MCQs

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What is the primary function of the CBC-MAC protocol in cybersecurity?
a. Confidentiality of communication
b. Message authentication
c. Key exchange
d. Traffic analysis
Answer: b. Message authenticationfiveable+2
What does CBC stand for in CBC-MAC?
a. Cipher Block Compression
b. Cipher Block Chaining
c. Code Block Check
d. Chained Byte Cipher
Answer: b. Cipher Block Chainingwikipedia+2
CBC-MAC is constructed using which type of cryptographic primitive?
a. Stream cipher
b. Block cipher
c. Hash function
d. Public-key algorithm
Answer: b. Block ciphercsrc.nist+2
In CBC-MAC, what is used as the MAC tag for authentication?
a. First encrypted block
b. Last encrypted block
c. XOR of all blocks
d. Hash of input
Answer: b. Last encrypted blocksciencedirect+2
Which statement is true regarding CBC-MAC and message length?
a. Secure for variable-length messages
b. Secure only for fixed-length messages
c. Only works for single-block messages
d. Length is not relevant
Answer: b. Secure only for fixed-length messagesyoutubewikipedia
What is a necessary input, in addition to the plaintext message, to calculate a CBC-MAC?
a. Initialization vector
b. Public key
c. Secret key
d. Nonce
Answer: c. Secret keyfiveable+1
Which cryptographic algorithm is commonly used in CBC-MAC implementations?
a. AES
b. RSA
c. ECC
d. SHA-256
Answer: a. AESsciencedirect+1
What happens if the same key is used with CBC-MAC for messages of varying lengths?
a. Increases security
b. No effect
c. Vulnerable to forgery attacks
d. Only affects performance
Answer: c. Vulnerable to forgery attackswikipediayoutube
Which component of CBC-MAC makes message blocks interdependent?
a. XOR operation
b. Permutation
c. Compression
d. Padding
Answer: a. XOR operationfiveable+1
Why is an initialization vector (IV) typically set to zero in CBC-MAC?
a. For security enhancement
b. To simplify computation
c. To ensure deterministic output
d. To randomize the MAC
Answer: c. To ensure deterministic outputwikipedia+1
Suppose two authentications are performed using CBC-MAC on different messages of different lengths; what risk arises?
a. Key exposure
b. Replay attack
c. Collision attack
d. Forgery attack
Answer: d. Forgery attackyoutubewikipedia
Which improvement modifies CBC-MAC for variable-length messages?
a. PadMAC
b. HMAC
c. CMAC
d. LMAC
Answer: c. CMACfiveable+1
What is the main advantage of using CBC-MAC over HMAC in embedded devices?
a. Less computation when block cipher is present
b. Stronger security
c. Larger tag size
d. Asymmetric key use
Answer: a. Less computation when block cipher is presentsciencedirect+1
The CBC-MAC fails to authenticate variable-length messages securely because:
a. Block ciphers can be reversed
b. Length information can be manipulated
c. MAC is independent of the full message
d. Key reuse weakens all encryption
Answer: b. Length information can be manipulatedyoutubewikipedia
In a CBC-MAC computation, if a message consists of n blocks, how many encryption operations are required?
a. n – 1
b. n
c. 1
d. 2n
Answer: b. nsciencedirect+1
Which cybersecurity protocol commonly uses MACs, including CBC-MAC, for data integrity?
a. SSL/TLS
b. DNSSEC
c. BGP
d. ARP
Answer: a. SSL/TLSlearnovita+1
What does the ‘chaining’ mean in Cipher Block Chaining?
a. Combining keys per round
b. Previous ciphertext output influencing next input block
c. Chaining of hash functions
d. Chaining random numbers
Answer: b. Previous ciphertext output influencing next input blockwikipedia+1
When verifying a CBC-MAC, what does the recipient do?
a. Regenerates the MAC using received message & key, compares with sent tag
b. Decrypts MAC with their private key
c. Applies a hash function to the message
d. Looks up the tag in a table
Answer: a. Regenerates the MAC using received message & key, compares with sent taglearnovita+1
Which block in a message serves as the starting input for CBC-MAC computation?
a. Last block of ciphertext
b. Initialization vector (often zero)
c. Middle block
d. Random block
Answer: b. Initialization vector (often zero)youtubewikipedia
What would be the output if a one-block message is input to CBC-MAC?
a. The message itself
b. Encrypted message block
c. Random number
d. Hash value
Answer: b. Encrypted message blockfiveable+1
Who are the communicating parties in CBC-MAC required to share?
a. Public key
b. Symmetric encryption key
c. Asymmetric key pair
d. Digital certificate
Answer: b. Symmetric encryption keylearnovita+1
Which mode is NOT appropriate as input for CBC-MAC construction?
a. Block cipher in ECB mode
b. Block cipher in CBC mode
c. Stream cipher in CFB mode
d. Block cipher in OFB mode
Answer: c. Stream cipher in CFB modecsrc.nist+1
If MAC tags are too short, what risk increases?
a. Integrity compromise
b. MAC forgery via brute-force
c. Loss of confidentiality
d. Performance degradation
Answer: b. MAC forgery via brute-forcelearnovita+1
What is the minimal requirement for the key used in CBC-MAC?
a. Long and reused
b. Strong and secret
c. Shared and public
d. One-time pad
Answer: b. Strong and secretlearnovita+1
CBC-MAC is especially well-suited for messages of what type?
a. Variable-length plaintexts
b. Dynamic emails
c. Fixed-length records
d. Streaming video
Answer: c. Fixed-length recordswikipediayoutube
Which mathematical property is vital for the block cipher used in CBC-MAC?
