University of South Florida Cryptography and Data Security Worksheet

Homework 5 (100 pts)Rule: Finish all of the following on your own.
1. Explain why we may need to use homomorphic encryption in some real-world
applications. What are the major differences between homomorphic encryption and
symmetric encryption (e.g., DES, AES).
2. During the basic operation for homomorphic encryption, why do we need to add a noise
2r+b?
3. Why it could be hard for the attacker to falsify the whole blockchain?
4. Why proof of work is necessary to build a blockchain? Suppose if we remove the proof
of work and design a simple rule: the first node in the network that finishes computing
a new block always wins and this new block will be immediately broadcast to the
network and all nodes must add this block to the chain. How will this simple (yet flawed)
design impact the security?
5. Suppose Alice is using her laptop at home to visit a commercial shopping website.
During this visit, what possible information the website could know about Alice?
6. If you are using the tor browser to visit a commercial website, which of the following
statement are true?
a. No tor node will know your source IP address
b. No tor node will know your destination IP address
c. No tor node will know your source and destination IP addresses at the same
time
d. The commercial website will not know your source IP address
e. The commercial website will know the visit is from the tor network
f. The commercial website may identify you if you choose to log in.