Submit: Turn in your codebreaker.c source file using the turnin command on morbius.mscsnet.mu.edu or one of the other Systems Lab machines.

Work is to be completed individually. Be certain to include your name in the file. You may submit multiple times, but only the last turnin will be kept. The automatic submission system will not accept work after the deadline. (See turnin instructional video HERE. Note that the video is for a different course number, and that Morbius's domain name has changed since the video was made: morbius.mscs.mu.edu -> morbius.mscsnet.mu.edu)

Include a comment block at the very top of each file that contains the following:

/**
 * COSC 3250 - Project #
 * Explain briefly the functionality of the program.
 * @author [your name]
 * Instructor [your instructor]
 * TA-BOT:MAILTO [your email address]
 */


Ring of Xinu

Unbeknownst to Lord Xinu (Dark Lord of the Mips), his super-secret, Wormhole X-Treme! cereal box decoder is actually a pretty lousy secret message channel. (Lord Xinu's specialty is embedded operating systems, not cryptography.)

Thwart Lord Xinu's insidious scheme to commandeer the world's home networking appliances by building a program that will crack his decoder ring's code.

Simple Cryptanalysis

The downfall of Lord Xinu's decoder ring scheme is that there are effectively only 27 possible key values. You can use any number as the key, but after the first modulus is taken, you're back to 27 real possibilities.

Code breakers often begin attacking such a weak cipher by trying all of the possibilities, hoping one of the results will obviously be the original message.

How will our program recognize when we've found the right key? Many algorithms exist, but one common technique is to examine the frequency of letters in the deciphered text. Written English has a known profile of letter frequencies. Let us assume that the correct key will result in the deciphered text that results in the most occurrences of the letter 'E'. This is a very close pass, but actually gives us the wrong key for most test inputs -- it gives us the incorrect key value that happens to decipher space characters from the original in the letter 'E'. Instead, we want the key that gives us the next largest number of E's in the deciphered text.

Construct a program that takes ciphertext output from the encoder-ring program, tries decoding it with all 27 possible keys, and outputs the key value that resulted in the second largest number of E's, followed by the deciphered text.

We have provided an example program for your reference, executable on Morbius as ~brylow/os/Projects/codebreaker.

Notes

• Unlike the first project, this project does not require command-line arguments. Unlike the first project, which could handle an arbitrary length text to encipher, your code breaker is only required to handle message lengths of up to 1024 characters.
• Whereas the first project could be implemented without use of arrays, this project will require them. You can implement this program using only statically-allocated arrays. That is, you need not use pointers and dynamic memory allocation at this point.
• This simple code breaker is by no means perfect -- it is only likely to work on longer messages in which 'E' is indeed the most common letter. A more elaborate crypt analysis engine would look at the relative frequencies of other letters, and probably search for common combinations of letter in English, such as "the", "of", or "is".
• In the case of a tie, your code breaker should return the lowest key value that produces the maximum number of E's.
• For reference, your assignments will be compiled and graded on Morbius.
• The Systems Lab (CU 310) is the primary development environment for COSC 3250. It is expected that you will learn to use the required tools for developing, testing, and submitting your projects. The Systems Lab machines are available locally in Cudahy 310, and can be accessed remotely via ssh using your CS Department credentials. (See D2L Laboratory Instructions for instructions for learning your credentials.) Free secure shell (ssh) clients are readily available for other platforms, including Windows and Mac at home or in the dorms. We recommend the built-in ssh client in found in Terminal for MacOS users, or in the Cmd.exe command interpreter in modern Windows. If you are fortunate enough to already have a Linux client at home, ssh is likely installed by default.
• The Systems Lab machines run the Linux operating system. If you need a refresher on using the UNIX environment, please see the UNIX Tutorial.
• The reference implementation is considered an integral part of the specifications for this assignment. Project grades will be based in significant part upon an automated comparison of behavior against the reference implementation. Devise test cases to discover the expected behavior, and do your best to match it precisely. Creativity will be valued in later assignments, but first one must master the tools.
• Use the electronic turnin system to submit your work. The command is "turnin", followed by the name(s) of files(s) you wish to submit. The command "turnin -v" will print a listing verifying the contents of your submission. See "turnin -h" for a list of other relevant options. The system does not accept work after the deadline, and neither does the professor.

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[Revised 2022 Feb 02 12:52 DWB]