Question

I’m trying to create a long string that is produced out of encrypted substrings. For the encryption I’m using AES128 and libmcrypt. The code is working, but I get a shorter output then I should and a beeping sound. I guess it’s because I’m using strlen, but I have no idea, how I can avoid that. I will be very grateful for some suggestions. Here is my code:

char *Encrypt( char *key, char *message){   
    static char *Res;
    MCRYPT mfd;
    char *IV;
    int i, blocks, key_size = 16, block_size = 16;
    blocks = (int) (strlen(message) / block_size) + 1;

    Res = calloc(1, (blocks * block_size));

    mfd = mcrypt_module_open(MCRYPT_RIJNDAEL_128, NULL, "ecb", NULL);
    mcrypt_generic_init(mfd, key, key_size, IV);

    strncpy(Res, message, strlen(message));
    mcrypt_generic(mfd, Res, block_size);
    //printf("the encrypted %s\n", Res);

    mcrypt_generic_deinit(mfd);
    mcrypt_module_close(mfd);

     return (Res);
}

char *mkline ( int cols) {
    int j;
    char seed[] = "thesecretmessage", key1[]="dontusethisinput", key2[]="abadinputforthis";

    char *encrypted, *encrypted2, *in = malloc(cols * 16);
    encrypted = Encrypt(key1, seed);
    sprintf(in, "%s", encrypted);
    encrypted2= Encrypt(key2, encrypted);
    printf("encrypted2 before for-loop %s\n", encrypted2);
    printf("encrypted2 before for loop len %d\n", strlen(encrypted2));
    for (j=1; j<cols; j++) {
        strcat(in, encrypted2);
        memmove(encrypted2, Encrypt(key2, encrypted2),strlen(seed));
        printf("encrypted2 %s on position %d\n" , encrypted2,j);
        printf("encrypted2 len %d\n", strlen(encrypted2));
    }
    free(encrypted);
    free(encrypted2);
    return in;
}

int main(int argc, char *argv[]) {
    char *line = mkline(15);
    printf("line %s\n", line);
    printf("line lenght %d\n", strlen(line));
    return 0;
}

Answer 1

You get the beep sound because you are printing control character.

Also strlen return the size until the first ‘\0’ character (because strings are zero terminated). That’s why you get length less than you expect since the encrypted message may contain zeroes.

You can do something like this to return the result length:

 char *Encrypt(const char *key, const char *message, int *result_len)
 {
   *result_len = blocks * block_size;
 }

Also

memmove(encrypted2, Encrypt(key2, encrypted2),strlen(seed));

This line should produce a memory leak since every time you call Encrypt you call calloc (allocate new memory) which you need to free after you are done.

You probably should use memcpy, memmove is primarly used if there is a chance destination and source may overlap.