SignedXml.CheckSignatureReturningKey(AsymmetricAlgorithm) Method

Definition

Namespace:

System.Security.Cryptography.Xml

Assembly:

System.Security.Cryptography.Xml.dll

Source:

SignedXml.cs

Determines whether the Signature property verifies using the public key in the signature.

C#

public bool CheckSignatureReturningKey(out System.Security.Cryptography.AsymmetricAlgorithm? signingKey);

Parameters

signingKey

AsymmetricAlgorithm

When this method returns, contains the implementation of AsymmetricAlgorithm that holds the public key in the signature. This parameter is passed uninitialized.

Returns

Boolean

true if the Signature property verifies using the public key in the signature; otherwise, false.

Exceptions

ArgumentNullException

The signingKey parameter is null.

CryptographicException

The SignatureAlgorithm property of the public key in the signature does not match the SignatureMethod property.

-or-

The signature description could not be created.

-or

The hash algorithm could not be created.

Examples

The following code example shows how to sign and verify an entire XML document using an enveloping signature.

C#

//
// This example signs an XML file using an
// envelope signature. It then verifies the 
// signed XML.
//
using System;
using System.Security.Cryptography;
using System.Security.Cryptography.Xml;
using System.Text;
using System.Xml;

public class SignVerifyEnvelope
{

    public static void Main(String[] args)
    {
        try
        {
            // Generate a RSA signing key.
            RSA RSAKey = RSA.Create();

            // Create an XML file to sign.
            CreateSomeXml("Example.xml");
            Console.WriteLine("New XML file created."); 

            // Sign the XML that was just created and save it in a 
            // new file.
            SignXmlFile("Example.xml", "SignedExample.xml", RSAKey);
            Console.WriteLine("XML file signed."); 

            // Verify the signature of the signed XML.
            Console.WriteLine("Verifying signature...");
            bool result = VerifyXmlFile("SignedExample.xml");

            // Display the results of the signature verification to \
            // the console.
            if(result)
            {
                Console.WriteLine("The XML signature is valid.");
            }
            else
            {
                Console.WriteLine("The XML signature is not valid.");
            }
        }
        catch(CryptographicException e)
        {
            Console.WriteLine(e.Message);
        }
    }

    // Sign an XML file and save the signature in a new file.
    public static void SignXmlFile(string FileName, string SignedFileName, RSA RSAKey)
    {
        // Create a new XML document.
        XmlDocument doc = new XmlDocument();

        // Format the document to ignore white spaces.
        doc.PreserveWhitespace = false;

        // Load the passed XML file using it's name.
        doc.Load(new XmlTextReader(FileName));

        // Create a SignedXml object.
        SignedXml signedXml = new SignedXml(doc);

        // Add the RSA key to the SignedXml document. 
        signedXml.SigningKey = RSAKey;

        // Create a reference to be signed.
        Reference reference = new Reference();
        reference.Uri = "";

        // Add an enveloped transformation to the reference.
        XmlDsigEnvelopedSignatureTransform env = new XmlDsigEnvelopedSignatureTransform();
        reference.AddTransform(env);

        // Add the reference to the SignedXml object.
        signedXml.AddReference(reference);

        // Add a RSAKeyValue to the KeyInfo (optional; helps recipient find key to validate).
        KeyInfo keyInfo = new KeyInfo();
        keyInfo.AddClause(new RSAKeyValue((RSA)RSAKey));
        signedXml.KeyInfo = keyInfo;

        // Compute the signature.
        signedXml.ComputeSignature();

        // Get the XML representation of the signature and save
        // it to an XmlElement object.
        XmlElement xmlDigitalSignature = signedXml.GetXml();

        // Append the element to the XML document.
        doc.DocumentElement.AppendChild(doc.ImportNode(xmlDigitalSignature, true));

        if (doc.FirstChild is XmlDeclaration)  
        {
            doc.RemoveChild(doc.FirstChild);
        }

        // Save the signed XML document to a file specified
        // using the passed string.
        XmlTextWriter xmltw = new XmlTextWriter(SignedFileName, new UTF8Encoding(false));
        doc.WriteTo(xmltw);
        xmltw.Close();
    }
    // Verify the signature of an XML file and return the result.
    public static Boolean VerifyXmlFile(String Name)
    {
        // Create a new XML document.
        XmlDocument xmlDocument = new XmlDocument();

        // Format using white spaces.
        xmlDocument.PreserveWhitespace = true;

        // Load the passed XML file into the document. 
        xmlDocument.Load(Name);

        // Create a new SignedXml object and pass it
        // the XML document class.
        SignedXml signedXml = new SignedXml(xmlDocument);

        // Find the "Signature" node and create a new
        // XmlNodeList object.
        XmlNodeList nodeList = xmlDocument.GetElementsByTagName("Signature");

        // Load the signature node.
        signedXml.LoadXml((XmlElement)nodeList[0]);

        // Check the signature and return the result.
        return signedXml.CheckSignature();
    }

    // Create example data to sign.
    public static void CreateSomeXml(string FileName)
    {
        // Create a new XmlDocument object.
        XmlDocument document = new XmlDocument();

        // Create a new XmlNode object.
        XmlNode  node = document.CreateNode(XmlNodeType.Element, "", "MyElement", "samples");
        
        // Add some text to the node.
        node.InnerText = "Example text to be signed.";

        // Append the node to the document.
        document.AppendChild(node);

        // Save the XML document to the file name specified.
        XmlTextWriter xmltw = new XmlTextWriter(FileName, new UTF8Encoding(false));
        document.WriteTo(xmltw);
        xmltw.Close();
    }
}

Applies to