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