How are .NET 4 GUIDs generated?

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Since Windows 2000 Microsoft uses a version 4 algorithm:

With Windows 2000, Microsoft switched to version 4 GUIDs, since embedding the MAC address was viewed as a security risk. 1

You can see that as well from a GUID generated in .NET (from Wikipedia):

Version 4 UUIDs have the form xxxxxxxx-xxxx-4xxx-yxxx-xxxxxxxxxxxx with any hexadecimal digits for x but only one of 8, 9, A, or B for y. e.g. f47ac10b-58cc-4372-a567-0e02b2c3d479.

A version 4 UUID consist of 122 significant bits, giving 2^122 distinct values which is a very large number. Given a set of H values, the expected number of values we have to choose before finding the first random collision with a 50% chance can be calculated as follows (see Birthday Attack on Wikipedia):

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The result (birthday bound) for 2^122 different values is approximately 2,89e+18. This assumes that the generated values are randomly distributed. Obviously, if the values are distributed unevenly, a random collision can be found faster. For further details also see Random UUID probability of duplicates.

1As a matter of fact, the author of the Melissa worm could be tracked down due to a GUID generated using a version 1 algorithm.

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