How are .NET 4 GUIDs generated?

Question

I am aware of the multitude of questions here as well as Raymond's excellent (as usual) post. However, since the algorithm to create GUIDs was changed apparently, I found it hard to get my hands on any up-to-date information. The MSDN seems to try and provide as few information as possible.

What is known about how GUIDs are generated in .NET 4? What was changed, and how does it affect the security ("randomness") and integrity ("uniqueness")?

One specific aspect I'm interested in: In v1, it seems to be about impossible to generate the same GUID on a single machine again since there was a timestamp and counter involved. In v4, this is no longer the case (I was told), so the chance to get the same GUID on a single machine ... increased?

Answer 1

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):

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.

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