HKDF.py

# MIT License
#
# Copyright (c) 2022 Mark Qvist / unsigned.io
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
# in the Software without restriction, including without limitation the rights
# to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
# copies of the Software, and to permit persons to whom the Software is
# furnished to do so, subject to the following conditions:
#
# The above copyright notice and this permission notice shall be included in all
# copies or substantial portions of the Software.
#
# THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
# IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
# FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
# AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
# LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
# OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
# SOFTWARE.

import hashlib
from math import ceil
from RNS.Cryptography import HMAC

def hkdf(length=None, derive_from=None, salt=None, context=None):
    hash_len = 32

    def hmac_sha256(key, data):
        return HMAC.new(key, data).digest()

    if length == None or length < 1:
        raise ValueError("Invalid output key length")

    if derive_from == None or derive_from == "":
        raise ValueError("Cannot derive key from empty input material")

    if salt == None or len(salt) == 0:
        salt = bytes([0] * hash_len)

    if context == None:
        context = b""

    pseudorandom_key = hmac_sha256(salt, derive_from)

    block = b""
    derived = b""

    for i in range(ceil(length / hash_len)):
        block = hmac_sha256(pseudorandom_key, block + context + bytes([i + 1]))
        derived += block

    return derived[:length]