Cryptography
This repo contains various small crypto related scripts, that I wrote during the course of Cryptography in NTUA. The viginere decypher was an exercise for that class.
Viginere decypher
The ciphertext is known to be encrypted using the viginere method. Find_length.py uses the index of coincidence(IC) to find the most probable length of the key. The partial output of this programm is:
[(46, 0.0644927536231884), (30, 0.06666666666666668),
(12, 0.06672113289760347), (6, 0.06721132897603486),
(18, 0.06969696969696969), (27, 0.05952380952380953), ...]
Under the asssumption that the key is a small phrase the most probable length is 6.
Find_key.py uses the chi-squared statistic to compare the distribution of each column shifted with the ceasar cipher(each column corresponds to a letter of the key) to the english language distribution. The number of shifts and the corresponding letter with the smallest chi-squared value is a letter of the key.
Finally, we decode the cypher and it is a speech by Martin Luther King and the key is LUTHER.