Review Questions

Anwers the following questions on a separate sheet during lectue and submit. Students should perform this task individually.

1. What are the essential ingredients of a symmetric cipher?

2. What are the two basic functions used in encryption algorithms?

3. How many keys are required for two people to communicate via a symmetric cipher?

4. What is the difference between a block cipher and a stream cipher?

5. What are the two general approaches to attacking a cipher?

6. Why do some block cipher modes of operation only use encryption while others use both encryption and decryption?

7. What is triple encryption?

8. Why is the middle portion of 3DES a decryption rather than an encryption?

Task 2: Problems

Anwers the following questions on a separate sheet during lectue and submit. Students should perform the steps in this task individually.

1. This problem uses a real-world example of a symmetric cipher, from an old U.S. Special Forces manual (public domain).
a) Using the two keys (memory words) cryptographic and network security, encrypt the following message:
Be at the third pillar from the left outside the lyceum theatre tonight at seven. If you are distrustful bring two friends.
Make reasonable assumptions about how to treat redundant letters and excess letters in the memory words and how to treat spaces and punctuation. Indicate what your assumptions are. Note: The message is from the Sherlock Holmes novel, The Sign of Four.
b) Decrypt the ciphertext. Show your work.
c) Comment on when it would be appropriate to use this technique and what its advantages are.

2. Eve has tricked Alice into decrypting a bunch of ciphertexts that Alice encrypted last month but forgot about. What type of attack is Eve employing?

3. Eve has an antenna that can pick up Alice's encrypted cell phone conversations. What type of attack is Eve employing?

4. Eve has given a bunch of messages to Alice for her to sign using the RSA signature scheme, which Alice does without looking at the messages and without using a one-way hash func-tion. In fact, these messages are ciphertexts that Eve constructed to help her _gure out Alice's RSA private key. What kind of attack is Eve using here?

5. Eve has bet Bob that she can _gure out the AES secret key he shares with Alice if he will simply encrypt 20 messages for Eve using that key. For some unknown reason, Bob agrees. Eve gives him 20 messages, which he then encrypts and emails back to Eve. What kind of attack is Eve using here?