Adds feature so that an IDP can choose the authentication scheme that will be challenged.

Author: gislikonrad

src/Solid.Identity.Protocols.Saml2p/Middleware/Idp/AcceptSsoEndpointMiddleware.cs

@@ -64,7 +64,10 @@ public override async Task InvokeAsync(HttpContext context)
 
             await Events.InvokeAsync(Options, partner, e => e.OnAcceptSso(context.RequestServices, ssoContext));
 
-            await ChallengeAsync(context, request, ssoContext.ReturnUrl);
+            if(ssoContext.AuthenticationScheme != null)
+                await ChallengeAsync(context, request, ssoContext.ReturnUrl, ssoContext.AuthenticationScheme);
+            else
+                await ChallengeAsync(context, request, ssoContext.ReturnUrl);
         }
     }
 }

src/Solid.Identity.Protocols.Saml2p/Middleware/Idp/InitiateSsoEndpointMiddleware.cs

@@ -70,7 +70,10 @@ public override async Task InvokeAsync(HttpContext context)
 
             await Events.InvokeAsync(Options, partner, e => e.OnInitiateSso(context.RequestServices, ssoContext));
 
-            await ChallengeAsync(context, request, ssoContext.ReturnUrl);
+            if (ssoContext.AuthenticationScheme != null)
+                await ChallengeAsync(context, request, ssoContext.ReturnUrl, ssoContext.AuthenticationScheme);
+            else
+                await ChallengeAsync(context, request, ssoContext.ReturnUrl);
         }
     }
 }

src/Solid.Identity.Protocols.Saml2p/Options/Saml2pIdentityProviderEvents.cs

@@ -15,13 +15,13 @@ public class Saml2pIdentityProviderEvents
     {
         /// <summary>
         /// This is run when an SP sends an <see cref="AuthnRequest"/> to your accept sso endpoint.
-        /// <para>You can manually replace <see cref="AcceptSsoContext.ReturnUrl"/> here.</para>
+        /// <para>You can manually set <see cref="AcceptSsoContext.AuthenticationScheme"/> and/or replace <see cref="AcceptSsoContext.ReturnUrl"/> here.</para>
         /// </summary>
         public Func<IServiceProvider, AcceptSsoContext, ValueTask> OnAcceptSso { get; set; } = (_, __) => new ValueTask();
 
         /// <summary>
         /// This is run before initiate SSO by sending a <see cref="SamlResponse"/> to an SP.
-        /// <para>You can manually replace <see cref="InitiateSsoContext.ReturnUrl"/> here.</para>
+        /// <para>You can manually set <see cref="AcceptSsoContext.AuthenticationScheme"/> and/or replace <see cref="InitiateSsoContext.ReturnUrl"/> here.</para>
         /// </summary>
         public Func<IServiceProvider, InitiateSsoContext, ValueTask> OnInitiateSso { get; set; } = (_, __) => new ValueTask();