Knowledge / crypto concepts hardening

AES-GCM

AES in Galois/Counter Mode. Confidentiality plus an integrity tag. This is the AES you actually want.

GCM is an authenticated-encryption mode specified by NIST in SP 800-38D. AES-GCM encrypts the payload and produces an authentication tag over the ciphertext and any associated data you choose not to encrypt (headers, for example). If the tag does not verify, you do not decrypt. That is the point. Confidentiality without integrity is a toy.

It is the AES mode you will meet in TLS 1.3, IPsec, and most current disk and object-storage encryption. Nonces must not repeat under the same key. A reused nonce in GCM is a class of failure, not a footnote. Libraries handle this if you call them the way they are documented. Rolling your own counter is how you get a paper.

When someone says they use AES, ask which mode. GCM (or another AEAD) is the adult answer. ECB is a screenshot of the penguin. CBC without a MAC is last decade.

Fact source: NIST SP 800-38D, GCM.