Encriptar/Desencriptar con C

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

main(int argc, char **argv)
{
    int i;
    char *string;
    char encrypted_string[strlen(string)];
    char decrypted_string[strlen(string)];
    char *key_ch;
    char key_int;

    string = argv[1];
    key_ch = argv[2];
    key_int = atoi(key_ch);

    if (strcmp(argv[3], "encrypt") == 0)
    {
        i = 0;
        while(i <= strlen(string)-1)
        {
            encrypted_string[i] = string[i]+ key_int;
            i++;
        }

        printf("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: ");
        i = 0;
        while(i <= strlen(string)-1)
        {
            printf("%c", decrypted_string[i]);
            i++;
        }

        printf("\n");
    }
}