/
keywrap.py
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/
keywrap.py
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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 annotations
import typing
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 _wrap_core(
wrapping_key: bytes,
a: bytes,
r: list[bytes],
) -> bytes:
# RFC 3394 Key Wrap - 2.2.1 (index method)
encryptor = Cipher(AES(wrapping_key), ECB()).encryptor()
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])
a = (
int.from_bytes(b[:8], byteorder="big") ^ ((n * j) + i + 1)
).to_bytes(length=8, byteorder="big")
r[i] = b[-8:]
assert encryptor.finalize() == b""
return a + b"".join(r)
def aes_key_wrap(
wrapping_key: bytes,
key_to_wrap: bytes,
backend: typing.Any = None,
) -> bytes:
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")
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)]
return _wrap_core(wrapping_key, a, r)
def _unwrap_core(
wrapping_key: bytes,
a: bytes,
r: list[bytes],
) -> tuple[bytes, list[bytes]]:
# Implement RFC 3394 Key Unwrap - 2.2.2 (index method)
decryptor = Cipher(AES(wrapping_key), ECB()).decryptor()
n = len(r)
for j in reversed(range(6)):
for i in reversed(range(n)):
atr = (
int.from_bytes(a, byteorder="big") ^ ((n * j) + i + 1)
).to_bytes(length=8, byteorder="big") + 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""
return a, r
def aes_key_wrap_with_padding(
wrapping_key: bytes,
key_to_wrap: bytes,
backend: typing.Any = None,
) -> bytes:
if len(wrapping_key) not in [16, 24, 32]:
raise ValueError("The wrapping key must be a valid AES key length")
aiv = b"\xA6\x59\x59\xA6" + len(key_to_wrap).to_bytes(
length=4, byteorder="big"
)
# pad the key to wrap if necessary
pad = (8 - (len(key_to_wrap) % 8)) % 8
key_to_wrap = key_to_wrap + b"\x00" * pad
if len(key_to_wrap) == 8:
# RFC 5649 - 4.1 - exactly 8 octets after padding
encryptor = Cipher(AES(wrapping_key), ECB()).encryptor()
b = encryptor.update(aiv + key_to_wrap)
assert encryptor.finalize() == b""
return b
else:
r = [key_to_wrap[i : i + 8] for i in range(0, len(key_to_wrap), 8)]
return _wrap_core(wrapping_key, aiv, r)
def aes_key_unwrap_with_padding(
wrapping_key: bytes,
wrapped_key: bytes,
backend: typing.Any = None,
) -> bytes:
if len(wrapped_key) < 16:
raise InvalidUnwrap("Must be at least 16 bytes")
if len(wrapping_key) not in [16, 24, 32]:
raise ValueError("The wrapping key must be a valid AES key length")
if len(wrapped_key) == 16:
# RFC 5649 - 4.2 - exactly two 64-bit blocks
decryptor = Cipher(AES(wrapping_key), ECB()).decryptor()
out = decryptor.update(wrapped_key)
assert decryptor.finalize() == b""
a = out[:8]
data = out[8:]
n = 1
else:
r = [wrapped_key[i : i + 8] for i in range(0, len(wrapped_key), 8)]
encrypted_aiv = r.pop(0)
n = len(r)
a, r = _unwrap_core(wrapping_key, encrypted_aiv, r)
data = b"".join(r)
# 1) Check that MSB(32,A) = A65959A6.
# 2) Check that 8*(n-1) < LSB(32,A) <= 8*n. If so, let
# MLI = LSB(32,A).
# 3) Let b = (8*n)-MLI, and then check that the rightmost b octets of
# the output data are zero.
mli = int.from_bytes(a[4:], byteorder="big")
b = (8 * n) - mli
if (
not bytes_eq(a[:4], b"\xa6\x59\x59\xa6")
or not 8 * (n - 1) < mli <= 8 * n
or (b != 0 and not bytes_eq(data[-b:], b"\x00" * b))
):
raise InvalidUnwrap()
if b == 0:
return data
else:
return data[:-b]
def aes_key_unwrap(
wrapping_key: bytes,
wrapped_key: bytes,
backend: typing.Any = None,
) -> bytes:
if len(wrapped_key) < 24:
raise InvalidUnwrap("Must be at least 24 bytes")
if len(wrapped_key) % 8 != 0:
raise InvalidUnwrap("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")
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)
a, r = _unwrap_core(wrapping_key, a, r)
if not bytes_eq(a, aiv):
raise InvalidUnwrap()
return b"".join(r)
class InvalidUnwrap(Exception):
pass