Are Java random UUID's predictable?

问题:

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?

回答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        */
        randomBytes[6]  |= 0x40;  /* set to version 4     */
        randomBytes[8]  &= 0x3f;  /* clear variant        */
        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...

回答2:

If you want to generate a secure random key, I suggest you use SecureRandom. This can generate a key of any number of bits you require. Its slower than Random, but much more secure.

回答3:

I have always thought that the 'cryptographically secure random number generator', (actually 'cryptographically strong pseudo random number generator') Javadoc note answers this.

http://download.oracle.com/javase/1,5.0/docs/api/java/util/UUID.html#randomUUID()

From What Wikipedia says http://en.wikipedia.org/wiki/Cryptographically_secure_pseudorandom_number_generator such prediction would be a non-polynomial algorithm.

If you need something 'truely' not just 'pseudo' random, you need to use something external, a hardware noise generator, random points generated after moving a mouse,...

EntropyPool seems to be helping with that, have not tried it yet http://random.hd.org/ The way I understand it, it let's you download some real world noise and use it in your Java app. It is not connected to java.util.UUID api, however, could probably be plugged in using the nameUUIDFromBytes (or other?) method.

Would be cool if you let us know which way you decided to go.