Security Program C++

qcoder · 11-28-2003, 03:59 PM

I would like to know how I can write a programm that just sits there and waits for the user to input a password. I want to make it so that the user can not exit the program by Ctrl-C. I also would like to have the password show up as "*" when the user types the password.

MartinN · 11-28-2003, 04:22 PM

This little program ignores the signals SIGINT (CTRL-C) and SIGTSTP (CTRL-Z) by adding a signal handler that does nothing (the predefined handler SIGIGN)

Code:

 #include <signal.h>
 #include <stdlib.h>
 #include <stdio.h>
 #include <unistd.h>
 
 int main(void) {

     struct sigaction sa;
     struct sigaction oldint;
     struct sigaction oldtstp;
 
     /* Ignore signal completely */
     sa.sa_handler = SIG_IGN;
     sigemptyset(&sa.sa_mask);
     sa.sa_flags = 0;
 
     /* Set up the signal handler for SIGINT (CTRL-C),
	remember old value, so we can restore it later */
     sigaction(SIGINT, &sa, &oldint);

     /* Same for SIGTSTP (CTRL-Z)*/
     sigaction(SIGTSTP, &sa, &oldtstp);
 
     /* Sleep for five seconds, to terminate the program */
     sleep(5);
 
     /* Restore the behavior of SIGINT and SIGTSTP, not really
	neccessary here. Just to show that it's possible*/
     sigaction(SIGINT, &oldint, NULL);
     sigaction(SIGTSTP, &oldtstp, NULL);
 
     return 0;
  };

Martin

MartinN · 11-28-2003, 05:21 PM

A small search came up with this code sample for character input from terminal:
http://www.pwilson.net/kbhit.html

It's generally not trivial to read character by character from a terminal since there is a buffer that gets flushed when the user presses return. But if you borrow some code from that page, you get a working program. Like this:

Code:

#include <stdio.h>
#include <termios.h>
#include <unistd.h>

#ifndef STDIN_FILENO
  #define STDIN_FILENO 0
#endif

static struct termios termattr, save_termattr;
static int ttysavefd = -1;
static enum 
{ 
  RESET, RAW, CBREAK 
} ttystate = RESET;



int
set_tty_raw(void) 
{
  int i;

  i = tcgetattr (STDIN_FILENO, &termattr);
  if (i < 0) 
  {
    printf("tcgetattr() returned %d for fildes=%d\n",i,STDIN_FILENO); 
    perror ("");
    return -1;
  }
  save_termattr = termattr;

  termattr.c_lflag &= ~(ECHO | ICANON | IEXTEN | ISIG);
  termattr.c_iflag &= ~(BRKINT | ICRNL | INPCK | ISTRIP | IXON);
  termattr.c_cflag &= ~(CSIZE | PARENB);
  termattr.c_cflag |= CS8;
  termattr.c_oflag &= ~(OPOST);
   
  termattr.c_cc[VMIN] = 1;
  termattr.c_cc[VTIME] = 0;

  i = tcsetattr (STDIN_FILENO, TCSANOW, &termattr);
  if (i < 0) 
  {
    printf("tcsetattr() returned %d for fildes=%d\n",i,STDIN_FILENO); 
    perror("");
    return -1;
  }
   
  ttystate = RAW;
  ttysavefd = STDIN_FILENO;

  return 0;
}

int
set_tty_cooked() 
{
  int i;
  if (ttystate != CBREAK && ttystate != RAW) 
  {
    return 0;
  }
  i = tcsetattr (STDIN_FILENO, TCSAFLUSH, &save_termattr);
  if (i < 0) 
  {
    return -1;
  }
  ttystate = RESET;
  return 0;
}

unsigned char
kb_getc_w(void) 
{
  unsigned char ch;
  size_t size;

  while (1)
  {

    usleep(20000);        /* 1/50th second: thanks, Floyd! */

    size = read (STDIN_FILENO, &ch, 1);
    if (size > 0)
    {
      break;
    }
  }
  return ch;
}


int main(void)
{

  unsigned char a;

  set_tty_raw();

  a = kb_getc_w();

  do
    {
      printf("*");
      fflush(stdout); 
      a = kb_getc_w();
    }
  while(a != '\r');

  set_tty_cooked();
  
  printf("\n");

  return 0;

}

Now, combine 1 and 2 to get a working program.

Good luck with your project!
Martin