Applied Cryptography
Which of the following does authenticated encryption (AEAD) such as AES-GCM provide in a single operation?
- AOnly the confidentiality of the data; detecting tampering requires a separate digital signature
- BThe confidentiality of the data and verification that the ciphertext and associated data have not been tampered with
- CNon-repudiation towards third parties, in addition to confidentiality and tamper detection
Show answer and explanation
Answer: B. The confidentiality of the data and verification that the ciphertext and associated data have not been tampered with
Authenticated encryption (AEAD: authenticated encryption with associated data) computes an authentication tag while encrypting and verifies the tag when decrypting; if the ciphertext has been tampered with, it returns an error without releasing any plaintext. Headers and other data that are not encrypted but must be protected against tampering can also be included in the tag computation as "associated data". GCM is a mode that combines encryption in CTR mode with authentication by GHASH, which uses multiplication in a Galois field, and TLS 1.3 uses only AEAD. Because the tag is made with a shared key, like a message authentication code it cannot provide non-repudiation.
- A. Authenticated encryption is precisely the scheme that includes tamper detection in the same operation.
- B. Correct. Confidentiality and integrity (that nothing has been tampered with) are checked together.
- C. The tag is made with a key shared by the sender and receiver, so the receiver can also create the same tag. Non-repudiation requires a digital signature.