diff --git a/phpseclib/File/Traits/Extensions.php b/phpseclib/File/Traits/Extensions.php
deleted file mode 100644
index 8705745d..00000000
--- a/phpseclib/File/Traits/Extensions.php
+++ /dev/null
@@ -1,64 +0,0 @@
-<?php
-
-/**
- * Pure-PHP X.509 Parser
- *
- * PHP version 5
- *
- * Encode and decode X.509 certificates.
- *
- * The extensions are from {@link http://tools.ietf.org/html/rfc5280 RFC5280} and
- * {@link http://web.archive.org/web/19961027104704/http://www3.netscape.com/eng/security/cert-exts.html Netscape Certificate Extensions}.
- *
- * Note that loading an X.509 certificate and resaving it may invalidate the signature.  The reason being that the signature is based on a
- * portion of the certificate that contains optional parameters with default values.  ie. if the parameter isn't there the default value is
- * used.  Problem is, if the parameter is there and it just so happens to have the default value there are two ways that that parameter can
- * be encoded.  It can be encoded explicitly or left out all together.  This would effect the signature value and thus may invalidate the
- * the certificate all together unless the certificate is re-signed.
- *
- * @category  File
- * @package   X509
- * @author    Jim Wigginton <terrafrost@php.net>
- * @copyright 2012 Jim Wigginton
- * @license   http://www.opensource.org/licenses/mit-license.html  MIT License
- * @link      http://phpseclib.sourceforge.net
- */
-
-namespace phpseclib3\File\Traits;
-
-trait Extensions
-{
-    /**
-     * @var array
-     * @access private
-     */
-    private static $extensions = [];
-
-    /**
-     * @var array
-     * @access private
-     */
-    private $extensionValues = [];
-
-    /**
-     * Register the mapping for a custom/unsupported extension.
-     *
-     * @param string $id
-     * @param array $mapping
-     */
-    public static function registerExtension($id, array $mapping)
-    {
-        self::$extensions[$id] = $mapping;
-    }
-
-    /**
-     * Register the mapping for a custom/unsupported extension.
-     *
-     * @param string $id
-     * @param mixed $value
-     */
-    public function setExtensionValue($id, $value)
-    {
-        $this->extensionValues[$id] = $value;
-    }
-}
diff --git a/phpseclib/File/X509.php b/phpseclib/File/X509.php
index 51bf4c81..57dbf052 100644
--- a/phpseclib/File/X509.php
+++ b/phpseclib/File/X509.php
@@ -54,8 +54,6 @@ use phpseclib3\Math\BigInteger;
  */
 class X509
 {
-    use Extensions;
-
     /**
      * Flag to only accept signatures signed by certificate authorities
      *
@@ -270,6 +268,12 @@ class X509
      */
     private $challenge;
 
+    /**
+     * @var array
+     * @access private
+     */
+    private $extensionValues = [];
+
     /**
      * OIDs loaded
      *
@@ -294,6 +298,12 @@ class X509
      */
     private static $disable_url_fetch = false;
 
+    /**
+     * @var array
+     * @access private
+     */
+    private static $extensions = [];
+
     /**
      * Default Constructor.
      *
@@ -4014,4 +4024,44 @@ class X509
 
         return false;
     }
+
+    /**
+     * Register the mapping for a custom/unsupported extension.
+     *
+     * @param string $id
+     * @param array $mapping
+     */
+    public static function registerExtension($id, array $mapping)
+    {
+        if (isset(self::$extensions[$id]) && self::$extensions[$id] !== $mapping) {
+            throw new \RuntimeException(
+                'Extension ' . $id . ' has already been defined with a different mapping.'
+            );
+        }
+
+        self::$extensions[$id] = $mapping;
+    }
+
+    /**
+     * Register the mapping for a custom/unsupported extension.
+     *
+     * @param string $id
+     *
+     * @return array|null
+     */
+    public static function getRegisteredExtension($id)
+    {
+        return isset(self::$extensions[$id]) ? self::$extensions[$id] : null;
+    }
+
+    /**
+     * Register the mapping for a custom/unsupported extension.
+     *
+     * @param string $id
+     * @param mixed $value
+     */
+    public function setExtensionValue($id, $value)
+    {
+        $this->extensionValues[$id] = $value;
+    }
 }
diff --git a/tests/Unit/File/X509/X509ExtensionTest.php b/tests/Unit/File/X509/X509ExtensionTest.php
index 781a75db..4613d9e3 100644
--- a/tests/Unit/File/X509/X509ExtensionTest.php
+++ b/tests/Unit/File/X509/X509ExtensionTest.php
@@ -21,12 +21,7 @@ class Unit_File_X509_X509ExtensionTest extends PhpseclibTestCase
         ];
         $customExtensionName = 'cust';
         $customExtensionNumber = '2.16.840.1.101.3.4.2.99';
-
-        ASN1::loadOIDs([
-            $customExtensionName => $customExtensionNumber,
-        ]);
-
-        X509::registerExtension($customExtensionName, [
+        $customExtensionMapping = [
             'type' => ASN1::TYPE_SEQUENCE,
             'children' => [
                 'toggle' => ['type' => ASN1::TYPE_BOOLEAN],
@@ -39,8 +34,14 @@ class Unit_File_X509_X509ExtensionTest extends PhpseclibTestCase
                     'children' => ['type' => ASN1::TYPE_OCTET_STRING],
                 ],
             ],
+        ];
+
+        ASN1::loadOIDs([
+            $customExtensionName => $customExtensionNumber,
         ]);
 
+        X509::registerExtension($customExtensionName, $customExtensionMapping);
+
         $privateKey = RSA::createKey();
 
         $publicKey = $privateKey->getPublicKey();
@@ -79,5 +80,38 @@ class Unit_File_X509_X509ExtensionTest extends PhpseclibTestCase
         $this->assertSame('3', (string) $customExtensionDecodedData['num']);
         $this->assertSame('Johnny', $customExtensionDecodedData['name']);
         $this->assertSame(['foo', 'bar'], $customExtensionDecodedData['list']);
+        $this->assertSame($customExtensionMapping, X509::getRegisteredExtension($customExtensionName));
+    }
+
+    public function testCustomExtensionRegisterTwiceTheSame()
+    {
+        $customExtensionMapping = [
+            'type' => ASN1::TYPE_SEQUENCE,
+            'children' => [
+                'toggle' => ['type' => ASN1::TYPE_BOOLEAN],
+                'num' => ['type' => ASN1::TYPE_INTEGER],
+                'name' => ['type' => ASN1::TYPE_OCTET_STRING],
+                'list' => [
+                    'type' => ASN1::TYPE_SEQUENCE,
+                    'min' => 0,
+                    'max' => -1,
+                    'children' => ['type' => ASN1::TYPE_OCTET_STRING],
+                ],
+            ],
+        ];
+
+        X509::registerExtension('foo', $customExtensionMapping);
+        X509::registerExtension('foo', $customExtensionMapping);
+
+        $this->assertSame($customExtensionMapping, X509::getRegisteredExtension('foo'));
+    }
+
+    public function testCustomExtensionRegisterConflict()
+    {
+        $this->expectException(\RuntimeException::class);
+        $this->expectExceptionMessage('Extension bar has already been defined with a different mapping.');
+
+        X509::registerExtension('bar', ['type' => ASN1::TYPE_OCTET_STRING]);
+        X509::registerExtension('bar', ['type' => ASN1::TYPE_ANY]);
     }
 }