Q2 Using the English alphabet (i.e., mod 26 arithmetic) let plaintext = {p1, p2,
ID: 3740977 • Letter: Q
Question
Q2 Using the English alphabet (i.e., mod 26 arithmetic) let plaintext = {p1, p2, pn} and corresponding cipher text = {c1, c2, cn}. Suppose the encryption function is ci = pi + 10 (mod 26). You receive the cipher text message EWEM SC K QBOKD CMRYYV What are the decryption function, and the decrypted/recovered plaintext? What type of cipher is this? Show your steps.
Q3 You are Alice. You have agreed with your friend Bob that you will use the Diffie-Hellman public-key algorithm to exchange secret keys. You and Bob have agreed to use the public base g = 7 and public modulus p = 1013. You have secretly picked the value SA = 17. You begin the session by sending Bob your calculated value of TA. Bob responds by sending you the value TB = 839. What is the value of your shared secret key? Can you guess Bob’s secret value SB and what it would be? Show each and every step of your calculations (for mod calculation, the following identity may be useful mod( X^n, p) = mod (mod(X,p)*mod(X^n-1, p), p) mod(X*Y,p) = mod(mod(X,p)*mod(Y,p),p)
Explanation / Answer
Ans:-
Given that,
plaintext = {p1, p2, pn}
corresponding cipher text = {c1, c2, cn}
cipher text message EWEM SC K QBOKD CMRYYV
ci = pi + 10 (mod 26)
ci + 16 = pi + 26 (mod 26)
ci + 16 = pi (mod 26)
We can probably decode the message by using this function-
Here the ciphertext message is
EWEM SC K QBOKD CMRYYV
We can describe the Plaintext by using congruence’s any cipher text is first expressed numerically by translating the characters of the text into digits by means of some correspondence such as the following:
A B C D E F G H I J K L M
00 01 02 03 04 05 06 07 08 09 10 11 12
N O P Q R S T U V W X Y Z
13 14 15 16 17 18 19 20 21 22 23 24 25
The cipher text is transformed into the plaintext
VNVD JT B HSFBU TDIPPM
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