GCM
acronymcryptography
Stands for: Galois/Counter Mode
An AEAD mode that turns AES into an authenticated cipher using a counter and a nonce.
Galois/Counter Mode (NIST SP 800-38D) provides both encryption and integrity in one pass and is the most common AEAD mode in TLS. Its one sharp edge: reusing a nonce with the same key collapses its security entirely.
Galois/Counter Mode encrypts and authenticates in one pass, producing ciphertext and an authentication tag together, which is why it became the default AEAD mode almost everywhere including TLS.
Its one hard requirement is that a nonce must never repeat under the same key. Reusing one does not merely leak plaintext relationships; it can expose the authentication key itself, which lets an attacker forge messages rather than only read them. That is a catastrophic and silent failure, and it is why random nonces need enough length to make collision implausible and why counter-based nonces must survive process restarts and cloned virtual machines.
Also known as: gcm