source/crypto/Hkdf.mc


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// Author:  cirakg & claude ai
// Summary: HKDF-SHA256 (RFC 5869) on the native HMAC, small enough for the glance.
//
// The key derivation SealedBox and KeyManager use for every sub-key. It
// lives apart from Kdf (PBKDF2, which needs the job system) so the glance
// can open the sealed favourite code (GlanceCode) with only the crypto it
// needs. Verified with RFC 5869 test cases 1 to 3 (test/crypto/KdfAesTest.mc)
// and in the start-up self-test.

import Toybox.Lang;

(:glance)
module Hkdf {

    const HASH_BYTES = 32;

    // Extract with salt (empty means 32 zero bytes), then expand to
    // `length` bytes with info. length <= 255 * 32.
    function derive(ikm as ByteArray, salt as ByteArray, info as ByteArray, length as Number) as ByteArray {
        if (length < 1 || length > 255 * HASH_BYTES) {
            throw new Lang.InvalidValueException("HKDF: bad key length");
        }
        var prk = Hmac.nativeSha256(salt.size() > 0 ? salt : new [HASH_BYTES]b, ikm);
        var okm = new [0]b;
        var block = new [0]b;
        for (var counter = 1; okm.size() < length; counter++) {
            var input = Bytes.concat(block, info);
            input.add(counter);
            Bytes.wipe(block);
            block = Hmac.nativeSha256(prk, input);
            okm.addAll(block);
        }
        Bytes.wipe(prk);
        Bytes.wipe(block);
        var key = okm.slice(0, length);
        Bytes.wipe(okm);
        return key;
    }
}