There is a clue as to what happens with User UUIDs, though, in that you can generate new ones on demand – or at least you could do so prior to High Sierra – in the Advanced Options for user accounts in the Users & Groups pane. Apple doesn’t document which UUID-like identifiers are generated using the normal randomising process, and which are the result of non-random mechanisms such as hashing other identifiers. For example, a Guest User UUID is invariably FFFFEEEE-DDDD-CCCC-BBBB-AAAA000000C9, and the HFS+ disk partition type is always 48465300-0000-11AA-AA11-00306543ECAC. Some are assigned quite strictly, and far from unique.

Hardware and User UUIDs are not as randomly-generated. For these, Apple’s operating systems use RFC 4122 version 4 random byte format, in which six bits are taken up with variant and version data, but the remaining 122 bits are randomly generated. MacOS and iOS use genuine UUIDs very extensively, in internal databases, logs, and pretty well everywhere that they might want to refer to across different subsystems or over time. The two which are known as UUIDs but are actually rather different are the device Hardware UUID, which uniquely identifies every Mac, iOS device, Watch, Apple TV, and most other hardware products supplied by Apple, and your User UUID, which is unique to you and different on each Mac, etc. I have explained in detail what a UUID is: a Universally Unique Identifier, a 16-byte number normally presented in a standard hex format, such as 1234a678-1b34-1c34-1d34-1234567890ab. I presume this same line of thinking led one reader to comment here that they recommend Mac users to generate themselves a new user UUID “to fix issues” – an act which can only be disruptive if not destructive for that user’s Mac account, because of its effects on Open Directory, and more. I won’t amplify their conspiracy theories with links here, but consider one of the many issues which they raise: that Apple generates device UUIDs from “a hash of the globally unique serial number, WiFi MAC address and, if applicable, IMEI, IMSI, ICCID, MSISDN etc” which is in turn linked by Apple with the credit card used for purchase, the purchaser’s name and email address.

Over the last few months, though, various websites and self-declared Mac experts have been expressing concerns about Apple’s extensive use of UUIDs on Macs and iOS devices. Who today needs conspiracy theories? Isn’t real life sufficiently conspiratorial yet?

Convert mac address to uuid