def try_keylen(keylen, ciphertext):
"""Try and find the most likely key for a given key length"""
chunks = enchunk(ciphertext, keylen)[:-1]
key = ''
# Transpose chunks to get sets of bytes xored with same key byte
nth_chars = zip(*chunks)
# Find the most likely key byte for each set
for nth_char in nth_chars:
best_k = None
max_score = 0
for k in range(26):
plaintext = rot(nth_char, -k)
score = freqs.score_freq(freqs.get_freqs(plaintext))
if best_k is None or score > max_score:
best_k = k
max_score = score
key += chr(ord('A') + best_k)
return key
# Find the most likely key byte for each set
for nth_char in nth_chars:
best_k = None
max_score = 0
for k in range(26):
plaintext = rot(nth_char, -k)
score = freqs.score_freq(freqs.get_freqs(plaintext))
if best_k is None or score > max_score:
best_k = k
max_score = score
key += chr(ord('A') + best_k)
return key
with open('cipher.txt') as f:
ciphertext = ''.join(c for c in f.read().upper() if c in ascii_uppercase)
for keylen in range(2, len(ciphertext) // 10):
k = try_keylen(keylen, ciphertext)
pt = vigenere_dec(ciphertext, k)
score = freqs.score_freq(freqs.get_freqs(pt))
print "Score: %8f Key length: %3d Likely key: %s" % (score, len(k), k)
print "Plaintext: %s" % pt
print ""
