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How do I generate an assertion token for Sharepoint Access on behalf of another user in C#

I have a web application with a number of modules. One of the modules grabs a number of excel files from SharePoint directories, and then combines the data in them. So far, I have been just mapping the folders to OneDrive and accessing them that way. But this always uses my OneDrive credentials, which need to be refreshed from time to time. The right way to do this is to access them directly from Sharepoint on behalf of the user logged into my web application. I have the delegated API permission things set up in Azure, and I have the client ID and secret, etc.. I've been reading a number of articles on how to do this. All of them talk about how to get the token on behalf of someone else. These articles also talk about the assertion token needing to be passed in order to get the on behalf of token. However, they don't tell you how to get the assertion token in the first place. Here is the code I currently have:
'''var client = new RestClient("https://login.microsoftonline.com/XXXX/oauth2/v2.0/token");
var request = new RestRequest();
request.Method = Method.Post;
request.AddHeader("Content-Type", "application/x-www-form-urlencoded");
request.AddParameter("client_id", "MYCLIENTID", ParameterType.GetOrPost);
request.AddParameter("client_secret", "MYSECRET", ParameterType.GetOrPost);
request.AddParameter("scope", "https://MYTenent.sharepoint.com/.default", ParameterType.GetOrPost);
request.AddParameter("grant_type", "urn:ietf:params:oauth:grant-type:jwt-bearer", ParameterType.GetOrPost);
request.AddParameter("requested_token_use", "on_behalf_of", ParameterType.GetOrPost);
RestResponse response = client.Execute(request);'''
The result of this is of course an error that the assertion was not supplied. I didn't supply any more code, because I can't even get passed this. The rest of my code takes the token and passes it to an auth provider, which is then used to instantiate the GraphServiceClient. Based on what I've read, that client is then used to get the lists, files, etc...
So, my question is, how do I get the assertion token in the first place? I'm hoping the code I have written so far is in the correct direction and all I'm missing is the assertion token.
UPDATE:
I've gotten one answer that really didn't help. I pretty much copied and pasted the code (replacing the clientID, etc..) and I received an error> I was going to copy and paste it from the solution comments provided in the answer, but I guess you can't do that while editing.
Someone also asked if I was able to get the auth code from the URL. The answer to that is no. We use 2 factor authentication, and I tried to manually look at the URLS as I was logging in, while using break points to slow things down a bit. And I did not see an auth code. I did put a break point directly after the line of code:
var info = await _signInManager.GetExternalLoginInfoAsync();
And when I look at the info variable, I can see 4 tokens. One of them is an access token and another is an ID token. The last one is an expiration date. I don't see an auth code, and from what I understand, by the time I see the access code, it's too late. The auth code was already used to get the access code.
UPDATE 2:
I know that OBO is not what I want. I also know that in order to use delegated permissions, I need to use the Auth Code flow and not client credentials. I can't seem to get the auth code from the users initial log in. And I don't know how to get it otherwise.
For those of you that might be thinking "Does he need to be spoon fed?", the answer is yes, I do. I need a simple code example that will get me the auth code, so I can use it in the rest of the code I already have. If anyone can paste that code into an answer and not provide a link, that would be great. I'm sorry, but the links I have been given, just go to microsoft learn sites that go through the explaination, but don't give any code samples.

The OBO flow is obviously not applicable in your context, and if you're going to get an access token on behalf of a logged in user, then you should focus on auth code flow or ROPC flow.
The corresponding C# code segment is:
using Microsoft.Graph;
using Azure.Identity;
var scopes = new[] { "https://{tenant-name}.sharepoint.com/.default" };
// Multi-tenant apps can use "common",
// single-tenant apps must use the tenant ID from the Azure portal
var tenantId = "tenant id";
// Values from app registration
var clientId = "client id";
var clientSecret = "client secret";
// For authorization code flow, the user signs into the Microsoft
// identity platform, and the browser is redirected back to your app
// with an authorization code in the query parameters
var authorizationCode = "authorization code ";
// using Azure.Identity;
var options = new TokenCredentialOptions
{
AuthorityHost = AzureAuthorityHosts.AzurePublicCloud
};
// https://learn.microsoft.com/dotnet/api/azure.identity.authorizationcodecredential
var authCodeCredential = new AuthorizationCodeCredential(
tenantId, clientId, clientSecret, authorizationCode, options);
var accessToken = await authCodeCredential.GetTokenAsync(new Azure.Core.TokenRequestContext(scopes) { });
Console.WriteLine(accessToken.Token);
//var graphClient = new GraphServiceClient(authCodeCredential, scopes);

