Computing 205/406 & IT8

Tutorial #18

1. What is a Vernam Cipher?

2. Why would you not use Vernam Cipher for large messages?

3. Simple cryptosystems (including Vernam Ciphers) make extensive use of the XOR (exclusive-OR) function. Demonstrate how XOR can be used to implement a simple Vernam Cipher.

4. More usable one-time pad systems (of the kind that real spies might use) don't necessarily use the XOR function. For example, one system uses a sequence of random numbers in the range of 0 to 25 which indicates how far each character of the message should be shifted in the alphabet. How would this work? Is it secure?

5. Describe some methods of distributing keys for single-key encryption. List their advantages and disadvantages.

6. What advantage does a single key encryption system have over a public key encryption system. What are the advantages of a public key system?

7. The DES uses a 56 bit key. How does this compare with the number of possible keys for a typical ATM card? What about typical Unix passwords chosen from the 96 character printable ASCII character set)?

8. Describe each of:
   • traffic padding
   • message authentication
   • digital signatures

Practical exercises:

• Investigate the Unix crypt utility.

• Even if you're not tackling Q.4 of assignment 3, you should be aware of the pgp utility available on the department Unix systems. Run pgp and see what it tells you in its usage message.

• Investigate skey, soon to be installed on our Unix systems. What problem does it attempt to solve.