§ 01
Definition
Encryption hides data, but it does not on its own stop an attacker from tampering with it. A MAC adds a short tag that only someone with the key can produce. Poly1305 is usually paired with the ChaCha20 cipher as ChaCha20-Poly1305, standardised in RFC 8439.
§ 02
How it works
For each message, a one-time key derived from the session key is used to compute a 16-byte tag over the ciphertext. The receiver recomputes the tag; if even one bit differs, the packet is dropped. This 'encrypt-then-authenticate' pairing is called AEAD (authenticated encryption with associated data).
§ 03
Why it matters
Poly1305 is part of WireGuard and of TLS 1.3's ChaCha20 cipher suite. It is a reason these protocols stay fast on phones and routers that lack AES hardware acceleration.