Captcha Number with Image Magic

by: fulscherPosted on 2006-09-22 at 03:18:51ID: 17576438

Since so many people use authentication with images, there are some clever algorithms around to actually OCR these images. If you use a completely different scheme, chances are that nobody will bother to crack it.

I've been using a simple scheme: I ask the user to add two numbers and enter the result in a field. If the result is correct, processing continues. If it isn't, it's most probably a robot. If the user can't add two numbers (it's a three digit number plus or minus a one-digit number), he's not supposed to use this site anyway. There are many variants to this scheme, for example, showing the numbers as text instead of figures ("four" instead of "4").

Just a thought...
J.

by: babuno5Posted on 2006-09-22 at 03:28:31ID: 17576478

well the two numbers which the user will see is again through images right ?

by: tolgaongPosted on 2006-09-22 at 14:49:48ID: 17581003

//newform.php
<?php
session_start();
$_SESSION["captcha"]=verificationNumber(4);

function verificationNumber($chars=6){
 $pass="";
 $harfler=array("a","b","c","d","e","f","g","i","j","k","l","m","n","o","p","q","r","s","t","u","v","y","z","0","1","2","3","4","5","6","7","8","9");
 for ($i=1;$i<=$chars;$i++){
 $pass.= $harfler[rand(0,count($harfler)-1)];
 }
 return $pass;
 }
echo '<img src="catchpa.php" border="0" />';
?>

//catchpa.php
<?php
/*
You can use backgrounds for it
and you can use imagettftext instead of imagestring to give some angles foreach letter
*/
 header("Content-type: image/png");
 $im = @imagecreate(100, 50) or die("Cannot Initialize new GD image stream");
 $background_color = imagecolorallocate($im, 255, 255, 255);
 $text_color = imagecolorallocate($im, 233, 14, 91);
 $vernum=$_SESSION["captcha"];
 imagestring($im, 4, 5, 5, $vernum , $text_color);
 imagepng($im);
 imagedestroy($im);
?>

by: babuno5Posted on 2006-09-23 at 00:01:24ID: 17582554

@tolgaong
My problem is that i cant install GD and have to complete this only using Image Magic
So your example will not work

by: tolgaongPosted on 2006-09-23 at 04:24:25ID: 17583318

<?php
session_start();
$_SESSION["captcha"]=verificationNumber(4);
function verificationNumber($chars=6){
$pass="";
$harfler=array("a","b","c","d","e","f","g","i","j","k","l","m","n","o","p","q","r","s","t","u","v","y","z","0","1","2","3","4","5","6","7","8","9");
for ($i=1;$i<=$chars;$i++){
$pass.= $harfler[rand(0,count($harfler)-1)];
}
return $pass;
}
?>
<html>
<head>
<meta http-equiv="Content-Type" content="text/html; charset=iso-8859-9" />
<title>Untitled Document</title>
</head>
<body>
<?php
include_once "catchpa.php";
?>
</body>
</html>

You need to define and upload every chars image as gif in this example.
<?php
$vernum=$_SESSION["captcha"];
for($i=0;$i<strlen($vernum);$i++){
echo '<img src="'.$vernum{$i}.'.gif" >';
}
?>

by: fulscherPosted on 2006-09-25 at 03:05:53ID: 17590809

Sorry - was offline for some time.

The two numbers would be actually text; no images are needed. If someone would want to submit the form using a software robot, they still would need to write quite a bit of code to extract the figures. It can be done without major effort, but I guess there are easier targets available...

Anyway, it works for me and its an easy and fast solution to the problem.

Here's the code I'm using:

// somewhere at the top or in a function library file.
 // Creates a piece of HTML which looks like a number to the user, but contains lots of other digits,
 // e.g. 992
// => 992
// could be improved by using hex codes instead of numbers, etc.

 function numberstring($n)
 {
 $res = '';
 while ($n > 0)
 {
 $rem = $n % 10;
 $n = ($n - $rem) / 10;
 $res = ''.$rem.'' . $res;
 }
 return $res;
 }

 // Form:

 <tr>
<td valign="top">Spam protection</td>
<td valign="top">Recently, we've received a lot of spam. Please prove that you're human by entering the result of the calculation into the following field:
<?php
$arg1 = rand(100, 999);
$arg2 = rand(1, 9);

if (rand(0, 10) < 5)
{
$res = $arg1 - $arg2;
$task = numberstring($arg1).' - '.numberstring($arg2);
}
else
{
$res = $arg1 + $arg2;
$task = numberstring($arg1).' + '.numberstring($arg2);
}

// save correct result
$_SESSION['res'] = $res;
?>
<?php echo $task; ?> = <input name="calcres" type="text" />
</td>
</tr>

// Evaluation of result:

 $calcres = safeREQUEST('calcres', '');
// safeREQUEST is a utility function, essentially a wrapped $_REQUEST['...']

 $savedOK = TRUE;
 // ... other validations

 if ($calcres == '')
 {
 $savedOK = FALSE;
 $errmsg = 'Please enter the result of the calculation below so we know that you're human.';
 }

 if ($_SESSION['res'] != $calcres)
 {
 $savedOK = FALSE;
 $errmsg = 'Sorry - we think that you've entered the wrong result for the calculation below. Please try again.';
 }
 // ... continue processing only if $savedOK == TRUE

HTH, J

by: oavsPosted on 2006-10-16 at 18:53:48ID: 17744400

Hi I am new to this. I try to use your code and all sorts of error messages I got. Some to do with 'you've' appostrophy. In any case I dook out the appostrophy and still not working. Is there anyway you could sample with a small complete form please so that I can see how it works.

Thanks for your contribution.

by: hujiPosted on 2006-11-09 at 11:51:49ID: 17908847

oavs, you need to ask your question yourself, in a separate thread. It is against EE rules to ask your questions in another one's. Also, you may not get an answer this way.
Wish I can help
Huji

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Captcha Number with Image Magic

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