CSE 539

Applied Cryptography

Assignment 2


Answer all questions, be brief

Please submit hard copy. Typed preferred.



1.  The problem with symmetric cryptosystems is distribution of keys. How do the public key algorithms solve this problem?


2.  What is a public key certificate? What data is at least part of a public key certificate?


3. Alice and Bob plan to use public keys to communicate with each other. Alice (ka1,ka2 ), Bob (Kb1,Kb2)

a)    Alice sends Bob her public key and bob uses Alice public key to encrypt the message and Alice gets back the message using her private key

b)     Instead of Alice sending her public key, Bob requests a trusted third party to send in the public key of Alice

c)       Alice and Bob use the public key of the trusted third party to get each others keys

Explain the vulnerabilities in these three protocols of operation.



4. Explain how SSL counters the following threats

a)      Brute force cryptanalytic attack : An exhaustive search of the key space

b)       Replay attack—An earlier SSL handshake messages are replayed

Man in the Middle attack: An attacker interposes during key exchange acting as a client to the server and as a server to the client

c)   IP spoofing: Uses forged IP address to fool a host into accepting bogus data



 5. About prime numbers,

a)   How are the prime numbers found?

b)   What is the largest prime number known?

c)   How are prime numbers tested?

d)   What is the importance of prime numbers in number theory?

e)   What are the different kinds of prime numbers?


6. We need to split a document among Alice, Bob and Carol, such that

  • Alice and Bob can recreate
  • Bob and Carol can recreate
  • Alice and Carol cannot recreate

Show how this can be done.


7. Using one-time pads it is possible to have two different plain text for the same cipher text? Show with an example how this possible.