Pset2 vigenere formula

Question

My formula for vigenere is not quite right, and I'm not sure which part is wrong, could someone leave me some bread crumbs please

int j = 0;
int upperk = key[j % klen] - 'A';
int lowerk = key[j % klen] - 'a';

for (int i = 0, l = strlen(plaintext); i < l; i++)
{
    if (isupper(plaintext[i]))
        {
            printf("%c", (plaintext[i] - 'A' + upperk) % 26 + 'A');
            j++;
        }
    else if (islower(plaintext[i]))
        {
            printf("%c", (plaintext[i] - 'a' + lowerk) % 26 + 'a');
            j++;

Answer 1

The code assumes that the case of the plaintext is the same as the case of the key. What happens when the plaintext is lowercase and the key is uppercase or vice versa? It breaks.

Calculating both upperk and lowerk is a novel approach (I kinda like it), but wouldn't it be easier to simply convert the key letter to a value between 0 and 25 inclusive and use that value later? Hint: look up the man page for toupper and tolower.

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