Thread: Encrypt/Decrypt Message in C

Hello. I am trying to make a C program that for argv 2 the user enters a word and for argv 3 the user MUST specify whether they want the program to decrypt or encrypt the message. Program compiles fine but when I try to run it, I am getting a "segmentation fault" any ideas?
Thank you

Code:

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


int main(int argc, char **argv)
{
    int i;
    const char string = argv[1];                                    //here I set the entered string for argv[2]
    const int length = strlen(string);


    char encrypted_string[length];                                    //length of encrypted value
    char decrypted_string[length];


    const char *key_ch = argv[2];
    char key_int = atoi(key_ch);




     if(argc < 3)                                            //argument checking
         {
            printf("Not enough arguments\n");
            exit (1);
            }


        
    if (strcmp(argv[3], "encrypt") == 0)                                //argv[3] sees if it is encrypt or decrypt
    {
        i = 0;
        while(i <= strlen(string)-1)
        {
            encrypted_string[i] = string[i] + key_int;
            i++;
        }
        
    //    printf("Encrypted string: ");                                //print encrypted string
        i = 0;
        while (i <= strlen(string) -1)
        {
            printf("%c", encrypted_string[i]);
            i++;
        }
        printf("\n");
    }
    if (strcmp(argv[3], "decrypt") == 0)
    {
        i = 0;
        while(i <= strlen(string) -1)
        {
            decrypted_string[i] = string[i] - key_int;
            i++;
        }
        
    //    printf("Decrypted String: ");                                //print decrypted string
        i = 0;
        while (i <= strlen(string) -1)
        {
            printf("%c", decrypted_string[i]);
            i++;
        }


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

If your program is "compiling fine" there's something terribly wrong! It should be spitting out numerous warnings. If you're using gcc you should use the following flags -W -Wall -pedantic.

Anyway, you're just missing the asterisk before "string" when you declare it. It should be a pointer, not just a char. Also, you need to make your arrays length+1 bytes long to account for the zero byte at the end of the string. Personally, I'd also make i a size_t (the return type of strlen, usually equivalent to an unsigned long) to remove any compiler complaints about comparing signed and unsigned integers (which gcc complains about with the -pedantic flag).