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?
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...