Simple XOR encoder

SimpleEncoder.java

public class SimpleEncoder {
    private static final byte[] SECRET = {
            (byte) 0x9e, (byte) 0x40, (byte) 0x5e, (byte) 0xdf,
            (byte) 0xd5, (byte) 0x92, (byte) 0x90, (byte) 0x60,
            (byte) 0xfe, (byte) 0x4a, (byte) 0xaf, (byte) 0x50,
            (byte) 0x74, (byte) 0xed, (byte) 0xcb, (byte) 0x74,
    };

    public static void xor(@NonNull byte[] input, @NonNull byte[] secret, boolean x) {
        if (input.length < 1 || secret.length < 1) {
            return;
        }

        int secretLength = secret.length;
        int secretIndex = input.length % secretLength;

        for (int i = 0, n = input.length; i < n; ++i) {
            int oldValue = (0xff & input[i]);

            input[i] ^= secret[secretIndex % secretLength];

            int newValue = (0xff & input[i]);

            secretIndex = (secretIndex + (x ? oldValue : newValue)) % secretLength;
        }
    }

    @NonNull
    public static String toHexString(@NonNull byte[] bytes) {
        StringBuilder sb = new StringBuilder(bytes.length * 2);

        for (byte b : bytes) {
            sb.append(String.format("%02x", b));
        }

        return sb.toString();
    }

    @NonNull
    public static byte[] hexToByteArray(@NonNull String input) {
        byte[] result = new byte[input.length() / 2];

        for (int i = 0, n = input.length(); i < n; i += 2) {
            result[i / 2] = (byte) (
                    (Character.digit(input.charAt(i), 16) << 4) + Character.digit(input.charAt(i + 1), 16)
            );
        }

        return result;
    }

    @NonNull
    public static String encode(@NonNull String input) {
        byte[] bytes = input.getBytes();
        xor(bytes, SECRET, true);
        return toHexString(bytes);
    }

    @NonNull
    public static String decode(@NonNull String input) {
        byte[] bytes = hexToByteArray(input);
        xor(bytes, SECRET, false);
        return new String(bytes);
    }
}