MadeEasy Test Series: Computer Networks - Network Security

Question

Answer 1

RSA

step 1 : n = p*q = 5*17 = 85

step 2: $\Phi (n)$ = (p-1)(q-1) = 4*16= 64

step3 : e*d mod $\Phi (n)$ = 1  [ use extended Euclidian Algorithm to find value of e ]
            e*13 mod 64 = 1 => e=5

Encryption

1. Obtains the recipient B's public key (n, e).
2. Represents the plaintext message as a positive integer m, 1 < m < n
3. Computes the ciphertext c = m^e mod n.
4. Sends the ciphertext c to RECEIVER

Deryption

1. Uses his private key (n, d) to compute m = c^d mod n.
2. Extracts the plaintext from the message representative m.

[  given Encoding  : A-1 , B-2 , C- 3....Z-26 ]

I -> 9 => 9⁵ mod 85 = 59 => (59-52 = 7) =>G
I -> 9 => 9⁵ mod 85 = 59 => (59-52 = 7) =>G
T->20 =>20⁵ mod 85 = 5 => E

Sum Of Integers In cipher Text  = 7+7+5 = 19

Good Read : https://gateoverflow.in/70939/ugcnet-aug2016-iii-30

Comments

I think we dont need to convert 59 to 7(G) further becoz they asked int corresponding to cipher text.And int corresponding to I(9) is 59.So it would be (59+59+5)=123

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@Kishalay_das Question doest clearly mentioned how to deal with that  end part .
Well , when we strictly follow RSA algorithm to encrypt a text there is so character for integer 59 ( according to given encoding ) The integer value of cipher text ' G ' is 7 .
So we cannot take 59 . 

According to Question , They have asked

Sum of Integers in Cipher Text

Cipher Text : GGE 

Integer Values Corresponding to Cipher Text : 7, 7 5

Sum of Integers in Cipher Text = 7+7+5

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@pc can you explain why did u subtract 52 from 59?

Answer 2

e=5 
encrypt key = p^e mod phi(n)
phi(n) = 4*16=64
I is number 9
9⁵mod64 = 41
T= 20
20⁵mod64 = 0
IIT= 41 41 0 = 41+41+0 = 82
 

Comments

@anusha .......i think u r wrong because for i its (9^5)mod 85 =  59
 and for T = (20^5)mod 85 =  5
 so its 59+59+5 = 123
 but u use phi(n) but for convert message in to cipher text it take n...plz check

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yeah correct.  its 123

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@anusha  plz check...https://gateoverflow.in/74859/made-eazy-2016-cn