🔐 CISSP Study Note: Encryption Mode That Provides Both Confidentiality and Integrity — GCM

 🔐 CISSP Study Note: Encryption Mode That Provides Both Confidentiality and Integrity — GCM

❓ Exam Question

What mode of encryption provides both confidentiality and integrity?
✅ GCM – Galois/Counter Mode

🔍 Definition

GCM (Galois/Counter Mode) is a symmetric encryption mode that extends CTR (Counter Mode) by adding integrity assurance using Galois field multiplication for authentication.

It provides:

  • Confidentiality: Keeps data private via encryption

  • Integrity: Ensures the data hasn’t been altered, using an authentication tag

In CISSP terms: GCM is an AEAD (Authenticated Encryption with Associated Data) mode.

🧠 Why It Matters in CISSP

Most block cipher modes (like ECB or CBC) only provide confidentiality—you need a separate function (like HMAC) for integrity.
GCM combines both into one efficient operation, reducing complexity and increasing performance, especially in network protocols and high-speed applications.

🔐 Technical Highlights

Feature Description
Encryption Base Uses AES in counter mode (CTR)
Authentication Adds a cryptographic tag generated by Galois field multiplication
Performance Highly parallelizable, efficient in hardware and software
Use Cases TLS 1.2+, IPSec, SSH, disk encryption, secure APIs

✅ Example (CISSP-Style)

Question: A company wants to secure data in transit while ensuring no unauthorized modification occurs. What encryption mode should they use?

  • A. ECB

  • B. CBC

  • C. CTR

  • D. GCM ✅

✅ Answer: GCM. Because it provides both confidentiality (via AES-CTR) and integrity (via authentication tag).

📖 Found In CISSP Domains

Domain Focus
📘 Domain 3: Security Architecture and Engineering Covers cryptographic systems, encryption modes, and integrity/confidentiality mechanisms.
📘 Domain 7: Security Operations Applies encryption standards to communications, file transfers, and operational safeguards.

🔑 Memory Hook

“GCM = Go Confidently with Message Integrity.”
GCM is your go-to mode for encryption when you need to protect the data and prove it hasn’t changed.


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