TRANSPOSITION CIPHER TRIAL

Is why we stop at around length 9 keywords. However, we have little hope of trying to enumerate all possible length 12 keywords, as it would take far too long.

TRANSPOSITION CIPHER TRIAL

Trying to test all possible combinations of a length 6 keyword is easy, 720 trial decryptions can be done very quickly. The number of possible keys for various length keywords is shown below: Key Length The number of possible rearrangements of a length N key is N! (N factorial). Text, then choose the text with the highest score as our best candidate. For every keyword permutation we score the deciphered Keywords up to a length of 9 or so, we don't have to wait very long. The first step in attacking a columnar transposition cipher is to try all possible short keywords. In this way we can rank different decryption keys, the decryption key we want is the one that produces deciphered text with the fewest rare sequences. 'QKPC' which are very rare in normal english. using the wrong key) message will probably contain sequences e.g. This method works by first determining the statistics of English text, then calculating the likelyhood that the ciphertext comes from the same distribution. For this we will use a fitness measure based on quadgram statistics. A piece of text very similar to english will get a high score (a high fitness), while a jumble of random characters will get a low score (a low fitness). This is called rating the 'fitness' of the text. We have a columnar transposition cipher, and we don't know the keyword length, there are several things we can try.įor the approaches described below, we need a way of determining how similar a piece of text is to english text. In addition, if we know the keyword length most of our work is done. Much weaker if shorter keywords are used. The columnar transposition is a suprisingly secure cipher when long keys are used (key words around length 20), but Solution' and the book by Sinkov 'Elementary Cryptanalysis' both describe at great length how toīreak columnar transposition ciphers by hand. The book by Helen Fouche Gains 'Cryptanalysis - a study of ciphers and their For cracking these ciphers by hand or with a crib,ĭifferent techniques can be used.

Thing we know about the plaintext is that it is English. This page deals with automated cracking of columnar transposition ciphers with no known crib. Cryptanalysis of the Columnar Transposition Cipherįor a recap of how the columnar transposition cipher works, see here.

Transposition cipher