php - Captcha encryption

Question

I have created a captcha image making program with PHP

<?php
function word($n) {
    $consonants = "bcdfghjklmnpqrstvwxyzBCDFGHJKLMNPQRSTVWXYZ";
    $vowels = "aeiou";
    $word[1] =  $consonants[rand(0, 41)];
    $word[2] .= $vowels[rand(0, 4)];
    $word[3] .= $consonants[rand(0, 41)];
    $word[4] .= $consonants[rand(0, 41)];
    return $word[$n];
}

    $xs = 550;
    $ys = 300;
    $im = imagecreatetruecolor($xs, $ys);
    $newim = imagecreatetruecolor($xs, $ys);
    imagettftext($im, $ys/5, rand(-10, 10), rand(0, 10), $ys/2, 0xFFFFFF, "geosans.ttf", word(1));
    imagettftext($im, $ys/4.5, rand(-10, 10), rand(50, 70), $ys/2, 0xFFFFFF, "geosans.ttf", word(2));
    imagettftext($im, $ys/4, rand(-10, 10), rand(100, 150), $ys/2, 0xFFFFFF, "geosans.ttf", word(3));
    imagettftext($im, $ys/3.5, rand(-10, 10), rand(185, 210), $ys/2, 0xFFFFFF, "geosans.ttf", word(4));
    for ($x=0; $x<=$xs;$x++){
        for ($y=0; $y<=$ys;$y++){
            $rgba = imagecolorsforindex($im, imagecolorat($im, $x, $y));
            $col = imagecolorallocate($newim, $rgba["red"], $rgba["green"], $rgba["blue"]);

            $distorted_y = ($y + round(45*sin($x/50)) + imagesy($im)) % imagesy($im);
            imagesetpixel($newim, $x, $distorted_y, $col);
        }
    }

    imagefilter($newim, IMG_FILTER_NEGATE);


    header("Content-type: image/png");
    imagepng($newim);
?>

But how can I apply it?

How does the official captcha website encrypt their's

This is an example

http://www.google.com/recaptcha/api/image?c=03AHJ_VutAc1sMxyCE0N98Kh2AfMGlGmu7_DzzFP3Rn1gLHdbDulOQYK0w-sVrxqHWSTBCfVBipmqY-ywmme2_cuClW5QBRzKdzRSJeMWyme1aoGZ-y0OluiSfn-uKDExfVCo2PGrTao2wWpBLultbUEsctlJ97JXKCQ

Overall, I'm asking how can I safely encrypt my data?

Answer 1

you could do it the same way as you could store passwords in your database. Hash them with MD5 with some random sort of SALT.

http://www.pixel2life.com/publish/tutorials/118/understanding_md5_password_encryption/