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Secret Decoder Ring

Submit: Turn in two C source files called "encoder-ring.c" and "decoder-ring.c" using the turnin command on the Systems Lab computers, as noted below.

Points for this project will be assigned primarily based upon automated testing of your submitted source code. It is imperative, therefore, that you name your files properly as directed above, (no changes in name, spelling, or capitalization,) and that your programs are ready to be compiled and run using gcc on Morbius. You should make liberal use of debugging output during development, but your submitted version should not be embellished with unspecified output such as additional prompts or commentary.

This assignment is to be completed individually.

Ring of Xinu

Whenever Lord Xinu (Dark Lord of the Mips) is communicating with his minions over long distances, he uses his super-secret, Wormhole X-Treme! cereal box decoder ring to properly encipher his messages.

Your task is to write two simple programs that automate the process of enciphering and deciphering messages without relying on the cheap plastic decoder ring.

You should use the getchar library function to read console input one character at a time. The use of any other library functions for input is not recommended at this time.

We have provided example programs for your reference, executable on Morbius as ~brylow/os/Projects/encoder-ring and ~brylow/os/Projects/decoder-ring.

Simple Autokey Cipher

Autokey ciphers are a class of polyalphabetic cipher algorithms that incorporate the text of the plaintext message into the cipher key.

Envision our decoder ring as a disk with the 26 upper-case alphabet letters arrayed around its edge in order, with a 27^th place between 'Z' and 'A' containing a space. Each of the 27 slots has a number associated with it: the space is 0, 'A' is 1, 'B' is 2, and so on, up to 'Z' = 26.

To encipher a message, Lord Xinu first chooses a positive integer key value. His minions will use the same key value later to decode the enciphered message. Next, he adds the key value to the value of the first letter of his message, which is equivalent to counting that many slots around the edge of the decoder ring. The symbol he lands on will be the first character he sends as his encrypted message.

If Lord Xinu followed this same algorithm -- adding the key value to each letter in turn -- he would have what is called a shift cipher, such as the infamous Caesar cipher or ROT13. This is too easy to break; to make this a more difficult autokey cipher, he instead adds the next letter value and key to his current position on the decoder ring.

For example:

Assume a key value of zero, and a message of "Hello world!". The first letter 'H' has value 8. The key is 0, and there is no previous value, so 'H' will be encoded as 'H' (8 + 0 + 0 = 8 = 'H'). The second letter is 'e', which has value 5. The previous value plus 'e' plus the key is 'M' (8 + 5 + 0 = 13 = 'M'.) The third letter of the message is 'L'. (Note that we immediately convert lower-case letters to upper-case for this code.) Previous value 13 plus 'L' (12) plus key (0) = 25 = 'Y'. The fourth character is also 'L'. Previous value (25) plus 'L' (12) plus key (0) = 37, which when wrapped around the decoder ring (modulo 27) is 10, or 'J'.

The enciphered result for "Hello world!" is thus "HMYJYYUI LPP" with a key of 0, or "IOANCDAQIV A" with a key of 1, or "JQDRHJHYREKM" with a key of 2, and so on.

Newline characters ('\n') are transmitted unencoded. Note that we treat spaces -- and all other non-alphabetic characters -- as value 0. They will be decoded as blank spaces.

This assignment can be completed using simple loop and conditional constructs, along with basic integer operations like addition and modulus. Your encoder-ring and decoder-ring programs should take an optional key value as a command-line argument. (See textbook section 5.10.) If no argument is given, the key is assumed to be 0. You may use the C library function atoi (textbook section B5), or parse the command-line integer yourself. The encoder-ring reads in plaintext messages line by line and outputs ciphertext encrypted with the specified key. The decoder-ring reads in ciphertext as input and outputs the plaintext message decrypted with the specified key.

Notes

• The Systems Lab (CU 310) is the primary development environment for COSC 3250 and COEN 4820. It is expected that you will learn to use the required tools for developing, testing, and submitting your projects. The are available locally in Cudahy 310, and can be accessed remotely via ssh using your MSCS password. Free secure shell (ssh) clients are readily available for other platforms, including Windows machines at home or in the dorms. See How to Get and Use Secure Shell (SSH)
• The run the Linux operating system. If you need a refresher on using the UNIX environment, please see the UNIX Tutorial. See also How to Get and Use Secure Shell (SSH), How to Run an X-Windows Session, or Brief UNIX intro
• This project can seem deceptively complex, but is quite tractable if you first take the time to design suitable helper functions, and test those function thoroughly before moving on to the overall project. We encourage the disciplined practice of test-driven development.
• 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.

We will return to encryption in Chapter 15 of the text in April.