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merged branch ircmaxell/master (PR #6510)

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This PR was merged into the master branch.

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c543116 Improve timing safe comparison function

Discussion
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Improve timing safe comparison function in Security bundle to not leak length information.

Improve the timing safe comparison function to better handle cases where input is of different length.

Note that it is now important to always pass any string that the user can directly control to the second parameter of the function. Otherwise, length information may be leaked.

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by ircmaxell at 2012-12-29T13:36:32Z

@apfelbox: No, for two reasons. First, you shouldn't be passing the password directly into this function (it should be hashed first).

Second, it depends only on the length of the user supplied input (the second parameter). So the execution time will vary, but 100% based on user input. No information about the stored string is leaked...

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by apfelbox at 2012-12-29T14:09:54Z

@ircmaxell yes, I just thought about it for a while and you are right. The `strlen($knownString)` is a constant factor and therefore the  execution time of the function does not vary with it (especially if it is hashed).
This commit is contained in:
Fabien Potencier 2012-12-29 21:07:20 +01:00
commit 11b0dae32d
1 changed files with 22 additions and 11 deletions
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33
src/Symfony/Component/Security/Core/Util/StringUtils.php
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@ -28,22 +28,33 @@ class StringUtils
*
* This method implements a constant-time algorithm to compare strings.
*
* @param string $str1 The first string
* @param string $str2 The second string
* @param string $knownString The string of known length to compare against
* @param string $userInput The string that the user can control
*
* @return Boolean true if the two strings are the same, false otherwise
*/
public static function equals($str1, $str2)
public static function equals($knownString, $userInput)
{
if (strlen($str1) !== $c = strlen($str2)) {
return false;
// Prevent issues if string length is 0
$knownString .= chr(0);
$userInput .= chr(0);
$knownLen = strlen($knownString);
$userLen = strlen($userInput);
// Set the result to the difference between the lengths
$result = $knownLen - $userLen;
// Note that we ALWAYS iterate over the user-supplied length
// This is to prevent leaking length information
for ($i = 0; $i < $userLen; $i++) {
// Using % here is a trick to prevent notices
// It's safe, since if the lengths are different
// $result is already non-0
$result |= (ord($knownString[$i % $knownLen]) ^ ord($userInput[$i]));
}
$result = 0;
for ($i = 0; $i < $c; $i++) {
$result |= ord($str1[$i]) ^ ord($str2[$i]);
}
return 0 === $result;
// They are only identical strings if $result is exactly 0...
return $result === 0;
}
}