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📝 Message integrity authentication digital signature (9 MCQs)

📖 From Data Communication and Networks • 31. Cryptography and Network Security • 9 questions available

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What is Message integrity authentication digital signature?

Message integrity ensures data has not been altered, authentication verifies the sender’s identity, and digital signatures combine both by using cryptographic techniques to prove origin and detect tampering in electronic messages.

Easy

Medium

Hard

📝 All Message integrity authentication digital signature MCQs

Q1. In a system that uses a MAC for message authentication, if the receiver detects a MAC mismatch, what logical conclusion is most justified?

A.The sender intentionally altered the message

B.The message integrity was compromised during transmission ✅

C.The encryption algorithm failed

D.The receiver's key is outdated

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: A MAC mismatch indicates that the computed tag does not match the transmitted tag, which directly points to a loss of integrity. This typically means the message was altered—either accidentally or maliciously—while in transit, rather than an issue with encryption or key freshness.

Q2. Suppose a sender computes a MAC using a secret key and sends both the ciphertext and MAC to a receiver. Which inference best explains why confidentiality is still protected despite the MAC revealing no secret information?

A.The MAC encrypts the plaintext

B.The MAC is generated from the ciphertext, not the plaintext

C.The MAC does not expose the secret key used for encryption ✅

D.The MAC replaces the need for encryption

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: A MAC is derived from the secret key and the message, but it does not disclose the key itself. Consequently, even though the MAC is transmitted in clear, an adversary cannot recover the encryption key or the plaintext, preserving confidentiality while ensuring integrity.

Q3. A digital signature is verified with a public key that does not correspond to the signer's private key. What is the most likely effect on the verification outcome?

A.Verification will succeed, but the signature will be considered non‑repudiable ✅

B.Verification will fail, indicating the signature is invalid

C.Verification will succeed, but confidentiality will be compromised

D.Verification will succeed, but integrity will be unaffected

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: Digital signatures rely on a matching key pair; using an incorrect public key prevents the mathematical verification from succeeding. The algorithm will output a failure, signaling that the signature cannot be trusted, while confidentiality and integrity of the original message remain unrelated to this mismatch.

Q4. Compare symmetric‑key and asymmetric‑key distribution methods. Which statement best captures their primary difference regarding scalability?

A.Symmetric‑key distribution requires a separate secret for each pair, limiting scalability ✅

B.Asymmetric‑key distribution needs a shared secret, making it less scalable

C.Symmetric‑key distribution uses public certificates, enhancing scalability

D.Asymmetric‑key distribution eliminates the need for any keys

💡 Difficulty: easy | ✅ Correct: A

📖 Explanation: Symmetric‑key systems demand a unique secret key for every communicating pair, leading to an exponential growth in keys as participants increase. Asymmetric systems, by contrast, use public keys that can be freely distributed, allowing the number of keys to grow linearly with participants, thereby offering superior scalability.

Q5. Evaluate the trade‑offs between using a MAC and a digital signature for entity authentication. Which assessment is most accurate?

A.MACs provide non‑repudiation, while digital signatures do not

B.Digital signatures guarantee confidentiality, whereas MACs do not

C.Digital signatures offer non‑repudiation and public verifiability, but MACs are faster and require shared secrets ✅

D.MACs are slower than digital signatures and need public‑key infrastructure

💡 Difficulty: medium | ✅ Correct: C

📖 Explanation: Digital signatures, built on asymmetric cryptography, enable anyone to verify the signer’s identity and provide non‑repudiation because only the private key holder could have created the signature. MACs, generated with a shared secret, are computationally cheaper but lack public verifiability and cannot prove who created the tag without revealing the secret.

Q6. Analyze a protocol that first encrypts a message with a symmetric key and then signs the ciphertext with a private key. Which vulnerability is most likely if the signer’s private key is compromised?

A.The attacker can decrypt the message but cannot forge signatures

B.The attacker can forge signatures and decrypt any future ciphertexts

C.The attacker can only alter the ciphertext without detection

D.The attacker can recover the symmetric key but cannot modify messages ✅

💡 Difficulty: hard | ✅ Correct: D

📖 Explanation: Compromise of the private signing key allows the attacker to generate valid signatures on any ciphertext, impersonating the original sender. However, without the symmetric key, the attacker cannot decrypt the ciphertext. If the protocol also transmits the symmetric key encrypted with the public key, the attacker could recover it, enabling decryption and further manipulation.

Q7. Apply key‑rollover principles: How does regularly updating symmetric keys improve message integrity in a long‑running communication session?

A.It reduces the chance of key exhaustion attacks

B.It ensures that old keys cannot be used to replay old messages ✅

C.It eliminates the need for MACs

D.It provides confidentiality without additional encryption

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: Frequent key updates limit the window in which an adversary could capture a key and reuse it to replay previously authenticated messages. By rotating keys, any replayed message would fail the MAC verification because the MAC would have been computed with a different key, thereby strengthening integrity.

Q8. Synthesize a security scheme that simultaneously ensures confidentiality, integrity, and entity authentication using both symmetric and asymmetric techniques. Which combination best meets all three goals?

A.Encrypt with a public key, then attach a MAC generated with the same public key

B.Encrypt with a symmetric key, encrypt the symmetric key with the receiver’s public key, and attach a digital signature from the sender ✅

C.Use only a MAC without encryption

D.Encrypt with a symmetric key and rely on a password‑based hash for authentication

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: The proposed hybrid approach encrypts the payload with an efficient symmetric algorithm, protects the symmetric key by encrypting it with the receiver’s public key (confidentiality), and adds a digital signature from the sender (entity authentication and non‑repudiation). The MAC can also be included to verify integrity, yielding a comprehensive solution.

Q9. Explain the relationship between a Message Authentication Code (MAC) and non‑repudiation. Which statement accurately reflects this relationship?

A.A MAC inherently provides non‑repudiation because only the sender can generate it

B.A MAC cannot provide non‑repudiation without additional mechanisms such as a digital signature ✅

C.A MAC guarantees both confidentiality and non‑repudiation

D.A MAC replaces the need for any form of authentication

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: A MAC is derived from a shared secret, meaning both the sender and receiver could produce it; thus, a receiver cannot prove that the sender, rather than themselves, generated the tag. Consequently, MACs alone do not furnish non‑repudiation, which requires an asymmetric mechanism like a digital signature to uniquely bind the sender to the message.

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