Set client credential style to PostBody only if client assertion is set

Client secrets are not recommended in the post body by RFC 6749. We should use post body (at least by default) only for client assertions.

Also added tests of this behavior.

Author: josephdecock

src/OidcClient/AuthorizeClient.cs

@@ -141,11 +141,15 @@ private async Task<PushedAuthorizationResponse> PushAuthorizationRequestAsync(st
                 ClientSecret = _options.ClientSecret,
                 ClientAssertion = await _options.GetClientAssertionAsync(),
 
-                ClientCredentialStyle = ClientCredentialStyle.PostBody,
 
                 Parameters = CreateAuthorizeParameters(state, codeChallenge, frontChannelParameters),
             };
 
+            if(par.ClientAssertion?.Value != null)
+            {
+                par.ClientCredentialStyle = ClientCredentialStyle.PostBody;
+            }
+
             return await http.PushAuthorizationAsync(par);
         }
 

test/OidcClient.Tests/CodeFlowResponseTests.cs

@@ -16,6 +16,7 @@
 using System.Threading.Tasks;
 using IdentityModel.Client;
 using Xunit;
+using System.Web;
 
 namespace IdentityModel.OidcClient.Tests
 {
@@ -486,5 +487,82 @@ public async Task Malformed_identity_token_on_token_response_should_fail()
             result.IsError.Should().BeTrue();
             result.Error.Should().Contain("invalid_jwt");
         }
+
+        [Fact]
+        public async Task Authorize_should_push_parameters_when_PAR_is_enabled()
+        {
+            // Configure the client for PAR, authenticating with a client secret
+            _options.ClientSecret = "secret";
+            _options.ProviderInformation.PushedAuthorizationRequestEndpoint = "https://this-is-set-so-par-will-be-used";
+            var client = new OidcClient(_options);
+
+            // Mock the response from the par endpoint
+            var requestUri = "mocked_request_uri";
+            var parResponse = new Dictionary<string, string>
+            {
+                { "request_uri", requestUri }
+            };
+            var backChannelHandler = new NetworkHandler(JsonSerializer.Serialize(parResponse), HttpStatusCode.OK);
+            _options.BackchannelHandler = backChannelHandler;
+
+            // Prepare the login to cause the backchannel PAR request
+            var state = await client.PrepareLoginAsync();
+
+            // Validate that the resulting PAR state is correct
+            var startUrl = new Uri(state.StartUrl);
+            var startUrlQueryParams = HttpUtility.ParseQueryString(startUrl.Query);
+            startUrlQueryParams.Should().HaveCount(2);
+            startUrlQueryParams.GetValues("client_id").Single().Should().Be("client");
+            startUrlQueryParams.GetValues("request_uri").Single().Should().Be(requestUri);
+
+            // Validate that the client authentication during the PAR request was correct
+            var request = backChannelHandler.Request;
+            request.Headers.Authorization.Should().NotBeNull();
+            request.Headers.Authorization.Scheme.Should().Be("Basic");
+            request.Headers.Authorization.Parameter.Should()
+                .Be(BasicAuthenticationOAuthHeaderValue.EncodeCredential("client", "secret"));
+        }
+        
+        [Fact]
+        public async Task Par_request_should_include_client_assertion_in_body()
+        {
+            // Configure the client for PAR, authenticating with a client assertion
+            var clientAssertion = "mocked_client_assertion";
+            var clientAssertionType = "mocked_assertion_type";
+            _options.ClientAssertion = new ClientAssertion
+            {
+                Type = clientAssertionType,
+                Value = clientAssertion
+            };
+            _options.ProviderInformation.PushedAuthorizationRequestEndpoint = "https://this-is-set-so-par-will-be-used";
+            var client = new OidcClient(_options);
+
+            // Mock the response from the par endpoint
+            var requestUri = "mocked_request_uri";
+            var parResponse = new Dictionary<string, string>
+            {
+                { "request_uri", requestUri }
+            };
+            var backChannelHandler = new NetworkHandler(JsonSerializer.Serialize(parResponse), HttpStatusCode.OK);
+            _options.BackchannelHandler = backChannelHandler;
+
+            // Prepare the login to cause the backchannel PAR request
+            var state = await client.PrepareLoginAsync();
+
+            // Validate that the resulting PAR state is correct
+            var startUrl = new Uri(state.StartUrl);
+            var startUrlQueryParams = HttpUtility.ParseQueryString(startUrl.Query);
+            startUrlQueryParams.Should().HaveCount(2);
+            startUrlQueryParams.GetValues("client_id").Single().Should().Be("client");
+            startUrlQueryParams.GetValues("request_uri").Single().Should().Be(requestUri);
+
+            // Validate that the client authentication during the PAR request was correct
+            var parRequest = backChannelHandler.Request;
+            var parContent = await parRequest.Content.ReadAsStringAsync();
+            var parParams = HttpUtility.ParseQueryString(parContent);
+            parParams.GetValues("client_assertion").Single().Should().Be(clientAssertion);
+            parParams.GetValues("client_assertion_type").Single().Should().Be(clientAssertionType);
+            parRequest.Headers.Authorization.Should().BeNull();
+        }
     }
 }