Information Security : exam 1

gistfile1.md

1. Follow

   1. What is a monoalphabetic cipher ?
   2. How large is the key space of the monoalphabetic cipher applied to English (just consider lower case letters) ?
   3. What is the main security weakness of a monoalphabetic cipher ?
   4. Please provide at least 4 approaches to prevent the above weakness.

2. Block ciphers process messages in blocks like an extremely large substitution, but this is very difficult to implement for a very big block size. What is Shannon's idea to realize a block cipher with big block size ?

3. The S-box of DES round operation is not invertable, but why DES can decrypt ?

4. Follow

   1. Please explain the OFB mode.
   2. In the OFB mode, we never reuse the same sequence (key+IV). If we reuse the key, then the attacker can recover message. Please describe very precisely (mathematiclly with symbols) how the attack works.

5. Please compare the CFB mode the OFB mode by providing a table indicating at least 5 of their advantage and corresponding disadvantage.

6. This question is about issues of random access of encryption and decryption. Define notations or symbols you need before providing your answer. Please describe how to use CFB mode and CTR mode to randomly encrypt and decrypt a file Why OFB mode is inappropriate to randomly encrypt and decrypt a file?

7. Describe the details (as much as you remember) of AES operation and describe how efficiently implement AES on an 8-bit CPU ?

8. How avalanche happens in AES within the first tow rounds ?