/**
 * Copyright (c) 2018 Jun Kurihara
 * Copyright 2020 The Matrix.org Foundation C.I.C.
 * 
 * MIT LICENSE, See https://github.com/junkurihara/jscu/blob/develop/packages/js-crypto-hkdf/LICENSE
 * Based on https://github.com/junkurihara/jscu/blob/develop/packages/js-crypto-hkdf/src/hkdf.ts
 */

// forked this code to make it use the cryptoDriver for HMAC that is more backwards-compatible
export async function hkdf(cryptoDriver, key, salt, info, hash, length) {
    length = length / 8;
    const len = cryptoDriver.digestSize(hash);

    // RFC5869 Step 1 (Extract)
    const prk = await cryptoDriver.hmac.compute(salt, key, hash);

    // RFC5869 Step 2 (Expand)
    let t = new Uint8Array([]);
    const okm = new Uint8Array(Math.ceil(length / len) * len);
    for(let i = 0; i < Math.ceil(length / len); i++){
        const concat = new Uint8Array(t.length + info.length + 1);
        concat.set(t);
        concat.set(info, t.length);
        concat.set(new Uint8Array([i+1]), t.length + info.length);
        t = await cryptoDriver.hmac.compute(prk, concat, hash);
        okm.set(t, len * i);
    }
    return okm.slice(0, length);
}