This may be an issue if you for-example use UUIDs as record IDs and use secrecy of the record ID as a means of controlling access (see for example "unlisted" videos on youtube). If your user uses a non-secure PRNG, then an attacker given a couple of UUIDs generated by a user may be able to reverse engineer the PRNG state, and hence predict other random numbers generated by that user.Since the UUIDs are being generated client side, you have to consider the possibility that a malicious client may deliberately modify the generator to generate colliding UUIDs.OTOH if the random number generator has 128 bits of state and is well seeded this is not really an issue. If the random number generator only has 32 bits of entropy, then the probability of collision is non-negligable even for relatively small numbers of UUIDs.Still that is sufficient to make accidental collisions unlikely unles youhave a ridiculous number of items. There are 128 bits in a UUID, but in a "variant 1 version 4" UUID, six of those bits are fixed, so you have at best 122 bits of entropy. this might especially be an issue on non-standard / embedded browsers.įirst off UUIDs do not have 128 bits of entropy. Beware of dragons though: "There is no minimum degree of entropy mandated by the Web Cryptography specification.". UUID/GUID comes from the acronym Universally/Globally Unique Identifier, which is a 128-bit number used to. If this is really something you require then there is subtle crypto now for JavaScript in browsers, use that cryptographically secure random number generator instead. This is an online tool to generate UUID / GUID. Try and avoid generating UID's at the client side entirely. If it is 32 bits taken from the system clock then it is really not entropy at all, to name just one common option. What is UUID You can use GUID codes as serial numbers for Windows & Mac software, or coupon codes for your online stores, etc. GUID is a 128-bit integer number used to identify resources uniquely. I'd also be concerned about the entropy source. You can generate UUIDs for free using our UUID Generator. This online tool allows you to generate the GUID ( Globally Unique Identifier ) strings. In that case all bets are off, because in the end the RNG is simply not cryptographically secure. Now generally you will likely not find dupes during regular use, but an adversary may simply try and see if a collision can be made. And in that case, there is a high likelihood of collisions due to the birthday bound. So if you require 128 - 4 = 124 bits and you input 32 bits, you can rest assured that the amount left is at most 32 bits. The entropy will never go higher than the amount you put in. to concatenate 4 bits of pseudo randomness at a time until you get a 128 bit number, will this really give a safe unique UUID of 128 bits? Or is the entropy in that resulting UUID much lower? getRandomHexChar() + getRandomHexChar() +. I am looking at a seemingly popular piece of JavaScript code to generate a UUID which is supposed to be a 128-bit number: function uuidv4() $ bits of entropy) like this e.g.
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