Which mode provides both confidentiality and integrity guarantees?

• A. CBC
• B. GCM
• C. CTR
• D. ECB

Answer: (B)

Authenticated encryption with associated data (AEAD) is the idea here: you want confidentiality plus a way to detect tampering in one primitive. Galois/Counter Mode (GCM) delivers exactly that by combining a counter-mode encryption for secrecy with a built-in authentication tag based on GHASH. As you encrypt, you also generate an authentication tag that covers both the ciphertext and any associated data you choose to include. On decryption, the tag is checked; if it doesn’t match, decryption fails and tampering is detected, providing integrity and authenticity in addition to confidentiality. Other modes like CBC, CTR, or ECB only offer encryption (confidentiality) by themselves and do not provide these built-in integrity guarantees, making them insufficient for scenarios where tamper detection is required without a separate MAC.