CODEWITHSUNDEEP
c#xmlx509sha256xml-signature
MichaelXanadu
MichaelXanadu

Reputation: 505

How to sign XML with SHA256 & X.509 correctly?

I want to sign a XML with SHA256 by using a self-signed X.509 certificate. I was inspired by many articles in Stackoverflow and use this code:

X509Certificate2 cert = new X509Certificate2();
X509Store store = new X509Store(StoreName.Root, StoreLocation.LocalMachine);
store.Open(OpenFlags.ReadOnly);
foreach (X509Certificate2 cert2 in store.Certificates)
{
    if (cert2.Subject == "CN=TestCerificate")
    {
        cert = cert2;
        break;
    }
}

store.Close();

CryptoConfig.AddAlgorithm(typeof(RSAPKCS1SHA256SignatureDescription), "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256");
var exportedKeyMaterial = cert.PrivateKey.ToXmlString(true);
var key = new RSACryptoServiceProvider(new CspParameters(24));
key.PersistKeyInCsp = false;
key.FromXmlString(exportedKeyMaterial);

XmlDocument doc = new XmlDocument();
doc.PreserveWhitespace = true;
doc.Load("test.xml");

SignedXml signedXml = new SignedXml(doc);
signedXml.SigningKey = key;
signedXml.SignedInfo.SignatureMethod = "http://www.w3.org/2001/04/xmldsig-more#rsa-sha256";

Reference reference = new Reference();
reference.AddTransform(new XmlDsigEnvelopedSignatureTransform());
reference.AddTransform(new XmlDsigExcC14NTransform());
reference.Uri = "";
reference.DigestMethod = "http://www.w3.org/2001/04/xmlenc#sha256"; 
signedXml.AddReference(reference);

KeyInfo keyInfo = new KeyInfo();
keyInfo.AddClause(new KeyInfoX509Data(cert));
signedXml.KeyInfo = keyInfo;
signedXml.ComputeSignature();

doc.Save("testSig.xml");

Afterwards, the generated signed XML part of "testSig.xml" looks like this:

<Signature xmlns="http://www.w3.org/2000/09/xmldsig#">
    <SignedInfo>
      <CanonicalizationMethod Algorithm="http://www.w3.org/TR/2001/REC-xml-c14n-20010315" />
      <SignatureMethod Algorithm="http://www.w3.org/2000/09/xmldsig#dsa-sha1" />
      <Reference URI="">
        <Transforms>
          <Transform Algorithm="http://www.w3.org/2000/09/xmldsig#enveloped-signature" />
        </Transforms>
        <DigestMethod Algorithm="http://www.w3.org/2000/09/xmldsig#sha1" />
        <DigestValue>ghOEPeYtAUs5Kb8VMOCIS3f2wIY=</DigestValue>
      </Reference>
    </SignedInfo>
    <SignatureValue>HANI0GrICbyc5tlmvtU9cB7txdxtuY4uDsntp5XVzaRQbts76ff3Qg==</SignatureValue>
  </Signature>

As you can see the XML says that "SHA1" was used to sign the XML instead (see nodes "SignatureMethod" & "DigestMethod"). But I would expect, that the SHA256-URLs have to be stated in these nodes. Is this assumption correct? If yes, does anybody know how to solve this problem? I use C# 4.6.0.

With best regards, Michael

Upvotes: 2

Views: 4510

Answers (1)

bartonjs
bartonjs

Reputation: 33108

While you successfully called signedXml.ComputeSignature() that just does the computation, it doesn't insert a signature element into the document (the XmlDocument you pass in the constructor is just what document GetXml()'s node will parent to).

You need to call signedXml.GetXml() (after ComputeSignature) to get the computed ds:Signature element, then insert that into your document.

The fact that your document has a signature node already in it is confusing. My first thought was that test.xml already had that signature in it, but your comment says that it was just some oops code from elsewhere. Lesson learned, I guess :).

Upvotes: 1

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