constant = expandNFold(constant, cipher.getBlockSize());
        }
        byte[] enc = constant;
        int klen = keybytes.length;
        byte[] dk = new byte[klen];
        for ( int n = 0; n < klen; ) {
            byte[] block = cipher.doFinal(enc);
            int len = Math.min(klen - n, block.length);
            System.arraycopy(block, 0, dk, n, len);
            n += len;
            enc = block;