Security token validation between Identity Server 4 and 3 - webforms

Question

I am trying to use a IdS4 server on .Net Core 2.0 with an IdS3 webforms client on .Net45.
As I login via the client I get this exception on the client browser.
[SecurityTokenSignatureKeyNotFoundException: IDX10500: Signature validation failed. Unable to resolve SecurityKeyIdentifier: 'SecurityKeyIdentifier
(
IsReadOnly = False,
Count = 2,
Clause[0] = X509ThumbprintKeyIdentifierClause(Hash = 0x6B7ACC520305BFDB4F7252DAEB2177CC091FAAE1),
Clause[1] = System.IdentityModel.Tokens.NamedKeySecurityKeyIdentifierClause
)
',
token: '{"alg":"RS256","kid":"6B7ACC520305BFDB4F7252DAEB2177CC091FAAE1","typ":"JWT",
"x5t":"a3rMUgMFv9tPclLa6yF3zAkfquE"}.{"nbf":1517303703,"exp":1517304003,
"iss":"http://localhost:5000","aud":"webforms","nonce":"636529004845229500.Mjg4YmMxMGEtZjk2MC00YWY5LWJiNTQtYmU0Njg0MDIwYTFhNzczN2Q1ZGMtN2YxYy00NGJmLWJhNzItNTM1ZDc0OTMyNzBj",
"iat":1517303703,"c_hash":"6Sty4gdTWGo4nEo0V_VSVQ","sid":"17936a127b0267d2588646052c4447c6",
"sub":"6498d093-8dc3-4d69-988e-3914d564f4d0","auth_time":1517303700,
"idp":"local","amr":["pwd"]}'.]
I first got this exception without Clause[0] and thought it was because the two samples I was using have different certificates embedded within them.
My attempt to fix this involved creating a new certificate following this guide.
In IdS4 Startup I have
services.AddIdentityServer()
.AddSigningCredential(GetSigningCredential())
and
private X509Certificate2 GetSigningCredential()
{
var store = new X509Store(StoreName.My, StoreLocation.LocalMachine);
store.Open(OpenFlags.ReadOnly);
var certs = store.Certificates.Find(X509FindType.FindBySerialNumber, "3506fe4f69dc22b340e9c2af500d4659", false);
store.Close();
return certs[0];
}
With the clients secret set to the X509 thumbprint.
This seems to be working. On the IdS3 client I cannot find a way to validate the security token, I assume this would be done by validating the certificate?
If anybody could help me understand my issue better that would be great, I cannot find any useful documentation or examples relating to my case so pretty much anything would be helpful.
Thanks in advance.

Answer 1

Turns out I was trying to validate in the wrong places. All i had to do was point to the certificate in the clients Startup.cs.
app.UseOpenIdConnectAuthentication(new OpenIdConnectAuthenticationOptions
{
Configuration = new OpenIdConnectConfiguration()
{
// Other Stuff...
SigningTokens = { new X509SecurityToken(GetX509Certificate2()) },
// More Stuff...
Where GetX509Certificate2() is:
private X509Certificate2 GetX509Certificate2()
{
var store = new X509Store(StoreName.TrustedPeople, StoreLocation.LocalMachine);
store.Open(OpenFlags.ReadOnly);
return cert = store.Certificates.Find(X509FindType.FindByThumbprint, "**thumbprint**", false)[0];
}