def test(key_length):
if args.verbose:
print(f"Testing key length {key_length}")
groups = []
for n in range(1, key_length + 1):
groups.append(subgroup(n, key_length))
a = ord('A')
key = ""
for n, group in enumerate(groups):
coef = utils.coincidence_index(group)
if args.all:
print(f"Subgroup {n + 1} (IC: {coef})\n{group}")
best_subkey = ('A', 0)
for i in range(MODULE):
shift = (MODULE - i)%MODULE
decrypt = caesar.caesar(group, shift)
frequencies = utils.most_frequent_chars(decrypt)
score = utils.match_score(''.join(map(lambda x: x[0], frequencies)))
subkey = chr(a + i)
if args.all:
print(f"Testing subkey '{subkey}' with match score {round(100 * (score/MAX_SCORE))}%")
if best_subkey[1] < score:
best_subkey = (subkey, score)
if args.all:
print(f"Best subkey is '{best_subkey[0]}' with match score {round(100 * (best_subkey[1]/MAX_SCORE))}%")
key += best_subkey[0]
decrypt = vigenere(text, key)
return (key, decrypt) if validator.is_valid(decrypt) else FAILED
def crack(text, terminal=True):
args.decrypt = True
frequencies = utils.most_frequent_chars(clean_text)
if args.all:
print(f"Frequencies: {frequencies}")
(coef, key_avg) = friedman(clean_text, frequencies)
if args.verbose:
print(f"Text IC (Index of Coincidence): {coef}")
PERMITTED_ERROR = 0.3 * (ENGLISH_IC - MIN_ENGLISH_IC)
if coef >= ENGLISH_IC - PERMITTED_ERROR:
if args.verbose:
print(
f"IC suggests that the text is encrypted with monoalphabetic cipher"
)
tryCaesar = caesar_crack(text)
if tryCaesar != FAILED:
return result(tryCaesar, terminal)
if key_avg > 0 and key_avg <= KEY_LENGTH_THRESHOLD:
if args.verbose:
print(f"Friedman test suggests a key length of {key_avg}")
decrypted = test(key_avg)
if decrypted != FAILED:
return result(decrypted, terminal)
if args.verbose:
print("Kasiki examination")
key_lengths = kasiki(text)
if key_avg in key_lengths:
key_lengths.remove(key_avg)
if args.all:
print("Kasiki possible key lengths (sorted by probability):")
print(key_lengths)
for key_length in key_lengths:
decrypted = test(key_length)
if decrypted != FAILED:
return result(decrypted, terminal)
if terminal:
validator.fail()
if not args.exhaustive:
print(
"If you want to try more keys execute this program again with the option --exhaustive. \
However, it is worth noting that the longer the key is the more errors can have the cracked key. \
In addition, this program may have difficulties to crack keys on smaller texts in comparison with the key length."
)
return FAILED