authenticatietags

An authentication tag, or auth tag, is a short data piece that accompanies a message to verify its integrity and origin. They are used in digital communications to ensure a message has not been altered and that it was created with a valid key. Authentication tags enable verification of authenticity as well as integrity, and may be part of an authenticated encryption scheme or used as separate MACs or signatures.

In cryptography, authentication tags are commonly produced by AEAD algorithms such as GCM or ChaCha20-Poly1305. After encrypting the plaintext (and any associated data), the algorithm computes a fixed-length tag using a secret key. The tag travels with the ciphertext; on decryption, the recipient recomputes and verifies it matches before accepting the message.

The tag length varies by algorithm, typically 8 to 16 bytes in modern schemes; longer tags reduce forgery risk. The security of the tag relies on a secret key and proper implementation. Tags do not reveal plaintext; they provide authenticity and integrity. Replays or tag reuse may require nonces or sequence numbers.

Outside cryptography, authentication tags appear in physical security and supply chains, including anti-counterfeit labels, RFID or NFC tags, holograms, and tamper-evident seals. Their goal is to help verify that an item is genuine and unaltered.

Related concepts include MACs, digital signatures, and authenticated encryption schemes. Common implementations and standards appear in TLS, IPsec, and other security protocols that rely on AEAD and MACs to provide data authenticity and integrity.