Vigenere cipher coding drama!

Question

your help is appreciated and please feel free to critique my coding approach. My concern is, not only that the code isn't working, but that I maybe complicating this more than it needs to be.

Working on the Vigenere cipher, I have the following:

• In this block of code, I wanted to change the ascii value of each of the alpha keyword chars to its alpha index value, so I can use the new value when encrypting the plain text.

if (isalpha(keyword[k]))
    {
        for (int k = 0, n = strlen(keyword); k < n; k++)
        {   
            // test - printf("The char in my keyword is %c, int value is %d\n", keyword[k], keyword[k]);

            if (islower(keyword[k]))
            {
                keyword[k] = keyword[k] - 'a';
            }

            if (isupper(keyword[k]))
            {
                keyword[k] = keyword[k] - 'A';
            }
         }
    }
    else
    {
        printf("Enter an alphabetic keyword\n");
        return 1;
    }

• This is the code block that is supposed to encrypt the plaintext:

for (int i = 0, n = strlen(plainText); i < n; i++)

     {
        if (isalpha(plainText[i]))

             for (k = 0, n = (keyword[k] % strlen(keyword)); k < n; k++)

                if (islower(plainText[i]))
                {
                    cipherText = (((plainText[i] - 'a') + keyword) % 26 + 'a');
                    //printf("%c", cipherText);       
                }

                if (isupper(plainText[i]))
                {
                    cipherText = (((plainText[i] - 'A') + keyword) % 26 + 'A');
                    //printf("%c", cipherText);
                }

         printf("%c", cipherText);

         }          
         else
         {
            printf("%c", plainText[i]);
         }

It seems to me part of the problem is the way I'm using the variable keyword. I tried adding a new variable, like key, but that doesn't seem to work. Using printf statements as tests, I don't think the keyword is iterating as it should. Thanks in advance for your response!

Answer 1

While I haven't tested it, most of your code looks on target at first glance. However, you have a serious problem:

for (k = 0, n = (keyword[k] % strlen(keyword)); k < n; k++)

Your goal is to wrap back to the beginning of the keyword array (your key string) after you use the last element. Let's say that you have a 3 letter key. You want k to go from 0 to 1 to 2 and then back to 0, so you know to use the % modulo operator. BUT, what are you applying the modulo to? Look at it another way. Say you had a character array called foo. It contains "abc". That means foo[2] = 'c'. foo[2] % 3 translates to 'c' % 3 or 99 % 3 = 0.

By now, you probably realize that you need a variation of the current code. Try this:

for (k = 0, n = (keyword[k % strlen(keyword) ]  ); k < n; k++)

I'm not saying that everything will work, but that should get you rolling.

If this answers your question, please accept this answer to remove the question from the unanswered question pool. Let's keep up on forum housekeeping. ;-)