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

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