ByteArrays.java

 public static List<byte[]> grouped(final byte[] input, final int length) {
            if (length <= 0) {
                throw new IllegalArgumentException("length must gt 0");
            }
            List<byte[]> ret = new ArrayList<byte[]>();

            int r = input.length / length;
            int from = 0;
            int to = 0;
            for (int i = 0; i < r; i++) {
                from = i * length;
                to = from + length;
                ret.add(Arrays.copyOfRange(input, from, to));
            }

            int y = input.length % length;
            if (y != 0) {
                from = to;
                to = input.length;
                ret.add(Arrays.copyOfRange(input, from, to));
            }
            return ret;
        }

        public static byte[] concat(final List<byte[]> bytes) {
            int length = 0;
            for (byte[] b : bytes) {
                length += b.length;
            }
            byte[] ret = new byte[length];
            int destPos = 0;
            for (byte[] b : bytes) {
                System.arraycopy(b, 0, ret, destPos, b.length);
                destPos += b.length;
            }
            return ret;
        }

Test.java

    @Test
    public void grouped() {
        byte[] input = { 1, 2, 3, 4, 5 };

        for (int i = 1; i < input.length * 2; i++) {
            printlnGrouped(input, 1);
            assertTrue(Arrays.equals(input, Crypto.Util.concat(Crypto.Util.grouped(input, i))));
        }
    }

    private void printlnGrouped(byte[] input, int len) {
        System.out.print(String.format("grouped(%s,%d) => ", Arrays.toString(input), len));
        List<byte[]> bytes = Crypto.Util.grouped(input, len);
        for (byte[] b : bytes) {
            System.out.print(", ");
            System.out.print(Arrays.toString(b));
        }
        println(" | " + Arrays.toString(Crypto.Util.concat(bytes)));
    }