Algorithm bugs that encrypt text

Question

I’m writing an algorithm that encrypts text using a word as a password, Vigenère cipher. Upper and lower case characters should encrypt, special characters and numbers should be ignored. My questions:

• When running the program, enter the password and later the text, error occurs Segmentation failure (recorded core image). For what?
• The code is not working, how to improve it?

code:

#include<cc50.h> // BIBLIOTECA DO CURSO QUE ESTOU FAZENDO.
#include<string.h>
#include<stdio.h>

int
main(int argc, char argv[])
{

    if (argc != 2)
    {
        printf("Erro 1. Digite uma palavra na linha de comando.\n");
        return 1;
    }

    printf("Texto a ser criptografado:\n");
    string texto = GetString();

    int k = 0;
    int l = strlen(texto);
    int m = strlen(argv);

    for (int i = 0, j = 0; i < l; i++)
    {
        k = atoi(argv[j]);
        if (j > m) // SE O CONTADOR J FOR MAIOR QUE A QUANTIDADE DE CARACTERES DA SENHA, REDEFINE J e K PARA 0.
            {
            j = 0;
            k = 0;
            }
        else if (texto[i] >= 65 && texto[i] <= 90)
            {
            texto[i] = (((texto[i] - 65) + k) % 26) + 65;
            j++;
            }
        else if (texto[i] >= 97 && texto[i] <= 122)
            {
            texto[i] = (((texto[i] - 97) + k) % 26) + 97;
            j++;
            }
        else;
        printf("%c", texto[i]);
    }

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

It doesn’t have to be a great answer, small tips that guide me the solution serves, because I’m already a few weeks stuck to this problem.

Answer 1

You are not going through every letter of the password, but trying to convert the entire password into a number. The line:

    k = atoi(argv[j]);

should be:

    k = (int)argv[1][j]; // Caractere j do argumento 1
                         // Não use atoi, pois o que você quer é o valor do caractere em si

The mistake is because, like the size of argv is 2, the third iteration of the loop attempts to read a memory position that goes beyond the limits of the array, so it gives the segmentation failure.

In the same way, the line:

int m = strlen(argv);

should be:

int m = strlen(argv[1]); // O tamanho da primeira palavra

Furthermore, your code seems correct. Note however that your k is not a number of 0 to 26 (as in the original Vigenère cipher), but rather the value of the character itself. This brings the collateral effect of the cipher being case sensitive (the original does not differentiate capitalization). If you want to do it in the standard way, you will need to normalize the value of k before using it. Example:

    k = (int)argv[1][j];
    if ( 65 <= k && k <= 90 )
        k -= 65;
    else if ( 97 <= k && k <= 122 )
        k -= 97;
    else
        k = 0; // Caractere inválido na senha, ignore

P.S. As pointed out by Isac in the comments, the way you are declaring argv is also incorrect - is to be an array of strings, not a single string.