🛡️GCM Mode

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An authenticated encryption mode combining counter-based confidentiality with Galois field hashing for integrity and authenticity.

DESCRIPTION

Galois/Counter Mode is a block cipher mode providing authenticated encryption with associated data. It encrypts using counter mode and computes an authentication tag via GHASH. GCM enables parallelizable operations and high throughput, making it popular in TLS and IPsec. Correct nonce management is critical; nonce reuse breaks security. GCM is recommended over CBC for modern protocols.

ALTERNATIVE DEFINITIONS

NIST

Galois/Counter Mode (GCM) is a mode of operation for block ciphers defined for authenticated encryption with associated data.

Formal description of authenticated encryption with associated data.

Source

Wikipedia

Galois/Counter Mode (GCM) is a mode of operation for symmetric-key cryptographic block ciphers widely used for its efficiency and performance.

Highlights efficiency and widespread adoption.

Source

RELATED TERMS

Rijndael Authenticated Encryption Associated Data CTR Mode GHASH Nonce Initialization Vector Authentication Tag SP 800-38D TLS

COMPETING TERMS

CBC Mode EAX

RELATIONS

• Broader: Mode

KNOWLEDGE AREAS

Data Security Software Engineering

TARGET AUDIENCE

Security Architect Cybersecurity Engineer Software Developer DevOps Engineer

EXTERNAL SOURCES

• Wikipedia - Galois/Counter Mode
• NIST - GCM specification
• IETF - TLS AES-GCM ciphersuites
• OpenSSL - AEAD API including GCM

TOP QUESTIONS

Why is GCM Mode preferred over CBC Mode?

GCM provides authenticated encryption, resists padding oracle attacks, and allows parallelization for better performance.

Category: Comparison

What happens if GCM nonces are reused?

Nonce reuse can reveal XOR relations between plaintexts and compromise authentication, breaking confidentiality and integrity.

Category: Risks

How does GHASH work in GCM Mode?

GHASH computes a polynomial hash over ciphertext and associated data in GF(2^128), producing an authentication tag.

Category: Mechanism

Is GCM secure without associated data?

Yes; associated data is optional but valuable to authenticate headers or context while keeping them unencrypted.

Category: Misunderstanding

What are typical GCM use cases?

TLS, IPsec, secure storage, and APIs needing integrity and confidentiality with high throughput.

Category: Use

What are best practices for GCM keys and IVs?

Use unique nonces per key, strong random IVs or counters, and rotate keys regularly.

Category: BestPractice

How does GCM performance compare to ChaCha20-Poly1305?

On AES-NI hardware, GCM excels; on mobile or older CPUs, ChaCha20-Poly1305 can outperform.

Category: Performance

Can GCM detect message tampering?

Yes; verification fails if the authentication tag does not match, signaling tampering or corruption.

Category: Capability

Where to learn GCM implementation details?

Study NIST SP 800-38D, TLS RFCs, and well-tested library examples from OpenSSL or libsodium.

Category: Learning