Why is the 13th charcater always a 4?

I noticed something odd regarding backend filenames.

I was assuming that filenames were completely random except for the prefix and extension, but it appears that the 13th “random” character of every DBLOCK and DINDEX file is always a 4!

This does not seem to be limited to any Duplicati version (DBLOCK and DINDEX files created years ago as well as recent ones have a 4 at pos.13).
There’s also no difference between used backends: in OneDrive, Local storage, WebDAV backends there’s a 4 at position 13 of every DINDEX and DBLOCK file.

Can anyone explain why this is happening?


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This is a GUID, you can read the answer on SO: SQL Server : why is the 15th char of a GUID always 4? - Stack Overflow

(15th position => 13th position due to removal of dashes)


That was very helpful, thanks for the link. I never realized that the filenames consist of UUID’s, I always thought it was just a bunch of random hex characters.