private static byte[] executeS(byte[] block) {

        byte[] helperBlock = splitInSextets(block);

        // execute S function on each byte
        // each resulting byte contains 4 bits
        for (int i = 0; i < helperBlock.length; i++) {
            ByteHelper.printByte(helperBlock[i]);
            helperBlock[i] = S(helperBlock[i], Feistel.s[i]);
        }

        return joinQuartetsToBytes(helperBlock);
    }