Add document signature support

@@ -3,6 +3,7 @@
 * Add Document ID
 * Add Document ID
 * Add encryption support
 * Add encryption support
 * Increase PDF version to 1.7
 * Increase PDF version to 1.7
+* Add document signature support
 
 
 ## 1.3.13
 ## 1.3.13
 
 

@@ -54,6 +54,7 @@ part 'src/page.dart';
 part 'src/point.dart';
 part 'src/point.dart';
 part 'src/polygon.dart';
 part 'src/polygon.dart';
 part 'src/rect.dart';
 part 'src/rect.dart';
+part 'src/signature.dart';
 part 'src/stream.dart';
 part 'src/stream.dart';
 part 'src/ttf_parser.dart';
 part 'src/ttf_parser.dart';
 part 'src/ttf_writer.dart';
 part 'src/ttf_writer.dart';

@@ -59,5 +59,10 @@ class PdfCatalog extends PdfObject {
     // the /PageMode setting
     // the /PageMode setting
     params['/PageMode'] =
     params['/PageMode'] =
         PdfStream.string(PdfDocument._PdfPageModes[pageMode.index]);
         PdfStream.string(PdfDocument._PdfPageModes[pageMode.index]);
+
+    if (pdfDocument.sign != null) {
+      params['/Perms'] = PdfStream.dictionary(
+          <String, PdfStream>{'/DocMDP': pdfDocument.sign.ref()});
+    }
   }
   }
 }
 }

@@ -89,6 +89,9 @@ class PdfDocument {
   /// Object used to encrypt the document
   /// Object used to encrypt the document
   PdfEncryption encryption;
   PdfEncryption encryption;
 
 
+  /// Object used to sign the document
+  PdfSignature sign;
+
   /// The PDF specification version
   /// The PDF specification version
   final String version = '1.7';
   final String version = '1.7';
 
 

@@ -40,6 +40,9 @@ class PdfOutput {
   /// This is used to track the /Encrypt object (encryption)
   /// This is used to track the /Encrypt object (encryption)
   PdfEncryption encryptID;
   PdfEncryption encryptID;
 
 
+  /// This is used to track the /Sign object (signature)
+  PdfSignature signatureID;
+
   /// This method writes a [PdfObject] to the stream.
   /// This method writes a [PdfObject] to the stream.
   ///
   ///
   /// @param ob [PdfObject] Object to write
   /// @param ob [PdfObject] Object to write
@@ -55,6 +58,10 @@ class PdfOutput {
     if (ob is PdfEncryption) {
     if (ob is PdfEncryption) {
       encryptID = ob;
       encryptID = ob;
     }
     }
+    if (ob is PdfSignature) {
+      assert(signatureID == null, 'Only one document signature is allowed');
+      signatureID = ob;
+    }
 
 
     offsets.add(PdfXref(ob.objser, os.offset));
     offsets.add(PdfXref(ob.objser, os.offset));
     ob._write(os);
     ob._write(os);
@@ -131,6 +138,10 @@ class PdfOutput {
     // end the trailer object
     // end the trailer object
     os.putDictionary(params);
     os.putDictionary(params);
     os.putString('\nstartxref\n$xref\n%%EOF\n');
     os.putString('\nstartxref\n$xref\n%%EOF\n');
+
+    if (signatureID != null) {
+      signatureID._writeSignature(os);
+    }
   }
   }
 
 
   /// Writes a block of references to the Pdf file
   /// Writes a block of references to the Pdf file

+/*
+ * 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;
+
+@immutable
+class PdfSignatureRange {
+  const PdfSignatureRange(this.start, this.end);
+
+  final int start;
+  final int end;
+}
+
+abstract class PdfSignature extends PdfObject {
+  PdfSignature(PdfDocument pdfDocument) : super(pdfDocument, '/Sig');
+
+  int _offsetStart;
+  int _offsetEnd;
+
+  void preSign();
+
+  void sign(PdfStream os, List<PdfSignatureRange> ranges);
+
+  @override
+  void _write(PdfStream os) {
+    preSign();
+
+    _offsetStart = os.offset;
+    super._write(os);
+    _offsetEnd = os.offset;
+  }
+
+  void _writeSignature(PdfStream os) {
+    assert(_offsetStart != null && _offsetEnd != null,
+        'Must reserve the object space before signing the document');
+
+    final List<PdfSignatureRange> ranges = <PdfSignatureRange>[
+      PdfSignatureRange(0, _offsetStart),
+      PdfSignatureRange(_offsetEnd, os.offset),
+    ];
+
+    sign(os, ranges);
+    final PdfStream signature = PdfStream();
+    super._write(signature);
+
+    assert(signature.offset == _offsetEnd - _offsetStart);
+    os.output().replaceRange(_offsetStart, _offsetEnd, signature.output());
+  }
+}