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#!/usr/bin/env python3
# Copyright (c) 2023 The Bitcoin Core developers
# Distributed under the MIT software license, see the accompanying
# file COPYING or http://www.opensource.org/licenses/mit-license.php.
"""Test-only HKDF-SHA256 implementation
It is designed for ease of understanding, not performance.
WARNING: This code is slow and trivially vulnerable to side channel attacks. Do not use for
anything but tests.
"""
import hashlib
import hmac
def hmac_sha256(key, data):
"""Compute HMAC-SHA256 from specified byte arrays key and data."""
return hmac.new(key, data, hashlib.sha256).digest()
def hkdf_sha256(length, ikm, salt, info):
"""Derive a key using HKDF-SHA256."""
if len(salt) == 0:
salt = bytes([0] * 32)
prk = hmac_sha256(salt, ikm)
t = b""
okm = b""
for i in range((length + 32 - 1) // 32):
t = hmac_sha256(prk, t + info + bytes([i + 1]))
okm += t
return okm[:length]
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