Secure Dynamic Messaging — NTAG 424 DNA

Every NFC tap generates a unique, cryptographically authenticated URL. This server decrypts the PICC data, derives per-tap session keys, decrypts the payload, and validates the short AES-CMAC — making replay attacks and card cloning detectable.

Tag Configuration

NDEF File	No. 02
SDM Feature	Enabled
PICC Data Enc.	Enabled · Key 3 (SDM Meta Read Key) · AES-128-CBC · zero IV
File Data Enc.	Enabled · Key 4 (SDM File Read Key, diversified per card) · AES-128-CBC
CMAC	Short 8-byte · odd bytes of full AES-CMAC · Key 4
UID Mirror	Enabled — inside PICC Data block
Counter Mirror	Enabled — 3 bytes little-endian, inside PICC Data
UID Length	7 bytes (standard NTAG424)
Payload Padding	ISO/IEC 9797-1 Method 2 · or NXP zero-fill for full blocks

NDEF URL Template

https://groupster.eu/tag
?data=[PICC_DATA · 32 hex]
&enc=[ENC_FILE_DATA · n×32 hex]
&cmac=[CMAC · 16 hex]

■ PICC_DATA — fixed 16 bytes

■ ENC — multiples of 16 bytes

■ CMAC — 8 bytes truncated

Key Structure

Key 3 on the card	SDM Meta Read Key — programmed on the card. The card firmware uses Key 3 to encrypt the PICC data block (UID + read counter) inside every SDM URL. The server decrypts this block with Key 3 to extract the UID and counter.
Master Key server only	Diversification Master — kept only on the server, never stored on the card. Used to derive a unique Key 4 for each card via: `AES-CMAC(Master Key, AID, UID, SYSID)`
Key 4 on the card	SDM File Read Key — the per-card diversified key, written to the card during personalisation. The server re-derives it per tap to decrypt the ENC payload and verify the CMAC.
SesEncKey per tap	Session encryption key derived per tap: `AES-CMAC(Key4, SV_ENC)`
SesMACKey per tap	Session MAC key derived per tap: `AES-CMAC(Key4, SV_MAC)`