AES Encryption

AES Encryption

AES (Advanced Encryption Standard) with 128-bit keys is the primary cryptographic mechanism used in modern NFC chips for secure authentication, encrypted data exchange, and anti-counterfeiting. AES-128 provides strong symmetric encryption that is considered computationally infeasible to break with current or foreseeable technology, making it the foundation for security-sensitive NFC applications.

AES in NFC Chips

Several NFC chip families implement hardware AES acceleration:

The AES engine runs on a dedicated hardware co-processor within the NFC chip, completing encryption and decryption operations without exposing key material to the external interface. Keys are stored in protected memory areas that cannot be read back — only used for cryptographic operations.

Mutual Authentication Flow

AES enables mutual authentication between reader and tag, ensuring both parties verify each other's identity before exchanging sensitive data:

1. Reader challenge: The reader sends a random number (RndA) to the tag.
2. Tag response: The tag generates its own random number (RndB), encrypts both RndA and RndB with the shared AES key, and returns the ciphertext.
3. Reader verification: The reader decrypts the response using its copy of the AES key, verifies that the returned RndA matches what it sent, and sends encrypted RndB back to the tag.
4. Tag verification: The tag verifies the returned RndB matches its original value.
5. Session keys: Both parties derive session keys from the exchanged random numbers for encrypting subsequent data transfer.

Advantages over Password Protection

Application: Secure Dynamic Messaging (SDM)

In NTAG 424 DNA, AES encryption powers the SDM feature. The tag generates a unique AES-CMAC (Cipher-based Message Authentication Code) on every tap, embedded in the NDEF URL as query parameters. The backend server verifies this CMAC using the shared key, confirming both tag authenticity and tap uniqueness. This makes cloning and replay attacks practically impossible.