Lab/Tutorial :
SEHH2238 : Computer Networking Session 10 : Network Security
A. Symmetric Cryptography
1. In symmetric-key cryptography, how many secret keys are needed, if every person in a group of 10 people needs to communicate with
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a) Every other person in another group of 10 people
b) Every other person in the same group
2. Encrypt the message “THIS IS AN EXERCISE” using a shift cipher with a key of 20. Ignore the space between words.
3. Encrypt “INTERNET” using a transposition cipher with the following key: 4 3 1 5 2.
B. Security
What are the DO’s and DON’Ts for handling accounts & passwords?
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4. (Optional) In RSA, given two prime numbers p=19 and q=23, find n and . Choose e = 5 and try to find d, such that e and d meet the criteria.
5. What is danger in choosing 2 as the public key e in RSA?
SEHH2238 Computer Networking Tutorial 10 Page 1
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