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41 lines
1.2 KiB
Python
41 lines
1.2 KiB
Python
"""Pure-python NIST P-224 Elliptic Curve cryptography. Used for some Apple algorithms."""
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from cryptography.hazmat.primitives import hashes
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from cryptography.hazmat.primitives.kdf.x963kdf import X963KDF
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ECPoint = tuple[float, float]
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P224_N = 0xFFFFFFFFFFFFFFFFFFFFFFFFFFFF16A2E0B8F03E13DD29455C5C2A3D
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def x963_kdf(value: bytes, si: bytes, length: int) -> bytes:
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"""Single pass of X9.63 KDF with SHA1."""
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return X963KDF(
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algorithm=hashes.SHA256(),
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sharedinfo=si,
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length=length,
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).derive(value)
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def bytes_to_int(value: bytes) -> int:
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"""Convert bytes in big-endian format to int."""
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return int.from_bytes(value, "big")
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def derive_ps_key(privkey: bytes, sk: bytes) -> bytes:
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"""
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Derive a primary or secondary key used by an accessory.
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:param privkey: Private key generated during pairing
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:param sk: Current secret key for this time period.
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Use SKN to derive the primary key, SKS for secondary.
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"""
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priv_int = bytes_to_int(privkey)
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at = x963_kdf(sk, b"diversify", 72)
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u = bytes_to_int(at[:36]) % (P224_N - 1) + 1
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v = bytes_to_int(at[36:]) % (P224_N - 1) + 1
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key = (u * priv_int + v) % P224_N
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return key.to_bytes(28, "big")
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