minor #11822 [Security] Use hash_equals for constant-time string comparison (again) (dunglas)

This PR was merged into the 2.3 branch.

Discussion
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[Security] Use hash_equals for constant-time string comparison (again)

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

Use the `hash_equals` function (introduced in PHP 5.6) for timing attack safe string comparison when available.

Add in the DocBlock that length will leak (https://github.com/symfony/symfony/pull/11797#issuecomment-53990712).

Commits
-------

3071557 [Security] Add more tests for StringUtils::equals
03bd74b [Security] Use hash_equals for constant-time string comparison

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

@@ -27,6 +27,7 @@ class StringUtils
      * Compares two strings.
      *
      * This method implements a constant-time algorithm to compare strings.
+     * Regardless of the used implementation, it will leak length information.
      *
      * @param string $knownString The string of known length to compare against
      * @param string $userInput   The string that the user can control
@@ -35,6 +36,13 @@ class StringUtils
      */
     public static function equals($knownString, $userInput)
     {
+        $knownString = (string) $knownString;
+        $userInput = (string) $userInput;
+
+        if (function_exists('hash_equals')) {
+            return hash_equals($knownString, $userInput);
+        }
+
         $knownLen = strlen($knownString);
         $userLen = strlen($userInput);
 
@@ -45,7 +53,7 @@ class StringUtils
         $result = $knownLen - $userLen;
 
         // Note that we ALWAYS iterate over the user-supplied length
-        // This is to prevent leaking length information
+        // This is to mitigate leaking length information
         for ($i = 0; $i < $userLen; $i++) {
             $result |= (ord($knownString[$i]) ^ ord($userInput[$i]));
         }

src/Symfony/Component/Security/Tests/Core/Util/StringUtilsTest.php

@@ -13,11 +13,49 @@ namespace Symfony\Component\Security\Tests\Core\Util;
 
 use Symfony\Component\Security\Core\Util\StringUtils;
 
+/**
+ * Data from PHP.net's hash_equals tests
+ */
 class StringUtilsTest extends \PHPUnit_Framework_TestCase
 {
-    public function testEquals()
+    public function dataProviderTrue()
     {
-        $this->assertTrue(StringUtils::equals('password', 'password'));
-        $this->assertFalse(StringUtils::equals('password', 'foo'));
+        return array(
+            array('same', 'same'),
+            array('', ''),
+            array(123, 123),
+            array(null, ''),
+            array(null, null),
+        );
+    }
+
+    public function dataProviderFalse()
+    {
+        return array(
+            array('not1same', 'not2same'),
+            array('short', 'longer'),
+            array('longer', 'short'),
+            array('', 'notempty'),
+            array('notempty', ''),
+            array(123, 'NaN'),
+            array('NaN', 123),
+            array(null, 123),
+        );
+    }
+
+    /**
+     * @dataProvider dataProviderTrue
+     */
+    public function testEqualsTrue($known, $user)
+    {
+        $this->assertTrue(StringUtils::equals($known, $user));
+    }
+
+    /**
+     * @dataProvider dataProviderFalse
+     */
+    public function testEqualsFalse($known, $user)
+    {
+        $this->assertFalse(StringUtils::equals($known, $user));
     }
 }