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# This file is dual licensed under the terms of the Apache License, Version
# 2.0, and the BSD License. See the LICENSE file in the root of this repository
# for complete details.
from __future__ import absolute_import, division, print_function
import struct
from cryptography.hazmat.primitives.ciphers import Cipher
from cryptography.hazmat.primitives.ciphers.algorithms import AES
from cryptography.hazmat.primitives.ciphers.modes import ECB
from cryptography.hazmat.primitives.constant_time import bytes_eq
def aes_key_wrap(wrapping_key, key_to_wrap, backend):
if len(wrapping_key) not in [16, 24, 32]:
raise ValueError("The wrapping key must be a valid AES key length")
if len(key_to_wrap) < 16:
raise ValueError("The key to wrap must be at least 16 bytes")
if len(key_to_wrap) % 8 != 0:
raise ValueError("The key to wrap must be a multiple of 8 bytes")
# RFC 3394 Key Wrap - 2.2.1 (index method)
encryptor = Cipher(AES(wrapping_key), ECB(), backend).encryptor()
a = b"\xa6\xa6\xa6\xa6\xa6\xa6\xa6\xa6"
r = [key_to_wrap[i:i + 8] for i in range(0, len(key_to_wrap), 8)]
n = len(r)
for j in range(6):
for i in range(n):
# every encryption operation is a discrete 16 byte chunk (because
# AES has a 128-bit block size) and since we're using ECB it is
# safe to reuse the encryptor for the entire operation
b = encryptor.update(a + r[i])
# pack/unpack are safe as these are always 64-bit chunks
a = struct.pack(
">Q", struct.unpack(">Q", b[:8])[0] ^ ((n * j) + i + 1)
)
r[i] = b[-8:]
assert encryptor.finalize() == b""
return a + b"".join(r)
def aes_key_unwrap(wrapping_key, wrapped_key, backend):
if len(wrapped_key) < 24:
raise ValueError("Must be at least 24 bytes")
if len(wrapped_key) % 8 != 0:
raise ValueError("The wrapped key must be a multiple of 8 bytes")
if len(wrapping_key) not in [16, 24, 32]:
raise ValueError("The wrapping key must be a valid AES key length")
# Implement RFC 3394 Key Unwrap - 2.2.2 (index method)
decryptor = Cipher(AES(wrapping_key), ECB(), backend).decryptor()
aiv = b"\xa6\xa6\xa6\xa6\xa6\xa6\xa6\xa6"
r = [wrapped_key[i:i + 8] for i in range(0, len(wrapped_key), 8)]
a = r.pop(0)
n = len(r)
for j in reversed(range(6)):
for i in reversed(range(n)):
# pack/unpack are safe as these are always 64-bit chunks
atr = struct.pack(
">Q", struct.unpack(">Q", a)[0] ^ ((n * j) + i + 1)
) + r[i]
# every decryption operation is a discrete 16 byte chunk so
# it is safe to reuse the decryptor for the entire operation
b = decryptor.update(atr)
a = b[:8]
r[i] = b[-8:]
assert decryptor.finalize() == b""
if not bytes_eq(a, aiv):
raise InvalidUnwrap()
return b"".join(r)
class InvalidUnwrap(Exception):
pass
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