Microsoft Graph The token contains no permissions, or permissions cannot be understood

I am working with Microsoft Graph and have created an app that reads mail from a specific user.
However, after getting an access token and trying to read the mailfolders, I receive a 401 Unauthorized answer. The detail message is:
The token contains no permissions, or permissions cannot be understood.
This seems a pretty clear message, but unfortunately I am unable to find a solution.
This is what I have done so far:
Registering the app on https://apps.dev.microsoft.com
Giving it
application permissions Mail.Read, Mail.ReadWrite
(https://learn.microsoft.com/en-us/graph/api/user-list-mailfolders?view=graph-rest-1.0)
Have gotten administrator consent.
The permissions are:
- Written the code below to acquire an access token:
// client_secret retrieved from secure storage (e.g. Key Vault)
string tenant_id = "xxxx.onmicrosoft.com";
ConfidentialClientApplication client = new ConfidentialClientApplication(
"..",
$"https://login.microsoftonline.com/{tenant_id}/",
"https://dummy.example.com", // Not used, can be any valid URI
new ClientCredential(".."),
null, // Not used for pure client credentials
new TokenCache());
string[] scopes = new string[] { "https://graph.microsoft.com/.default" };
AuthenticationResult result = client.AcquireTokenForClientAsync(scopes).Result;
string token = result.AccessToken;
So far so good. I do get a token.
Now I want to read the mail folders:
url = "https://graph.microsoft.com/v1.0/users/{username}/mailFolders";
handler = (HttpWebRequest)WebRequest.Create(url);
handler.Method = "GET";
handler.ContentType = "application/json";
handler.Headers.Add("Authorization", "Bearer " + token);
response = (HttpWebResponse)handler.GetResponse();
using (StreamReader sr = new StreamReader(response.GetResponseStream()))
{
returnValue = sr.ReadToEnd();
}
This time I receive a 401 message, with the details:
The token contains no permissions, or permissions cannot be understood.
I have searched the internet, but can’t find an answer to why my token has no permissions.
Thanks for your time!
update 1
If I use Graph Explorer to read the mailfolders, then it works fine. Furthermore: if I grap the token id from my browser en use it in my second piece of code, then I get a result as well. So, the problem is really the token I receive from the first step.

To ensure this works like you expect, you should explicitly state for which tenant you wish to obtain the access token. (In this tenant, the application should, of course, have already obtained admin consent.)
Instead of the "common" token endpoint, use a tenant-specific endpoint:
string url = "https://login.microsoftonline.com/{tenant-id}/oauth2/v2.0/token";
(Where {tenant-id} is either the tenant ID of the tenant (a Guid), or any verified domain name.)
I would also strongly recommend against building the token request on your own, as you show in your question. This may be useful for educational purposes, but will tend to be insecure and error-prone in the long run.
There are various libraries you can use for this instead. Below, an example using the Microsoft Authentication Library (MSAL) for .NET:
// client_secret retrieved from secure storage (e.g. Key Vault)
string tenant_id = "contoso.onmicrosoft.com";
ConfidentialClientApplication client = new ConfidentialClientApplication(
client_id,
$"https://login.microsoftonline.com/{tenant_id}/",
"https://dummy.example.com", // Not used, can be any valid URI
new ClientCredential(client_secret),
null, // Not used for pure client credentials
new TokenCache());
string[] scopes = new string[] { "https://graph.microsoft.com/.default" };
AuthenticationResult result = client.AcquireTokenForClientAsync(scopes).Result
string token = result.AccessToken;
// ... use token

