conv

Catégorie: Crytpo Difficulté: Moyen Flag: CTF{89c5cce663fce1500d22c2ef5112dc2885c491d37d3503118251bdd516b4dcc0}

Challenge

Description

---

Time for convolutions.

def conv(array1:bytes, array2:bytes) -> bytes:
    len1, len2 = len(array1), len(array2)
    res = [0]*(len1 + len2 - 1)
    for i in range(len1 + len2 - 1):
        csum = 0
        for j in range(max(0, i - len2 + 1), min(len1, i + 1)):
            csum += array1[j] * array2[i - j]
        res[i] = csum % 256
    return res

key = b'\xab\xec\xe9<\xaaC\x7fr\xeb\x8dgQ\xc0\x94\x01\x1d\xc03\x14\x97\xe2\x91\x97\xcf\x8b\x13?\x1d24w|'
cip = conv(plain1,key)
print(bytes(cip).hex())

Output:

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

Plaintext is ascii printable.

Solution

Si on lance la fonctionplusieurs fois avec la même clé mais un input différent :

def conv(array1:bytes, array2:bytes) -> bytes:
    len1, len2 = len(array1), len(array2)
    res = [0]*(len1 + len2 - 1)
    for i in range(len1 + len2 - 1):
        csum = 0
        for j in range(max(0, i - len2 + 1), min(len1, i + 1)):
            csum += array1[j] * array2[i - j]
        res[i] = csum % 256
    return res

key = b'\xab\xec\xe9<\xaaC\x7fr\xeb\x8dgQ\xc0\x94\x01\x1d\xc03\x14\x97\xe2\x91\x97\xcf\x8b\x13?\x1d24w|'

print(bytes(conv(b'a', key)).hex())
print(bytes(conv(b'aa', key)).hex())
print(bytes(conv(b'aaa', key)).hex())
print(bytes(conv(b'aaaa', key)).hex())

# cb6c49bc6a631f320b6d07b1c01461fdc0539437a2f1376fab33dffdf2b417fc
# cb37b50526cd82513d7874b871d4755ebd13e7cbd99328a61ade12dcefa6cb13fc
# cb3780716f89ecb45caa7f25788535721e10a71e6dcaca97514dbd0fcea3bdc713fc
# cb37803cdbd2a81ebfc9b130e58ce6323271a4dec05e013942842cba0182bab9c713fc

On s'aperçoit qu'un caractère chiffré n'influe pas sur les précédents. Autrement dit, on peut fixer le début au fur et à mesure du déchiffrement.

Il faut donc tester tous les caractères possibles à la première position jusqu'à trouver le même résultat que la première position de l'output, puis refaire la même opération avec le second etc...

import string

def conv(array1:bytes, array2:bytes) -> bytes:
    len1, len2 = len(array1), len(array2)
    res = [0]*(len1 + len2 - 1)
    for i in range(len1 + len2 - 1):
        csum = 0
        for j in range(max(0, i - len2 + 1), min(len1, i + 1)):
            csum += array1[j] * array2[i - j]
        res[i] = csum % 256
    return res

key = b'\xab\xec\xe9<\xaaC\x7fr\xeb\x8dgQ\xc0\x94\x01\x1d\xc03\x14\x97\xe2\x91\x97\xcf\x8b\x13?\x1d24w|'
result = bytes.fromhex("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")

charset = string.printable
decrypted = ''
for _ in range(len(result)):
  for c in charset:
    attempt = decrypted + c 
    cip = conv(attempt.encode(), key)
    if cip[len(decrypted)] == result[len(decrypted)]:
      decrypted = attempt
print(decrypted)

Elit cybernetica fusce stratagemata enigma penetratio exsertus. CTF{89c5cce663fce1500d22c2ef5112dc2885c491d37d3503118251bdd516b4dcc0} Combinatio complexus networkus quantum facilis vectura obfuscatus. Latitudo cripto diversus et preditus, securitas hexadecimale detectus phantasma scriptum. Insidiae infiltratio breviaria kernel status, protus obscura administratio.

Le flag est dans notre résultat.