Alex Rivera | Logout

How Random is System.Guid.NewGuid()?

Asked 2009-01-21T22:20:24.390
55

I know this may sounds like a pointless question, but hear me out...

I basically want to know if I can trust the GUID to generate a value which will be unique 100% of the time and impossible to predict.

I'm basically rolling my on login system for a website and want to know if the GUID is secure enough for session cookies.

Any background on how the GUID is generated would be much appreciated in evaluating the answers.

Thanks for the links to duplicate questions, however, my question is specific to the .Net framework.

.net random guid
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2 Answers

13

No fixed-length value can ever guarantee to be 100% unique (just call it enough times, give or take the universe ending ;-p) - but it can be very, very, very unlikely to duplicate.

answered 2009-01-21T22:25:06.237
5

I can't speak to the predictability of sequential numbers but it will be unique. I think you'd be better off using a random number generator from System.Security.Cryptography, though. Tie a random number with a monotonically increasing value (time) to generate your unique key and you can be sure that it is unique and not predictable.

answered 2009-01-21T22:25:08.343

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