Guid Smash

[twiss]

> The chances of generating two GUIDs that are the same is astronomically small.

> The odds are 1 in 2^122 — that’s approximately 1 in 5,000,000,000,000,000,000,000,000,000,000,000,00.

This is true if you only generate two GUIDs, but if you generate very many GUIDs, the chance of generating two identical ones between any of them increases. E.g. if you generate 2^61 GUIDs, you have about a 1 in 2 chance of a collision, due to the birthday paradox.

2^61 is still a very large number of course, but much more feasible to reach than 2^122 when doing a collision attack. This is the reason that cryptographic hashes are typically 256 bits or more (to make the cost of collision attacks >= 2^128).

[Xelbair]

i have seen in my life two guid collisions already. and i'm not that old.

One of them was genuine - generated by different systems, and it was caught when loading data from one to another - object had same ID, but different underlying type.

Other one was due to 'error' - two systems(by different companies, supporting the same data exchange standard) used magic hardcoded guid that turned out to be the same.

Both of those systems have full audit trail - each change created new row in database and IDs were formatted as {NAMESPACE}.{GUID}.{TIMESTAMP}. Mutation of an object created new entry with different {TIMESTAMP} part. Namescapes are mandated by standard, so different systems can have the same namespace value.

[habibur]

The birthday paradox simplified : if you generate n bits of random data, you can at most generate n/2 bits of random numbers before clashes start to occur. That's square root of number's range.

So if you need 1000 random numbers, generate from 1 to 1 million.

[Retr0id]

2^61 isn't even that large, well within the compute budget of mere mortals.

[somat]

2^61 guids is... 36 exabytes, if my napkin math is correct. when storing them in binary format(16 bytes each) if doing the javascript thing and storing them as strings... (shudders) I don't even want to think about it.

Anyhow that was my first thought when you mentioned 2^61 guids, where are you even going to put them? second thought, I don't think enumerating 2^61 guids is trivial, in fact, I suspect it would take longer than anyone would be willing to spend, and if you are not storing them why are you generating them?

And what even is a guid collision attack? it is not like they are a hash, and since they tend to be public identifiers it turns out despite their stated use to prevent collisions, you can't really use guids generated by others(if they wanted collisions they would straight up just copy yours) so you end up regenerating them anyway.

[NoahZuniga]

* not the birthday paradox, but the birthday bound.

[amingilani]

Instead of picking a target UUID and evaluating new UUIDs against it, a better experiment would be finding duplicates in all the UUIDs you have generated.

This plays nicely with the birthday paradox.

[whyever]

It would require a lot more memory, because you have to remember every generated UUID. And how would you do the partial match? You are not going to observe any collisions.

[8organicbits]

Note that this only considers UUIDv4, the random UUID. Other forms can generate UUIDs that are much closer together. For UUIDv7, UUIDs generated within the same millisecond will have identical 48 bit prefixes (or up to 60 when the monotonic counter from section 6.2 is used).

https://www.rfc-editor.org/rfc/rfc9562.html#monotonicity_cou...

[e1g]

You need to be generating >100M of them within the same millisecond before even remembering that collisions can theoretically happen.

[nopassrecover]

Reminds me of a problem I ran into once where someone had wanted unique but short codes as identifiers for relatively small counts, and picked a substring of a UUID:

http://mattmitchell.com.au/birthday-problems-friendly-identi...

[kr2]

> However, the overall takeaway was: Don’t use the MongoDB Increment value as a Unique Identifier.

However, the overall takeaway should be, as always: don't use MongoDB. Period. Every time I learn something new about it I'm baffled about why people continue to use it.

[franky47]

Easy, it should be listed there: https://everyuuid.com/

[Joel_Mckay]

Most just pack down:

epoch time + MAC Address + transaction counter (catch NTP skew) + Thread PID + new Pointer address = GUID

Then increment global transaction counter, complete some ops, and check to ensure current epoch time is in the future before the transaction frees the memory locations.

This is often robust in highly concurrent distributed systems even under network degradation, or corrupted sync states. Has other interesting use-cases too. =3

[ahmedfromtunis]

The proximity measure seems to be flawed.

If you want to see how close to a non-ordinal 123456 a random generator can get, you also need to look for stuff like 923456 or 123956, etc.

Also, would 223456 be considered a closer match compared to 323456? (It shouldn't in my opinion because, again, these are non-ordinal strings).

[gammalost]

If its a random ID then I'd argue that all of them are equally close to each other. With that said, I do not know how GUIDs are generated

[webstrand]

This is the chance that given a specific guid, that you'll find a collision for it. Utterly minuscule chance. However birthday paradox controls, if you generate 2^62.60 guids the chance that you've generated a collision is around 99%. Still enormously unlikely, but way smaller than 2^122.

At a rate of comparing 400,000 guids per second, you have a 99% chance of seeing a collision within the next 553,750 years.

[jonathrg]

You would need a little more memory to see/detect that collision.

[ivanjermakov]

Reminds me of SHAllenge: https://news.ycombinator.com/item?id=40683564

[nesk_]

Nice experiment. Is the code available somewhere?

[867-5309]

please may all the death huggers go hug a tree. thanks

[RS-232]

UUID > GUID.

Microsoft’s GUID standard is garbage.

[lionkor]

Oh, why?