Question

i wonder if there is a way to generate valid GUIDs/UUIDs where the first (or any part) part is a user-selected prefix.

I.e., the GUID has the format AAAAAAAA-BBBB-CCCC-DDDD-DDDDDDDDDDDD, and I want to set any part to a pre-defined value (ideally the AAA’s). The goal is to have GUIDs still globally unique, but they do not need to be cryptographically safe.

Answer 1

Sorry, you want too much from a GUID. Summarizing from both your question and your own answer/update, you want it to

• 1 be a GUID
• 2 not collide with any other GUID (be globally unique)
• 3 Ignore the standard on the interpretation of the first bits, using a reserved value
• 4 Use a personal scheme for the remaining bits

This is impossible, proof: If it was possible, I could generate a GUID G1 and you could generate another GUID G2. Since we both ignore the standard and use the same reserved prefix, and my personal scheme for the other bits is outside your control, my GUID G1 can clash with your GUID G2. The non-collision propery of GUIDs follows from sticking to the GUID standard.

The mechanisms to prevent collisions are indeed inherently privacy-sensitive. If I generate at random a GUID G1, I can guarantee that random GUID is unique if two conditions are satisfied:

• 1 It’s a member of the subset of GUIDs under my control and
• 2 I didn’t generate the GUID before.

For GUIDs outside the subset under your control, you cannot guarantee (2). But how do you assign non-overlapping subsets of GUIDs to a single person? Using the MAC of a NIC is a simple, effective way. Other means are also possible. But in any case, the mere existence of such a subset is privacy-implicating. It’s got to belong to someone, and I must be able to determine whether that’s me or someone else. It’s a bit harder to prove whether two random GUIDs G1 and G2 belong to the same subset (ie. person) but the current schemes (which you object to) do not try to hide that.