Differences between AcquireTokenAsync and LoginAsync in Xamarin Native

TL;DR
What is the difference between authenticating users with AuthenticationContext.AcquireTokenAsync() and MobileServiceClient.LoginAsync() ?
Can I use the token from the first method to authenticate a user in the second?
Long Version
I've been trying to authenticate users via a mobile device (iOS) for a mobile service in Azure with Xamarin Native (not Forms).
There are enough tutorials online to get you started but in the process, I got lost and confused...
What's working at the moment is the following; which has the user enter his credentials in another page and returns a JWT token which (if decoded here1) has the claims listed here2.
Moreover, this token is authorized in controllers with the [Authorize] attribute in requests with an Authorization header and a Bearer token.
Note: the following constants are taken from the registered applications in Active Directory (Native and Web App / API).
public const string Authority = #"https://login.windows.net/******.com";
public const string GraphResource = #"https://*******.azurewebsites.net/********";
public const string ClientId = "046b****-****-****-****-********0290";
public const string Resource = #"https://******.azurewebsites.net/.auth/login/done";
var authContext = new AuthenticationContext(Authority);
if (authContext.TokenCache.ReadItems().Any(c => c.Authority == Authority))
{
authContext = new AuthenticationContext(authContext.TokenCache.ReadItems().First().Authority);
}
var uri = new Uri(Resource);
var platformParams = new PlatformParameters(UIApplication.SharedApplication.KeyWindow.RootViewController);
AuthenticationResult authResult = await authContext.AcquireTokenAsync(GraphResource, ClientId, uri, platformParams);
Another working authentication flow I tried is the following; which does the same with the difference that it informs the user that the app requires permissions to access some resources.
If allowed, a JWT token (with less characters than the previous one) is returned with less payload data. This token though, won't pass the authorization attribute just like the previous one.
public const string AadResource = #"https://******.azurewebsites.net/.auth/aad";
var client = new MobileServiceClient(AadResource);
var rootView = UIApplication.SharedApplication.KeyWindow.RootViewController;
MobileServiceUser user = await client.LoginAsync(rootView, "aad");
Obviously, the return type is different, but, what is the main difference between these two authentication methods?
Additionally, another headache comes from trying to achieve this3 at the very end of the article. I already have the token from the first aforementioned method but when I try to follow the client flow with the token in client.LoginAsync() the following error is returned:
The resource you are looking for has been removed, had its name changed, or is temporarily unavailable.
Link References:
https://jwt.io/
https://learn.microsoft.com/en-us/azure/active-directory/develop/active-directory-token-and-claims
https://adrianhall.github.io/develop-mobile-apps-with-csharp-and-azure/chapter2/enterprise/
https://www.reddit.com/r/xamarindevelopers/comments/6dw928/differences_between_acquiretokenasync/
Edit (30 May 2017)
The Why are they different? has been answered on this4 reddit post by the same person (pdx mobilist / saltydogdev) and the simple answer is claims.

Yes. You can insert a token into the MobileServicesClient and then use it had been authenticated directly. That's the beauty of bearer tokens.
Just set the MobileServiceClient CurrentUser:
MobileServiceclient Client;
...
Client.CurrentUser = new MobileServiceUser(username)
{ MobileServiceAuthenticationToken = authtoken};
Edit:
The reason they are different is because each library is requesting a different set of claims. The reason they still work is that the basic information for authenticating/validating the token is there. I'm not sure what are the specific required claims. At a minimum it would be the user id AND that the signature is valid. They are doing the same basic thing, MobileServiceClient just requests less claims.
I believe that the MobileServicesClient can authenticate against Azure AD, if you set up the mobile service correctly. So you should be able to just use the MobileServiceClient.
Here is the document that describes how this works: https://learn.microsoft.com/en-us/azure/app-service-mobile/app-service-mobile-how-to-configure-active-directory-authentication

