Help with ECDSA Verification in .NET

SignedXml in .NET does not natively support ECDSA verification for XML signatures, especially with algorithms like ECDSA-SHA384. You’ll need to extend or customize the verification logic.

Here's a breakdown of the issue and how to address it:

 Why ECDSA Signatures Fail with SignedXml.CheckSignature()

• The SignedXml class in .NET is designed around XMLDSIG standards, which historically focused on RSA and DSA.
• While newer versions of .NET support ECDSA keys, the default SignedXml implementation does not automatically support ECDSA algorithms like http://www.w3.org/2001/04/xmldsig-more#ecdsa-sha384.
• The failure occurs because SignedXml doesn’t recognize or handle the ECDSA signature algorithm URI properly during verification.

 How to Verify ECDSA-SHA384 Signatures in XML

To verify ECDSA signatures, you’ll need to:

1. Extend SignedXml to support ECDSA

Create a custom class that inherits from SignedXml and override the GetSignatureMethod() and GetHashAlgorithm() methods to handle ECDSA URIs.

public class EcdsaSignedXml : SignedXml
{
    public EcdsaSignedXml(XmlDocument doc) : base(doc) {}

    public override AsymmetricAlgorithm GetPublicKey()
    {
        return base.Signature.Key;
    }

    protected override AsymmetricSignatureDeformatter GetSignatureFormatter(AsymmetricAlgorithm key)
    {
        if (key is ECDsa)
        {
            var formatter = new ECDsaSignatureDeformatter((ECDsa)key);
            formatter.SetHashAlgorithm("SHA384");
            return formatter;
        }
        return base.GetSignatureFormatter(key);
    }

    protected override AsymmetricSignatureDeformatter GetSignatureDeformatter(AsymmetricAlgorithm key)
    {
        if (key is ECDsa)
        {
            var deformatter = new ECDsaSignatureDeformatter((ECDsa)key);
            deformatter.SetHashAlgorithm("SHA384");
            return deformatter;
        }
        return base.GetSignatureDeformatter(key);
    }
}

2. Use the custom class for verification

var signedXml = new EcdsaSignedXml(xmlDoc);
signedXml.LoadXml(signatureNode);
bool result = signedXml.CheckSignature(publicKey);

Make sure publicKey is an instance of ECDsa loaded from the certificate.

Helpful Resources

• Microsoft Docs on SignedXml
• Stack Overflow discussion on ECDSA signature verification in C#
• Example code for ECDSA signing and verification