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Mcrypt helper function for padding AES encryption keys to ensure PHP 5.6-7.1 is backwards compatible with PHP <= 5.5.
<?php
/**
* Pad an AES encrypted key to ensure it's of valid size according
* to specification. This function is backwards compatible with
* PHP <= 5.5 and is intended for usage with PHP 5.6+.
*/
function pad_aes($str) {
$c = strlen($str);
$validLengths = array(16, 24, 32);
foreach ($validLengths as $l) {
if ($c === $l) return $str;
while ($c < $l) {
$str .= "\0";
$c++;
}
}
return $str;
}
<?php
/**
* Example of using the pad_aes() helper function to ensure your
* mcrypt_encrypt() and mcrypt_decrypt() functions use appropriately
* sized key lengths.
*/
// invalid key of length 15
$key = 'abcdefghijklmni';
// our secret message for two-way encryption
$message = 'hello, world!';
// demonstration of encryption and decryption
$iv = mcrypt_create_iv(16, MCRYPT_DEV_URANDOM);
$encrypted = mcrypt_encrypt(MCRYPT_RIJNDAEL_128, pad_aes($key), $message, MCRYPT_MODE_CBC, $iv);
$decrypted = mcrypt_decrypt(MCRYPT_RIJNDAEL_128, pad_aes($key), $encrypted, MCRYPT_MODE_CBC, $iv);
// example demonstrating that the decrypted message still has padding
var_dump('SECRET: ' . $message);
var_dump('DECRYPTED WITH PADDING: ' . $decrypted);
// strip off null byte padding
$stripped = rtrim($decrypted, "\0");
var_dump('DECRYPTED: ' . $stripped);
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