Euler Problem 59

28 Sep 2013 - Lorentz Vedeler

This is my solution to Euler problem 59 in python. It is pretty straight forward and relies on the heuristics of the problem. Initially it filters out every key that will only produce printable characters for all its corresponding positions in the encrypted text. It then proceeds to combine these keys and check for actual, common English words in the text.

from math import ceil
import string

def test():
    asciis = open('cipher1.txt', 'r').read()
    encCodes = [int(s) for s in asciis.split(',')]

    asciiSum = 0
    
    pKeys = plausibleKeys(encCodes, 3)
    for k0 in pKeys[0]:
        for k1 in pKeys[1]:
            for k2 in pKeys[2]:
                text = "".join(applyKey([k0,k1,k2], encCodes))
                if(properStringProbability(text)):
                    print(text)
                    asciiSum = sum([ord(c) for c in text])
    return asciiSum

def plausibleKeys(encCodes, keyLen):
    pKeys = {
        0: [x for x in range(255)],
        1: [y for y in range(255)],
        2: [z for z in range(255)]
        }

    for i, c in enumerate(encCodes):
        for k in pKeys[i % keyLen]:
            if chr(c ^ k) not in string.printable:
                pKeys[i % keyLen].remove(k)
    return pKeys

def properStringProbability(string):
    cnt = 0
    for word in ["the", "and", "have", "that", "you"]:
        cnt += string.count(word)        
    return cnt > 5


def applyKey(key, asciiText):
    return [chr(x ^ int(y)) for (x,y) in zip(key * int(ceil(len(asciiText) / 3)),asciiText)]