HW2 - HW #2: Private Key Cryptography CS 392/6813: Computer...

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HW #2: Private Key Cryptography CS 392/6813: Computer Security Fall 2009 [100pts] DUE 09/28/2009 (midnight) Problem 1 [10pts] An encryption scheme is said to be “deterministic” if encryption of the same plaintext value always results in the same ciphertext value. In other words, first time one encrypts P, the resulting ciphertext is C; the next time one encrypts P, the resulting ciphertext is still C, and so on. Is a deterministic encryption scheme secure against the “Known-Plaintext Attack”, we discussed in class? Explain why or why not. Give an example to illustrate your answer. Problem 2 [15pts] In the class, we studied the Caesar’s cipher. A similar cipher is called affine cipher which works as follows. Encryption : each plaintext letter P is encrypted to obtain the ciphertext letter C such that C = aP + b (mod 26), (where a,b are numbers between 0,1,….,25, and represent the secret key). Decryption : each ciphertext letter C is decrypted to obtain the plaintext letter such that P = (C-b)a -1 (mod 26). I need to send a message to the class
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HW2 - HW #2: Private Key Cryptography CS 392/6813: Computer...

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