(PHP 5 >= 5.6.0, PHP 7, PHP 8)
hash_equals — Timing attack safe string comparison
$known_string, #[\SensitiveParameter] string $user_string): bool
Checks whether two strings are equal without leaking information about the
contents of known_string via the execution time.
This function can be used to mitigate timing attacks. Performing a regular
comparison with === will take more or less time to execute
depending on whether the two values are different or not and at which
position the first difference can be found, thus leaking information about
the contents of the secret known_string.
It is important to provide the user-supplied string as the second parameter, rather than the first.
Example #1 hash_equals() example
<?php
$secretKey = '8uRhAeH89naXfFXKGOEj';
// Value and signature are provided by the user, e.g. within the URL
// and retrieved using $_GET.
$value = 'username=rasmuslerdorf';
$signature = '8c35009d3b50caf7f5d2c1e031842e6b7823a1bb781d33c5237cd27b57b5f327';
if (hash_equals(hash_hmac('sha256', $value, $secretKey), $signature)) {
echo "The value is correctly signed.", PHP_EOL;
} else {
echo "The value was tampered with.", PHP_EOL;
}
?>The above example will output:
The value is correctly signed.
Note:
Both arguments must be of the same length to be compared successfully. When arguments of differing length are supplied,
falseis returned immediately and the length of the known string may be leaked in case of a timing attack.