CS 2011, A Term 1999
Introduction to Machine Organization and Assembly Language
Homework 5 (due Oct. 1)

Objective

Instructions

1. Write an assembly language procedure that accepts an unsigned integer as a parameter (seed) and generates a pseudorandom number using the linear congruence method described below. Debug and test this procedure using td.
2. Write assembly language instructions that fill a mask buffer with characters obtained using repeated calls to your pseudorandom number generator procedure. The characters in the mask buffer will be used to encrypt characters stored in a separate data buffer.
3. Define a data buffer separate from the mask buffer. Write a procedure that encrypts the character sequence in the data buffer by replacing each character with the result of XOR-ing that character with the character in the corresponding position of the mask buffer.
4. Write a procedure that reads character data from a previously existing input file and stores it in the data buffer. Use a text editor to create a test file and store a message in the file. Debug your code using td, making sure that your message is being correctly stored in memory.
5. Write a procedure that writes the characters in the data buffer to an output file. Debug your code to check that the message from the input file is transferred correctly to the output file when the output procedure is called after the input procedure from the preceding step.
6. Add console I/O to your program so that the user can specify the names of the input and output files. You may use appropriate functions from the Irvine library.
7. Combine the code from the above steps and debug it. The resulting program should prompt the user to provide the names of the input and output files; it should then read the data from the input file, encrypt it by XOR-ing each data character with the corresponding mask character, and write the encrypted version to the output file.
8. Test your program for various input files. Notice that the XOR masking method has the property that if the encrypted input is used as the new input for the same encryption scheme, then the original input is recovered. Make sure your program behaves in this way.

Deliverables

1. Submit a floppy disk containing the assembly source code and documentation (follow the WPI CS documentation standard described at http://www.cs.wpi.edu/Help/documentation-standard.html), together with two different examples of text files and their encryptions generated by your program. Give complete DOS command-line information needed to assemble and link your program using tasm and tlink.
2. Include detailed descriptions of all parameter values used, in particular the values of the various parameters used to generate the pseudorandom masking sequence in each case. Also include detailed instructions allowing the TA to feed an input file containing a brief message such as "Assembly rules!" to your program and to inspect both the encrypted and decrypted versions of the message in the respective output files.
3. Explain why the sequences produced by your pseudorandom number generator aren't really "random". If someone showed you a long sequence of characters generated using a linear congruence generator as described below, how could you tell that the sequence was produced by a non-random process? Explain carefully.
4. Submit hardcopies of the above items.

Background Information

XOR encoding

(x XOR y) XOR y = x.

Why? Just check all the cases. The y bit is either 0 or 1. Suppose y is 0. In this case, x XOR y = x XOR 0 = x by the XOR truth table, and XOR-ing with y again we get x again as claimed. Now suppose y is 1. Then x XOR y = x XOR 1 = ~x (the negation, or toggle, of x). XOR-ing this with y just toggles the result back to x. So (x XOR y) XOR y = x in all cases.

The above XOR formula might seem like a complicated way of doing nothing, since we get x back again at the end. But suppose that we only know the intermediate result x XOR y. Then, unless we know the value of y, we can't recover x. So we have an encryption method. Keep y in a safe place. Use y to XOR an incoming bit x. Store x XOR y, but don't reveal y. When you need x again, whip out y and XOR the stored result to recover x.

Pseudorandom sequences via the linear congruence method

newseed = remainder in the integer division (multiplier*seed + increment) / modulus

Many different values of the defining parameters may be considered. For starters try:

• multiplier = 40
• increment = 3641
• modulus = 729
• seed = 1

File I/O

function 3Dh to open file
function 3Fh to read from file
function 40h to write to file
function 3Eh to close file