March 24th, 2011

How can I generate a consistent but unique value that can coexist with GUIDs?

A customer needed to generate a GUID for each instance of a hardware device they encounter:

The serial number for each device is 20 bits long (four and a half bytes). We need to generate a GUID based on each device, subject to the constraints that when a device is reinserted, we generate the same GUID for it, that no two devices generate the same GUID, and that the GUIDs we generate not collide with GUIDs generated by other means. One of our engineers suggested just running uuidgen and substituting the serial number for the final nine hex digits. Is this a viable technique?

This is similar to the trap of thinking that half of a GUID is just as unique as the whole thing. Remember that all the pieces of a GUID work together to establish uniqueness. If you just take parts of it, then the algorithm breaks down. For this particular case, you’re in luck. The classic Type 1 GUID uses 60 bits to encode the timestamp and 48 bits to identify the location (computer). You can take a network card, extract the MAC address, then smash the card with a hammer. Now you have a unique location. Put your twenty bits of unique data as the timestamp, and you have a Type 1 GUID that is guaranteed never to collide with another GUID.

If you have more than 60 bits of unique data, then this trick won’t work. Fortunately, RFC4122 explains how to create a so-called name-based UUID, which is a UUID that can be reliably regenerated from the same source data. Section 4.3 explains how it’s done. The result is either a type 3 or type 5 UUID, depending on which variant of the algorithm you chose.

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Raymond has been involved in the evolution of Windows for more than 30 years. In 2003, he began a Web site known as The Old New Thing which has grown in popularity far beyond his wildest imagination, a development which still gives him the heebie-jeebies. The Web site spawned a book, coincidentally also titled The Old New Thing (Addison Wesley 2007). He occasionally appears on the Windows Dev Docs Twitter account to tell stories which convey no useful information.

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