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@aczid
Last active Dec 18, 2015
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Bruteforcer for BostonKeyParty CTF level 'MITM'.
import base64
import struct
from Crypto.Cipher import AES
# reduce 2^48 to 2^27-ish using time-memory tradeoff
def crack(plaintext1, ciphertext1, plaintext2, ciphertext2):
print "Cracking plaintext '%s'" % plaintext1
zeroes = "\x00"*29
for table in range(0,7):
print "Generating table %s of 8 with 2^21 candidates" % (table+1)
ciphertext_table1 = {}
ciphertext_table2 = {}
key_value = init = 2**21*table
while key_value < init + 2**21:
key = zeroes + struct.pack('>I', key_value)[1:]
cipher = AES.new(key, AES.MODE_ECB)
ciphertext_table1[cipher.encrypt(plaintext1)] = key
ciphertext_table2[cipher.encrypt(plaintext2)] = key
key_value += 1
if key_value % 100000 == 0:
print "generated %s ciphertexts for key1 candidates" % key_value
key_value = 0
while key_value < 2**24:
key = zeroes + struct.pack('>I', key_value)[1:]
cipher = AES.new(key, AES.MODE_ECB)
if (ciphertext_table1.get(cipher.decrypt(ciphertext1)) and
ciphertext_table2.get(cipher.decrypt(ciphertext2)) and
(ciphertext_table1[cipher.decrypt(ciphertext1)] ==
ciphertext_table2[cipher.decrypt(ciphertext2)])):
return (ciphertext_table1[cipher.decrypt(ciphertext1)], key)
key_value += 1
if key_value % 100000 == 0:
print "tried %s key2 candidates" % ((table+1)*key_value)
# Challenge
mess1 = base64.b64decode("QUVTLTI1NiBFQ0IgbW9kZSB0d2ljZSwgdHdvIGtleXM=")
cypr1 = base64.b64decode("THbpB4bE82Rq35khemTQ10ntxZ8sf7s2WK8ErwcdDEc=")
mess2 = base64.b64decode("RWFjaCBrZXkgemVybyB1bnRpbCBsYXN0IDI0IGJpdHM=")
cypr2 = base64.b64decode("01YZbSrta2N+1pOeQppmPETzoT/Yqb816yGlyceuEOE=")
final = base64.b64decode("s5hd0ThTkv1U44r9aRyUhaX5qJe561MZ16071nlvM9U=")
print mess1
print mess2
# Hax
key1, key2 = crack(mess1, cypr1, mess2, cypr2)
#key1 = base64.b64decode("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAACa6Ac=")
#key2 = base64.b64decode("AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAD/P0U=")
# Done
cipher = AES.new(key2, AES.MODE_ECB)
decrypted = cipher.decrypt(final)
cipher = AES.new(key1, AES.MODE_ECB)
decrypted = cipher.decrypt(decrypted)
print decrypted
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