Vigenere Cipher Black Hawk Down

Question

I cannot figure out why this thing doesn't scramble correctly. I read some other posts on this cipher and as far as I can tell I'm using the exact same algorithm as they are...

The areas commented out are tests I tried to make sure everything was passing through correctly. I believe it all goes through correctly then fails in the algorithm.

#include <stdio.h>
#include <cs50.h>
#include <string.h>
#include <ctype.h>
#include <stdlib.h>

string get_message(void);
string scramble(string key, string message);

int main(int argc, string argv[])
{
    if(argc == 2)
    {
        string key;
        string message;

        key = argv[1]; 
        //printf("KEY: %s<<",key); 
        message = get_message();
        scramble(key, message);
    }
    else
    {
        printf("Please enter 2 arguments.\n");
        return 1;
    }
}

string get_message(void)
{   
    string message = "";
    do
    {
        message = GetString();
    }
    while(strlen(message) < 1);
    return message;
}

string scramble(string key,string message)
{       
    for(int i = 0, len = strlen(message), key_len = strlen(key); i < len; i++)
    {   
        int letter = message[i];
        //int Tkey = atoi(key[i % key_len]); 
        //printf("KEY: %d<<\n",Tkey);

        if(islower(letter))
        {
            //printf("KEY(%d)",(key[i % key_len]-97));
            letter = (((letter - 97) + (key[i % key_len])) % 26 + 97);
            //printf("(%d)",(letter - 97) + (key[i % key_len])%26);
            printf("%c",letter);
        }       
        else if(isupper(letter))
        {
            //printf("(%d)", key[i]);
            //printf("(%c)",letter); WE HAVE CORRECT LETTER
            letter = (((letter - 65) + (key[i % key_len])) % 26 + 65);
            printf("%c",letter);
        }
    }
    printf("\n");
    return 0;
}

Answer 1

I think your calculation is wrong:

You currently have

encryptedLetter = (letter - firstLetterOffset) + key[position % keyLength] % 26 + firstLetterOffset

by check the C operator precedence table we notice that % is evaluated before - or +, meaning that your code actually mean :

encryptedLetter = (letter - firstLetterOffset) + ( key[position % keyLength] % 26 ) + firstLetterOffset

Where you wanted :

encryptedLetter = ( (letter - firstLetterOffset) + key[position % keyLength] ) % 26 + firstLetterOffset

Conclusion : you need to put more parenthese to specify in which order you which to evaluate your expression.

---

In addition you took the letter number for the text character but not for the key !

Correct expression

encryptedLetter = ( (letter - firstLetterOffset) + key[position % keyLength] - firstLetterOffset ) % 26 + firstLetterOffset

Demonstration in javascript