Index: src/filesystem/Filesystem.php
===================================================================
--- src/filesystem/Filesystem.php
+++ src/filesystem/Filesystem.php
@@ -393,37 +393,32 @@
    */
   public static function readRandomBytes($number_of_bytes) {
 
-    if (phutil_is_windows()) {
-      if (!function_exists('openssl_random_pseudo_bytes')) {
-        if (version_compare(PHP_VERSION, '5.3.0') < 0) {
-          throw new Exception(
-            'Filesystem::readRandomBytes() requires at least PHP 5.3 under '.
-            'Windows.');
-        }
-        throw new Exception(
-          'Filesystem::readRandomBytes() requires OpenSSL extension under '.
-          'Windows.');
-      }
+    if (function_exists('openssl_random_pseudo_bytes')) {
       $strong = true;
-      return openssl_random_pseudo_bytes($number_of_bytes, $strong);
-    }
+      $data = openssl_random_pseudo_bytes($number_of_bytes, $strong);
+    } else {
+      $urandom = @fopen('/dev/urandom', 'rb');
+      if (!$urandom) {
+        throw new FilesystemException(
+          '/dev/urandom',
+          'Failed to open /dev/urandom for reading!');
+      }
 
-    $urandom = @fopen('/dev/urandom', 'rb');
-    if (!$urandom) {
-      throw new FilesystemException(
-        '/dev/urandom',
-        'Failed to open /dev/urandom for reading!');
+      $data = @fread($urandom, $number_of_bytes);
+      @fclose($urandom);
+      if (strlen($data) != $number_of_bytes) {
+        throw new FilesystemException(
+          '/dev/urandom',
+          'Failed to read random bytes!');
+      }
     }
 
-    $data = @fread($urandom, $number_of_bytes);
     if (strlen($data) != $number_of_bytes) {
-      throw new FilesystemException(
-        '/dev/urandom',
-        'Failed to read random bytes!');
+      throw new Exception(
+        'Filesystem::readRandomBytes() requires at least PHP 5.3 or
+         /dev/urandom');
     }
 
-    @fclose($urandom);
-
     return $data;
   }
 
Index: src/filesystem/__tests__/FilesystemTestCase.php
===================================================================
--- src/filesystem/__tests__/FilesystemTestCase.php
+++ src/filesystem/__tests__/FilesystemTestCase.php
@@ -60,4 +60,10 @@
     $this->assertEqual(true, ($f != $g));
   }
 
+  public function testReadRandomBytes() {
+    $number_of_bytes = 1024;
+    $data = Filesystem::readRandomBytes($number_of_bytes);
+    $this->assertEqual(true, (strlen($data) == $number_of_bytes));
+  }
+
 }