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from Cryptodome.Util.number import long_to_bytes
from Cryptodome.PublicKey.ECC import EccKey
def _compute_ecdh(key_priv, key_pub):
# See Section 5.7.1.2 in NIST SP 800-56Ar3
pointP = key_pub.pointQ * key_priv.d
if pointP.is_point_at_infinity():
raise ValueError("Invalid ECDH point")
z = long_to_bytes(pointP.x, pointP.size_in_bytes())
return z
def key_agreement(**kwargs):
"""Perform a Diffie-Hellman key agreement.
Keywords:
kdf (callable):
A key derivation function that accepts ``bytes`` as input and returns
``bytes``.
static_priv (EccKey):
The local static private key. Optional.
static_pub (EccKey):
The static public key that belongs to the peer. Optional.
eph_priv (EccKey):
The local ephemeral private key, generated for this session. Optional.
eph_pub (EccKey):
The ephemeral public key, received from the peer for this session. Optional.
At least two keys must be passed, of which one is a private key and one
a public key.
Returns (bytes):
The derived secret key material.
"""
static_priv = kwargs.get('static_priv', None)
static_pub = kwargs.get('static_pub', None)
eph_priv = kwargs.get('eph_priv', None)
eph_pub = kwargs.get('eph_pub', None)
kdf = kwargs.get('kdf', None)
if kdf is None:
raise ValueError("'kdf' is mandatory")
count_priv = 0
count_pub = 0
curve = None
def check_curve(curve, key, name, private):
if not isinstance(key, EccKey):
raise TypeError("'%s' must be an ECC key" % name)
if private and not key.has_private():
raise TypeError("'%s' must be a private ECC key" % name)
if curve is None:
curve = key.curve
elif curve != key.curve:
raise TypeError("'%s' is defined on an incompatible curve" % name)
return curve
if static_priv is not None:
curve = check_curve(curve, static_priv, 'static_priv', True)
count_priv += 1
if static_pub is not None:
curve = check_curve(curve, static_pub, 'static_pub', False)
count_pub += 1
if eph_priv is not None:
curve = check_curve(curve, eph_priv, 'eph_priv', True)
count_priv += 1
if eph_pub is not None:
curve = check_curve(curve, eph_pub, 'eph_pub', False)
count_pub += 1
if (count_priv + count_pub) < 2 or count_priv == 0 or count_pub == 0:
raise ValueError("Too few keys for the ECDH key agreement")
Zs = b''
Ze = b''
if static_priv and static_pub:
# C(*, 2s)
Zs = _compute_ecdh(static_priv, static_pub)
if eph_priv and eph_pub:
# C(2e, 0s) or C(2e, 2s)
if bool(static_priv) != bool(static_pub):
raise ValueError("DH mode C(2e, 1s) is not supported")
Ze = _compute_ecdh(eph_priv, eph_pub)
elif eph_priv and static_pub:
# C(1e, 2s) or C(1e, 1s)
Ze = _compute_ecdh(eph_priv, static_pub)
elif eph_pub and static_priv:
# C(1e, 2s) or C(1e, 1s)
Ze = _compute_ecdh(static_priv, eph_pub)
Z = Ze + Zs
return kdf(Z)
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