Are Java random UUID’s predictable?

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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…

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