Are Java random UUID's predictable?

Question

I would like to use a cryptographically secure primary key for sensitive data in a database - this cannot be guessable/predictable and it cannot be generated by the database (I need the key before the object is persisted).

I understand Java uses a type 4 UUID with a cryptographically secure random number generator, however I know the UUID isn't completely random so my question is how safe is it to assume that uuids cannot be predicted from a set of existing ones?

Answer 1

Well if you want to know how random a UUID is you have to look onto the source.

The following code section is taken from OpenJDK7 (and it is identical in OpenJDK6):

public static UUID randomUUID() {
        SecureRandom ng = numberGenerator;
        if (ng == null) {
            numberGenerator = ng = new SecureRandom();
        }

        byte[] randomBytes = new byte[16];
        ng.nextBytes(randomBytes);
        randomBytes[6]  &= 0x0f;  /* clear version (set highest 4 bits to zero)       */
        randomBytes[6]  |= 0x40;  /* set to version 4     */
        randomBytes[8]  &= 0x3f;  /* clear variant (set highest 2 bits to zero)       */
        randomBytes[8]  |= 0x80;  /* set to IETF variant  */
        return new UUID(randomBytes);
    }

As you can see only 2 of 16 bytes are not completely random. In the sixth byte you lose 4 of 8 bits and on byte 8 you loose 2 bits of randomness.

Therefore you will get an 128 bit value with 122 bit randomness.

The only problem that may arise from the manipulation is that with a high chance your data can be identified as an UUID. Therefore if you want to hide it in other random data this will not work...