caesar_break.py

import urllib2, random

response = urllib2.urlopen("http://vip.udel.edu/crypto/mobydick.txt")
mobytext = response.read()

onlyletters = filter(lambda x: x.isalpha(), mobytext)
loweronly = onlyletters.lower()

def shiftBy(c, n):
    return chr(((ord(c) - ord('a') + n) % 26) + ord('a'))

caesar_key = random.randint(1,25)
print "shhh the secret caesar key is: ", caesar_key
encrypted = map(lambda x: shiftBy(x, caesar_key), loweronly)

normal_freqs = {'a': 0.080642499002080981, 'c': 0.026892340312538593, 'b': 0.015373768624831691, 'e': 0.12886234260657689, 'd': 0.043286671390026357, 'g': 0.019625534749730816, 'f': 0.024484713711692099, 'i': 0.06905550211598431, 'h': 0.060987267963718068, 'k': 0.0062521823678781188, 'j': 0.0011176940633901926, 'm': 0.025009719347800208, 'l': 0.041016761327711163, 'o': 0.073783151266212627, 'n': 0.069849754102356679, 'q': 0.0010648594165322703, 'p': 0.017031440203182008, 's': 0.063817324270355996, 'r': 0.06156572691936394, 'u': 0.027856851020401599, 't': 0.090246649949305979, 'w': 0.021192261444145363, 'v': 0.010257964235274787, 'y': 0.01806326249861108, 'x': 0.0016941732664605912, 'z': 0.0009695838238376564}

frequency = {}
for ascii in range(ord('a'), ord('a')+26):
    frequency[chr(ascii)] = float(encrypted.count(chr(ascii)))/len(loweronly)

sum_freqs_squared = 0.0
for ltr in frequency:
    sum_freqs_squared += frequency[ltr]*frequency[ltr]

print "Will be near .065 despite Caesar: " + str(sum_freqs_squared)

for possible_key in range(1, 26):
    sum_f_sqr = 0.0
    for ltr in normal_freqs:
        caesar_guess = shiftBy(ltr, possible_key)
        sum_f_sqr += normal_freqs[ltr]*frequency[caesar_guess]
    if abs(sum_f_sqr - .065) < .005:
        print "Key is probably: ", possible_key, " f_sqr is ",sum_f_sqr