Computer Networks — Test 15

Correct Answer: B. B. Ith bit of the hash code
Explanation: “m” is the number of n-bit blocks in the input, bij is the ith bit in the jth block, ci is the ith bit of the hash code. Operation between the bits is exclusive OR and this operation is performed on every input block.

Correct Answer: D. D. When x != y
Explanation: Hash function’s primary objective is to provide integrity. No two inputs should have the same hash value, which causes the collision. Collisions are undesirable in hash functions.

Correct Answer: A. A. 2b-n
Explanation: In the hash value h = H(x), x is the preimage of the hash value. 2b-n formula finds the total number of possible collisions for a hash value. Here b > n and 2b is the total number of possible messages, 2n is the total number of possible hash values.

Correct Answer: D. D. Preimage resistant
Explanation: Hash function will produce fixed lengths of outputs and accepts any size of blocks as input. It is efficient to calculate. If these three requirements need to be met to implement hash function in practical applications.

Correct Answer: D. D. Collision resistant
Explanation: Hash function is referred as weak, when it only satisfies the Efficiency, preimage resistant, second image resistant, fixed length output, variable length input. It is referred as strong if it satisfies the collision resistant along with all of the above.

Correct Answer: B. B. False
Explanation: The attacks that are on hash functions are brute force attack and crypt-analysis. Brute force attack mainly concentrates on the bit length of the hash function, whereas crypt-analysis concentrates on cryptographic algorithms.

Correct Answer: B. B. Total break
Explanation: Total break is finding private key and Universal forgery is finding signing algorithm. Selective forgery is doing forgery of signature for particular message. Atleast one message can have forgery signature.

Correct Answer: D. D. Computationally feasible
Explanation: If a digital signature is computationally feasible, then it is very easy for a hacker to attack the signing scheme. He may forge the signature for the new message or change the signature for the present message.

Correct Answer: A. A. Source and destination
Explanation: In general, there will be a third party, who authenticate the digital signatures. But there is no rule that everyone should believe the third party. So, the direct digital signature scheme involves sender and receiver. And receiver knows from where the message is coming using public key of the sender.

Correct Answer: A. A. Signature key is of signing and public key for verification
Explanation: In digital signature, private key is the signature key, that is used to create digital signature. And this private key belongs to the signer. Public key belongs to the receiver who receives the message. These two keys are used in digital signature algorithm.