Computer Networks

Tutorial #17

1. The simplest cryptosystems are monoalphabetic substitution cyphers. Give an example of such a system, and explain why it is not regarded as being very secure.

2. Just about the simplest subsitution cypher to implement in software is a byte-wise XOR of each byte in the data stream with a single byte key. How does this work? Investigate the XOR function and explain why it is particularly useful for this function.

3. The DES uses a 56 bit key.
   1. How does this compare with the number of "bits" in the key for a typical ATM card? NB: think about how many bits are needed to represent each digit.
   2. What is the key size in bits for typical Unix passwords chosen from the 96 character printable ASCII character set?
   3. What if the Unix password was only chosen from the set of upper and lowercase letters and the 10 digits?

4. The following string of bits is ciphertext which has been encrypted using a Vernam cipher. The key which must be used for decryption is also shown. Use your cryptographic knowledge to crack the code and discover the plaintext message. Some (possibly useful) ASCII codes are given below to convert the resulting plaintext bit string into English text.

       Ciphertext: 0001010 0001001 0000010
   
              Key: 1001011 1000101 1000111
   

   Some useful ASCII character codes:

   A:  1000001 B:  1000010 C:  1000011 D:  1000100 E:  1000101
   F:  1000110 G:  1000111 H:  1001000 I:  1001001 J:  1001010
   K:  1001011 L:  1001100 M:  1001101 N:  1001110 O:  1001111
   

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

6. More usable one-time pad systems (of the kind that real spies might use) can use a variety of encryption functions. For example, one system uses a sequence of random numbers in the range of 0 to 25 as the key. How would this work? Is it secure?

7. One of the biggest problems with single key encryption is to do with key management. Propose some methods of distributing keys for single-key encryption. List their advantages and disadvantages.

8. What are the advantages of a public key cryptosystem compared to a single key system? Why is the problem of key management as difficult in public key systems as it is in single key systems?

9. What is meant by each of the terms:
   • key escrow
   • authentication
   • digital signatures