Cryptography and Network Security BEU Question paper solution

(a) Advanced Encryption Standard (AES) has three different configurations with respect to number of rounds and

(i) data size
(ii) round size
(iii) key size
(iv) encryption size

(b) Which of the following is also known as key exchange algorithm?

(1) RSA
(ii) DES
(iii) DH
(iv) ECC

(c) RC4 is an example of

(i)hash algorithm
(ii) stream algorithm
(iii) block cipher
(iv) None of the above

(d) Hash function is

(i) used to produce fingerprint of a file
(ii) useful for message authentication
(iii) Both (i) and (ii)
(iv) None of the above

(e) Which one of the following is not a higher-layer SSL protocol?

(i) Alert protocol
(ii) Handshake protocol
(iii) Alarm protocol
(iv) Change cipher spec protocol

(j) in which of the following encryption key is used to encrypt and decrypt the data?

(1) Public key
(ii) Private key
(iii) Symmetric key
(iv) Asymmetric key

  1. (a) What are the two general approaches to attack a cipher? List and briefly define the types of cryptanalytic attacks based on what is known to the attacker.

(b) What is the difference between a mono- alphabetic cipher and a polyalphabetic cipher? Explain with examples.

3. Explain the key generation process of AES cipher. We try now to take advances in computer technology into account. Predicting the future tends to be tricky but the estimate usually applied is Moore’s law, which states that the computer power doubles every 18 months while the costs of integrated circuits stay constant. How many years do we have to wait until a key-search machine can be built for breaking AES with 128 bits with an average search time of 24 hours? Again, assume a budget of $1 million (do not take inflation into account).

4. Design a security service that provides dataintegrity, data confidentiality and non-repudiation using public-key cryptography ina two-party communication system over aninsecure channel. Give a rationale that dataintegrity, confidentiality and nonrepudiationare achieved by your solution. (Recommen-dation: Consider the corresponding threatsin your argumentation.)

5. In an RSA system, the public key of a givenuser is e = 65 n = 2 8 8 1. What is the privatekey of this user? Hint: First use trial-and-error to determine p and q; then use theextended Euclidean algorithm to find themultiplicative inverse of 31 modulo (n). 14

(a)What types of attacks are addressed bymessage authentication? Describe thebasic uses of message authenticationcode.

(b) Differentiate between the symmetric keycryptography and public key crypto-graphy.

  1. Explain the following:

(a) X.509 Format of a certificate

(b) Public key distribution

  1. (a) Explain the working of Diffie-Hellman key exchange technique.

(b) Alice and Bob use the Diffie-Hellman key exchange technique with a common prime q-23 and a primitive root α=5.

(i) If Bob has a public key Y-10, what is Bob’s private key Y?

(2) If Alice has a public key Y, -8, what is the shared key K with Bob? 3

9. List the main features of the SHA-512 cryptographic hash. What kind of compression function is used in SHA-512? Compare the compression function of SHA-512 without the last operation (Final Padding) with a Feistel cipher of 80 rounds. Show the similarities and differences.



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