QUESTION 2 [23 Marks (a) Ahmad and Ben want to have a secured communication betw
ID: 3874060 • Letter: Q
Question
QUESTION 2 [23 Marks (a) Ahmad and Ben want to have a secured communication between them. Before sending any email, the messages will be encrypted with double encryption. Both of encryption. Given the encrypted message received by Ben is, Ct"BNY them have agreed to apply Playfair cipher for the first encryption and Vigenere cipher as the second YB XZHMZ HOFIE FVREI LKJQx c", knowing that it was obtained by applying first the Playfair cipher using the key, K LOST TIME IS NEVER FOUND AGAIN, and then using the Vigenere cipher with the key, K" JYXYVI". ) Ahmad has encrypted K to make sure the integrity of the delivered messages. Given the encrypted K2 is JYXYVI, Ben is required to decrypt K2 using Caesar Cipher. Write the plaintext which will be generated from the encryption key [3 Marks] (ii) Write the plaintext messages from the above cryptogram, C after successfully [9 Marks] i hat you are obtaining the K2 in (i). Reference provided in Appendix A. (b) You want to establish a secure communication w th Alice. In order to do t going to use Diffie-Hellman key arrangement protocol with the following step. 1. You and Alice agree on using prime number, n 467 and root number, g 2. Alice sends you the public key which is keypabb 30 3. You choose keypi ag 9 asyour private key mpute your public key and the common key for Diffei-Hellman arrangemen [8 Marks] Assume that it was not Alice but Oscar who sent you his public key, keypb bo G) Co protocol. 30 in Step 2. Name the attack and give justification whether it is successful or (3 Marks] not.Explanation / Answer
i.
The substitution of cipher known as the “shift cipher” , shifts the letter of the letters of one alphabet against another alphabet to create the secret message.
For encryption the formula is:
C= (P+K) mod 26
Where C is the cipher text,
P in the plain text,
K is the key having default value 3.
But in our above question we need to decrypt K2 = “JYXYVI”, So the equation for decryption is
P = (C-K) mod 26
So firstly we need to write down the alphabet and assign them with numbers.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26
So for the letter count the number shifted by 1
J ; P = (9-3) mod 26 ; P = 6 mod 26 ; P = 6=F
Y ; P= (24-3)mod 26 ; P = 21 mod 26 ; P = 21 = U
X; P = (23-3)mod 26 ; P = 20 = T
Y; P = U (already generated)
V; P= (21-3) mod 26; P = 18 = R
I; P = (8-3) mod 26 ; P = 5 = E
So the plain text to be generated from encrypted key will be “FUTURE”
ii.
AS the given C1 value is BNYYB XZHMZ HOFIB FVREI LKJQX C we will get the plain text using key k2 is
SELLS KNOWN OFAYS AMBRY THICK