SharePoint 2013 Cross-Domain Library Scenario: Authentication Mechanism for Remote App - sharepoint

Question

I have a SharePoint provider-hosted app that exposes a Web API endpoint. I'm using this endpoint as a middle man to call a secured external web service. I want to make calls to my Web API endpoint via javascript in a SharePoint Page (a Publishing Page) in my host web. Since this is a cross-domain call, I'm utilizing SharePoint's cross domain library (SP.RequestExecutor.js). I followed the steps in this article to create my custom proxy page that is required by the cross-domain library. Everything works fine. I can call my service via SP.RequestExecutor with no issues. Now, I just want to require authentication for accessing the Web API endpoint.
The article that I reference states that I'm responsible for the authentication mechanism. I just can't seem to come up with a really secure one and there are literally no examples on the web. I'd really like to leverage the identity of the SharePoint user somehow since only SharePoint users will be hitting the Web API endpoint, I just can't figure out how. SP.RequestExecutor won't let me pass the SPHostUrl querystring parameter when hitting the endpoint, so that's why I can't use the trust relationship between SharePoint and remote app. Does anybody have some ideas for authentication in this scenario that would work well when using SP.RequestExecutor to call my endpoint?

Answer 1

To summarize, you have the following scenario:
You have a SharePoint add in (SharePoint App).
A page on the add in web (app web) needs to call an external service.
You have an external service implemented using ASP.NET Web Api.
The external service needs authentication.
The first issue you need to address is the Same Origin Policy. To overcome this issue, Microsoft documentation describes three options as you know:
The SharePoint Cross Domain Library
The SharePoint Web Proxy
Create a custom proxy page.
However, I think the best bet is to use CORS because it is a W3C recommendation, it's simpler, easier to use, comprehensive, hack-free, and specially: ASP.NET Web API 2 supports CORS.
The other issue to address is authentication. Unfortunatelly Microsoft documentation doesn't provide any example nor hint, it simply tells you it's your responsability. Searching the web doesn't provide any example or hint either. So I conclude: You need to invent an authentication mechanism. Several authentication protocols are based on chalenges such as NTLM authentication. Email address validation uses also a chalenge, it chalenges you to read an email sent to the emails address. I propose you a mechanism based on the same paradigm. I chalenge the user to create a specific list item on the SharePoint App Web (add in). So we need a list on the App Web called AutenticationChalenges with the following fields:
ID: autoincrement built in field.
ChanlengeValue: single line of text.
CreatedBy: user built in field.
The authentication proccess has the following steps:
1.- JavaScript on a App Web page calls https://myexternalservice.mycompay.com/create-chalenge web api endpoint with the following payload:
{
"UserId": "3432" // the SharePoint UserId
"AppWebUrl": "https://mysharpointonline-e849d5bbe0ddc2.sharepoint.com/MySharePointApp"
"HostWebUrl": "https://mysharepointonline.sharepoint.com/MySharePointApp"
}
2.- The external server generates two 16-32 bytes random values: ChalengeValue and CorrelationToken, and it inserts them along with the payload into some storage such a table like the following:
CREATE SEQUENCE authentication_chalenges_authentication_chalenge_id_seq
START WITH 1;
CREATE TABLE authentication_chalenges
(
authentication_chalenge_id int NOT NULL DEFAULT NEXT VALUE FOR authentication_chalenges_authentication_chalenge_id_seq
CONSTRAINT authentication_chalenges_authentication_chalenge_id_seq PRIMARY KEY,
user_id int NOT NULL,
correlation_token binary(16) NOT NULL,
chalenge_value binary(16) NOT NULL,
app_web_url varchar(4000) NOT NULL,
host_web_url varchar(4000) NULL,
created_timestamp datetime NOT NULL
)
Then, the server returns the following result:
{
"ChalengeId": 31232, // the value of authentication_chalenge_id column of the table
"CorrelationToken" : "95AE040FE6844345B36B5E33BE03437F",
"ChalengeValue" : "E38A022B7F744D3BA8C676259AECD607"
}
3.- JavaScript on the App Web page inserts an item into the AuthenticationChanlenges list setting ChalengeValue column = "E38A022B7F744D3BA8C676259AECD607" and calls https://myexternalservice.mycompay.com/login web api endpoint with the following payload:
{
"ChalengeItemId" : 4133, // the ID column of the AuthenticationChalenges SharePoint list
"ChalengeId" : 31232,
"CorrelationToken" : "95AE040FE6844345B36B5E33BE03437F",
"ChalengeValue" : "E38A022B7F744D3BA8C676259AECD607"
}
4.- The external services server look for the row on chalenges table:
SELECT * FROM authentication_chalenges WHERE authentication_chalenge_id = 31232
If the query returns a row and CorrelationToken and ChanlengeValue match, and it has not expired yet, the server connects to sharepoint looking for the item with ID = 4133 on the AuhenticationChalenges list, ands checks that ChalengeValue is equals to E38A022B7F744D3BA8C676259AECD607, and finally it checks that CreatedBy user id is equals to 3432. If all checks success, then it responds with and ok response and sets the authentication cookie. If any of the checks fails then it responds with 401 result.