uuid types 3 and 5

[Fraser Gordon]

On 09/01/2014 20:20, Martin Baxter wrote:
> I have read that type 3 is deprecated, on the basis that md5 is not
> considered secure these days, although I think that was one thing that
> confused me because uuid is nothing to do with encryption or obfuscation
> as far as I was aware.
I believe the idea is to map a hierarchical system of names (like a
filesystem) to fixed-length identifiers:

foo/bar - UUID = hash(bar + hash(foo))
foo/baz/bar - UUID = hash(bar + hash(baz + hash(foo)))
baz/bar - UUID = hash(bar + hash(baz))

will all (with overwhelming probability) have unique identifiers but the
identifiers will be consistent whenever the UUID of a particular name is
generated.
> Is it perhaps that the "namespace" has to be obfuscated so that
> "unspecified unauthorised persons" don't also deliberately create uuids
> based on the same namespace?
>
> If that is the case, you wouldn't want to use this in an open-source
> context, because the namespace would be a secret?
UUIDs are not really intended to provide security - their purpose is for
uncoordinated parties to generate identifiers without (realistic)
probability of overlap.

The collision resistance is for a situation where you have
filesystem-like paths that resolve to UUIDs that are then looked up in a
table (this is a fairly normal hash table but based on the full path
rather than hash components).  With such "unique" identifiers, you might
decide not to allow UUID collisions. Now imagine a system where a number
of people create records (e.g. a census) which have a UUID assigned
based on some of the recorded information, e.g.:

/Country/State/City/Street/Building/Surname/FirstName/DateOfBirth

Some malicious person might deliberately change their name in such a way
that their UUID will clash with another name - this will cause problems
when the records are centralised and combined into the full data set.

This isn't a terribly realistic example but should hopefully indicate
the motivation. I have no idea if anyone actually does this but similar
hashing schemes exist in modern filesystems for things like metadata
integrity checking.

Regards,
Fraser