vigenere cipher formula

Question

I am having trouble with the cipher formula. I am working on just getting lowercase character right using the keyword "bacon" and plain text string "test". I get a result of "nxna". Any help is appreciated. Here is the relevant code.

for (int i = 0,  pl = strlen(plaintext), kl = strlen(keyword), index = 0; i < pl; i++)

{
    char current = plaintext[i];

    if (isalpha (current))
    {
        if (islower(current))

        current = (current - 'a' + keyword[index % kl]) % 26 + 'a';

            printf("%c" , current);

        index++;
    }

Answer 1

First of all, I got the result:
nxnaz instead of nxna (You missed a letter)

Second of all: What is wrong? Look at your code:
If the character is lower case. current = (current - 'a' + keyword[index % kl]) % 26 + 'a'; You seem to realize that you need to do current - 'a' because the ascii value for a is not '1' (or 0), but in fact 97.

However you forgot to do this for your keyword value.
Replace: keyword[index%k1]) % 26 with (keyword[index % kl] - 'a') % 26)

If you're converting your plaintext letter from ASCII to position in the alphabet, you should do so with the keyword letter too.

Answer 2

The current character in your keyword should have a value in [0 - 25] too. And it's preferable to always use braces to delimit the body of an if statement.