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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 30th, 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 [4 pts] Consider the four basic modes of operations of block ciphers (ECB, CBC, OFB, CFB). For all four modes of operation analyze the effect on the decryption of remaining blocks if, for the sequence of ciphertext blocks c1,c 2...,c n, one ciphertext block cj (0≤j <n) is erroneous. Specify which plaintext blocks xjxj+1,... are computed correctly. Question 2 [2 pts] One of the recommendations for a proper use of “One-time pad” (to ensure perfect secrecy) is to never reuse the same key for encryption of two different messages. The simplest way to implement it is to generate a random key sequence of the same length as message and to encrypt using, Ci =Mi +Ki mod 26 where Ki are random key symbols and M =K =C = Z26. Explain how the knowledge of two different ciphertext sequences C1C2... and C′ 1C′ 2... obtained by applying the same secret key, can compromise the security of the system. Question 3 [4 pts] Digital signature, developed using public key cryptography, is a means for communication coun- terparts in cyberspace to authenticate themselves to each other. Briefly describe the trust model…