vigenere ciphering formula

Question

my vigenere program isn't doing quite what it's supposed to so I was wondering if anyone could tell me if my formula is correct. I don't need the answer, just which part (if any) of the code I post is wrong. Thanks!

for (int i = 0, pl = strlen(plaintext), kl = strlen(argv[1]), index = 0 ; i < pl; i++)
{
    char  current = plaintext[i];

        if (islower(current))
        {
            int k = argv[1][index % kl] - 97;
        current = (current + k) % 26;
        current = current + 97;

        }

Answer 1

First, you should consider ciphering uppercase characters too!

Second, the formula assumes that each of your alphabetical characters has a value in [0, 25].
For example, when ciphering, the letter A should be equivalent to 0, the letter B should be equivalent to 1 ... Z should be equivalent to 25.
Similarly, a should be equivalent to 0, b should be equivalent to 1 ... z should be equivalent to 25.

At the same time, after ciphering, each character should have its original ASCII value back.
For example, if the original char was ciphered to A (i.e., 0), it should become 65. If, rather, it was ciphered to B (i.e., 1), it should become 66 and so on.

Same thing applies for lowercase chars.

Third, the ciphering formula per the pset specification page is

c = (p + k) % 26

Where c is the ciphered char, p is the current plain char in [0, 25] and k is the current key. And since k must be an alphabetical char, it has to be in [0, 25] either.

---

What you're doing wrong is

1. You're never getting the value of your plain char (i.e., current) to be in [0, 25].

2. You're not considering uppercase chars either in the plaintext or in the keyword.

3. You're adding 97 to current at the end without subtracting 97 from it at the first place.

Answer 2

int k = argv[1][index % kl] - 97;

What if argv[1][0] is 'B'? Do you want k to be ('B' - 97)?

Shouldn't you be zero-basing your key? So that 'A' = 0? 'B' = 1? etc..

Your code will have 'A' = -32. Make sure you consider that the argv[1] can include both upper and lower case characters.