Problem 2-6: Vigenère

This course’s philosophy on academic honesty is best stated as "be reasonable." The course recognizes that interactions with classmates and others can facilitate mastery of the course’s material. However, there remains a line between enlisting the help of another and submitting the work of another. This policy characterizes both sides of that line.

The essence of all work that you submit to this course must be your own. Collaboration on problems is not permitted (unless explicitly stated otherwise) except to the extent that you may ask classmates and others for help so long as that help does not reduce to another doing your work for you. Generally speaking, when asking for help, you may show your code or writing to others, but you may not view theirs, so long as you and they respect this policy’s other constraints. Collaboration on quizzes and tests is not permitted at all. Collaboration on the final project is permitted to the extent prescribed by its specification.

Below are rules of thumb that (inexhaustively) characterize acts that the course considers reasonable and not reasonable. If in doubt as to whether some act is reasonable, do not commit it until you solicit and receive approval in writing from your instructor. If a violation of this policy is suspected and confirmed, your instructor reserves the right to impose local sanctions on top of any disciplinary outcome that may include an unsatisfactory or failing grade for work submitted or for the course itself.

Your work on this problem set will be evaluated along four axes primarily.

To obtain a passing grade in this course, all students must ordinarily submit all assigned problems unless granted an exception in writing by the instructor.

We’d recommend you get started by watching a few short videos; in particular, these three on functions, command-line arguments, and the Vigenère cipher. If you happen to see (and are confused by!) char * in these and other shorts, know for now that char * simply means string. But more on that soon!

Be sure you’re reasonably comfortable answering the below questions before moving too far!

Log into your CS50 IDE workspace and execute

within a terminal window to make sure your workspace is up-to-date. If you somehow closed your terminal window (and can’t find it!), make sure that Console is checked under the View menu, then click the green, circled plus (+) in CS50 IDE’s bottom half, then select New Terminal. If you need a hand, do just ask via the channels noted at the top of this specification.

Now execute

to move yourself into (i.e., open) that directory. Your prompt should now resemble the below.

If so, you’re ready to go!

If you recall from Problem 2-5, the Caesar cipher was hardly secure. Fortunately, Nate’s short on Vigenère’s cipher, there’s a more sophisticated algorithm out there. Suffice it to say it’s French, per http://en.wikipedia.org/wiki/Vigen%C3%A8re_cipher. Though do not be mislead by the article’s discussion of a tabula recta. Each c_i can be computed with relatively simple arithmetic! You do not need a two-dimensional array.

Vigenère’s cipher improves upon Caesar’s by encrypting messages using a sequence of keys (or, put another way, a keyword). In other words, if p is some plaintext and k is a keyword (i.e., an alphbetical string, whereby A and a represent 0, while Z and z represent 25), then each letter, c_i, in the ciphertext, c, is computed as:

c_i = (p_i + k_j) % 26

Whoa! Well… actually, that’s not too different from the formula for Caesar’s cipher, right? Note this cipher’s use of k_j as opposed to just k. And recall that, if k is shorter than p, then the letters in k must be reused cyclically as many times as it takes to encrypt p.

Your goal for this problem is to write, in vigenere.c, a program that encrypts messages using Vigenère’s cipher. This program must accept a single command-line argument: a keyword, k, composed entirely of alphabetical characters. If your program is executed without any command-line arguments, with more than one command-line argument, or with one command-line argument that contains any non-alphabetical character, your program should complain and exit immediately, with main returning 1 (thereby signifying an error that our own tests can detect). Otherwise, your program must proceed to prompt the user for a string of plaintext, p, which it must then encrypt according to Vigenère’s cipher with k, ultimately printing the result and exiting, with main returning 0.

As for the characters in k (the keyword), you must treat A and a as 0, B and b as 1, … , and Z and z as 25. In addition, your program must only apply Vigenère’s cipher to a character in p if that character is a letter. All other characters (numbers, symbols, spaces, punctuation marks, etc.) must be outputted unchanged. Moreover, if your code is about to apply the j^th character of k to the i^th character of p, but the latter proves to be a non-alphabetical character, you must wait to apply that j^th character of k to the next alphabetical character in p; you must not yet advance to the next character in k. Finally, your program must preserve the case of each letter in p. (Uppercase letters in p must remain uppercase in the output, etc.)

Not sure where to begin? As luck would have it, this program’s pretty similar to caesar! In fact, you may wish to use caesar as a starting point, as by way of:

This time though, you need to decide which character in k to use as you iterate from character to character in p. Perhaps this could be an opportunity for you to break some of your code into functions, allowing you to repurpose some of what you’ve previously done to make the amount of work you have to do with this problem slightly less?

Here’s Zamyla with some tips:

It’s quite likely you’ll find some handy functions documented at https://reference.cs50.net/ under ctype.h and stdlib.h. For instance, isalpha seems promising for checking out that k consists only of alphabetic characters. And, with regard to wrapping around from Z to A (or z to a) when necessary, don’t forget about %, C’s modulo operator. You might also want to check out http://asciitable.com/, which reveals the ASCII codes for more than just alphabetical characters, just in case you find yourself printing some characters accidentally.

So that we can automate some tests of your code, your program must behave per the below; highlighted in bold are some sample inputs.

How to test your program, besides predicting what it should output, given some input? Well, recall that we’re nice people. And so we’ve written a program called devigenere that also takes one and only one command-line argument (a keyword) but whose job is to take ciphertext as input and produce plaintext as output.

To use our program, execute

at your prompt, where k is some keyword. Presumably you’ll want to paste your program’s output as input to our program; be sure, of course, to use the same key. Note that you do not need to implement devigenere yourself, only vigenere.

If you’d like to check the correctness of your program with check50, you may execute the below.

And if you’d like to play with the staff’s own implementation of vigenere, you may execute the below.

This was Problem 2-6.