Why is an iPhone Device ID 40 characters?

Question

UUIDs are usually 36 characters.

Solution 1

Because UDID ≠ UUID. The iPhone Unique Device Identifier (UDID) is a hash of several different hardware identifiers pulled from the chips on the phone. It's not a software-generated identifier for a software object.

It's 160 bits, not 128 bits, so it takes 40 hex characters to represent, not 32 + 4 hyphens.

Solution 2

It's not a UUID, it's a UDID. A UUID is generated on the fly to be unique, and then stored. However, the UDID is calculated based off of the hardware on the phone, so it's never generated. The 40 character length might imply it's a SHA1 hash of a few pieces of hardware info, or it could just be a coincidence.