Problem: Vigenère


Implement a program that encrypts messages using Vigenère’s cipher, per the below.

$ ./vigenere ABC
plaintext:  HELLO
ciphertext: HFNLP

Academic Honesty

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.


  • Communicating with classmates about problems in English (or some other spoken language).

  • Discussing the course’s material with others in order to understand it better.

  • Helping a classmate identify a bug in his or her code, such as by viewing, compiling, or running his or her code, even on your own computer.

  • Incorporating snippets of code that you find online or elsewhere into your own code, provided that those snippets are not themselves solutions to assigned problems and that you cite the snippets' origins.

  • Reviewing past years' quizzes, tests, and solutions thereto.

  • Sending or showing code that you’ve written to someone, possibly a classmate, so that he or she might help you identify and fix a bug.

  • Sharing snippets of your own solutions to problems online so that others might help you identify and fix a bug or other issue.

  • Turning to the web or elsewhere for instruction beyond the course’s own, for references, and for solutions to technical difficulties, but not for outright solutions to problems or your own final project.

  • Whiteboarding solutions to problems with others using diagrams or pseudocode but not actual code.

  • Working with (and even paying) a tutor to help you with the course, provided the tutor does not do your work for you.

Not Reasonable

  • Accessing a solution to some problem prior to (re-)submitting your own.

  • Asking a classmate to see his or her solution to a problem before (re-)submitting your own.

  • Decompiling, deobfuscating, or disassembling the staff’s solutions to problems.

  • Failing to cite (as with comments) the origins of code, writing, or techniques that you discover outside of the course’s own lessons and integrate into your own work, even while respecting this policy’s other constraints.

  • Giving or showing to a classmate a solution to a problem when it is he or she, and not you, who is struggling to solve it.

  • Looking at another individual’s work during a quiz or test.

  • Paying or offering to pay an individual for work that you may submit as (part of) your own.

  • Providing or making available solutions to problems to individuals who might take this course in the future.

  • Searching for, soliciting, or viewing a quiz’s questions or answers prior to taking the quiz.

  • Searching for or soliciting outright solutions to problems online or elsewhere.

  • Splitting a problem’s workload with another individual and combining your work (unless explicitly authorized by the problem itself).

  • Submitting (after possibly modifying) the work of another individual beyond allowed snippets.

  • Submitting the same or similar work to this course that you have submitted or will submit to another.

  • Using resources during a quiz beyond those explicitly allowed in the quiz’s instructions.

  • Viewing another’s solution to a problem and basing your own solution on it.


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


To what extent is your code consistent with our specifications and free of bugs?


To what extent is your code written well (i.e., clearly, efficiently, elegantly, and/or logically)?


To what extent is your code readable (i.e., commented and indented with variables aptly named)?

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.

Starting Shortly

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!

  • What’s a function?

  • Why bother writing functions when you can just copy and paste code as needed?

  • What makes Vigenère’s cipher more secure than Caesar’s?

  • What does undeclared identifier usually indicate if outputted by make (or, really, clang)?

Now execute

cd ~/workspace/unit2

to move yourself into (i.e., open) that directory. Make a new directory called vigenere, and you’re good to go!


Vigenère’s cipher improves upon Caesar’s cipher 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 represents 0, B represents 1, C represents 2, …​, and Z represents 25), then each letter, ci, in the ciphertext, c, is computed as:

\[c_i = (p_i + k_j) \bmod 26\]

Note this cipher’s use of kj as opposed to just k. And if k is shorter than p, then the letters in k must be reused cyclically as many times as it takes to encrypt p.

In other words, if Vigenère himself wanted to say HELLO to someone confidentially, using a keyword of, say, ABC, he would encrypt the H with a key of 0 (i.e., A), the E with a key of 1 (i.e., B), and the first L with a key of 2 (i.e., C), at which point he’d be out of letters in the keyword, and so he’d reuse (part of) it to encrypt the second L with a key of 0 (i.e., A) again, and the O with a key of 1 (i.e., B) again. And so he’d write HELLO as HFNLP.

Table 1. Encrypting HELLO with a keyword of ABC (reused cyclically as ABCAB) yields HFNLP.







+ key











= ciphertext







Design and implement a program that encrypts messages using Vigenère’s cipher.

  • Implement your program in a file called vigenere.c in a directory called vigenere.

  • Your 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 print an error (of your choice) and exit immediately, with main returning 1 (thereby signifying an error).

  • Otherwise, your program must proceed to prompt the user for a string of plaintext, p, (as by a prompt for plaintext:) which it must then encrypt according to Vigenère’s cipher with k, ultimately printing the result (prepended with ciphertext: and ending with a newline) and exiting, with main returning 0.

  • With respect to the characters in k, you must treat A and a as 0, B and b as 1, …​ , and Z and z as 25.

  • 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 jth character of k to the ith character of p, but the latter proves to be a non-alphabetical character, you must wait to apply that jth character of k to the next alphabetical character in p; you must not yet advance to the next character in k.

  • Your program must preserve the case of each letter in p.



Your program should behave per the examples below. Assumed that the underlined text is what some user has typed.

$ ./vigenere 13
Usage: ./vigenere k
$ ./vigenere
Usage: ./vigenere k
$ ./vigenere bacon and eggs
Usage: ./vigenere k
$ ./vigenere bacon
plaintext: Meet me at the park at eleven am
ciphertext: Negh zf av huf pcfx bt gzrwep oz


To help you test vigenere, we’ve written a program called devigenere for you 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

~cs50/unit2/devigenere k

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.


check50 cs50/2018/ap/vigenere


style50 vigenere.c

Staff’s Solution



Not sure where to begin? As luck would have it, this program’s pretty similar to caesar! Only this time, you need to decide which character in k to use as you iterate from character to character in p.

How to Submit

Step 1 of 3

Execute update50 again to ensure that your IDE is up-to-date.

Step 2 of 3

  • Recall that you were asked to implement vigenere

    • Be sure that vigenere.c is in ~/workspace/unit2/vigenere/, as with:

      cd ~/workspace/unit2/vigenere/

Step 3 of 3

  • To submit vigenere, execute:

    cd ~/workspace/unit2/vigenere/
    submit50 cs50/2018/ap/vigenere

If you run into any trouble, email!

You may resubmit any problem as many times as you’d like.

Your submission should be graded for correctness within 2 minutes, at which point your score will appear at!

This was Vigenère.