# Cryptography

1. Hash (30 points)
Create in Notepad a txt file called mygroup.txt for the Hash exercise.
Type your group members name and save
Full Name1,Full Name2,Full Name3
For this exercise, we suggest to use certutil tool if you are using windows or md5sum and sha1sum, sha256sum in Linux. You can use other tools if you desire.

For all your response, you must provide commands used and the output.
a. Question 1
▪ Provide the md5 hash of the file. (3 points)
b. Question 2
▪ Modify the file content and add just one character (.) at the end of all names (Full Name1,Full Name2,Full Name3.) and provide a new md5 hash of the modified file.
▪ Do the output changed? Why? ( 3 points)
c. Question 3
▪ Provide the SHA1 of the modified file. (3 points)
▪ The hash is the same as which obtained in the previous question (question2) ? Why? (3 points)
▪ What you can conclude about the security of sha1 compared to md5?
(3 points)
d. Question 4
▪ With the same modified file, provide sha256 hash. (4 points)
▪ Is the new hash longer or shorter? Why? (4 points)
▪ What you can conclude about the security of sha256 compared to sha1?
(2 points)
e. Question 5 (5 points)
Find two protocols where hashing algorithms are used and explain how hashing algorithms are applied to secure the protocols.

1. RSA (35 points) a. Question 1 (10 points)
If we choose p as 19 and q as 17, what would be the n, e and d? Show all your calculations.

b. Question 2 (20 points)
If we choose RSA to encrypt a message (“Secure”) use public, private key from the above question 2(a).
▪ Show how the message will be encrypted by using the public key (n,e)
▪ Show the decryption will work by using the private key (n,d). You need to show all the steps for encryption and decryption. (15 points)
▪ You need also to provide the ciphertext obtained. (5 points)
Message = CMIS
E(Message) = CypherText
D (CypherText) = Message

c. Question 3 (5 points)
Explain clearly and briefly, how RSA can defeat “man in the middle” attack.
You can use the following scenario:
Imagine a sender S wants to share a secret key K to the receiver R. What exactly the sender S has to send to the receiver R?

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