A UDID is a unique alphanumeric identifier assigned to Apple devices, which is combined with the IMEI and Serial Number to authenticate the hardware during activation.
When an iPhone powers up and finds a connection for the first time, the setup assistant sends a cryptographic handshake containing the Serial Number, IMEI, and Chip ID (ECID) to Apple's servers. This ping securely authenticates the OS, permanently registers the hardware on the network, and officially starts the 1-year limited warranty countdown.
