Using DES with Cipher Block Chaining to Send a Message to Alice
Added on -2019-09-25
Explain your answer to each of the following: (a) Assume that an error occurs during the transmission of the first ciphertext block c1, so that Alice receives an incorrect block c 0 1 instead of c1. One day you discover that your private RSA key, (n1, d1), happens to be the same as the public RSA key of your friend, (n2, e2).
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1.You are using DES with cipher block chaining to send a message (containing plaintext blocks m1, m2, m3, . . . ) to your friend, Alice. Explain your answer to each of the following: (a) Assume that an error occurs during the transmission of the first ciphertext block c1, so that Alice receives an incorrect block c 0 1 instead of c1. After Alice decrypts the message, which of her plaintext blocks will be incorrect? (b) Assume that an error occurs inside your computer during the encryption of the second plaintext block (m2) and you end up computing c2 incorrectly as c 0 2 . Assume no errors occur during transmission. After Alice decrypts the message, which of her plaintext blocks will be incorrect? 2. It was shown during the lecture that Double DES is vulnerable to chosen plaintext attack using the meet-in-the-middle approach in which the number of steps is of order O(263). If you were toconduct a similar chosen plaintext attack on Triple DES in the most efficient way possible, what is the order of the number of steps you will require? 3.Users A and B are using the Diffie-Hellman key exchange. You are an interceptor who notes that they are using the common prime p = 71 and generator g = 7. You observe that A next sends B her public key X = 51, and B sends A his public key Y = 47. What is the shared secret key? Hint: Use Python if your calculator is not able to help you here. 4.Consider a user communicating using the RSA public key cryptography. You have just intercepted the ciphertext C = 6, sent to the user whose public key is e = 31, n = 247. What do you think is the plaintext? 5.Suppose you know that one of your nosy friends is interested in the content of your encrypted messages you send and receive. One day you discover that your private RSA key, (n1, d1), happens to be the same as the public RSA key of your friend, (n2, e2). That is, n1 = n2 and d1 = e2. Should you change your public and private keys? Explain your answer. 6.Decrypt the following message shown below in exactly 70 characters per line (excepton the last line). nhjQHktQSktQUwSjQ?hjgQ,hjQMjx,QwqQ,hjQkgESkdQUMjk,EwgQSktQkUc.EMjQ,hwxjQMEPh,xQ?hEUhQgjejMQUw.dHQhkejQ jjgQ?E,hhwdHjgQqMwSQ,hjSQ .,Q tQ,hjQhkgHQwqQ,tMkggtVQnhjQmMjgUhQhkejQkdMjkHtQHExUwejMjHQ,hk,Q,hjQ dkUfgjxxQwqQ,hjQxfEgQExQgwQMjkxwgQ?htQkQh.SkgQ jEgPQxhw.dHQ jQk kgHwgjHQ?E,hw.,QMjHMjxxQ,wQ,hjQUkJMEUjQwqQkQ,wMSjg,wMVQG,QSktQwgjQHktQUwSjQ,wQ jQMjUwPgEYjHQ,hk,Q,hjQg.S jMQwqQdjPxyQ,hjQeEddwxE,tQwqQ,hjQxfEgyQwMQ,hjQ,jMS
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