UID (Unique Identifier)

UID (Unique Identifier)

The UID (Unique Identifier) is a factory-programmed, read-only serial number permanently stored in every NFC chip. Each UID is intended to be globally unique, assigned during the IC manufacturing process and written to non-volatile memory before the chip leaves the fabrication facility. The UID serves as the primary mechanism for anti-collisionanti-collisionProtocol for selecting individual tags from multiple in RF fieldView full → (distinguishing multiple tags in the same RF field) and as a basic identifier for inventory and tracking systems.

UID Formats

UID length depends on the underlying standard and chip type:

For 7-byte UIDs (the most common in modern NFC tags), the first byte identifies the IC manufacturer (NXP = 0x04, STMicroelectronics = 0x02), and the remaining 6 bytes are sequential serial numbers.

UID in the Anti-Collision Process

During the NFC-A anti-collision procedure defined in ISO 14443 Part 3, the reader broadcasts ANTICOLLISION commands and tags respond with their UIDs. When multiple tags respond simultaneously, the reader detects collision bits and narrows the search by specifying known UID bits in subsequent commands. This binary tree walk isolates each tag's complete UID, enabling the reader to SELECT a specific tag for further communication.

Security Considerations

A UID alone is not a security credential. UIDs are transmitted in plaintext during anti-collision and can be easily read, recorded, and programmed into writable chips (MIFARE Classic clones). Relying on UID-based access control without additional cryptographic authenticationauthenticationIdentity verification of NFC tags/readers via passwords or cryptographyView full → is a significant security vulnerability. Secure applications should use AES encryption, mutual authentication, or SUN messages instead of or in addition to UID checks.

UID Mirroring

Some chips (NTAG 213/215/216) support UID mirroringUID mirroringAutomatic UID insertion into NDEF messages for trackingView full →: the chip automatically writes its UID into a specified location within user memory. This allows the UID to appear as part of an NDEF URL payload — for example, a tag might open https://example.com/verify?uid=04A1B2C3D4E5F6 where the UID portion is dynamically inserted by the chip on every read.