Getting "Signature is invalid." when using Artifact Binding during the artifact consumption step - itfoxtec-identity-saml2

Question

I have an IdP and an SP setup using the ITfoxtec SAML2 libraries, and everything works great when not using artifact binding, or when not validating signatures. When using artifact binding and validating signatures I'm getting a "Signature is invalid." exception in the ACS when trying to retrieve and bind the actual response/assertion.
It seems to unbind the artifact response fine, then when it goes to retrieve and unbind the artifact from the ArtifactResolutionService it fails, specifically on the last line of this block:
var soapEnvelope = new Saml2SoapEnvelope();
saml2AuthnResponse = new Saml2AuthnResponse(config);
await soapEnvelope.ResolveAsync(httpClient, saml2ArtifactResolve, saml2AuthnResponse);
I've checked that my signature validation certificate is correct and I've dug through the source code but am scratching my head. I've tried to validate the "saml2p:ArtifactResponse" myself but there isn't much out there.
If I put this line before the chunk above everything works as expected as it no longer validates the signature:
config.SignatureValidationCertificates.Clear();
One thing I noticed is that in the 'saml2p:ArtifactResponse' there is a signature inside of that node but not inside the contained 'saml2p:Response' node. Is it possible that the saml2p:Response is being isolated and then a signature check is being performed? I tried to see if it was supposed to be signing the response/assertion in the artifact cache on the IdP side (artifactSaml2AuthnResponseCache), but it doesn't sign response at all. I'm doing this before putting it in the cache just like in the example and just like I do when using POST binding:
var token = saml2AuthnResponse.CreateSecurityToken(relyingParty.Issuer, subjectConfirmationLifetime: 5, issuedTokenLifetime: 60);
artifactSaml2AuthnResponseCache[saml2ArtifactResolve.Artifact] = saml2AuthnResponse;`
EDIT: I have determined that the ArtifactResponse just isn't signed properly. Another tool claims the digest in the XML doesn't match the computed value. This is after stepping through the source and grabbing the XML that the code is trying to validate directly. I can see that the ArtifactResolve is being signed and validated properly (and I checked with the external tool) but the ArtifactResponse isn't. Even in the code it fails at the final validation of the signature (and not at any checks before it).
EDIT 2: Found the problem in the source. The .ToXmlDocument() extension is breaking the signed XML. The final test was done by 'replacing' it in the spot with a new method that just returns the string directly with "envelope.ToString(SaveOptions.DisableFormatting)":
protected virtual XmlDocument ToSoapXml()
{
var envelope = new XElement(Saml2Constants.SoapEnvironmentNamespaceX + Saml2Constants.Message.Envelope);
envelope.Add(GetXContent());
return envelope.ToXmlDocument();
}
protected string ToSoapXmlString()
{
var envelope = new XElement(Saml2Constants.SoapEnvironmentNamespaceX + Saml2Constants.Message.Envelope);
envelope.Add(GetXContent());
return envelope.ToString(SaveOptions.DisableFormatting);//.ToXmlDocument();
}
And directly save that to the SoapResponseXml of the Saml2SoapEnvelope:
protected override Saml2SoapEnvelope BindInternal(Saml2Request saml2Request, string messageName)
{
if (!(saml2Request is Saml2ArtifactResponse))
throw new ArgumentException("Only Saml2ArtifactResponse is supported");
BindInternal(saml2Request);
SoapResponseXml = ToSoapXmlString();// ToSoapXml().OuterXml;
return this;
}
I would initiate a pull request for this change but honestly I'm not that up to speed with Git. I'm also not sure if this is the best way to fix the issue.

Answer 1

Thank you for your question and code to solve the problem. I'll look into the problem.
EDIT: I'm trying to reproduce the error but no luck. The sample is both an IdP an RP, what have you changed to get the error?