Encrypts data and detects any tampering, in one step.
The idea: Two jobs at once. A cipher turns the key and a never-repeating number (the nonce) into a random-looking stream that is mixed into the message, and a one-time MAC stamps the result with a tag. The receiver checks the tag before decrypting anything and rejects the message if a single bit changed. Reusing a nonce repeats the stream, which leaks the messages and, with AES-GCM, lets attackers forge tags.
Why it matters: Protects almost every HTTPS, SSH and VPN connection you make.
On the map: authenticated encryption (also AES-CBC + HMAC)
Authenticated encryption encrypts and detects tampering in one step[1]. AES-GCM, fast with hardware support[2], and ChaCha20-Poly1305, fast in software[3], protect nearly all TLS, SSH, QUIC and WireGuard traffic. Older encrypt-then-MAC constructions such as AES-CBC with HMAC, used by Signal, also count.
Security comes from the cipher behaving like a random function plus a one-time MAC. Reusing a nonce with the same key is catastrophic[4].
If any of these breaks, this breaks too.
If this breaks, these break with it.