2016 09 14nosol

Pagina 1

Cryptography and Architectures for Computer Security Exam Code: 095947 (old 090959), A.Y. 2015–2016, Semester: 2 Prof. G. Pelosi September 14th, 2016 – 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 [2 pts] Consider a chosen plaintext attack scenario, where the adversary can submit a plaintext of her choice for symmetric-encryption. We adopt Kerckhoffs’ principle and assume that the adversary knows the type of system used. Which choice of plaintext would he make if the crypto-system is: (a) a shift cipher? (b) a Vigen´ ere cipher? (c) a simple substitution cipher? (d) a transposition cipher? Question 2 [2 pts] Consider a a symmetric block cipher with a 128 bit block size, and a 128 bit key size, having a fully linear (non-singular) round function. Describe how it is possible to retrieve the secret key, under the assumption of a known plaintext attack stating also which are the requirements in terms of the quantity of plaintext and ciphertext to be known. The method should not resort to exhaustive key search. Justify what is the change in the requirements for the attacker if the same block cipher is changed to employ a 256 bit key, while keeping all its other features the same. Question 3 [4 pts] Consider the case of Linear Feedback Shift Registers (LFSRs), employed to generate a…