CODEWITHSUNDEEP
c++guid
mharran
mharran

Reputation: 159

Creating a GUID using My Own Algorithm

I need to create a document GUID for an app that will insert Xmp data into a file. Written in C++, the app will be cross platform so I want to avoid using libraries that might create compatibility problems.

What I'm thinking of doing is to create a string by concatenating the name of my app plus the number of seconds since epoch plus the appropriate number of characters from a SHA256 hash calculated for the full file (I will be doing that anyway for another reason).

Would the result produced be enough to guarantee that collision would be "sufficiently improbable in practice"?

Upvotes: 0

Views: 230

Answers (1)

rhashimoto
rhashimoto

Reputation: 15869

Unless you are expecting to generate insanely high numbers of documents, using SHA256 all by itself is overwhelmingly likely to avoid any collisions. If your app generates fewer than 10^29 documents over its lifetime then the chance of any collisions is less than 1 in 10^18 (assuming SHA256 is well-designed).

This means that roughly everyone in the world (7e9) could use your app to generate 1e9 documents per second for 1,000 years before there was a 0.0000000000000001% chance of a collision.

You can add more bits (like name and time) if you like but that will almost certainly be overkill for the purpose of avoiding collisions.

Upvotes: 2

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