borg/darc/oaep.py

72 lines
2.5 KiB
Python

from Crypto.Util.number import long_to_bytes
from Crypto.Hash import SHA
from .helpers import IntegrityError
def _xor_bytes(a, b):
return ''.join(chr(ord(x[0]) ^ ord(x[1])) for x in zip(a, b))
def MGF1(seed, mask_len, hash=SHA):
"""MGF1 is a Mask Generation Function based on hash function
"""
T = ''.join(hash.new(seed + long_to_bytes(c, 4)).digest()
for c in range(1 + mask_len / hash.digest_size))
return T[:mask_len]
class OAEP(object):
"""Optimal Asymmetric Encryption Padding
"""
def __init__(self, k, hash=SHA, MGF=MGF1):
self.k = k
self.hash = hash
self.MGF = MGF
def encode(self, msg, seed, label=''):
# FIXME: length checks
if len(msg) > self.k - 2 * self.hash.digest_size - 2:
raise ValueError('message too long')
label_hash = self.hash.new(label).digest()
padding = '\0' * (self.k - len(msg) - 2 * self.hash.digest_size - 2)
datablock = '%s%s\1%s' % (label_hash, padding, msg)
datablock_mask = self.MGF(seed, self.k - self.hash.digest_size - 1, self.hash)
masked_db = _xor_bytes(datablock, datablock_mask)
seed_mask = self.MGF(masked_db, self.hash.digest_size, self.hash)
masked_seed = _xor_bytes(seed, seed_mask)
return '\0%s%s' % (masked_seed, masked_db)
def decode(self, ciphertext, label=''):
if len(ciphertext) < self.k:
ciphertext = ('\0' * (self.k - len(ciphertext))) + ciphertext
label_hash = self.hash.new(label).digest()
masked_seed = ciphertext[1:self.hash.digest_size + 1]
masked_db = ciphertext[-(self.k - self.hash.digest_size - 1):]
seed_mask = self.MGF(masked_db, self.hash.digest_size, self.hash)
seed = _xor_bytes(masked_seed, seed_mask)
datablock_mask = self.MGF(seed, self.k - self.hash.digest_size - 1, self.hash)
datablock = _xor_bytes(masked_db, datablock_mask)
label_hash2 = datablock[:self.hash.digest_size]
data = datablock[self.hash.digest_size:].lstrip('\0')
if (ciphertext[0] != '\0' or
label_hash != label_hash2 or
data[0] != '\1'):
raise IntegrityError('decryption error')
return data[1:]
def test():
from Crypto.Hash import SHA256
import os
import random
oaep = OAEP(256, SHA256)
for x in range(1000):
M = os.urandom(random.randint(0, 100))
EM = oaep.encode(M, os.urandom(32))
assert len(EM) == oaep.k
assert oaep.decode(EM) == M
if __name__ == '__main__':
test()