Term::ReadPassword - Asking the user for a password

Term-ReadPassword documentation Contained in the Term-ReadPassword distribution.

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NAME

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Term::ReadPassword - Asking the user for a password

SYNOPSIS

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  use Term::ReadPassword;
  while (1) {
    my $password = read_password('password: ');
    redo unless defined $password;
    if ($password eq 'flubber') {
      print "Access granted.\n";
      last;
    } else {
      print "Access denied.\n";
      redo;
    }
  }

DESCRIPTION

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This module lets you ask the user for a password in the traditional way, from the keyboard, without echoing.

This is not intended for use over the web; user authentication over the web is another matter entirely. Also, this module should generally be used in conjunction with Perl's crypt() function, sold separately.

The read_password function prompts for input, reads a line of text from the keyboard, then returns that line to the caller. The line of text doesn't include the newline character, so there's no need to use chomp.

While the user is entering the text, a few special characters are processed. The character delete (or the character backspace) will back up one character, removing the last character in the input buffer (if any). The character CR (or the character LF) will signal the end of input, causing the accumulated input buffer to be returned. Control-U will empty the input buffer. And, optionally, the character Control-C may be used to terminate the input operation. (See details below.) All other characters, even ones which would normally have special purposes, will be added to the input buffer.

It is not recommended, though, that you use the as-yet-unspecified control characters in your passwords, as those characters may become meaningful in a future version of this module. Applications which allow the user to set their own passwords may wish to enforce this rule, perhaps with code something like this:

    {
      # Naked block for scoping and redo
      my $new_pw = read_password("Enter your new password: ");
      if ($new_pw =~ /([^\x20-\x7E])/) {
        my $bad = unpack "H*", $1;
	print "Your password may not contain the ";
	print "character with hex code $bad.\n";
	redo;
      } elsif (length($new_pw) < 5) {
        print "Your password must be longer than that!\n";
	redo;
      } elsif ($new_pw ne read_password("Enter it again: ")) {
	print "Passwords don't match.\n";
	redo;
      } else {
        &change_password($new_pw);
	print "Your password is now changed.\n";
      }
    }

The second parameter to read_password is the optional idle_timeout value. If it is a non-zero number and there is no keyboard input for that many seconds, the input operation will terminate. Notice that this is not an overall time limit, as the timer is restarted with each new character.

The third parameter will optionally allow the input operation to be terminated by the user with Control-C. If this is not supplied, or is false, a typed Control-C will be entered into the input buffer just as any other character. In that case, there is no way from the keyboard to terminate the program while it is waiting for input. (That is to say, the normal ability to generate signals from the keyboard is suspended during the call to read_password.)

If the input operation terminates early (either because the idle_timeout was exceeded, or because a Control-C was enabled and typed), the return value will be undef. In either case, there is no way provided to discover what (if anything) was typed before the early termination, or why the input operation was terminated.

So as to discourage users from typing their passwords anywhere except at the prompt, any input which has been "typed ahead" before the prompt appears will be discarded. And whether the input operation terminates normally or not, a newline character will be printed, so that the cursor will not remain on the line after the prompt.

BUGS

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Windows users will want Term::ReadPassword::Win32.

This module has a poorly-designed interface, and should be thoroughly rethought and probably re-united with the Windows version.

Users who wish to see password characters echoed as stars may set $Term::ReadPassword::USE_STARS to a true value. The bugs are that some terminals may not erase stars when the user corrects an error, and that using stars leaks information to shoulder-surfers.

SECURITY

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You would think that a module dealing with passwords would be full of security features. You'd think that, but you'd be wrong. For example, perl provides no way to erase a piece of data from memory. (It's easy to erase it so that it can't be accessed from perl, but that's not the same thing as expunging it from the actual memory.) If you've entered a password, even if the variable that contained that password has been erased, it may be possible for someone to find that password, in plaintext, in a core dump. And that's just one potential security hole.

In short, if serious security is an issue, don't use this module.

LICENSE

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This program is free software; you may redistribute it, modify it, or both, under the same terms as Perl itself.

AUTHOR

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Tom Phoenix <rootbeer@redcat.com>. Copyright (C) 2007 Tom Phoenix.

SEE ALSO

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Term::ReadLine, crypt in perlfunc, and your system's manpages for the low-level I/O operations used here.

Term-ReadPassword documentation Contained in the Term-ReadPassword distribution. 
package Term::ReadPassword;

use strict;
use Term::ReadLine;
use POSIX qw(:termios_h);
my %CC_FIELDS = (
	VEOF => VEOF,
	VEOL => VEOL,
	VERASE => VERASE,
	VINTR => VINTR,
	VKILL => VKILL,
	VQUIT => VQUIT,
	VSUSP => VSUSP,
	VSTART => VSTART,
	VSTOP => VSTOP,
	VMIN => VMIN,
	VTIME => VTIME,
    );

use vars qw(
    $VERSION @ISA @EXPORT @EXPORT_OK
    $ALLOW_STDIN %SPECIAL $SUPPRESS_NEWLINE $INPUT_LIMIT
    $USE_STARS $STAR_STRING $UNSTAR_STRING
);

require Exporter;

@ISA = qw(Exporter);
@EXPORT = qw(
	read_password 
);
$VERSION = '0.11';

