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Cryptography and Architectures for Computer Security Exam Code: 095947 (old 090959), A.Y. 2014–2015, Semester: 2 Prof. G. Pelosi September 9th, 2015 – Exam Session Name: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Surname: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Student ID: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .Signature: . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Time: 2h:30’. Use of textbooks, notes, phones or Internet connected devices is not allowed. Prior to turn in your paper, write your name on any additional sheet and sign it. Question 1 [6 pts] (a) What are the choices of RSA and DSA key lengths to provide a sufficient security to be employed with a 128-bit symmetric cipher? What are the ones for a 256-bit symmetric cipher? (b) Discuss if and when re-encrypting a message with the same block cipher employing different keys improves the security of system. (c) Discuss if re-encrypting a message with an RSA cryptosystem employing different public ex- ponents and the same public modulus improves the security of system. Solution: RSA – public modulus size: 3072-bit ( ≡ AES-128 security) 15360 ( ≡ AES-256 security) DSA – group and pub. key sizes: 3072-bit; private key size: 256-bit ( ≡ AES-128 security) DSA – group and pub. key sizes: 15360-bit; private key size: 512-bit ( ≡ AES-256 security), See lectures... Question 2 [2 pts] (a) To define a cryptographic hash function, assume the input message m to be divided into l blocks of length L bits: m= ⟨m1,m 2, · · ·,m l⟩. Let h(x) =m1 ⊕m2 ⊕ · · · ⊕ml. Is h(·) a good cryptographic hash function? (b) Describe the Merkle–Damg˚ ard design of hash functions and show how to realize a proper…