break_simplesub.py

from pycipher import SimpleSubstitution as SimpleSub
import random
import re
from ngram_score import ngram_score
fitness = ngram_score('quadgrams.txt') # load our quadgram statistics

ctext='pmpafxaikkitprdsikcplifhwceigixkirradfeirdgkipgigudkcekiigpwrpucikceiginasikwduearrxiiqepcceindgmieinpwdfprduppcedoikiqiasafmfddfipfgmdafmfdteiki'
ctext = re.sub('[^A-Z]','',ctext.upper())

maxkey = list('ABCDEFGHIJKLMNOPQRSTUVWXYZ')
maxscore = -99e9
parentscore,parentkey = maxscore,maxkey[:]
print "Substitution Cipher solver, you may have to wait several iterations"
print "for the correct result. Press ctrl+c to exit program."
# keep going until we are killed by the user
i = 0
while 1:
    i = i+1
    random.shuffle(parentkey)
    deciphered = SimpleSub(parentkey).decipher(ctext)
    parentscore = fitness.score(deciphered)
    count = 0
    while count < 1000:
        a = random.randint(0,25)
        b = random.randint(0,25)
        child = parentkey[:]
        # swap two characters in the child
        child[a],child[b] = child[b],child[a]
        deciphered = SimpleSub(child).decipher(ctext)
        score = fitness.score(deciphered)
        # if the child was better, replace the parent with it
        if score > parentscore:
            parentscore = score
            parentkey = child[:]
            count = 0
        count = count+1
    # keep track of best score seen so far
    if parentscore>maxscore:
        maxscore,maxkey = parentscore,parentkey[:]
        print '\nbest score so far:',maxscore,'on iteration',i
        ss = SimpleSub(maxkey)
        print '    best key: '+''.join(maxkey)
        print '    plaintext: '+ss.decipher(ctext)