Briefy answer the following questions:

1. Explain the intuition behind the two rules in the Bell-LaPadula model for manda-tory access control. 

2. Give an example of how covert channels can be used to defeat the Bell-LaPadula model. 

3. Give an example of polyinstantiation. 

4. Describe a scenario in which mandatory access controls prevent a breach of security that cannot be prevented through discretionary controls. 

5. Describe a scenario in which discretionary access controls are required to enforce a security policy that cannot be enforced using only mandatory controls. 

6. If a DBMS already supports discretionary and mandatory access controls, is there a need for encryption? 

7. Explain the need for each of the following limits in a statistical database system: 

(a) A maximum on the number of queries a user can pose. 

(b) A minimum on the number of tuples involved in answering a query. 

(c) A maximum on the intersection of two queries (i.e., on the number of tuples that both queries examine). 

8. Explain the use of an audit trail, with special reference to a statistical database system. 

9. What is the role of the DBA with respect to security? 

10. Describe AES and its relationship to DES. 

11. What is public-key encryption? How does it di?er from the encryption approach taken in the Data Encryption Standard (DES), and in what ways is it better than DES?

12. Explain how a company ofering services on the Internet could use encryption-based techniques to make its order-entry process secure. Discuss the role of DES, AES, SSL, SET, and digital signatures. Search the Web to find out more about related techniques such as electronic cash.