**CSE 539**

**Applied
Cryptography**

# Assignment 2

** **

Answer all questions, be brief

Please submit hard copy. Typed preferred.

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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 (k_{a1},k_{a2} ),
Bob (K_{b1},K_{b2})

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.