Add Document ID

+## 1.3.14
+
+* Add Document ID
+
 ## 1.3.13
 
 * Do not modify the TTF font streams

@@ -21,6 +21,7 @@ import 'dart:convert';
 import 'dart:math' as math;
 import 'dart:typed_data';
 
+import 'package:crypto/crypto.dart';
 import 'package:meta/meta.dart';
 import 'package:utf/utf.dart';
 import 'package:vector_math/vector_math_64.dart';
@@ -34,21 +35,21 @@ part 'src/color.dart';
 part 'src/colors.dart';
 part 'src/compatibility.dart';
 part 'src/document.dart';
-part 'src/font.dart';
 part 'src/font_descriptor.dart';
 part 'src/font_metrics.dart';
+part 'src/font.dart';
 part 'src/formxobject.dart';
 part 'src/graphics.dart';
 part 'src/image.dart';
 part 'src/info.dart';
 part 'src/names.dart';
-part 'src/object.dart';
 part 'src/object_stream.dart';
+part 'src/object.dart';
 part 'src/outline.dart';
 part 'src/output.dart';
-part 'src/page.dart';
 part 'src/page_format.dart';
 part 'src/page_list.dart';
+part 'src/page.dart';
 part 'src/point.dart';
 part 'src/polygon.dart';
 part 'src/rect.dart';

@@ -97,6 +97,20 @@ class PdfDocument {
   /// This holds the current fonts
   final Set<PdfFont> fonts = Set<PdfFont>();
 
+  /// Generates the document ID
+  List<int> _documentID;
+  List<int> get documentID {
+    if (_documentID == null) {
+      final math.Random rnd = math.Random();
+      _documentID = sha256
+          .convert(DateTime.now().toIso8601String().codeUnits +
+              List<int>.generate(32, (_) => rnd.nextInt(256)))
+          .bytes;
+    }
+
+    return _documentID;
+  }
+
   /// Creates a new serial number
   int _genSerial() => _objser++;
 

@@ -106,6 +106,8 @@ class PdfOutput {
     // the /Root catalog indirect reference (REQUIRED)
     if (rootID != null) {
       params['/Root'] = rootID.ref();
+      final PdfStream id = PdfStream.binary(rootID.pdfDocument.documentID);
+      params['/ID'] = PdfStream.array(<PdfStream>[id, id]);
     } else {
       throw Exception('Root object is not present in document');
     }

@@ -122,6 +122,22 @@ class PdfStream {
     putString(value ? 'true' : 'false');
   }
 
+  /// Returns the ASCII/Unicode code unit corresponding to the hexadecimal digit
+  /// [digit].
+  int _codeUnitForDigit(int digit) =>
+      digit < 10 ? digit + 0x30 : digit + 0x61 - 10;
+
+  void putBinary(List<int> s) {
+    _stream.add(0x3c);
+    for (int byte in s) {
+      _stream.add(_codeUnitForDigit((byte & 0xF0) >> 4));
+      _stream.add(_codeUnitForDigit(byte & 0x0F));
+    }
+    _stream.add(0x3e);
+  }
+
+  static PdfStream binary(List<int> s) => PdfStream()..putBinary(s);
+
   void putArray(List<PdfStream> values) {
     putString('[');
     for (PdfStream val in values) {

@@ -4,7 +4,7 @@ description: A pdf producer for Dart. It can create pdf files for both web or fl
 homepage: https://github.com/DavBfr/dart_pdf/tree/master/pdf
 repository: https://github.com/DavBfr/dart_pdf
 issue_tracker: https://github.com/DavBfr/dart_pdf/issues
-version: 1.3.13
+version: 1.3.14
 
 environment:
   sdk: ">=2.1.0 <3.0.0"
@@ -13,6 +13,7 @@ dependencies:
   meta: "^1.1.5"
   vector_math: "^2.0.0"
   utf: "^0.9.0"
+  crypto: "^2.0.6"
 
 dev_dependencies:
   test: any