Password Protection

Password Protection

Password protection is a basic access control mechanism in NFC tags that uses a fixed-length password to restrict read and/or write operations on specified memory blocks. The password is a 32-bit (4-byte) value stored in a configuration page of the tag, and a reader must transmit this password using the PWD_AUTH command before accessing protected memory areas. While simple to implement, password protection has significant security limitations compared to cryptographic authentication.

Implementation in NTAG 21x

In NTAG 213, NTAG 215, and NTAG 216, password protection is configured through three parameters:

• PWD (Password): A 4-byte value stored in the chip's configuration pages (page 43 for NTAG 213, page 133 for NTAG 215, page 229 for NTAG 216).
• PACK (Password Acknowledge): A 2-byte value the tag sends back upon successful authentication. Allows the reader to verify it is talking to the expected tag.
• AUTH0: The page address from which password protection begins. Pages before AUTH0 remain freely accessible. For example, setting AUTH0 = 4 protects all user memory pages; setting AUTH0 = 10 leaves pages 4-9 unprotected.

Access control modes: - Write-only protection (PROT bit = 0): Reading is always allowed; writing to protected pages requires authentication. - Read-and-write protection (PROT bit = 1): Both reading and writing require authentication.

Security Limitations

Password protection has critical weaknesses that make it unsuitable for security-sensitive applications:

1. Plaintext transmission: The 32-bit password is sent from reader to tag without encryption over the RF field. An attacker with an NFC sniffer can capture the password from the air interface.
2. Small keyspace: A 32-bit password has only 4.3 billion possible values — feasible to brute-force with dedicated hardware, though NTAG chips limit authentication attempts (AUTHLIM counter) to slow down attacks.
3. No session keys: After authentication, subsequent commands are not encrypted. The session is vulnerable to replay and man-in-the-middle attacks.
4. Static credential: The password does not change between sessions, so a captured password grants permanent access.

When to Use Password Protection

Despite its limitations, password protection is appropriate for:

• Write protection of publicly readable data: Preventing unauthorized overwrites of NDEF URLs on marketing tags while keeping the content freely readable. Eavesdropping risk is low because the protected content is public.
• Low-value access gating: Fitness badges, simple loyalty cards, or internal asset labels where the threat model does not include sophisticated RF attacks.

For security-critical applications (payments, brand protection, access control), use AES encryption and mutual authentication via chips like NTAG 424 DNA or MIFARE DESFire EV3.