[mmoskal]

Counting to 2^61 probably is.

To actually find a collision in 128b cryptographic hash function it would take closer to 2^65 hashes. Back of the envelope calculations suggest that with Pollard's rho it would cost a few million dollars of CPU time at Hetzner's super-low prices. Not nearly mere mortals budget, but not that far off I guess.

[Retr0id]

A GUID is not a cryptographic hash function.

In any case, in 2023 I back-of-the-envelope estimated that you could compute 2^64 SHA256 for ~$100K, using rented GPU capacity https://www.da.vidbuchanan.co.uk/blog/colliding-secure-hashe...

[8organicbits]

That's great analysis. As you call out in the post, the 2^64 value is used to attack SHA256-128 (SHA256 truncated to 128 bits). NIST recommends at least SHA-224, which makes sense given your conclusions.