/*********************************************************************
  auth.c
  # gcc auth.c -o auth -lcrypt
  # chmod 4755 auth
  reads two lines containing login and password
  returns 0 on success, or sleeps 3 seconds and returns 1
*********************************************************************/

#define _XOPEN_SOURCE           /* define before 1st #include */

#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>             /* crypt() */

#define SKIP  20                /* skip this many lines at the
                                   beginning of the shadow file */

char user[129], pass[513], line[1024];

int auth(char *user, char *pass)
{
    FILE *f;
    int len;
    int val = 1;
    int count = 0;

    if ((f = fopen("/etc/shadow", "r")) == NULL) return 1;
    strcat(user, ":");
    len = strlen(user);
    while (fgets(line, sizeof line, f)) {
        if (++count > SKIP && strncmp(line, user, len) == 0) {
            int cmplen = 13;
            if (strncmp(&line[len], "$1$", 3) == 0) cmplen = 34;
            if (strncmp(crypt(pass, &line[len]), &line[len], cmplen) == 0) val = 0;
            break;
        }
    }
    fclose(f);
    return val;                 /* 0=good, 1=bad */
}

void mygets(char *s, int size)
{
    int i;

    fgets(s, size, stdin);
    for (i = 0; i < size; i++) {
        if (s[i] == '\n' || s[i] == '\r') {
            s[i] = '\0';
            break;
        }
    }
}

int main()
{
#ifdef SOLARIS
    /* On Solaris 7, it seems necessary to explicitly setuid(0) */
    setuid(0);
#endif
    mygets(user, sizeof(user) - 1); /* keep room to append ":" */
    mygets(pass, sizeof(pass));
    if (auth(user, pass) != 0) {
        sleep(3);
        return 1;               /* bad */
    }
    return 0;                   /* good */
}