Base64 UUID with space for some flags

RichUuid.java

package dev.richst.libs.richuuid;

import java.io.*;
import java.util.Base64;
import java.util.UUID;

public class RichUuid {
    static byte[] uuidToByteArray(UUID uuid) {
        try {
            ByteArrayOutputStream bos = new ByteArrayOutputStream();
            DataOutputStream dos = new DataOutputStream(bos);
            dos.writeLong(uuid.getMostSignificantBits());
            dos.writeLong(uuid.getLeastSignificantBits());
            dos.write(2);
            dos.flush();
            return bos.toByteArray();
        } catch (IOException ex) {
            // this should never be thrown, and there is no sensible way to handle it
            throw new IllegalStateException(
                    "IO exception occurred working on memory backed streams", ex);
        }
    }

    static UUID byteArrayToUuid(byte[] uuidBytes) {
        try {
            ByteArrayInputStream bis = new ByteArrayInputStream(uuidBytes);
            DataInputStream dis = new DataInputStream(bis);
            long msb = dis.readLong();
            long lsb = dis.readLong();
            return new UUID(msb, lsb);
        } catch (IOException ex) {
            // this should never be thrown, and there is no sensible way to handle it
            throw new IllegalStateException(
                    "IO exception occurred working on memory backed streams", ex);
        }
    }

    public static String fromUUID(UUID uuid) {
        byte[] uuidBytes = uuidToByteArray(uuid);
        byte[] uuidBytesWithChecksum = new byte[18];
        int checksum = 0;
        for (int ii = 0; ii < 16; ii++) {
            uuidBytesWithChecksum[ii] = uuidBytes[ii];
            checksum += uuidBytes[ii] & 0xFF;
        }
        uuidBytesWithChecksum[16] = 0;
        uuidBytesWithChecksum[17] = (byte) (checksum % 256);
        return Base64.getUrlEncoder().encodeToString(uuidBytesWithChecksum);
    }

    public static String generate() {
        return fromUUID(UUID.randomUUID());
    }

    private static boolean validateChecksum(byte[] uuidBytesWithChecksum) {
        int checksum = 0;
        for (int ii = 0; ii < 17; ii++) {
            checksum += uuidBytesWithChecksum[ii] & 0xFF;
        }
        return ((byte) checksum) == uuidBytesWithChecksum[17];
    }

    private static void assertChecksumValid(byte[] uuidBytes) {
        if (!validateChecksum(uuidBytes)) {
            throw new InvalidRichUuidException("checksum invalid");
        }
    }

    public static UUID toUUID(String richUuid) {
        byte[] uuidBytesWithChecksum;
        try {
            uuidBytesWithChecksum = Base64.getUrlDecoder().decode(richUuid);
        } catch (IllegalArgumentException ex) {
            throw new InvalidRichUuidException("Could not base64 decode richUuid", ex);
        }
        if (uuidBytesWithChecksum.length != 18) {
            throw new InvalidRichUuidException("UUID length invalid");
        }
        assertChecksumValid(uuidBytesWithChecksum);
        return byteArrayToUuid(uuidBytesWithChecksum);
    }
}