[Source] : encryptor challenge

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

int loccc(char *s, int n){
    div_t qr;
    int pos;

    if(n == 0) return 0;

    qr = div(n, 10);
    pos = loccc(s, qr.quot);
    s[pos] = qr.rem + '0';
    return pos + 1;
}

int crypt(char* filename){
    FILE *fp;
    FILE *fp2;
    int i;
    int sz;
    unsigned char k[500];
    int cc=2;
    unsigned char c[500]="";
    fp=fopen(filename, "r");
    fread(k, 500, 1, fp);
    fseek(fp, 0L, SEEK_END);
    sz = ftell(fp);
    fp2=fopen(filename, "w+");
    for (i=0; i<sz; i++){
        cc+=(cc^(i+3));
        c[i] = (k[i]+i) ^ (cc+i);
   }
   fwrite(c, 1, strlen(c)/1, fp2);
   fclose(fp);
   fclose(fp2);
}

char *str_replace(char *orig, char *rep, char *with) {
    char *result;
    char *ins;
    char *tmp;
    int len_rep;
    int len_with;
    int len_front;
    int count;

    if (!orig || !rep)
        return NULL;
    len_rep = strlen(rep);
    if (len_rep == 0)
        return NULL;
    if (!with)
        with = "";
    len_with = strlen(with);

    ins = orig;
    for (count = 0; tmp = strstr(ins, rep); ++count) {
        ins = tmp + len_rep;
    }

    tmp = result = malloc(strlen(orig) + (len_with - len_rep) * count + 1);

    if (!result)
        return NULL;

    while (count--) {
        ins = strstr(orig, rep);
        len_front = ins - orig;
        tmp = strncpy(tmp, orig, len_front) + len_front;
        tmp = strcpy(tmp, with) + len_with;
        orig += len_front + len_rep;
    }
    strcpy(tmp, orig);
    return result;
}
char** str_split(char* a_str, const char a_delim)
{
    char** result    = 0;
    size_t count     = 0;
    char* tmp        = a_str;
    char* last_comma = 0;
    char delim[2];
    delim[0] = a_delim;
    delim[1] = 0;

    while (*tmp)
    {
        if (a_delim == *tmp)
        {
            count++;
            last_comma = tmp;
        }
        tmp++;
    }

    count += last_comma < (a_str + strlen(a_str) - 1);

    count++;

    result = malloc(sizeof(char*) * count);

    if (result)
    {
        size_t idx  = 0;
        char* token = strtok(a_str, delim);

        while (token)
        {
            assert(idx < count);
            *(result + idx++) = strdup(token);
            token = strtok(0, delim);
        }
        assert(idx == count - 1);
        *(result + idx) = 0;
    }

    return result;
}
char* ctyhh(char *output_buff, int num){
    char *p = output_buff;
    if(num < 0){
        *p++ = '-';
        num *= -1;
    } else if(num == 0)
        *p++ = '0';
    p[loccc(p, num)]='\0';
    return output_buff;
}
int main(int argc, char const *argv[])
{
    char *what=malloc(500);
here:
    printf("\nwhat you want to do ? ");
    fgets(what, 500, stdin);
    what[strlen (what) - 1] = '\0';
    if(!strcmp("help",what)){
        printf("\nyou can encrypt file by : encrypt filename\n");
        goto here;
    }else if(!strcmp("decrypt flag",what)){
        printf("\nreally ? do u really think i'm fu**ing stupid ! :(\n");
        goto here;
    }else if(strstr(what, "encrypt")!=NULL){
        what = str_replace(what,"encrypt ","");
        if(access(what,F_OK)!=-1) {
            crypt(what);
            printf("\ndone.\n");
        } else {
            printf("\nFile not found !\n");
        }
        goto here;
    }else if(strstr(what, "quit")!=NULL){
        return 0;
    }else{
        printf("\nuse : help to help :)\nor quit to quit ;)\n");
        goto here;
    }
    return 0;
}