PHP : Encryption and Security - [14.4] Verifying Data with Hashes

14.4.1 Problem

You want to make sure users don't alter data you've sent them in a cookie or form element.

14.4.2 Solution

Along with the data, send an MD5 hash of the data with a secret word. When you receive the data back, compute the hash of the received value with the same secret word. If they don't match, the user has altered the data.

Here's how to print a hash in a hidden form field:

$secret_word = 'flyingturtle';
$id = 2836;
$hash = md5($secret_word . $id);

print<<<_HTML_
<input type="hidden" name="id" value="$id">
<input type="hidden" name="idhash" value="$hash">
_HTML_;

Here's how to verify the hidden form field data when it's submitted:

$secret_word = 'flyingturtle';

if (md5($secret_word . $_REQUEST['id']) == $_REQUEST['idhash']) {
    $id = $_REQUEST['id'];
} else {
    die("Invalid data in $_REQUEST[id]");
}

14.4.3 Discussion

When processing the submitted form data, compute the hash of the submitted value of $_REQUEST['id'] and the secret word. If it matches the submitted hash, the value of $_REQUEST['id'] has not been altered by the user. If the hashes don't match, you know that the value of $_REQUEST['id'] you received is not the same as the one you sent.

To use a verification hash with a cookie, add the hash to the cookie value with join( ) :

$secret_word = 'flyingturtle';
$cookie_value = 'Ellen';
$hash = md5($secret_word . $id);

setcookie('name',join('|',array($cookie_value,$hash)));

Parse the hash from the cookie value with explode( ):

$secret_word = 'flyingturtle';
list($cookie_value,$cookie_hash) = explode('|',$_COOKIE['name'],2);
if (md5($secret_word . $cookie_value) == $cookie_hash) {
    $name = $cookie_value;
} else {
    die('Invalid data in $_COOKIE[name]');
}

Using a data-verification hash in a form or cookie obviously depends on the secret word used in hash computation. If a malicious user discovers your secret word, the hash offers no protection. Aside from guarding the secret word zealously, changing it frequently is a good idea. For an additional layer of protection, use different secret words, choosing the specific word to use in the hash based on some property of the $id value (10 different words selected by $id%10, for example). That way, damage is controlled if one of the words is compromised.

If you have the mhash module installed, you're not limited to MD5 hashes. mhash supports a number of different hash algorithms. For more information about mhash, see the mhash material in the online PHP manual or the mhash home page at http://mhash.sourceforge.net/.