code: final byte[] hmac(final byte[] input, final int offset, final int len) {
  		hMac.update(input, offset, len);
  		return hMac.doFinal();
  	}
docs: Calculate HMAC of input
  <p>
  <a href="http://en.wikipedia.org/wiki/HMAC">HMAC</a>
  
  @param input
  @param offset
  @param len
  @return