hash_equals

Timing attack safe string comparison

&reftitle.description; `bool` `hash_equals` `string` `known_string` `string` `user_string`

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.

&reftitle.parameters;

`known_string`

- The known &string; that must be kept secret.

`user_string`

- The user-supplied &string; to compare against.

&reftitle.returnvalues;

Returns &true; when the two strings are equal, &false; otherwise.

&reftitle.examples;

### `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;
    }
    ?>

&example.outputs;

    The value is correctly signed.

&reftitle.notes;

Both arguments must be of the same length to be compared successfully. When arguments of differing length are supplied, &false; is returned immediately and the length of the known string may be leaked in case of a timing attack.

&reftitle.seealso;

- `hash_hmac`