Add encryption support

 ## 1.3.14
 
 * Add Document ID
+* Add encryption support
 
 ## 1.3.13
 

@@ -35,6 +35,7 @@ part 'src/color.dart';
 part 'src/colors.dart';
 part 'src/compatibility.dart';
 part 'src/document.dart';
+part 'src/encryption.dart';
 part 'src/font_descriptor.dart';
 part 'src/font_metrics.dart';
 part 'src/font.dart';

@@ -86,6 +86,9 @@ class PdfDocument {
   /// No compression by default
   final DeflateCallback deflate;
 
+  /// Object used to encrypt the document
+  PdfEncryption encryption;
+
   /// These map the page modes just defined to the pagemodes setting of Pdf.
   static const List<String> _PdfPageModes = <String>[
     '/UseNone',

+/*
+ * Copyright (C) 2017, David PHAM-VAN <dev.nfet.net@gmail.com>
+ *
+ * Licensed under the Apache License, Version 2.0 (the 'License');
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an 'AS IS' BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+part of pdf;
+
+abstract class PdfEncryption extends PdfObject {
+  PdfEncryption(PdfDocument pdfDocument) : super(pdfDocument, null);
+
+  List<int> encrypt(List<int> input, PdfObject object);
+}

@@ -54,6 +54,9 @@ class PdfObjectStream extends PdfObject {
       // This is a non-deflated stream
       _data = buf.output();
     }
+    if (pdfDocument.encryption != null) {
+      _data = pdfDocument.encryption.encrypt(_data, this);
+    }
     params['/Length'] = PdfStream.intNum(_data.length);
   }
 

@@ -37,6 +37,9 @@ class PdfOutput {
   /// This is used to track the /Info object (info)
   PdfObject infoID;
 
+  /// This is used to track the /Encrypt object (encryption)
+  PdfEncryption encryptID;
+
   /// This method writes a [PdfObject] to the stream.
   ///
   /// @param ob [PdfObject] Object to write
@@ -49,6 +52,9 @@ class PdfOutput {
     if (ob is PdfInfo) {
       infoID = ob;
     }
+    if (ob is PdfEncryption) {
+      encryptID = ob;
+    }
 
     offsets.add(PdfXref(ob.objser, os.offset));
     ob._write(os);
@@ -117,6 +123,11 @@ class PdfOutput {
       params['/Info'] = infoID.ref();
     }
 
+    // the /Encrypt reference (OPTIONAL)
+    if (encryptID != null) {
+      params['/Encrypt'] = encryptID.ref();
+    }
+
     // end the trailer object
     os.putDictionary(params);
     os.putString('\nstartxref\n$xref\n%%EOF\n');