HKDF.py

# Reticulum License
#
# Copyright (c) 2016-2025 Mark Qvist
#
# Permission is hereby granted, free of charge, to any person obtaining a copy
# of this software and associated documentation files (the "Software"), to deal
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#   dataset, including but not limited to any use that contributes to the
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import hashlib
from math import ceil
from RNS.Cryptography import HMAC

# Precomputed byte-permutation tables used to key the inner
# and outer hashing states of the HMAC construction
trans_5C = bytes((x ^ 0x5C) for x in range(256))
trans_36 = bytes((x ^ 0x36) for x in range(256))

# Precomputed HKDF counter bytes
_counters = [bytes([(i + 1) % 0x100]) for i in range(0x100)]

def hkdf(length=None, derive_from=None, salt=None, context=None):
    hash_len = 32
    hash_block_size = 64

    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.new(salt, derive_from).digest()

    # The pseudorandom key is always a 32-byte SHA-256 digest, so the
    # HMAC key schedule below never needs the long-key hashing branch.
    key = pseudorandom_key.ljust(hash_block_size, b"\x00")

    # Precompute the keyed inner and outer hashing states of the
    # HMAC construction once. Each expansion block then only requires
    # two cheap hash-object copies instead of constructing a complete
    # HMAC object per 32 bytes of output. This is possible since the
    # PRK is **always** 32-byte SHA-256 HMAC output.
    inner_state = hashlib.sha256(key.translate(trans_36))
    outer_state = hashlib.sha256(key.translate(trans_5C))

    # Expand
    has_context = len(context) > 0
    block = b""
    derived = bytearray()

    for i in range(ceil(length / hash_len)):
        inner = inner_state.copy()
        inner.update(block)
        if has_context: inner.update(context)
        inner.update(_counters[i & 0xFF])

        outer = outer_state.copy()
        outer.update(inner.digest())

        block = outer.digest()
        derived += block

    return bytes(derived[:length])

def hkdf_legacy(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)%(0xFF+1)]))
        derived += block

    return derived[:length]