muhash.py

# Copyright (c) 2020 Pieter Wuille
# Distributed under the MIT software license, see the accompanying
# file COPYING or http://www.opensource.org/licenses/mit-license.php.
"""Native Python MuHash3072 implementation."""

import hashlib
import unittest

def rot32(v, bits):
    """Rotate the 32-bit value v left by bits bits."""
    bits %= 32  # Make sure the term below does not throw an exception
    return ((v << bits) & 0xffffffff) | (v >> (32 - bits))

def chacha20_doubleround(s):
    """Apply a ChaCha20 double round to 16-element state array s.

    See https://cr.yp.to/chacha/chacha-20080128.pdf and https://tools.ietf.org/html/rfc8439
    """
    QUARTER_ROUNDS = [(0, 4, 8, 12),
                      (1, 5, 9, 13),
                      (2, 6, 10, 14),
                      (3, 7, 11, 15),
                      (0, 5, 10, 15),
                      (1, 6, 11, 12),
                      (2, 7, 8, 13),
                      (3, 4, 9, 14)]

    for a, b, c, d in QUARTER_ROUNDS:
        s[a] = (s[a] + s[b]) & 0xffffffff
        s[d] = rot32(s[d] ^ s[a], 16)
        s[c] = (s[c] + s[d]) & 0xffffffff
        s[b] = rot32(s[b] ^ s[c], 12)
        s[a] = (s[a] + s[b]) & 0xffffffff
        s[d] = rot32(s[d] ^ s[a], 8)
        s[c] = (s[c] + s[d]) & 0xffffffff
        s[b] = rot32(s[b] ^ s[c], 7)

def chacha20_32_to_384(key32):
    """Specialized ChaCha20 implementation with 32-byte key, 0 IV, 384-byte output."""
    # See RFC 8439 section 2.3 for chacha20 parameters
    CONSTANTS = [0x61707865, 0x3320646e, 0x79622d32, 0x6b206574]

    key_bytes = [0]*8
    for i in range(8):
        key_bytes[i] = int.from_bytes(key32[(4 * i):(4 * (i+1))], 'little')

    INITIALIZATION_VECTOR = [0] * 4
    init = CONSTANTS + key_bytes + INITIALIZATION_VECTOR
    out = bytearray()
    for counter in range(6):
        init[12] = counter
        s = init.copy()
        for _ in range(10):
            chacha20_doubleround(s)
        for i in range(16):
            out.extend(((s[i] + init[i]) & 0xffffffff).to_bytes(4, 'little'))
    return bytes(out)

def data_to_num3072(data):
    """Hash a 32-byte array data to a 3072-bit number using 6 Chacha20 operations."""
    bytes384 = chacha20_32_to_384(data)
    return int.from_bytes(bytes384, 'little')

class MuHash3072:
    """Class representing the MuHash3072 computation of a set.

    See https://cseweb.ucsd.edu/~mihir/papers/inchash.pdf and https://lists.linuxfoundation.org/pipermail/bitcoin-dev/2017-May/014337.html
    """

    MODULUS = 2**3072 - 1103717

    def __init__(self):
        """Initialize for an empty set."""
        self.numerator = 1
        self.denominator = 1

    def insert(self, data):
        """Insert a byte array data in the set."""
        data_hash = hashlib.sha256(data).digest()
        self.numerator = (self.numerator * data_to_num3072(data_hash)) % self.MODULUS

    def remove(self, data):
        """Remove a byte array from the set."""
        data_hash = hashlib.sha256(data).digest()
        self.denominator = (self.denominator * data_to_num3072(data_hash)) % self.MODULUS

    def digest(self):
        """Extract the final hash. Does not modify this object."""
        val = (self.numerator * pow(self.denominator, -1, self.MODULUS)) % self.MODULUS
        bytes384 = val.to_bytes(384, 'little')
        return hashlib.sha256(bytes384).digest()

class TestFrameworkMuhash(unittest.TestCase):
    def test_muhash(self):
        muhash = MuHash3072()
        muhash.insert(b'\x00' * 32)
        muhash.insert((b'\x01' + b'\x00' * 31))
        muhash.remove((b'\x02' + b'\x00' * 31))
        finalized = muhash.digest()
        # This mirrors the result in the C++ MuHash3072 unit test
        self.assertEqual(finalized[::-1].hex(), "10d312b100cbd32ada024a6646e40d3482fcff103668d2625f10002a607d5863")

    def test_chacha20(self):
        def chacha_check(key, result):
            self.assertEqual(chacha20_32_to_384(key)[:64].hex(), result)

        # Test vectors from https://tools.ietf.org/html/draft-agl-tls-chacha20poly1305-04#section-7
        # Since the nonce is hardcoded to 0 in our function we only use those vectors.
        chacha_check([0]*32, "76b8e0ada0f13d90405d6ae55386bd28bdd219b8a08ded1aa836efcc8b770dc7da41597c5157488d7724e03fb8d84a376a43b8f41518a11cc387b669b2ee6586")
        chacha_check([0]*31 + [1], "4540f05a9f1fb296d7736e7b208e3c96eb4fe1834688d2604f450952ed432d41bbe2a0b6ea7566d2a5d1e7e20d42af2c53d792b1c43fea817e9ad275ae546963")