Using SHA256 with the SignedXml Class

Article
11/14/2015

With the industry moving away from SHA1 including Microsoft (see https://social.technet.microsoft.com/wiki/contents/articles/32288.windows-enforcement-of-authenticode-code-signing-and-timestamping.aspx), many developers should start using SHA2 in their code. There are resources on the internet that describe how to use SHA256 with SignedXml. This blog is meant to summarize it in one place.

First of all, we need to register a SignatureDescription class that defines the DigestAlgorithm as SHA256. The .NET Cryptography namespace implements a class called RSAPKCS1SHA1SignatureDescription that supports SHA1. So we need a similar class called RSAPKCS1SHA256SignatureDescription that supports SHA256.

If your code is based on .NET 4.5 or higher, there is an RSAPKCS1SHA256SignatureDescription class you can register. You have to reference the System.Deployment assembly in your project. The full namespace is System.Deployment.Internal.CodeSigning.RSAPKCS1SHA256SignatureDescription. You must call CryptoConfig.AddAlgorithm to register the class.

Here's the MSDN SignedXml sample modified to use SHA256:

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Security.Cryptography;
using System.Security.Cryptography.X509Certificates;
using System.Security.Cryptography.Xml;
using System.Xml;
using System.Deployment.Internal.CodeSigning;

namespace SignVerify
{
    public class SignVerifyEnvelope
    {
        public static void Main(String[] args)
        {
            try
            {
                CryptoConfig.AddAlgorithm(typeof(RSAPKCS1SHA256SignatureDescription), "https://www.w3.org/2001/04/xmldsig-more#rsa-sha256");

                // Generate a signing key.
                RSACryptoServiceProvider Key = new RSACryptoServiceProvider();

                // 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", Key);
                Console.WriteLine("XML file signed.");

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

                // 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. This method does not
        // save the public key within the XML file. This file cannot be verified unless
        // the verifying code has the key with which it was signed.
        public static void SignXmlFile(string FileName, string SignedFileName, RSA Key)
        {
            // Create a new XML document.
            XmlDocument doc = new XmlDocument();

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

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

            // Add the key to the SignedXml document.
            signedXml.SigningKey = Key;
            signedXml.SignedInfo.SignatureMethod = "https://www.w3.org/2001/04/xmldsig-more#rsa-sha256";

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

            // Add an enveloped transformation to the reference.
            reference.AddTransform(new XmlDsigEnvelopedSignatureTransform());
            reference.AddTransform(new XmlDsigExcC14NTransform());
            reference.DigestMethod = "https://www.w3.org/2001/04/xmlenc#sha256";

            // 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.
            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 against an asymetric
        // algorithm and return the result.
        public static Boolean VerifyXmlFile(String Name, RSA Key)
        {
            // Create a new XML document.
            XmlDocument xmlDocument = new XmlDocument();

            // 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(Key);
        }

        // 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();
        }
    }

}

If your code is based on .NET 4.0 or you prefer not to depend on System.Deployment you can implement your own RSAPKCS1SHA256SignatureDescription class.

Shawn Neal at GitHub provides a good implementation - https://gist.github.com/sneal/f35de432115b840c4c1f\#file-rsapkcs1sha256signaturedescription.

One thing though, Microsoft recommends that you avoid using any managed classes. You should change SHA256Managed to SHA256CryptoServiceProvider.

.NET versions below 4.0 won't work because the CryptoConfig class did not provide the AddAlgorithm method to add additional algorithms to the internal algorithm table.

Using SHA256 with the SignedXml Class

Additional resources