cryptoscope-crack.py

#!/usr/bin/env python

import random

# Here is our plaintext message
message = 'THISISATESTITISONLYATEST'

# Insert random spaces into the message
while len(message) < 72:
    i = random.randrange(len(message))
    message = message[:i] + ' ' + message[i:]

# Write the message across the six wheels
wheels = [[' '] * 72 for _ in range(6)]
for i, m in enumerate(message):
    if m == ' ':
        for w in wheels:
            w[i] = random.choice('ABCDEFGHIJKLMNOPQRSTUVWXYZ')
    else:
        random.choice(wheels)[i] = m

# Generate a key
key = [ random.randrange(72) for _ in wheels ]

# Rotate the wheels the opposite direction
for w, k in zip(wheels, key):
    w[:] = w[72 - k:] + w[:72 - k]

print('Scrambled wheels:')
for w in wheels:
    print(' '.join(w))
print()

###########################

# Now let's try to brute force how wheel[i] aligns to wheel[0]
found_key = [ 0 for _ in wheels ]
for i, w in enumerate(wheels[1:]):
    scores = []
    for k in range(72):
        collide = 0
        gap = 0
        letter = 0
        for a, b in zip(wheels[0], w[k:] + w[:k]):
            if a == ' ' and b == ' ':
                gap += 1
            elif a == ' ' or b == ' ':
                letter += 1
            else:
                collide += 1
        scores.append((k, collide, gap, letter))

    # Select the rotation with the lowest letter count
    best_score = min(scores, key=lambda score: score[3])
    found_key[i + 1] = best_score[0]

# Rotate all the wheels according to our found key
for w, k in zip(wheels, found_key):
    w[:] =  w[k:] + w[:k]

print('Unscrambled wheels:')
for w in wheels:
    print(' '.join(w))
print()

# We can now combine wheels and drop out any colliding characters:
decoded = ''
for i, chars in enumerate(zip(*wheels)):
    if sum(1 for x in chars if x != ' ') > 1:
        decoded += ' '
    else:
        decoded += next(x for x in chars if x != ' ')

# We don't know the orientation of the first wheel, but we can show all possible
# de-scramblings
print('Possible keys and plaintexts:')
for k in range(72):
    out_key = [ (f + k) % 72 for f in found_key ]
    print(out_key, ''.join(decoded[k:] + decoded[:k]).replace(' ', ''), end='')
    print('  <--' if out_key == key else '')