ITunes backups - how's the UID generated?

I'm struggling with an issue with an encrypted backup (see https://apple.stackexchange.com/q/192153/7488 and https://apple.stackexchange.com/q/192157/7488 if you want the details) and I'm trying to understand how things got so messed up. For that I'd need to understand how are the backup UIDs created. Are they "hardware" based? I don't think so as if expect to have a lot of "iPhone Backup" keys in Keychain as I've been "evolving" my devices over time... So where do they come from? **EDIT**: To make clearer what I mean, when I say UID I'm referring to the number that's in the folder where the backup is stored. The location of the backups is on: Mac: ~/Library/Application Support/MobileSync/Backup/{UID} Windows Vista and Windows 7: \Users\\(username)\AppData\Roaming\Apple Computer\MobileSync\Backup\\{UID} And the UID itself is a long string of hexadecimal characters like: 59cf77659e76a55e206192c325f732xxxxxxxxxx (last 10 digits removed in case it's some "magic" number) Thanks a lot!