diff --git a/phpseclib/File/X509.php b/phpseclib/File/X509.php
index ba6a0481..639a0233 100644
--- a/phpseclib/File/X509.php
+++ b/phpseclib/File/X509.php
@@ -267,6 +267,12 @@ class X509
      */
     private $challenge;
 
+    /**
+     * @var array
+     * @access private
+     */
+    private $extensionValues = [];
+
     /**
      * OIDs loaded
      *
@@ -291,6 +297,12 @@ class X509
      */
     private static $disable_url_fetch = false;
 
+    /**
+     * @var array
+     * @access private
+     */
+    private static $extensions = [];
+
     /**
      * Default Constructor.
      *
@@ -558,6 +570,10 @@ class X509
         $filters['distributionPoint']['fullName']['directoryName']['rdnSequence']['value'] = $type_utf8_string;
         $filters['directoryName']['rdnSequence']['value'] = $type_utf8_string;
 
+        foreach (self::$extensions as $extension) {
+            $filters['tbsCertificate']['extensions'][] = $extension;
+        }
+
         /* in the case of policyQualifiers/qualifier, the type has to be \phpseclib3\File\ASN1::TYPE_IA5_STRING.
            \phpseclib3\File\ASN1::TYPE_PRINTABLE_STRING will cause OpenSSL's X.509 parser to spit out random
            characters.
@@ -641,6 +657,13 @@ class X509
      */
     private function mapOutExtensions(&$root, $path)
     {
+        foreach ($this->extensionValues as $id => $value) {
+            $root['tbsCertificate']['extensions'][] = [
+                'extnId' => $id,
+                'extnValue' => $value,
+            ];
+        }
+
         $extensions = &$this->subArray($root, $path);
 
         if (is_array($extensions)) {
@@ -850,6 +873,10 @@ class X509
             return true;
         }
 
+        if (isset(self::$extensions[$extnId])) {
+            return self::$extensions[$extnId];
+        }
+
         switch ($extnId) {
             case 'id-ce-keyUsage':
                 return Maps\KeyUsage::MAP;
@@ -3996,4 +4023,44 @@ 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
new file mode 100644
index 00000000..4613d9e3
--- /dev/null
+++ b/tests/Unit/File/X509/X509ExtensionTest.php
@@ -0,0 +1,117 @@
+<?php
+/**
+ * @author    Jim Wigginton <terrafrost@php.net>
+ * @copyright 2014 Jim Wigginton
+ * @license   http://www.opensource.org/licenses/mit-license.html  MIT License
+ */
+
+use phpseclib3\Crypt\RSA;
+use phpseclib3\File\ASN1;
+use phpseclib3\File\X509;
+
+class Unit_File_X509_X509ExtensionTest extends PhpseclibTestCase
+{
+    public function testCustomExtension()
+    {
+        $customExtensionData = [
+            'toggle' => true,
+            'num' => 3,
+            'name' => 'Johnny',
+            'list' => ['foo', 'bar'],
+        ];
+        $customExtensionName = 'cust';
+        $customExtensionNumber = '2.16.840.1.101.3.4.2.99';
+        $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],
+                ],
+            ],
+        ];
+
+        ASN1::loadOIDs([
+            $customExtensionName => $customExtensionNumber,
+        ]);
+
+        X509::registerExtension($customExtensionName, $customExtensionMapping);
+
+        $privateKey = RSA::createKey();
+
+        $publicKey = $privateKey->getPublicKey();
+
+        $subject = new X509();
+        $subject->setDNProp('id-at-organizationName', 'phpseclib CA cert');
+        $subject->setPublicKey($publicKey);
+
+        $issuer = new X509();
+        $issuer->setPrivateKey($privateKey);
+        $issuer->setDN($subject->getDN());
+
+        $x509 = new X509();
+        $x509->setExtensionValue($customExtensionName, $customExtensionData);
+        $x509->makeCA();
+
+        $result = $x509->sign($issuer, $subject);
+
+        $certificate = $x509->saveX509($result);
+
+        $x509 = new X509();
+
+        $decodedData = $x509->loadX509($certificate);
+        $customExtensionDecodedData = null;
+
+        foreach ($decodedData['tbsCertificate']['extensions'] as $extension) {
+            if ($extension['extnId'] === $customExtensionName) {
+                $customExtensionDecodedData = $extension['extnValue'];
+
+                break;
+            }
+        }
+
+        $this->assertTrue($customExtensionDecodedData['toggle']);
+        $this->assertInstanceOf('phpseclib3\Math\BigInteger', $customExtensionDecodedData['num']);
+        $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]);
+    }
+}