How to debug a Vigenere cipher in C?

Question

I am trying to make a Vigenere cipher. My problem is that I'm not getting the expected output. When running the program it gives this output: HFNLP WPTLE. The correct output should be: HFNLP YOSND.

I think that the problem resides in the bad use of modulo (mod). When I try to wrap around the key (ABC) with the variable i, the space (" ") in the plainText also wraps, affecting directly the result of the wrapping. I don't what to do in order to get the right output.

string plainText = "HELLO WORLD";   
string keyword =   "ABC";

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

    int wrap =  (int) strlen( keyword) % (int) strlen(plainText);

     if(isalpha(plainText[i]))
     {

     int upper = 'A' + (plainText[i] + (toupper(keyword[i % wrap]))) % 26;
     printf("%c", upper);

     }

Answer 1

Index of keys on non-alphabetic characters must not increase.

An example of fix:

char *keyp = keyword;
char ch;
for(int i = 0; ch = plainText[i]; i++){  
    if(isalpha(ch)){
        putchar('A' + (toupper(ch) - 'A' + toupper(*keyp++) - 'A') % 26);
        if(!*keyp)
            keyp = keyword;
    } else
        putchar(ch);
}