def x25519_key_derivation(
self,
public_key: 'OKP',
context: CoseKDFContext = b'',
alg: Optional[CoseAlgorithms] = None,
curve: Optional[CoseEllipticCurves] = None) -> Tuple[bytes, bytes]:
self._check_key_conf(alg, KeyOps.DERIVE_KEY, public_key, curve)
try:
alg_cfg = config(CoseAlgorithms(self.alg))
except KeyError as err:
raise CoseIllegalAlgorithm(err)
p = X25519PublicKey.from_public_bytes(public_key.x)
d = X25519PrivateKey.from_private_bytes(self.d)
shared_secret = d.exchange(p)
derived_key = alg_cfg.kdf(
algorithm=alg_cfg.hash(),
length=int(context.supp_pub_info.key_data_length / 8),
salt=None,
info=context.encode(),
backend=default_backend()).derive(shared_secret)
return shared_secret, derived_key
def hmac_key_derivation(self,
context: CoseKDFContext,
alg: Optional[CoseAlgorithms] = None,
salt: bytes = b'') -> bytes:
"""
HMAC-based key derivation based on secret bytes and a CoseKDFContext object.
:param context: A CoseKDFContext object which contains necessary input for the KDF algorithm.
:param alg: An optional CoseAlgorithm parameter.
:param salt: And optional salt parameter.
:return: A cryptographic key with a length specified in the CoseKDFContext object.
"""
self._check_key_conf(alg, KeyOps.DERIVE_KEY)
try:
alg_cfg = config(CoseAlgorithms(self.alg))
except KeyError as err:
raise CoseIllegalAlgorithm(err)
derived_key = alg_cfg.kdf(
algorithm=alg_cfg.hash(),
length=int(context.supp_pub_info.key_data_length / 8),
salt=salt,
info=context.encode(),
backend=default_backend()).derive(self.k)
return derived_key
def _prepare_cipher(self):
alg_cfg = config(CoseAlgorithms(self.alg))
if alg_cfg.tag_length is not None:
cipher: Union[AESGCM, AESCCM] = alg_cfg.primitive(
self.k, tag_length=alg_cfg.tag_length)
else:
cipher = alg_cfg.primitive(self.k)
return cipher
def key_unwrap(self,
wrapped_key: bytes,
alg: Optional[CoseAlgorithms] = None) -> bytes:
self._check_key_conf(alg, KeyOps.UNWRAP)
try:
alg_cfg = config(CoseAlgorithms(self.alg))
except KeyError as err:
raise CoseIllegalAlgorithm(err)
return alg_cfg.primitive.aes_key_unwrap(self.k, wrapped_key,
default_backend())
def compute_tag(self,
to_be_maced: bytes,
alg: Optional[CoseAlgorithms] = None) -> bytes:
"""
Compute the MAC over the payload.
:param to_be_maced: The payload over which the authentication tag will be calculated.
:param alg: An optional CoseAlgorithm for computing the authentication tag.
:return: The authentication tag.
"""
self._check_key_conf(alg, KeyOps.MAC_CREATE)
iv = unhexlify(b"".join([b"00"] * 16))
try:
alg_cfg = config(CoseAlgorithms(self.alg))
except KeyError as err:
raise CoseIllegalAlgorithm(err)
if self.alg in {
CoseAlgorithms.AES_MAC_128_128, CoseAlgorithms.AES_MAC_128_64,
CoseAlgorithms.AES_MAC_256_64, CoseAlgorithms.AES_MAC_256_128
}:
encryptor = Cipher(alg_cfg.primitive(self.k),
modes.CBC(iv),
backend=default_backend()).encryptor()
while len(to_be_maced) % 16 != 0:
to_be_maced += unhexlify(b"00")
ciphertext = encryptor.update(to_be_maced) + encryptor.finalize()
if alg_cfg.tag_length is not None:
# truncate the result to the first 64 bits
ciphertext = ciphertext[:-8]
digest = ciphertext[-8:]
else:
digest = ciphertext[-16:]
else:
h = alg_cfg.primitive(self.k,
alg_cfg.hash(),
backend=default_backend())
h.update(to_be_maced)
digest = h.finalize()
if CoseAlgorithms(self.alg) == CoseAlgorithms.HMAC_256_64:
# truncate the result to the first 64 bits
digest = digest[:8]
return digest
def compute_hash(self) -> bytes:
try:
alg_cfg = config(CoseAlgorithms(self.alg))
except KeyError as err:
raise CoseIllegalAlgorithm(err)
h = Hash(algorithm=alg_cfg.hash(), backend=openssl.backend)
h.update(self.certificate)
digest = h.finalize()
if alg_cfg.tag_length is not None:
digest = digest[:8]
return digest
def key_wrap(self,
plaintext_key: bytes,
alg: Optional[CoseAlgorithms] = None) -> bytes:
self._check_key_conf(alg, KeyOps.WRAP)
try:
alg_cfg = config(CoseAlgorithms(self.alg))
except KeyError as err:
raise CoseIllegalAlgorithm(err)
if self.alg in {
CoseAlgorithms.A128KW, CoseAlgorithms.A192KW,
CoseAlgorithms.A256KW
}:
return alg_cfg.primitive.aes_key_wrap(self.k, plaintext_key,
default_backend())
elif self.alg == CoseAlgorithms.DIRECT:
return b''
else:
raise CoseIllegalAlgorithm(
f"Key wrap requires one of the following algorithms: \
{(CoseAlgorithms.A256KW, CoseAlgorithms.A192KW, CoseAlgorithms.A128KW, CoseAlgorithms.DIRECT)}"
)