Talk:Vigenère cipher/Cryptanalysis

I'm not completely sure how algorithmically feasible this task is, but it at least sounds interesting. (Well, I'm not sure how feasible it is to do this completely automatically.) I'll try to add an example soon. MagiMaster 05:37, 31 May 2011 (UTC)

C++ code added, but I'm not sure about how robust it is. There seems like a decent chance that the first part will pass a too-short key, and the second part doesn't have any checks on the final output. Another chi-squared check doesn't work, which also makes me suspect that the chi-squared check in the first part isn't the best solution either, even if it does ok. MagiMaster 11:28, 31 May 2011 (UTC)

There. I changed the chi-squared test to something that maximizes the correlation instead. Smaller pieces increase the correlation artificially, so I added a weight against them. It seems to work on nearly all the ciphertext I've tried. Long keys are more likely to cause errors, but it still gets the length and most of the characters. A second pass to try and correct any errors by looking for words or common trigrams might not be a bad idea, but I don't think I'll do that myself. MagiMaster 20:26, 31 May 2011 (UTC)

Difficulty and suggested improvements

This is a huge amount of text which makes the task much easier. As a hobby I used to solve these by hand and we would typically get cipher texts of 15-20 5 letter groups. This cipher text has 165 groups!

I suggest to make it more interesting, that we try our algorithms on successively shorter texts (halving the text each time) and see how low we can drive it.--Dgamey 12:28, 3 June 2011 (UTC)