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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 “KnownPlaintext 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
= (Cb)a
1
(mod 26).
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 Fall '09
 Saxena
 Computer Security

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