). So the method used is to take the ciphertext, try decrypting it with each key, then see which decryption looks the best.

References

Wikipedia has a good description of the encryption/decryption process, history and cryptanalysis of this algorithm

Simon Singh's 'The Code Book' is an excellent introduction to ciphers and codes.
Singh, Simon (2000). The Code Book: The Science of Secrecy from Ancient Egypt to Quantum Cryptography. ISBN 0-385-49532-3. 

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