break_caesar.py

import re
from ngram_score import ngram_score
fitness = ngram_score('quadgrams.txt') # load our quadgram statistics
from pycipher import Caesar
      
def break_caesar(ctext):
    # make sure ciphertext has all spacing/punc removed and is uppercase
    ctext = re.sub('[^A-Z]','',ctext.upper())
    # try all possible keys, return the one with the highest fitness
    scores = []
    for i in range(26):
        scores.append((fitness.score(Caesar(i).decipher(ctext)),i))
    return max(scores)
    
# example ciphertext
ctext = 'YMJHFJXFWHNUMJWNXTSJTKYMJJFWQNJXYPSTBSFSIXNRUQJXYHNUMJWX'
max_key = break_caesar(ctext)

print 'best candidate with key (a,b) = '+str(max_key[1])+':'
print Caesar(max_key[1]).decipher(ctext)