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QUESTION 3 a ) In a public key cryptosystem, you do not need to keep the encryption key secret. Why is it essential to keep
QUESTION
a In a public key cryptosystem, you do not need to keep the encryption key secret. Why is it essential to keep the encryption key secret in a symmetric key cryptosystem?
b In a symmetric key cryptosystem, what do the terms blocksize and keysize mean? Why is it important for a symmetric cryptosystem to have a large blocksize and a large keysize?
c What is the main difference in the way in which data is encrypted by a stream cipher and a block cipher?
d Why are stream ciphers used in implementations where a lot of interference might be expected such as mobile telephone communications?
e A block cipher should have the properties of diffusion, confusion and completeness. Explain each of these properties and why they are essential for a secure block cipher.
f Triple DES and Rijndael are two widely used symmetric key ciphers. State one advantage and one disadvantage of Rijndael when compared to Triple DES.
QUESTION
a
i Describe the key generation protocol for the RSA public key cryptosystem.
ii Suppose that an algorithm is found that can efficiently factorise a large number. Explain how a cryptanalyst could use this algorithm to break the RSA cryptosystem.
b Below is a simple method for blocking and encoding uppercase letters for encryption.
Group letters into blocks of two starting at the leftmost letter. Add a space character at the rightmost side if necessary to make an even number of characters.
Encode letters using dots, space
Convert each block of two characters into a single decimal number by multiplying the value of the first character by and then adding the value of the second character.
Thus, the word STARS is partially encoded as follows:
tablecharactersS spacecharacter values,block value,table
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