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The plain text message $BAHI$ encrypted with $RSA$ algorithm using $e = 3, d = 7$ and $n = 33$ and the characters of the message are encoded using the values $00$ to $25$ for letters A to Z. Suppose character by character encryption was implemented. Then, the Cipher Text message is _____.

  1. ABHI
  2. HAQC
  3. IHBA
  4. BHQC
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Best answer
6 votes
6 votes

RSA
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 = me mod n.
  4. Sends the ciphertext c to RECEIVER

Deryption

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

For BAHI
A-1 ,B-2, C-3 , D-4,E-5,.... etc  (Message  Encoded Using 01-26)

B=>2=> 27 mod 33 = 8 =>H
A => 1=>17 mod 33 = 1 =>A
H =>8=> 87 mod 33 = 17 =>Q
I=> 9=>97 mod 33 = 3 => C
Cipher Text Message : HAQC


A-0 ,B-1, C-2 , D-3,E-4,.... etc  ( Message Encoded Using 00-25 as per question)

B=>1=> 17 mod 33 = 0 =>B
A => 0=>07 mod 33 = 1 =>A
H =>7=> 77 mod 33 = 13 =>N
I=> 8=>87 mod 33 = 17 => R
Cipher Text Message : BANR 

[ No options will be matched taking encoding scheme 00-25 as per the question ]

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5 votes

question needs some minor correction (  A-Z  values must be from 1-26  )   else no option is correct 

formula for ciphertext is 

C=Pe mod n

where P=plain text (here its value is =2  as B is 2nd  alphabet )  here n=33 is given 

now e=3 is also given so

for B =2

C=23 mod33 =8

now  alphabet with value 8 is  so B is mapped to H

for A =1

 C=13 mod33 =1

so  A with value =1 is  mapped to A

for H  =8 

C=83 mod33 => 512mod33=17  which corresponds to Q

and finally I with value 9 is 

C=93 mod 33 =729 mod33 =3 map to C 

so ans will be option B  HAQC 

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