Azure AD authentication with asp.net Identity for authorisation

I tried to look for all over internet but couldn't see how I can achieve what I was asked to. Here is my enterprise app which uses Asp.net Identity for form based authentication. I had extended User and Role along with Groups to provide authorization in my code. (note: not using any group/role directives).
Now I was asked to look at possibility of changing code to accommodate Azure Active Directory authentication. I tried reading on how you can register app, send user to Azure site for authentication, get back token etc. However I'm stuck at 'what-afterwards?' I have authenticated user How can I use my existing Asp.net Identity model where user was stored in sql database. How to use this token to relate the existing user.
Moreover, when I change my project to allow Azure AD, it removes Aspnet.Identity package as its not compatible with Azure AD !!
I even tried manually keeping both packages side by side, I got to point where user is sent to authenticate on Azure, diverted back to home page and again to login on Azure AD in never ending loop.
to summarize the question, How can I authenticate user from AAD and keep using existing Roles and groups authorization.
Edit:
I tried creating separate web service which will authenticate user and send JWT token. which works find if I call it directly on browser, however, when I tried to call this service from my web app I get weird error
Application with identifier 'a2d2---------------' was not found in the directory azurewebsites.net
Weird part here is name of directory is 'azurewebsites.net' and not the default directory I'm using.
Update
Here is code which throws error
public async Task<ActionResult> Login(string returnUrl)
{
try
{
// get the access token
AuthenticationContext authContext = new AuthenticationContext(authority, new TokenCache());
var clientCredential = new ClientCredential(clientId, password);
//Error on below line
AuthenticationResult result = await authContext.AcquireTokenAsync(resourceId, clientCredential);
// give it to the server to get a JWT
HttpClient httpClient = new HttpClient();
httpClient.DefaultRequestHeaders.Authorization = new AuthenticationHeaderValue("Bearer", result.AccessToken);
......

try this:
var client = new RestClient("https://login.microsoftonline.com/{tenant-
Id}/oauth2/v2.0/token");
var request = new RestRequest(Method.POST);
request.AddHeader("cache-control", "no-cache");
request.AddHeader("content-type", "application/x-www-form-urlencoded");
request.AddHeader("grant_type", "password");
request.AddParameter("application/x-www-form-urlencoded",
"grant_type=password&client_id={client-Id}&client_secret={client-
secret}&scope={scopeurl}&userName={username}&password={password}",
ParameterType.RequestBody);
IRestResponse response = client.Execute(request);
var json = response.Content;
var JSONObject = JObject.Parse(json);
var token = (string)JSONObject["access_token"];

I had a similar issue so I created an Office 365 owin security plugin. I shared the code on github. It's based on the katana project at codeplex.
You can find the source code at https://github.com/chadwjames/wakizashi.
You will need to register your application here. When registering the application set the call back uri to https://yourdomain/signin-office365
The Application ID is your Client Id and the Password is your Client Secret.
Once you have it registered you can modify the Startup.Auth.cs and add something like this to the ConfigureAuth method.
//setup office 365
var office365Options = new Office365AuthenticationOptions
{
ClientId = ConfigurationManager.AppSettings["ada:ClientId"],
ClientSecret = ConfigurationManager.AppSettings["ada:ClientSecret"],
Provider = new Office365AuthenticationProvider
{
OnAuthenticated = async context =>
{
await
Task.Run(
() => context.Identity.AddClaim(new Claim("Office365AccessToken", context.AccessToken)));
}
},
SignInAsAuthenticationType = DefaultAuthenticationTypes.ExternalCookie
};
office365Options.Scope.Add("offline_access");
app.UseOffice365Authentication(office365Options);
When I have more time I hope to create a nuget package for this.

Why the same Access Token works for Media but doesn't work for Feed?

I'm using InstaSharp to get all medias from my account. This code works without any error:
var user = new InstaSharp.Endpoints.Media.Authenticated(config, authInfo);
var media = user.Popular();
Now if I do the same with Users:
var user = new InstaSharp.Endpoints.Users.Authenticated(config, authInfo);
var feed = user.Feed("self");
This returns:
OAuthParameterException: The access_token provided is invalid.
Why the same access token works in one place but doesn't work in another?
Note: I'm passing all scopes (basic, comments, likes, relationships) while getting the access token.I have also tried with default (basic) and it didn't work either.
Note2: Since I'm trying this in a ConsoleApplication, the way I'm getting the access token is, generate the link, copy/paste to the browser, confirm the authorization and get it from url. I'm not sure it makes any difference but it's worth noting...

I figure out the problem. It seems that the generated link was requesting code, not access_token.So that's why it was invalid.
And I realized that I was using older version of InstaSharp even though I downloaded it from it's website. First, I installed the latest version via NuGet. Then, I get the access token and the media like this:
var config = new InstagramConfig(clientId, clientSecret);
var AuthLink = OAuth.AuthLink(config.OAuthUri + "/authorize",
config.ClientId,
config.RedirectUri,
scopes,
OAuth.ResponseType.Code);
var authInfo = new OAuth(config);
// get the code from the link then pass it to RequestToken
var response = await authInfo.RequestToken(code);
var user = new Users(config, response);
var media = await user.RecentSelf(count: 30);
And all worked very well.