gistfile1.py

#!/usr/bin/env python3
# Heaviliy influenced by: http://tinyurl.com/qe2ht4w
import operator
import collections
import numpy

from functools import partial
from string import ascii_lowercase as alpha

NORMALISED_FREQUENCIES = [
    0.64297,0.11746,0.21902,0.33483,1.00000,0.17541,  
    0.15864,0.47977,0.54842,0.01205,0.06078,0.31688,0.18942,
    0.53133,0.59101,0.15187,0.00748,0.47134,0.49811,0.71296,
    0.21713,0.07700,0.18580,0.01181,0.15541,0.00583
]
  
def translator(text, alphabet, key):
    return text.translate(str.maketrans(alphabet, key))  
  
def caesar_decode(ciphertext, s):  
    return translator(ciphertext, alpha, alpha[-s:] + alpha[:-s])  

def get_frequencies(text):
    array = numpy.zeros(len(alpha), numpy.float64)
    
    for c in text:
        ord_c = ord(c)
        if ord_c >= 97 and ord_c <= 122:
            array[ord_c - 97] += 1.0
    
    #print(max(array))
    array /= max(array)
    return array

def _get_error(frequencies, i):
    return abs(frequencies[i] - NORMALISED_FREQUENCIES[i])

def get_error(text):
    frequencies = get_frequencies(text)
    return sum(map(partial(_get_error, frequencies), range(len(frequencies))))

def get_minimal_error(text):
    texts = (caesar_decode(text, rot) for rot in range(len(alpha) - 1))
    return min(texts, key=get_error)

def get_cipher_texts(fd, N):
    texts = collections.defaultdict(list)
    for i, c in enumerate(fd.read()):
        texts[i % N].append(c)
    return { i: "".join(chars) for i, chars in texts.items() }

if __name__ == '__main__':
    import sys

    N = int(sys.argv[1])
    cipher = open("cipher.txt")
    texts = get_cipher_texts(cipher, N)
    
    partial_solutions = [iter(get_minimal_error(texts[i])) for i in range(N)]

    exhausted = False
    while not exhausted:
        for n in range(N):
            try:
                print(next(partial_solutions[n]), end="")            
            except StopIteration:
                exhausted = True