It is not very secure, but it is great for learning about cryptography. It is obvious that if a word consists of seven letters which locates in the alphabet in the same sequence order as the letters in the word “DECRYPT”, this word can also be used as a key with the help of which one can decrypt an encrypted message (ciphertex). For instance, “BCAEGDF” or “CHAPTOR”, etc.
When we have got a cipher text, the first thing we can do with it is to examine frequency of individual symbols. We know that transposition does not affect the frequency of individual symbols. If the ciphertext exhibits a frequency distribution very similar to ordinary plaintext, it is most likely a transposition. It is well known fact that certain letters occur with varying frequencies. Moreover, there is a characteristic distribution of letters that is roughly the same for the majority of samples of plaintext. For instance, “E” tends to be very common, while “X” is very rare. Thought, if encryption technique was transposition we can immediately detect it.
Then we can use anagramming for investigation ciphertext. Anagramming is sliding pieces of ciphertext around, then looking for sections that look like anagrams of English words, and solving the anagrams. Once such anagrams have been found, they reveal information about the transposition pattern, and can consequently be extended.
There is a historical evidence for the fact that such encryption is not safe enough. Thus, during World War I, the German military used a double columnar transposition cipher. The system was regularly solved by the French, naming it Übchi, who were typically able to find the key in a matter of days after a new one had been introduced. However, the French success became widely-known and, after a publication in Le Matin, the Germans changed to a new system on 18 November 1914.
Transposition is often combined with other techniques. For example, a simple substitution cipher combined with a columnar transposition avoids the weakness of both. Replacing high frequency ciphertext symbols with high frequency plaintext letters does not reveal chunks of plaintext because of the transposition. Anagramming the transposition does not work because of the substitution. A disadvantage is that such ciphers are considerably more laborious and error prone than simpler ciphers.
References
- Kak, Avi Lecture 2: Classical Encryption Techniques. Lecture Notes on “Introduction to Computer Security” Jan 13, 2006 Retrieved Feb 08, 2006 from http://shay.ecn.purdue.edu/~ece495f/Lecture/Lecture_2.pdf
- Murphy, Sean and Piper, Fred. (2002) Cryptography: A Very Short Introduction Oxford: Oxford University Press.
