This chapter is from the book
This chapter is from the book
This chapter is from the book
EXERCISES
Suppose you are a customs inspector. You are responsible for checking suitcases for secret compartments in which bulky items such as jewelry might be hidden. Describe the procedure you would follow to check for these compartments.
Your boss hands you a microprocessor and its technical reference manual. You are asked to check for undocumented features of the processor. Because of the number of possibilities, you cannot test every operation code with every combination of operands. Outline the strategy you would use to identify and characterize unpublicized operations.
Your boss hands you a computer program and its technical reference manual. You are asked to check for undocumented features of the program. How is this activity similar to the task of the previous exercises? How does it differ? Which is the most feasible? Why?
Could a computer program be used to automate testing for trapdoors? That is, could you design a computer program that, given the source or object version of another program and a suitable description, would reply Yes or No to show whether the program had any trapdoors? Explain your answer.
A program is written to compute the sum of the integers from 1 to 10. The programmer, well trained in reusability and maintainability, writes the program so that it computes the sum of the numbers from k to n. However, a team of security specialists scrutinizes the code. The team certifies that this program properly sets k to 1 and n to 10; therefore, the program is certified as being properly restricted in that it always operates on precisely the range 1 to 10. List different ways that this program can be sabotaged so that during execution it computes a different sum, such as 3 to 20.
One means of limiting the effect of an untrusted program is confinement: controlling what processes have access to the untrusted program and what access the program has to other processes and data. Explain how confinement would apply to the earlier example of the program that computes the sum of the integers 1 to 10.
List three controls that could be applied to detect or prevent salami attacks.
The distinction between a covert storage channel and a covert timing channel is not clear-cut. Every timing channel can be transformed into an equivalent storage channel. Explain how this transformation could be done.
List the limitations on the amount of information leaked per second through a covert channel in a multiaccess computing system.
An electronic mail system could be used to leak information. First, explain how the leakage could occur. Then, identify controls that could be applied to detect or prevent the leakage.
Modularity can have a negative as well as a positive effect. A program that is overmodular-ized performs its operations in very small modules, so a reader has trouble acquiring an overall perspective on what the system is trying to do. That is, although it may be easy to determine what individual modules do and what small groups of modules do, it is not easy to understand what they do in their entirety as a system. Suggest an approach that can be used during program development to provide this perspective.
You are given a program that purportedly manages a list of items through hash coding. The program is supposed to return the location of an item if the item is present, or return the location where the item should be inserted if the item is not in the list. Accompanying the program is a manual describing parameters such as the expected format of items in the table, the table size, and the specific calling sequence. You have only the object code of this program, not the source code. List the cases you would apply to test the correctness of the program's function.
You are writing a procedure to add a node to a doubly linked list. The system on which this procedure is to be run is subject to periodic hardware failures. The list your program is to maintain is of very high importance. Your program must ensure the integrity of the list, even if the machine fails in the middle of executing your procedure. Supply the individual statements you would use in your procedure to update the list. (Your list should be fewer than a dozen statements long.) Explain the effect of a machine failure after each instruction. Describe how you would revise this procedure so that it would restore the integrity of the basic list after a machine failure.
Explain how information in an access log could be used to identify the true identity of an impostor who has acquired unauthorized access to a computing system. Describe several different pieces of information in the log that could be combined to identify the impostor.
Several proposals have been made for a processor that could decrypt encrypted data and machine instructions and then execute the instructions on the data. The processor would then encrypt the results. How would such a processor be useful? What are the design requirements for such a processor?