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Security Fundamentals & Crypto · Intermediate

Galois/Counter Mode (GCM)

Authenticated block cipher mode combining counter mode encryption with Galois field authentication (GHASH).

Definition

GCM provides both confidentiality and integrity in one pass, producing ciphertext and an authentication tag. It is fast, parallelizable, and widely used in TLS 1.2/1.3, IPsec, and SSH. A nonce must never repeat under the same key, or authentication and confidentiality can both fail.

Example

TLS_AES_256_GCM_SHA384 is a TLS 1.3 cipher suite that uses AES-GCM.

Exam tip

GCM is an AEAD mode; if asked for a mode that provides both encryption and integrity, pick GCM.

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