Generating secure Guids

Guids are used extensively throughout Microsoft systems and developers tend to turn to Guid.NewGuid() whenever they need to create a value to uniquely identify something. Guids might also be used as keys or identifiers in security critical operations — under the assumption that they are hard to guess for an attacker. I've been looking around the Internet to see if I could find some guidance on Guid security along with details on how they are generated in the .NET framework. I couldn't find much information, but I did find that Eric Lippert from the C# team recently raised some concerns about the Guids on his blog. So I started digging around to see what more I could find out.

First of all a quick background. Microsoft's Guid is their implementation of the Universally Unique IDentifier (UUID) outlined in RFC 4122. UUIDs are 128 bits, and the Guid class generates version 4 UUIDs, meaning that all bits except those defining the version and variant of the UUID are "random." Please note that 4 bits are used for the version number, and two bits are used for the variant — so it's not a 128 bit random number, it's a 122 bit random number.