A Unique Device Identifier, or UDID, is a 40-character hexadecimal number that is specific to each individual iOS device. This identifier serves as a serial number for the device, distinguishing it from all others. It was commonly used by developers for device tracking, ad targeting, and beta testing before being deprecated by Apple. For example, a developer might use the UDID to register a specific iPhone for access to a pre-release version of an application.
The UDIDs prevalence stemmed from its ease of access and reliability in identifying devices, which simplified the process of managing application distribution and testing. However, its deprecation was driven by privacy concerns. The UDID provided a persistent and easily accessible method for tracking users across applications, which raised significant ethical and security considerations. The move away from UDIDs reflects an industry-wide shift towards prioritizing user privacy and adopting more secure, anonymized methods of device identification.