How to: Work with Digital Signatures

Applies to: InfoPath 2010 | InfoPath Forms Services | Office 2010 | SharePoint Server 2010 | Visual Studio | Visual Studio Tools for Microsoft Office

In this article
Digital Signature Features
Overview of the Digital Signatures Object Model
Working with Digital Signatures Programmatically

The object model of the Microsoft.Office.InfoPath namespace provides features for working with digital signatures programmatically.

Digital Signature Features

The digital signatures features provided by InfoPath enable you to:

• Enable signatures for the entire form, or for specific sets of data in the form that can be signed separately.

• Specify, for each set of data that can be signed, whether a single signature or multiple signatures are allowed and what their relationship will be. For example, you can specify whether they are parallel co-signatures or whether each new signature countersigns all the earlier signatures.

• Specify a message to be shown to form users as they sign the form.

• Insert and see a signature in the document.

• View verifiable non-repudiation information that has been added to each signature for increased security. This additional information, which includes a view of the form as it was presented to each signer, is part of the signature and cannot be removed without invalidating the signature. At any time, you can recall this data by clicking on the signature in the form to display the Verify Digital Signature dialog box.

• Take advantage of an object model for working with digital signatures. Add custom information to the signature block in fully trusted forms through the digital signature object model.

Overview of the Digital Signatures Object Model

The Sign Event

The object model for digital signatures provides the following event.

The SignEventArgs Object

An event handler for the Sign event can work with the SignEventArgs event object, which provides the following properties.

Collections and Objects

The object model for digital signatures provides the following collections.

The object model for digital signatures provides the following objects.

Working with Digital Signatures Programmatically

The object model of the Microsoft.Office.InfoPath namespace provides members for interacting with digital signatures programmatically. In particular, fully-trusted forms can add custom information to the signature block before it is committed, according to the following timeline:

1. User chooses to add a digital signature to a form.

2. The Digital Signatures dialog box is displayed, the user clicks Add, and then selects the data to sign.

3. The Sign event for the selected data represented by the SignedDataBlock object is raised, and the Sign() method of SignedDataBlock object and the CreateSignature method of the SignatureCollection collection are executed.

4. Any optional custom actions are executed.

5. The Sign() method of the Signature object is executed.

6. The Sign dialog box is displayed for typing a name (or selecting a signature image), selecting a certificate to sign with, and to enter comments.

7. When the Sign button is clicked, the signature is added to the collection of signatures for the form and the non-repudiation information is captured and saved with the signature (which can be viewed later by clicking View Signed Form on the Digital Signatures dialog box, and then clicking See the additional signing information that was collected).

The following example invokes the Sign() method when the user signs the selected data and countersigns the signature with a timestamp value retrieved from a trusted timestamp service.

public void FormEvents_Sign(object sender, SignEventArgs e)
{
   // Add a new Signature object to the SignedDataBlockCollection.
   Signature thisSignature = 
      e.SignedDataBlock.Signatures.CreateSignature();

   // Write the XML digital signature block and compute hash
   // for signed data.
   thisSignature.Sign();

   // Countersign the signature with a trusted timestamp.

   // Get the XML node storing the signature block.
   XPathNavigator oNodeSig = thisSignature.SignatureBlockXmlNode;
   XPathNavigator oNodeSigValue = 
      oNodeSig.SelectSingleNode(
      ".//*[local-name(.)='signatureValue']");

   // Get timestamp from a trusted timestamp service (fictitious).
   MyTrustedTimeStampService s = new MyTrustedTimeStampService();
   string strVerifiedTimeStamp = s.AddTimeStamp(oNodeSigValue.Value);

   // Add the value returned from the timestamp service to the 
   // unsigned part of the signature block.
   XPathNavigator oNodeObj = 
      oNodeSig.SelectSingleNode(".//*[local-name(.)='Object']");
   XPathNavigator oNode = oNodeObj.Clone();
   oNode.SetValue(strVerifiedTimeStamp);
   oNodeObj.MoveToParent();
   oNodeObj.AppendChild(oNode);

   e.SignatureWizard = false;
}

Public Sub FormEvents_Sign(ByVal sender As Object, _
   ByVal e As SignEventArgs)
   ' Add a new Signature object to the SignedDataBlockCollection.
   Dim thisSignature As Signature = 
      e.SignedDataBlock.Signatures.CreateSignature

   ' Write the XML digital signature block and compute hash
   ' for signed data.
   thisSignature.Sign()

   ' Countersign the signature with a trusted timestamp.

   ' Get the XML node storing the signature block
   Dim oNodeSig As XPathNavigator  = _
      thisSignature.SignatureBlockXmlNode
   Dim oNodeSigValue As XPathNavigator  = _
      oNodeSig.SelectSingleNode(_
      ".//*[local-name(.)='signatureValue']")

   ' Get timestamp from a trusted timestamp service (fictitious).
   Dim s As MyTrustedTimeStampService = New MyTrustedTimeStampService()
   Dim strVerifiedTimeStamp As String  = _
      s.AddTimeStamp(oNodeSigValue.Value)

   ' Add the value returned from the timestamp service to the 
   ' unsigned part of the signature block.
   Dim oNodeObj As XPathNavigator  = 
      oNodeSig.SelectSingleNode(".//*[local-name(.)='Object']")
   Dim oNode As XPathNavigator  = oNodeObj.Clone()
   oNode.SetValue(strVerifiedTimeStamp)
   oNodeObj.MoveToParent()
   oNodeObj.AppendChild(oNode)

   e.SignatureWizard = False