# The special characters in the input stream
%SPECIAL = (
    "\x03"	=> 'INT',	# Control-C, Interrupt
    "\x15"	=> 'NAK',	# Control-U, NAK (clear buffer)
    "\x08"	=> 'DEL',	# Backspace
    "\x7f"	=> 'DEL',	# Delete
    "\x0d"	=> 'ENT',	# CR, Enter
    "\x0a"	=> 'ENT',	# LF, Enter
);

# The maximum amount of data for the input buffer to hold
$INPUT_LIMIT = 1000;

sub read_password {
    my($prompt, $idle_limit, $interruptable) = @_;
    $prompt = '' unless defined $prompt;
    $idle_limit = 0 unless defined $idle_limit;
    $interruptable = 0 unless defined $interruptable;

    # Let's open the TTY (rather than STDIN) if we can
    local(*TTY, *TTYOUT);
    my($in, $out) = Term::ReadLine->findConsole;
    die "No console available" unless $in;
    if (open TTY, "+<$in") {
        # Cool
    } elsif ($ALLOW_STDIN) {
        open TTY, "<&STDIN"
	    or die "Can't re-open STDIN: $!";
    } else {
        die "Can't open '$in' read/write: $!";
    }

    # And let's send the output to the TTY as well
    if (open TTYOUT, ">>$out") {
	# Cool
    } elsif ($ALLOW_STDIN) {
	# Well, let's allow STDOUT as well
	open TTYOUT, ">>&STDOUT"
	    or die "Can't re-open STDOUT: $!";
    } else {
	die "Can't open '$out' for output: $!";
    }

    # Don't buffer it!
    select( (select(TTYOUT), $|=1)[0] );
    print TTYOUT $prompt;

    # Okay, now remember where everything was, so we can put it back when
    # we're done 
    my $fd_tty = fileno(TTY);
    my $term = POSIX::Termios->new();
    $term->getattr($fd_tty);
    my $original_flags = $term->getlflag();
    my %original_cc;
    for my $field_name (keys %CC_FIELDS) {
        $original_cc{$field_name} = $term->getcc($CC_FIELDS{$field_name});
    }

    # What makes this setup different from the ordinary?
    # No keyboard-generated signals, no echoing, no canonical input
    # processing (like backspace handling)
    my $flags = $original_flags & ~(ISIG | ECHO | ICANON);
    $term->setlflag($flags);
    if ($idle_limit) {
	# $idle_limit is in seconds, so multiply by ten
	$term->setcc(VTIME, 10 * $idle_limit);
	# Continue running the program after that time, even if there
	# weren't any characters typed
	$term->setcc(VMIN, 0);
    } else {
	# No time limit, but...
	$term->setcc(VTIME, 0);
	# Continue as soon as one character has been struck
	$term->setcc(VMIN, 1);
    }

    # Optionally echo stars in place of password characters. The 
    # $unstar_string uses backspace characters.
    my $star_string = $USE_STARS ? ($STAR_STRING || '*') : '';
    my $unstar_string = $USE_STARS ? ($UNSTAR_STRING || "\b*\b \b") : '';

    # If there's anything already buffered, we should throw it out. This
    # is to discourage users from typing their password before they see
    # the prompt, since their keystrokes may be echoing on the screen. 
    #
    # So this statement supposedly makes sure the prompt goes out, the
    # unread input buffer is discarded, and _then_ the changes take
    # effect. Thus, everything they typed ahead is (probably) echoed.
    $term->setattr($fd_tty, TCSAFLUSH);

    my $input = '';
    my $return_value;
KEYSTROKE:
    while (1) {
        my $new_keys = '';
	my $count = sysread(TTY, $new_keys, 99);
	# We're here, so either the idle_limit expired, or the user typed
	# something.
	if ($count) {
	    for my $new_key (split //, $new_keys) {
	        if (my $meaning = $SPECIAL{$new_key}) {
		    if ($meaning eq 'ENT') {
		        # Enter/return key
			# Return what we have so far
			$return_value = $input;
			last KEYSTROKE;
		    } elsif ($meaning eq 'DEL') {
		        # Delete/backspace key
			# Take back one char, if possible
			if (length $input) {
			    $input = substr $input, 0, length($input)-1;
			    print TTYOUT $unstar_string;
			}
		    } elsif ($meaning eq 'NAK') {
		        # Control-U (NAK)
		        # Clear what we have read so far
			for (1..length $input) {
			    print TTYOUT $unstar_string;
			}
		        $input = '';
		    } elsif ($interruptable and $meaning eq 'INT') {
			# Breaking out of the program
			# Return early
			last KEYSTROKE;
		    } else {
		        # Just an ordinary keystroke
			$input .= $new_key;
			print TTYOUT $star_string;
		    }
		} else {
		    # Not special
		    $input .= $new_key;
		    print TTYOUT $star_string;
		}
 	    }
	    # Just in case someone sends a lot of data
	    $input = substr($input, 0, $INPUT_LIMIT)
	        if length($input) > $INPUT_LIMIT;
	} else {
	    # No count, so something went wrong. Assume timeout.
	    # Return early
	    last KEYSTROKE;
	}
    }

    # Done with waiting for input. Let's not leave the cursor sitting
    # there, after the prompt.
    print TTYOUT "\n" unless $SUPPRESS_NEWLINE;

    # Let's put everything back where we found it.
    $term->setlflag($original_flags);
    while (my($field, $value) = each %original_cc) {
        $term->setcc($CC_FIELDS{$field}, $value);
    }
    $term->setattr($fd_tty, TCSAFLUSH);
    close(TTY);
    close(TTYOUT);
    $return_value;
}

1;
__END__