hash_equals

(PHP 5 >= 5.6.0)

hash_equals — 可防止时序攻击的字符串比较

说明

bool hash_equals ( string $known_string , string $user_string )

比较两个字符串，无论它们是否相等，本函数的时间消耗是恒定的。

本函数可以用在需要防止时序攻击的字符串比较场景中，例如，可以用在比较 crypt() 密码哈希值的场景。

参数

known_string: 已知长度的、要参与比较的 string
user_string: 用户提供的字符串

返回值

当两个字符串相等时返回 TRUE ，否则返回 FALSE 。

错误／异常

如果所提供的 2 个参数中任何一个不是字符串，会导致 E_WARNING 消息。

范例

Example #1 例程

  <?php
$expected   =  crypt ( '12345' ,  '$2a$07$usesomesillystringforsalt$' );
 $correct    =  crypt ( '12345' ,  '$2a$07$usesomesillystringforsalt$' );
 $incorrect  =  crypt ( 'apple' ,   '$2a$07$usesomesillystringforsalt$' );

 var_dump ( hash_equals ( $expected ,  $correct ));
 var_dump ( hash_equals ( $expected ,  $incorrect ));
 ?>

以上例程会输出：

bool(true)
bool(false)

注释

Note:

要想成功进行比较，那么所提供的 2 个参数必须是相同长度的字符串。如果所提供的字符串长度不同，那么本函数会立即返回 FALSE ，在时序攻击的场景下，已知字符串的长度可能会被泄露。

Note:

非常重要的一点是，用户提供的字符串必须是第二个参数。

用户评论:

[#1] David Grudl [2015-11-25 17:55:56]

Very short timing attack safe string comparison for PHP < 5.6

<?php function hash_equals($a, $b) { return substr_count($a ^ $b, "\0") * 2 === strlen($a . $b); } ?>

[#2] Cedric Van Bockhaven [2015-04-15 09:01:34]

Our server does not support the hash_equals function. We are using the following snippet which also has support for strings of different length:

<?php if(!function_exists('hash_equals')) { function hash_equals($a, $b) { $ret = strlen($a) ^ strlen($b); $ret |= array_sum(unpack("C*", $a^$b)); return !$ret; } } ?>

[#3] Markus P. N. [2014-09-04 21:46:24]

I don't know why asphp at dsgml dot com got that many downvotes, the function seems to work.

I extended it a bit to support strings of diffent length and to handle errors and ran some tests:

The test results and how to reproduce them: http://pastebin.com/mLMXJeva

The function:
<?php if (!function_exists('hash_equals')) { function hash_equals($known_string, $user_string) { if (func_num_args() !== 2) { // handle wrong parameter count as the native implentation trigger_error('hash_equals() expects exactly 2 parameters, ' . func_num_args() . ' given', E_USER_WARNING); return null; } if (is_string($known_string) !== true) { trigger_error('hash_equals(): Expected known_string to be a string, ' . gettype($known_string) . ' given', E_USER_WARNING); return false; } $known_string_len = strlen($known_string); $user_string_type_error = 'hash_equals(): Expected user_string to be a string, ' . gettype($user_string) . ' given'; // prepare wrong type error message now to reduce the impact of string concatenation and the gettype call if (is_string($user_string) !== true) { trigger_error($user_string_type_error, E_USER_WARNING); // prevention of timing attacks might be still possible if we handle $user_string as a string of diffent length (the trigger_error() call increases the execution time a bit) $user_string_len = strlen($user_string); $user_string_len = $known_string_len + 1; } else { $user_string_len = $known_string_len + 1; $user_string_len = strlen($user_string); } if ($known_string_len !== $user_string_len) { $res = $known_string ^ $known_string; // use $known_string instead of $user_string to handle strings of diffrent length. $ret = 1; // set $ret to 1 to make sure false is returned } else { $res = $known_string ^ $user_string; $ret = 0; } for ($i = strlen($res) - 1; $i >= 0; $i--) { $ret |= ord($res[$i]); } return $ret === 0; } } ?>

[#4] asphp at dsgml dot com [2014-08-28 19:48:47]

To transparently support this function on older versions of PHP use this:

<?php if(!function_exists('hash_equals')) { function hash_equals($str1, $str2) { if(strlen($str1) != strlen($str2)) { return false; } else { $res = $str1 ^ $str2; $ret = 0; for($i = strlen($res) - 1; $i >= 0; $i--) $ret |= ord($res[$i]); return !$ret; } } } ?>