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Computer Science

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Question 1: a. Consider the following plaintext “hello world”. What is the cipher text using classical Caesar cipher? b. Consider the following plaintext “The boy has the bag” and key is “vig”. What is the cipher text using Vigenere cipher? c. Consider the following plaintext “attack from east at dawn” and the key 35214. What is the cipher text using row-column cipher? Question 2: Consider p=3 and q=11 a. Compute n and (n) Bob selects eB=7 and dB=3 and Alice selects eA=9 and dA=9. Bob would like to send a message HELLO to Alice that is encrypted and signed. What is the resulting cipher text.
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Running head: Computer Securities

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COMPUTER SECURITIES

Question 1:
a. Consider the following plaintext “hello world”. What is the cipher text using
classical Caesar cipher?
Encryption (cipher Text) Ci=(p+3) mod 26.
Decryption(plain text) Pi=(Ci-3) mod 26.
Given plaintext: “h e l l o w o r l

d”

Cipher text: k h o o r z r u o g
Plain text : H cipher text is : k
P=7
C=(7+3) mod 26 =10 mod 26 =10( 10 is remainder) 10 th letter is k
Plain text: E cipher text is : h
P=4
C= (4+3) mod 26 =7 mod 26 =7 .7 th letter is

h

Plain text = L cipher text : O
C=(11+ 3) mod 26 =14 mod 26=14 . 14 th letter is o
Plain text = o cipher text : r
C=(14+ 3) mod 26 =17 mod 26=17 . 17 th letter is r
Plain text =w cipher text : z
C=(22+3) mod 26 = 25 mod 26 =25 . 25th letter is =z
Plain text = o cipher text : r
C=(14+ 3) mod 26 =17 mod 26=17 . 17 th letter is r
Plain text = r cipher text : u
C=(17+ 3) mod 26 =20 mod 26=20 . 14 th letter is
Plain text =L cipher text :o

u

2

COMPUTER SECURITIES

3

C=(11+3) mod 26 = 14 mod 26 =14 . 14th letter is =o
Plain text = d cipher text : g
C=(3+ 3) mod 26 =6 mod 26=6 . 6 th letter is g
Final cipher text is = k h o o r z u r o g

PLAIN
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b. Consider the following plaintext “The boy has the bag” and key is “vig”. What is the
cipher text using Vigenere cipher?
plaintext : “ T h e b o y h a s t h e b a g”
KEY

:“ v i g v i g v i g v i g v i g "

cipher text: o p k ...


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