minor symfony#14028 [Security] [Core] String utils refactor (sarciszewski, ircmaxell)

This PR was submitted for the 2.7 branch but it was merged into the 2.3 branch instead (closes symfony#14028).

Discussion
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[Security] [Core] String utils refactor

| Q             | A
| ------------- | ---
| Bug fix?      | yes
| New feature?  | no
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets |
| License       | MIT
| Doc PR        |

This supersedes symfony#13984 (it includes it, but also includes additional refactoring).

Since length information is leaked in any case, preventing unnecessary duplication of secrets is important. Since casting will *always* make a copy, we only cast if absolutely necessary. Additionally, appending will create a new copy of the secret, so we avoid doing that, but instead only iterate over the minimum of the two strings.

Commits
-------

45cfb44 Change behavior to mirror hash_equals() returning early if there is a length mismatch
8269589 CS fixing
bdea4ba Prevent modifying secrets as much as possible
76b36d3 Update StringUtils.php
7221efc Whitespace
56ed71c Update StringUtils.php

Author: fabpot

src/Symfony/Component/Security/Core/Util/StringUtils.php

@@ -38,29 +38,55 @@ private function __construct()
      */
     public static function equals($knownString, $userInput)
     {
-        $knownString = (string) $knownString;
-        $userInput = (string) $userInput;
+        // Avoid making unnecessary duplications of secret data
+        if (!is_string($knownString)) {
+            $knownString = (string) $knownString;
+        }
+
+        if (!is_string($userInput)) {
+            $userInput = (string) $userInput;
+        }
 
         if (function_exists('hash_equals')) {
             return hash_equals($knownString, $userInput);
         }
 
-        $knownLen = strlen($knownString);
-        $userLen = strlen($userInput);
+        $knownLen = self::safeStrlen($knownString);
+        $userLen = self::safeStrlen($userInput);
 
-        // Extend the known string to avoid uninitialized string offsets
-        $knownString .= $userInput;
+		if ($userLen != $knownLen) {
+            return false;
+        }
 
-        // Set the result to the difference between the lengths
-        $result = $knownLen - $userLen;
+        $result = 0;
 
-        // Note that we ALWAYS iterate over the user-supplied length
-        // This is to mitigate leaking length information
-        for ($i = 0; $i < $userLen; $i++) {
+        for ($i = 0; $i < $knownLen; $i++) {
             $result |= (ord($knownString[$i]) ^ ord($userInput[$i]));
         }
 
         // They are only identical strings if $result is exactly 0...
         return 0 === $result;
     }
+
+    /**
+     * Return the number of bytes in a string
+     *
+     * @param string $string The string whose length we wish to obtain
+     * @return int
+     */
+    public static function safeStrlen($string)
+    {
+        // Premature optimization
+        // Since this cannot be changed at runtime, we can cache it
+        static $func_exists = null;
+        if ($func_exists === null) {
+            $func_exists = function_exists('mb_strlen');
+        }
+
+        if ($func_exists) {
+            return mb_strlen($string, '8bit');
+        }
+
+        return strlen($string);
+    }
 }