Which block cipher mode is designed for authenticated encryption with associated data?

• A. ECB
• B. CBC
• C. CFB
• D. GCM

Answer: (D)

Authenticated encryption with associated data aims to provide both secrecy and data integrity, including parts of the message that you want authenticated but not encrypted. The mode that does this in one integrated operation is GCM. It combines Counter mode for encryption with a Galois field-based authentication tag, so you get confidentiality plus a strong integrity check. Importantly, it can include associated data in the authentication calculation, meaning headers or other non-encrypted data can be protected from tampering without being encrypted.

Other common block cipher modes focus on confidentiality alone. ECB reveals patterns in plaintext, CBC provides encryption but not built-in authentication, and CFB offers confidentiality without an intrinsic authentication mechanism. So, for authenticated encryption with associated data, this mode is the appropriate design.