Glossary · Cryptography & authentication
AES-256-GCM
AES-256-GCM is authenticated encryption: the AES block cipher with 256-bit keys in Galois/Counter Mode, providing confidentiality and integrity in one pass — tampered ciphertext fails authentication instead of decrypting to garbage.
GCM matters because encryption without authentication is exploitable: attackers can flip ciphertext bits and observe how systems react. GCM computes an authentication tag over the ciphertext (and optional associated data), so any modification is detected before a single byte is accepted.
Its critical operational rule is nonce uniqueness — reusing a nonce under the same key is catastrophic for both confidentiality and the ability to forge tags. Correct implementations derive fresh nonces per message (randomly, or via counters) and rotate keys well before birthday bounds.
AES-GCM is hardware-accelerated on effectively all modern CPUs and is the workhorse of TLS 1.3 and client-side encryption, including the vault encryption in Ciphera services.