cryptohack.org/challenges

enc2.py

# py2

# for each character of plaintext we could observe many variants of ciphertext value (XORed)
# but we'll never observe the actual value (no leaks!)
# so we could collect enough data and exclude what we have observed
# we end up with something we've never observed - the actual bytes of the key

import base64, socket, json

conn = socket.socket(socket.AF_INET, socket.SOCK_STREAM)
conn.connect(('socket.cryptohack.org', 13370))
conn.recv(100)  # "No leaks"

req = json.dumps({"msg": "request"}) + '\n'  # const request

chars = [set(range(256)) for _ in range(12)]  # 12 = length of the actual flag

for i in range(10000):  # will break earlier
    conn.send(req)
    resp = json.loads(conn.recv(50))
    if 'error' in resp:
        continue

    ct = base64.b64decode(resp['ciphertext'])
    ct = ct[7:-1]  # omit 'crypto{' and '}'
    for pos in range(12):
        chars[pos].discard(ord(ct[pos]))

    # show progress
    print i, [len(cc) for cc in chars]

    # this can be optimized to the target character set to run faster
    if all(len(cc) == 1 for cc in chars):
        print '** took', i, 'iterations'
        break

print 'flag: crypto{{{}}}'.format(''.join(chr(list(c)[0]) for c in chars))
conn.close()

# >>> chars
# [{117}, {110}, {114}, {52}, {110}, {100}, {48}, {109}, {95}, {48}, {55}, {112}]

# ** took 2125 iterations
# flag: crypto{unr4nd0m_07p}