GCM
Galois/Counter Mode
An authenticated encryption mode providing both confidentiality and integrity.
GCM is an authenticated encryption mode that combines counter mode confidentiality with a Galois field authentication tag, delivering both encryption and integrity in a single efficient operation. Because it detects tampering and supports parallel processing and hardware acceleration, it is widely used in TLS and IPSec cipher suites such as AES-GCM. The exam contrast is with CBC, which provides confidentiality but requires a separate mechanism for integrity. A critical pitfall is that reusing a nonce with the same key in GCM can catastrophically break its security guarantees.
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