How to: Sign XML Documents with Digital Signatures
You can use the classes in the System.Security.Cryptography.Xml namespace to sign an XML document or part of an XML document with a digital signature. XML digital signatures (XMLDSIG) allow you to verify that data was not altered after it was signed. For more information about the XMLDSIG standard, see the World Wide Web Consortium (W3C) recommendation XML Signature Syntax and Processing.
The code example in this procedure demonstrates how to digitally sign an entire XML document and attach the signature to the document in a <Signature> element. The example creates an RSA signing key, adds the key to a secure key container, and then uses the key to digitally sign an XML document. The key can then be retrieved to verify the XML digital signature, or can be used to sign another XML document.
For information about how to verify an XML digital signature that was created using this procedure, see How to: Verify the Digital Signatures of XML Documents.
To digitally sign an XML document
Generate an asymmetric key using the RSACryptoServiceProvider class. The key is automatically saved to the key container when you pass the CspParameters object to the constructor of the RSACryptoServiceProvider class. This key will be used to sign the XML document.
Add an XmlDsigEnvelopedSignatureTransform object to the Reference object. A transformation allows the verifier to represent the XML data in the identical manner that the signer used. XML data can be represented in different ways, so this step is vital to verification.
This example assumes that a file named test.xml exists in the same directory as the compiled program. You can place the following XML into a file called test.xml and use it with this example.
<root> <creditcard> <number>19834209</number> <expiry>02/02/2002</expiry> </creditcard> </root>
Imports System Imports System.Security.Cryptography Imports System.Security.Cryptography.Xml Imports System.Xml Module SignXML Sub Main(ByVal args() As String) Try ' Create a new CspParameters object to specify ' a key container. Dim cspParams As New CspParameters() cspParams.KeyContainerName = "XML_DSIG_RSA_KEY" ' Create a new RSA signing key and save it in the container. Dim rsaKey As New RSACryptoServiceProvider(cspParams) ' Create a new XML document. Dim xmlDoc As New XmlDocument() ' Load an XML file into the XmlDocument object. xmlDoc.PreserveWhitespace = True xmlDoc.Load("test.xml") ' Sign the XML document. SignXml(xmlDoc, rsaKey) Console.WriteLine("XML file signed.") ' Save the document. xmlDoc.Save("test.xml") Catch e As Exception Console.WriteLine(e.Message) End Try End Sub ' Sign an XML file. ' This document cannot be verified unless the verifying ' code has the key with which it was signed. Sub SignXml(ByVal xmlDoc As XmlDocument, ByVal Key As RSA) ' Check arguments. If xmlDoc Is Nothing Then Throw New ArgumentException("xmlDoc") End If If Key Is Nothing Then Throw New ArgumentException("Key") End If ' Create a SignedXml object. Dim signedXml As New SignedXml(xmlDoc) ' Add the key to the SignedXml document. signedXml.SigningKey = rsaKey ' Create a reference to be signed. Dim reference As New Reference() reference.Uri = "" ' Add an enveloped transformation to the reference. Dim env As New XmlDsigEnvelopedSignatureTransform() reference.AddTransform(env) ' Add the reference to the SignedXml object. signedXml.AddReference(reference) ' Compute the signature. signedXml.ComputeSignature() ' Get the XML representation of the signature and save ' it to an XmlElement object. Dim xmlDigitalSignature As XmlElement = signedXml.GetXml() ' Append the element to the XML document. xmlDoc.DocumentElement.AppendChild(xmlDoc.ImportNode(xmlDigitalSignature, True)) End Sub End Module
Compiling the Code
.NET Framework Security
Never store or transfer the private key of an asymmetric key pair in plaintext. For more information about symmetric and asymmetric cryptographic keys, see Generating Keys for Encryption and Decryption.
Never embed a private key directly into your source code. Embedded keys can be easily read from an assembly using the Ildasm.exe (IL Disassembler) or by opening the assembly in a text editor such as Notepad.