a. Pseudorandom permutation
b. Perfect secrecy
c. Hash collision resistance
d. Polynomial time computability
Answer: a. Pseudorandom permutationucdavis+1youtube
Which attack is CBC-MAC provably secure against for fixed message length?
a. Man-in-the-middle
b. Chosen-message attack
c. Key-recovery attack
d. Replay attack
Answer: b. Chosen-message attackucdavisyoutube
Which process MUST be prevented to protect CBC-MAC from replay attacks?
a. Key reuse
b. Message reuse
c. Tag duplication detection
d. Random IV usage
Answer: c. Tag duplication detectionfiveable+1
What is appended to a message before using CBC-MAC for variable-length message protection (in a modified scheme)?
a. Hash of the message
b. Message length
c. Authenticator’s public key
d. User ID
Answer: b. Message lengthyoutubewikipedia
Which enhanced CBC-MAC construction is standardized for AES?
a. HMAC
b. NMAC
c. CMAC
d. PMAC
Answer: c. CMACwikipedia+1
CBC-MAC with a secure block cipher and fixed message lengths is:
a. Information-theoretically secure
b. Computationally secure
c. Universally secure
d. Not secure
Answer: b. Computationally securecsrc.nist+1
For a one-block message, CBC-MAC is equivalent to:
a. Applying the block cipher to the plaintext
b. XORing with a random key
c. Double encrypting the block
d. Using a stream cipher
Answer: a. Applying the block cipher to the plaintextfiveable+1
Which MAC mode is preferable for variable message lengths to avoid CBC-MAC vulnerabilities?
a. HMAC
b. Classic CBC-MAC
c. Direct encryption
d. ECB mode
Answer: a. HMACwikipedia+1
What happens if the secret key in CBC-MAC is exposed?
a. Only future messages are at risk
b. All previous authenticated messages are compromised
c. No risk, tags are random
d. MACs remain valid
Answer: b. All previous authenticated messages are compromisedlearnovita+1
What is the core difference between digital signatures and CBC-MAC?
a. Key type: symmetric vs asymmetric
b. Encryption algorithm used
c. Output tag size
d. Operating platform
Answer: a. Key type: symmetric vs asymmetriclearnovita+1
The best practice for CBC-MAC keys is:
a. Fixed key for all uses
b. Separate key for each unique message length
c. Using public keys
d. Key derived from the message
Answer: b. Separate key for each unique message lengthcsrc.nist+1
Which operation occurs between a plaintext block and the ciphertext of the previous block in CBC-MAC?
a. Addition
b. XOR
c. Multiplication
d. Subtraction
Answer: b. XORfiveable+1
The security of CBC-MAC ultimately depends MOST on:
a. The cipher’s randomness
b. The key schedule
c. The unpredictability of MAC tags
d. The security of the underlying block cipher
Answer: d. The security of the underlying block ciphersciencedirect+1
Which standard MAC method is based on block ciphers like CBC-MAC but addresses its weaknesses?
a. CMAC
b. SHA-MAC
c. S/MIME
d. RC4-MAC
Answer: a. CMACcsrc.nist+1
The MAC output length should be:
a. Equal to block cipher size
b. Greater than key size
c. Randomly selected
d. Shorter than block cipher block
Answer: a. Equal to block cipher sizewikipedia+1
Which of the following could cause two different messages to produce the same CBC-MAC tag given protocol weaknesses?
a. Tag collision
b. IV reuse
c. Cipher block chaining
d. Padding oracle
Answer: a. Tag collisionyoutubewikipedia
Which cryptographic property does the MAC provide?
a. Confidentiality only
b. Integrity and authenticity
c. Non-repudiation only
d. Availability
Answer: b. Integrity and authenticitysciencedirect+2
The term ‘existential forgery’ in CBC-MAC context refers to:
a. Forging a valid MAC for at least one new message
b. Brute-forcing the secret key
c. Cracking the ciphertext
d. Intercepting the tag
Answer: a. Forging a valid MAC for at least one new messageyoutubewikipedia
Which approach is not sufficient to secure CBC-MAC for variable length messages?
a. Length-prepending
b. Encrypting the last block
c. Using unique keys for each length
d. Adding a random nonce
Answer: d. Adding a random noncewikipedia
Which is true about HMAC compared to CBC-MAC?
a. Independent of block ciphers
b. Uses hash functions instead of block ciphers
c. Requires asymmetric keys
d. Only suited for fixed-length messages
Answer: b. Uses hash functions instead of block cipherslearnovita+1
For which applications is CBC-MAC most commonly preferred?
a. Bulk encryption
b. File transfer authentication for fixed size packets
c. Random number generation
d. Digital signatures
Answer: b. File transfer authentication for fixed size packetssciencedirect+1
The block cipher in CBC-MAC must be:
a. A publicly known permutation
b. Secure against brute-force
c. Weak for quick computation
d. Used with public key cryptography
Answer: b. Secure against brute-forcesciencedirect+1
To prevent MAC forgery in CBC-MAC, best practice is:
a. Use with variable message lengths directly
b. Restrict to fixed-length messages or use modifications like CMAC
c. Rotate keys frequently
d. Decrease tag size for speed
Answer: b. Restrict to fixed-length messages or use modifications like CMACcsrc.nist+1
Which is NOT true for CBC-MAC?
a. Provides message integrity
b. Uses symmetric keys
c. Secure for fixed-length messages with a strong cipher
d. Always appropriate for variable-length data
Answer: d. Always appropriate for variable-length datayoutubewikipedia
If a protocol migrates from CBC-MAC to CMAC, it gains:
a. Security for variable-length messages
b. Better encryption speed
c. Non-repudiation
d. Elimination of symmetric keys
Answer: a. Security for variable-length messagescsrc.nist+1

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