Mini Shell

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)