15. User Interfaces

//------------------------------------------------------------------
// /!\ this function is deprecated
#include <unistd.h> // getpass()

...

char * password = getpass( "password : " );

...

//------------------------------------------------------------------
// gcc -Wall -o checkuser checkuser.c -lcrypt
#define _GNU_SOURCE

#include <stdio.h> // gnu version of getline()
#include <stdlib.h> // at_exit()
#include <string.h>
#include <unistd.h> // getlogin()
#include <termios.h> // struct termios, tcgetattr(), tcsetattr()
#include <pwd.h> // struct passwd, fgetpwent()
#include <crypt.h>  // crypt() 

static struct termios stored_settings;

void restore_term_setting( void ) {
  /*
    restore terminal setting
  */
  tcsetattr( 0, TCSANOW, &stored_settings);
}

int main( void ) {

  struct termios new_settings;
  char * cryptpw; // our encrypted password
  char * my_login = getlogin(); // our login

  /*
    we ask for password
  */
  fprintf( stdout, "Password : " );
  
  /*
    we disable echo input
  */
  
  // we first set an exit handler to restore terminal setting
  // * if something go wrong
  // * on the final return
  if( atexit( restore_term_setting ) != 0 ) {
    fprintf( stderr, "error on atexit\n" );
    exit(1);
  }

  
  tcgetattr( 0, &stored_settings ); // save terminal setting

  new_settings = stored_settings;
  new_settings.c_lflag &= (~ECHO); // disable echo input characters
  tcsetattr( 0, TCSANOW, &new_settings );

  /*
    we get the plain password
  */
  char * passwd = NULL;
  size_t size = 0;
  ssize_t ret;

  if( (ret = getline( &passwd, &size, stdin )) == -1 )
    exit(1);

  // we "chomp" the plain password
  char *p = strchr( passwd, '\n' );
  *p = 0;

  /*
    we get our login encrypted password
    /!\ most of the Linux system use shadow password
    we have to use /etc/shadow instead of /etc/passwd
    /!\ we have to be root in order to read /etc/shadow
  */
  FILE * fp;

  if( (fp = fopen( "/etc/shadow", "r" )) == NULL ) {
    perror("fopen");
    exit(1);
  }
  
  struct passwd * shadow_pwd_ent;
  cryptpw = NULL;
  
  while( (shadow_pwd_ent = fgetpwent(fp)) != NULL ) {
    // we search our login in the entry returned
    if( strcmp( my_login, shadow_pwd_ent->pw_name ) == 0 ) {
      cryptpw = shadow_pwd_ent->pw_passwd;
      break;
    }
  }

  fclose(fp);
    
  /*
    password matching
  */
  int match = strcmp( crypt(passwd, cryptpw), cryptpw );
    
  // we hurry to free the plain version of the password
  free(passwd), passwd = NULL;
  
  puts( "" );
  if( match == 0 )
    printf( "Welcome, %s\n", my_login );
  else 
    printf( "You are not %s\n", my_login );
  
  return 0;
}