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phpencryption
John Rey Flores
John Rey Flores

Reputation: 187

PHP mcrypt_decrypt will return garbage data when I tried to alter the encrypted text

How can I detect if a variable contains garbage?

I have encrypted text generated from mcrypt_encrypt(). I attempt to edit this (assuming that someone will alter the encrypted text). I want to return an empty value after decryption instead of something like "Ú0úà¡ßQ9õ÷rŒi¾¼v’Ó}çTc~Žk"

Upvotes: 0

Views: 332

Answers (1)

r3mainer
r3mainer

Reputation: 24587

If I understand you correctly, you're looking for a way of encrypting data in such a way that it will be possible to detect if anyone has tampered with it.

The easiest way to achieve this would be to incorporate a checksum into the data before you encrypt it.

For example, you could encrypt the data as follows:

$plaintext = "Lorem ipsum dolor sit amet";
$hash = md5($plaintext);
$output = base64_encode(mcrypt_encrypt(CIPHER, KEY, $hash.$plaintext, MODE));

and then the decryption process would look like this:

$input = mcrypt_decrypt(CIPHER, KEY, base64_decode($output, MODE));
$hash = substr($input, 0, 32);
$text = substr($input, 32);
if (md5($text) != $hash) {
   die("Invalid data");
}
/* If you get this far, the encrypted data is fine */
  :
echo $text;
  :

Upvotes: 2

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