Vigenere cipher code

C
#1

I am trying to write an encryption code for the Vigenere cipher and I can’t work out what is wrong with my code - I think there is a problem with indexing into the key and wrapping, but I am not sure what:

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

int shift(char c);

int main(int argc, string argv[])
{
    // Checks to make sure user enters correct number of command-line arguments
    if (argc != 2)
    {
        printf("Usage: ./vigenere keyword\n");
        return 1;
    }
    else
    {
        // Checks that user has only entered letters as a second command-line argument
        for (int i = 0, n = strlen(argv[1]); i < n; i++)
        {    
            if (isalpha(argv[1][i]) == 0)
            {
                printf("Usage: ./vigenere keyword\n");
                return 1;
            }
            else
            {
                // Gets plaintext from user
                string s = get_string("plaintext: ");
                printf("ciphertext: ");
                                
                
                // Converts plain text to cipher text
                for (int l = 0, o = strlen(s); l < o; l++)
                {
                        // Get key for this letter
                        m = shift(s[l] % strlen(argv[1])); 
                        printf("%i\n", m);
                        if (islower(s[l]))
                        {
                            char c = (s[l] + m - 97) % 26 + 97;
                            printf("%i\n", m);
                            printf("%c", c);
                        }
                        else if (isupper(s[l]))
                        {
                            char c = (s[l] + m - 65) % 26 + 65; 
                            printf("%c", c);
                        }
                        else
                        {
                            printf("%c", s[l]);

                        }
                }
                printf("\n");
                return 0;
            }
        }
    }
}
    
int shift(char c)
    
{  
    // Converts alphabetical character to a numerical key
    if (islower(c))
    {
        c = c - 97;
    }
    else
    {
        c = c - 65;
    }
    return c;
}
#2

I have resolved this issue:

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

int shift(char c);

int main(int argc, string argv[])
{
    // Checks to make sure user enters correct number of command-line arguments
    if (argc != 2)
    {
        printf("Usage: ./vigenere keyword\n");
        return 1;
    }
    else
    {
        // Checks that user has only entered letters as a second command-line argument (key)
        for (int i = 0, n = strlen(argv[1]); i < n; i++)
        {
            if (isalpha(argv[1][i]) == 0)
            {
                printf("Invalid keyword\n");
                return 1;
            }
        }
        // Gets plaintext from user
        string s = get_string("plaintext: ");
        printf("ciphertext: ");
                
        string key = argv[1];
        int t = strlen(key);
                
        int m;
        // Converts plain text to cipher text
        for (int l = 0, o = strlen(s); l < o; l++)
        {
            // Obtains value of index for key for this letter
            m = (l % t); 

            // Checks if plaintext letter is alphabetical character 
            if (isalpha(s[l]) == 0)
            {
                char c = s[l];
                printf("%c", c);
            }
            else
            {
                if (islower(s[l]))
                {
                    char c = ((s[l] + shift(key[m]) - 97) % 26 + 97);
                    printf("%c", c);
                    m++;
                }
                else if (isupper(s[l]))
                {
                    char c = (s[l] + shift(key[m]) - 65) % 26 + 65; 
                    printf("%c", c);
                    m++;
                }
                
            }
                            

        }  

         } 

    printf("\n");
    return 0;
      }
    

        
    

    
int shift(char c)
    
{  
    // Converts alphabetical character to a numerical key
    if (islower(c))
    {
        c = c - 97;
    }
    else
    {
        c = c - 65;
    }
    return c;
}
#3

I think I replied to another version of this post, so do me a favor and delete that thread then go with this one.

So, as I said, open this in gdb like this:

gdb cipher

Then you can use gdb to set break points at key places. You can also print things out with printf to make sure they’re changing and doing what you suspect. I suspect the lines where you use shift() are wrong.