ownership.py

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#!/usr/bin/env python3
 
# Copyright (c) 2015-2022 Clearmatics Technologies Ltd
#
# SPDX-License-Identifier: LGPL-3.0+
 
from __future__ import annotations
 
from zeth.core.utils import hex_extend_32bytes, digest_to_binary_string, \
    encode_abi
 
from Crypto import Random
from hashlib import blake2s
from typing import NewType
 
 
# Secret key for proving ownership
OwnershipSecretKey = NewType('OwnershipSecretKey', bytes)
 
 
# Public key for proving owenership
OwnershipPublicKey = NewType('OwnershipPublicKey', bytes)
 
 
class OwnershipKeyPair:
    """
    Key-pair for ownership proof. This represents the 'payment key' (apk)
    from the 'payment address' and the 'spending key' (ask) from the
    'private address'. These are components of ZethAddress, used in
    note commitments in the joinsplit statement.
    """
    def __init__(self, a_sk: OwnershipSecretKey, a_pk: OwnershipPublicKey):
        self.a_sk: OwnershipSecretKey = a_sk
        self.a_pk: OwnershipPublicKey = a_pk
 
 
def ownership_key_as_hex(a_sk: bytes) -> str:
    """
    Convert either a secret or public ownership key to hex representation of the
    underlying 32-byte object.
    """
    return hex_extend_32bytes(a_sk.hex())
 
 
def ownership_public_key_from_hex(key_hex: str) -> OwnershipPublicKey:
    """
    Read an ownership public key from a hex string.
    """
    return OwnershipPublicKey(bytes.fromhex(key_hex))
 
 
def ownership_secret_key_from_hex(key_hex: str) -> OwnershipSecretKey:
    """
    Read an ownership public key from a hex string.
    """
    return OwnershipSecretKey(bytes.fromhex(key_hex))
 
 
def gen_ownership_keypair() -> OwnershipKeyPair:
    a_sk = OwnershipSecretKey(Random.get_random_bytes(32))
    a_pk = _derive_a_pk(a_sk)
    keypair = OwnershipKeyPair(a_sk, a_pk)
    return keypair
 
 
def _derive_a_pk(a_sk: OwnershipSecretKey) -> OwnershipPublicKey:
    """
    Returns a_pk = blake2s(1100 || [a_sk]_252 || 0^256)
    """
    binary_a_sk = digest_to_binary_string(a_sk)
    first_252bits_ask = binary_a_sk[:252]
    left_leg_bin = "1100" + first_252bits_ask
    left_leg_hex = "{0:0>4X}".format(int(left_leg_bin, 2))
    zeroes = "0000000000000000000000000000000000000000000000000000000000000000"
    a_pk = blake2s(
        encode_abi(
            ["bytes32", "bytes32"],
            [bytes.fromhex(left_leg_hex), bytes.fromhex(zeroes)])
    ).digest()
    return OwnershipPublicKey(a_pk)