Tutorial Activity 4

# Tutorial Activity 4 - MH 8300 Tutorial Activity 4 RSA Set A...

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MH 8300 Tutorial Activity 4 RSA Set A Group Members Name NTU Email ID Marks : /2

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MH 8300 Tutorial Activity 4 RSA Set A The following describes the communication between Alice and Bob via RSA public key encryption. Eve is eavesdropping on their communication and trying to decrypt the message sent. Question 1. Bob prepares the RSA protocol by choosing the public keys and calculating the private key. Bob chooses two primes p = 29 , q = 53 and then computes n = 1537 . (a) What is ϕ ( n ) ? (b) Which one of the following options is a good choice for e ? 5 , 7 , 13 (c) Based on your choice in Qn 1(b), find e - 1 ( mod ϕ ( n )) . (d) What should Bob publicize? Question 2. Suppose P is any integer between 2 P 1536 . Alice wants to communicate the plaintext message P with Bob via RSA encryption. She encrypts P into ciphertext C and sends it to Bob. (a) Write down C in terms of P. (b) Bob receives 989 . What is P? Question 3. Alice would like to communicate with Bob again, hence Bob decides to change his public key. But he was lazy and hence decides to only change the value for e while keeping n = 1537 . Which one of the following is a good choice for e ? 8 , 11 , 26 Question 4. Alice, being absent-minded, could not recall if she sent P to Bob. So she re-encrypt the same message P into C using the new public key and sends it to Bob. What does Alice send to Bob this time? Write down C in terms of P.
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