def p12():
target = b64decode(
'Um9sbGluJyBpbiBteSA1LjAKV2l0aCBteSByYWctdG9wIGRvd24gc28gbXkg'
'aGFpciBjYW4gYmxvdwpUaGUgZ2lybGllcyBvbiBzdGFuZGJ5IHdhdmluZyBq'
'dXN0IHRvIHNheSBoaQpEaWQgeW91IHN0b3A/IE5vLCBJIGp1c3QgZHJvdmUg'
'YnkK')
block_size = _detect_block_size(target)
tmp_ctxt = _encrypt('A' * block_size * 4)
assert detect_ecb_mode(tmp_ctxt)
return _crack_blocks(target, 'A' * block_size, block_size)
def _find_offset(data, block_size, prefix):
ecb_detect = 'A' * block_size * 3
ctxt = _encrypt(prefix+ecb_detect+data)
while detect_ecb_mode(ctxt):
ecb_detect = ecb_detect[:-1]
ctxt = _encrypt(prefix+ecb_detect+data)
ecb_detect += 'A'
ctxt = _encrypt(prefix+ecb_detect+data)
pad_offset = len(ecb_detect) % block_size
for i in range(len(ctxt) / block_size):
n_0, n_1, n_2 = map(lambda x: block_size * x, range(i, i + 3))
cur_block, next_block = ctxt[n_0:n_1], ctxt[n_1:n_2]
if cur_block == next_block:
return n_0 - pad_offset