Assignment-8 with Answer Hints:
1. Digital signature is applicable for which cryptosystem? A) Symmetric-key B) Asymmetric-key C) Both (A) or (B) D) Neither (A) nor (B)
ANSWER: (B) Hints: See the definition of Digital Signature.
2. Digital signature provides A) Authentication B) Nonrepudiation C) Both (A) and (B) D) Neither (A) nor (B)
ANSWER: (C) Hints: A digital signature gives a receiver reason to believe that the message was created by a known sender (authentication), that the sender cannot deny having sent the message (non- repudiation).
3. Authentication means ______A) Verification of user's identification B) Verification of the data C) Both (A) or (B) D) None of the above
ANSWER: (A) Hints: See the definition of Authentication.
4. To authenticate the data origin, one needs a A) Message Detection Code (MDC) B) Message Authentication Code (MAC) C) Both (A) or (B) D) Neither (A) nor (B)
ANSWER: (B) Hints: In MAC, the receiver validate the code by using the secret key shared between them (sender and receiver). In MDC, there is no such shared secret key is used.
5. Hash function is a function which usually takes an arbitrary size of data and A) creates a small flexible size of data. B) creates a small, fixed size of data. C) creates a permutation on input data. D) none of the mentioned
ANSWER: (B) Hints: See the definition of Hash function. 6. ______key exchange protocol based on discrete logarithm problem enables two parties to establish a common secret session key over unsecure communication channel. A) Micali-Schnorr B) ElGamal-Shamir C) Diffie-Hellman D) Miller-Rabin
ANSWER: (C) Hints: See the Diffie-Hellman key exchange protocol.
7. ______can endanger the security of the Diffie-Hellman key exchange protocol if two parties are not authenticated to each other? A) Man-in-the-middle attack B) Ciphertext attack
C) Plaintext attack D) None of the above
ANSWER: (A) Hints: Man-in-the middle attack is infeasible when two parties authenticate to each other.
8. Key distribution center plays a role A) to distribute ciphertexts among users. B) to reduce the risk in exchanging keys. C) Both A) and B) D) none of the above
ANSWER: (B) Hints: Key Distribution Center is part of a cryptosystem which plays role to reduce the risks inherent in exchanging keys.
9. Certification authority issues the digital certificate which must include A) the signer’s private key and identity B) the signer’s public key and identity C) the certificate authority’s private key D) a certificate revocation list
ANSWER: (B) Hints: See about Certificate Authority.
10. When prime p used in ElGamal signature and composite n used in RSA signature are of same size then which of the following is true? A) Elgamal signatures are shorter than RSA signatures B) Elgamal signatures are more efficient to verify than RSA signatures C) RSA signature is faster than Elgamal signature D) None of the above ANSWER: (C) Hints: ElGamal signature contains a pair (r=gk mod p,s=k-1(m-xr) mod (p-1)) whereas RSA signature is just c=md (mod n) (where symbol are conventional).