Accessing Function App via App Service Authentication from Dynamics 365 Plugin - azure

Question

I am trying to access an Azure Function from Dynamics 365 Plugin via service to service call: https://learn.microsoft.com/en-us/azure/active-directory/develop/active-directory-protocols-oauth-service-to-service
This function is protected via App Service Authentication.
I created a Function App and enabled App Service Authentication
under Platform Features -> Authentication/Authorisation.
I enabled Azure Active Directory as the Auth Provider and set Management mode to Express
I then got the generated Client ID and Client Secret from the Advanced Mode:
Apparently this is all that is needed to make a token request for the Azure function based, based on article I need 4 required parameters:
Client ID
Client Secret
Grant Type
Resource
I make the following request to generate a token from a Dynamics 365 Plugin but get the following error:
Invalid Plugin Execution Exception: Microsoft.Xrm.Sdk.InvalidPluginExecutionException: {"error":"invalid_client","error_description":"AADSTS70002: Error validating credentials. AADSTS50012: Invalid client secret is provided.\r\nTrace ID: 06ddda7f-2996-4c9b-ab7e-b685ee933700\r\nCorrelation ID: d582e2f2-91eb-4595-b44b-e95f42f2f071\r\nTimestamp: 2018-05-23 06:30:58Z","error_codes":[70002,50012],"timestamp":"2018-05-23 06:30:58Z","trace_id":"06ddda7f-2996-4c9b-ab7e-b685ee933700","correlation_id":"d582e2f2-91eb-4595-b44b-e95f42f2f071"}-The remote server returned an error: (401) Unauthorized.
My code is :
var tokenendpoint = "https://login.microsoftonline.com/de194c13-5ff7-4085-91c3-ac06fb869f28/oauth2/token";
var reqstring = "client_id=" + Uri.EscapeDataString("5f315431-e4da-4f68-be77-4e257b1b9295");
reqstring += "&client_secret=" + Uri.EscapeDataString("/oK7nh8pl+LImBxjm+L7WsQdyILErysOdjpzvA9g9JA=");
reqstring += "&resource=" + Uri.EscapeUriString("https://keyvaultaccess.azurewebsites.net");
reqstring += "&grant_type=client_credentials";
//Token request
WebRequest req = WebRequest.Create(tokenendpoint);
req.ContentType = "application/x-www-form-urlencoded";
req.Method = "POST";
byte[] bytes = System.Text.Encoding.ASCII.GetBytes(reqstring);
req.ContentLength = bytes.Length;
System.IO.Stream os = req.GetRequestStream();
os.Write(bytes, 0, bytes.Length);
os.Close();
//Token response
HttpWebResponse resp = (HttpWebResponse)req.GetResponse();
StreamReader tokenreader = new StreamReader(resp.GetResponseStream());
string responseBody = tokenreader.ReadToEnd();
I have made sure that I have the correct Client Secret and have also encoded it as I have read somewhere that '+' and '/' are no good.
I am getting the same error in Postman
Any ideas??

Answer 1

reqstring += "&resource=" + Uri.EscapeUriString("https://keyvaultaccess.azurewebsites.net");
Since you set resource parameter to https://keyvaultaccess.azurewebsites.net, I assumed that you have set the App ID URI of your AAD app (clientId equals 5f315431-xxxxxxx-4e257b1b9295) to https://keyvaultaccess.azurewebsites.net. I assume that you could retrieve the access_token, but when accessing your azure function endpoint with the access_token, you got the 401 status code.
You need to modify your advanced management mode for Active Directory Authentication, add https://keyvaultaccess.azurewebsites.net to ALLOWED TOKEN AUDIENCES or change the resource parameter to your AAD clientID when sending the token request for acquiring the access_token.
Active Directory Authentication configuration:
TEST:
Docode the JWT token to check the aud property:
Access my Azure Function endpoint:
Note: You need to take care of the Authorization level of your function as follows:
If you also enable function level authentication, your request sent to azure function needs to have the relevant code parameter in the querystring or set the header x-functions-key with the value of your function key, or you may just set the authorization level to Anonymous.