[SignedXml].CheckSignature() do not pass with

Inclusive canonicalization (REC-xml-c14n-20010315) can cause SignedXml.CheckSignature() to fail in .NET 8 due to namespace handling differences, especially when default or unused namespaces are present. Exclusive canonicalization avoids these issues by omitting unused namespaces.

 Key Differences Between Inclusive and Exclusive Canonicalization

• Inclusive Canonicalization (REC-xml-c14n-20010315):

  • Includes all namespace declarations in the canonicalized output, even if they’re not used in the signed node.
  • This can lead to mismatches between the digest value and the actual canonicalized content during verification.

• Exclusive Canonicalization (xml-exc-c14n#):

  • Includes only the namespaces that are actually used in the signed node.
  • This results in a cleaner digest and fewer verification issues.

 Why It Fails in .NET 8

• .NET’s SignedXml implementation is sensitive to namespace context and whitespace.
• If the inclusive canonicalization includes extra namespaces or whitespace that weren’t present during signing, the computed digest during verification will differ from the original.
• This mismatch causes CheckSignature() to return false.

 How to Fix or Work Around It

1. Prefer Exclusive Canonicalization:

   • Use http://www.w3.org/2001/10/xml-exc-c14n# in your and elements.
   • This is especially important when signing nested elements or working with SOAP messages.

2. Ensure PreserveWhitespace = true:

   • Before loading the XML:

     XmlDocument doc = new XmlDocument();
     doc.PreserveWhitespace = true;
     doc.Load(xmlStream);
     

3. Avoid unnecessary namespaces:

   • Clean up unused or default namespaces in the signed portion of the XML.

4. Use XmlDsigExcC14NTransform explicitly:

   • When creating the signature:

     var transform = new XmlDsigExcC14NTransform();
     reference.AddTransform(transform);