Galois/Counter Mode (GCM)

Definition

Galois/Counter Mode (GCM) is an authenticated encryption mode that combines the Counter mode of operation with the Galois field multiplication to provide both confidentiality and data integrity. It uses symmetric key cryptography for encryption while also offering authentication through a unique tag generated from the encrypted data. GCM is efficient and widely used in network security protocols, making it essential for secure data transmission.

5 Must Know Facts For Your Next Test

1. GCM provides both encryption and authentication in a single operation, making it more efficient than using separate processes for each.
2. The mode is designed to work with block ciphers that operate on fixed-size blocks, commonly using AES as its cipher.
3. In GCM, the nonce must be unique for each encryption operation with the same key to prevent vulnerabilities and ensure security.
4. GCM can process data in parallel, allowing for high-speed encryption and decryption, which is beneficial for performance in network applications.
5. The length of the authentication tag can vary but is often set to 128 bits, ensuring a robust level of security against forgery.

Review Questions

• How does GCM enhance both confidentiality and integrity in data encryption?
  • GCM enhances confidentiality by encrypting data using a block cipher like AES, while also providing integrity through an authentication tag. The tag verifies that the data has not been tampered with during transmission. By combining these two functions into one process, GCM effectively ensures that the data remains secure and authentic, reducing the risk of attacks such as unauthorized modifications or eavesdropping.
• What are the implications of using a nonce in GCM, and why is it important to keep it unique?
  • The nonce in GCM is critical for maintaining security because it ensures that the same plaintext will yield different ciphertexts when encrypted multiple times. This uniqueness prevents replay attacks and other vulnerabilities where an attacker could reuse intercepted ciphertexts. If the nonce were reused with the same key, it would compromise both confidentiality and integrity by allowing potential recovery of plaintext or authentication tag collisions.
• Evaluate how the efficiency of GCM impacts its application in modern network protocols compared to traditional modes of operation.
  • GCM's efficiency significantly enhances its application in modern network protocols, as it allows for parallel processing of data during encryption and decryption. Unlike traditional modes that may require sequential processing, GCM can handle large amounts of data quickly without compromising security. This high-speed performance makes it suitable for high-throughput applications like HTTPS, VPNs, and other secure communications where timely delivery is crucial. Consequently, GCM has become a preferred choice over older modes such as CBC (Cipher Block Chaining) due to its ability to maintain security while improving speed and resource utilization.

Related terms

AES: Advanced Encryption Standard (AES) is a symmetric encryption algorithm widely used for securing data, and it serves as the underlying block cipher in GCM.

Authentication Tag: An authentication tag is a short piece of information used to verify the authenticity and integrity of the encrypted message in GCM.

Nonce: A nonce is a number that is used only once in cryptographic communication to ensure that the same plaintext encrypts differently each time, which is crucial for security in modes like GCM.