/**
     * @param sessionKey
     * @param label
     * @param context
     */
    private static byte[] derive ( byte[] sessionKey, byte[] label, byte[] context ) {
        KDFCounterBytesGenerator gen = new KDFCounterBytesGenerator(new HMac(new SHA256Digest()));

        int r = 32;
        byte[] suffix = new byte[label.length + context.length + 5];
        // per bouncycastle