Inside Bitcoin

1#!/usr/bin/env python3
2# Copyright (c) 2023-present The Bitcoin Core developers
3# Distributed under the MIT software license, see the accompanying
4# file COPYING or http://www.opensource.org/licenses/mit-license.php.
5
6"""Test-only HKDF-SHA256 implementation
7
8It is designed for ease of understanding, not performance.
9
10WARNING: This code is slow and trivially vulnerable to side channel attacks. Do not use for
11anything but tests.
12"""
13
14import hashlib
15import hmac
16
17
18def hmac_sha256(key, data):
19    """Compute HMAC-SHA256 from specified byte arrays key and data."""
20    return hmac.new(key, data, hashlib.sha256).digest()
21
22
23def hkdf_sha256(length, ikm, salt, info):
24    """Derive a key using HKDF-SHA256."""
25    if len(salt) == 0:
26        salt = bytes([0] * 32)
27    prk = hmac_sha256(salt, ikm)
28    t = b""
29    okm = b""
30    for i in range((length + 32 - 1) // 32):
31        t = hmac_sha256(prk, t + info + bytes([i + 1]))
32        okm += t
33    return okm[:length]
34