Make a unique filesystem path, without creating the file

[Steven D'Aprano]

On Tue, 23 Feb 2016 05:48 am, Marko Rauhamaa wrote:

> Jon Ribbens <jon+usenet at unequivocal.co.uk>:
> 
>> I was under the impression that the point of UUIDs is that you can be
>> *so* confident that there won't be a collision that for all practical
>> purposes it's indistinguishable from being certain.
> 
> Yes, if you generate a random 128-bit number, it will be unique --


If you generate a second random 128 bit number, you have a chance of 1 in
2**128 of a collision. All you can say is that it will be *very probably*
unique. (I might even allow "almost certainly" unique.)

If you generate 2**128 + 1 such numbers, you are *guaranteed* to have at
least one collision.

If I can arrange matters so that I am using the same seed as you, then I can
generate the same UUIDs as you.

If I know you are using the Mersenne Twister PRNG, and I can get hold of (by
memory) 128 consecutive UUIDs, I can reconstruct the seed you are using and
generate all future (and past) UUIDs the same as yours. (Well, when I
say "I can", I don't mean *me*, I mean some attacker who is smarter than
me, but not that much smarter.)



> unless someone clones it.
> 
> Cloning will be a practical issue when you clone virtual machines, for
> example.

This is certainly a practical issue that people have to be aware of.




-- 
Steven