SAML with a SKI instead of certificate in X509Data node - x509certificate

Question

I have already implemented authentication mechanism based on SAML protocol. Project use SAML2 library. Everything worked fine until change on the server has ocurred. The server used to respond with <ds:X509Certificate> node:
<ds:KeyInfo><ds:X509Data><ds:X509Certificate>Here was certificate</ds:X509Certificate></ds:X509Data></ds:KeyInfo>
But it has changed to:
<ds:X509SKI>Here is Subject Key Identifier</ds:X509SKI>
SAML2 library has CheckSignature method which can be applied on server response:
/// <summary>
/// Checks the signature.
/// </summary>
/// <returns>True of the signature is valid, else false.</returns>
public bool CheckSignature()
{
return XmlSignatureUtils.CheckSignature(Document);
}
It points here:
/// <summary>
/// Verifies the signature of the XmlDocument instance using the key enclosed with the signature.
/// </summary>
/// <param name="doc">The doc.</param>
/// <returns><code>true</code> if the document's signature can be verified. <code>false</code> if the signature could
/// not be verified.</returns>
/// <exception cref="InvalidOperationException">if the XmlDocument instance does not contain a signed XML document.</exception>
public static bool CheckSignature(XmlDocument doc)
{
CheckDocument(doc);
var signedXml = RetrieveSignature(doc);
if (signedXml.SignatureMethod.Contains("rsa-sha256"))
{
// SHA256 keys must be obtained from message manually
var trustedCertificates = GetCertificates(doc);
foreach (var cert in trustedCertificates)
{
if (signedXml.CheckSignature(cert.PublicKey.Key))
{
return true;
}
}
return false;
}
return signedXml.CheckSignature();
}
And finally GetCertificates method looks like that:
/// <summary>
/// Gets the certificates.
/// </summary>
/// <param name="doc">The document.</param>
/// <returns>List of <see cref="X509Certificate2"/>.</returns>
private static List<X509Certificate2> GetCertificates(XmlDocument doc)
{
var certificates = new List<X509Certificate2>();
var x509CertificateNodeList = doc.GetElementsByTagName("ds:X509Certificate");
if (x509CertificateNodeList.Count == 0)
{
x509CertificateNodeList = doc.GetElementsByTagName("X509Certificate");
}
foreach (XmlNode xn in x509CertificateNodeList)
{
try
{
var xc = new X509Certificate2(Convert.FromBase64String(xn.InnerText));
certificates.Add(xc);
}
catch
{
// Swallow the certificate parse error
}
}
return certificates;
}
As you can see the library checks only certificates not subject key identifiers. I believe I can implement SKI comparison between installed certificate and provided element on my own, but I'm not sure if this is legal way to do it.
Here Thomas Pornin wrote:
The Subject Key Identifier does not play a role in validation, at
least not in the algorithm which makes up section 6 of RFC 5280. It is
meant to be an help for path building
His statement suggest I can't do validation by comparing SKI from server response and installed certificate.
RFC 5280 Suggests the same, but I don't have enough time to read it carefully, so I'm searching for your help.
Is comparison of subject key identifier of installed X509 certificate and those in SAML response right way to verify response?

Answer 1

No, as already mentioned, SKI is used only to bind certificates in the chain (when key match is used). It doesn't provide enough information about the certificate and its details.
However, if client have a full certificate preinstalled, client could use SKI to locate the right certificate and use the cert for validation procedures.