Background:

To integrate OpenId Connect (henceforth OIDC) in a new ASP.NET MVC application, we can easily use ASP.NET Identity which relies on OWIN.

What to do, when you have a legacy ASP.NET MVC application with custom Forms Authentication and identity management taken care by a legacy backend service(SOAP)?

I’ll try to describe the process of achieving this kind of integration.

### Basics

Before we get to the nuts and bolts of things promised in this post, lets quickly define some key concepts:

OIDC: is an authentication (identity) layer on top of OAuth 2.0. The main actors in OIDC are OpenId Provider (OP) and Relying Party (RP). OP is an OAuth 2.0 compliant server, capable of authenticating an user for RP. RP on the other hand is again an OAuth 2.0 application which relies (hence the name) on OP for user authentication.

So, in our case, our classic/legacy web application is the RP and we can use any OP which is OAuth 2.0 compliant! For exploration purposes Auth0, Okta, OneLogin, etc are pretty good implementations of an OP. For this post, I’ll be using okta OP, you can replace it with an OP of your choice.

OIDC allows three types of flows for authentication and these pretty much dictate how the OP will handle them. Each flow is for a specific use case. Auth0 and Okta have some fantastic documentation on OIDC in general and in this case of which flow to use in each use case, see Auth0 & Okta. Following is a list and brief on each flow.

Authorization Code Flow: As the name says, it returns an authorization code, that will be used/exchanged for an identity token and/or an access token. Which further requires your client id and secret to request these tokens. This flow is used on Server-side (web applications), as the user performs authentication (via a url redirection), the server application is provided with an authorization code (via a configured callback/redirect url), and the server on a back-channel will communicate with the OP (this is where client id and secret are shared with OP) to authenticate.

Implicit Flow: This flow, as the name suggests, is used to obtain access token or identity token along with the redirection response from OP. No back-channel request. Mostly used by SPA or mobile apps.

Hybrid Flow: This combines the other two flows. It allows use cases where an application can immediately use an identity token for user information and authorization code to be exchanged for access token. Rarely used!

We modified the backend and enabled it to store, 2 additional pieces of data along with a user identity. An OpenId Provider (OP) name and an user identification token. And updated the service interfaces. With this change, the backend can now allow a login, based on an email, user identification token and OP name, all of which will/can be read from OP.

We’ll be using Authorization code flow in this case, as it is a web application.

As we are done with changes to backend, the real work can start in the web application. The first thing we’ll do is to enable OP based login; for which we’ll update the login UI. Notice the additional “Sign in with Okta” button.

We’ll need the following nuget packages for the entire OIDC implementation:

• Install-Package System.IdentityModel.Tokens.Jwt -Version 5.4.0
• Install-Package Microsoft.IdentityModel.JsonWebTokens -Version 5.4.0
• Install-Package Microsoft.IdentityModel.Logging -Version 5.4.0
• Install-Package Microsoft.IdentityModel.Protocols -Version 5.4.0
• Install-Package Microsoft.IdentityModel.Protocols.OpenIdConnect -Version 5.4.0
• Install-Package Microsoft.IdentityModel.Tokens -Version 5.4.0

We now need an action, which will be executed, when the user presses the Okta sign in button.

[AllowAnonymous]
public ActionResult Authorize()
{
var queryParams = new NameValueCollection
{
{"client_id", settings.ClientId},       //client id provided by OP
{"redirect_uri", settings.CallbackUrl}, //for OP to return auth code to RP
{"response_type", "code"},              //authorization code
{"scope", "openid profile email"}       //openid for id_token
};

// sample authorize url can be as follows
var authUrl = settings.AuthorizeUrl;
var urlBuilder = new UrlBuilder(authUrl)
{
Query = ToQueryString(queryParams)
};

// The resultant redirect url may look like:
// c38HLL1ef0h7&response_type=code&scope=openid&redirect_uri=http%3A%2F%2Flocal
// host%3A8080%3Acallback
return Redirect(urlBuilder.ToString());
}

private static string ToQueryString(NameValueCollection paramsCollection)
{
return string.Join("&",
paramsCollection.AllKeys.Select(o =>
$"{HttpUtility.UrlEncode(o)}={HttpUtility.UrlEncode(paramsCollection[o])}")); }  The Okta signin button will POST to above Authorize MVC action and a redirect to Okta hosted sign-in page is done. Here, the user can supply credentials to authenticate himself/herself. Upon successful authentication, OP (Okta) will redirect with an authorization code (generally a hash string as a GET parameter named as code). Lets see how we’ll handle this authorization code response from OP via another MVC action. public async Task<ActionResult> Callback(string code) { var tokenResp = await RequestOpenIdToken(code); var idToken = ValidateToken(tokenResp.IdToken, tokenIssuer, settings.ClientId, tokenSigningKeys); // Leaving implementation details of "GetUserIdentifier" & "GetUserEmail" // for sake of brevity var userIdentifierHashOrToken = GetUserIdentifier(idToken); var userEmailAddress = GetUserEmail(idToken); // Now call our backend service... if(await AuthenticationSvc.Login(settings.OidcProviderName, userIdentifierHashOrToken, userEmailAddress)) { return RedirectToAction("Secure", "Home"); } return RedirectToAction("Login", "Account"); } private static readonly HttpClient httpClient = new HttpClient(); private async Task<OpenIdConnectTokenResponse> RequestOpenIdToken(string code) { var requestParams = new Dictionary<string, string> { { "client_id", settings.ClientId }, { "client_secret", settings.ClientSecret }, { "code", code }, { "grant_type", "authorization_code" }, { "redirect_uri", settings.CallbackUrl } }; var postContent = new FormUrlEncodedContent(requestParams); var tokenUrl = settings.TokenUrl; // This is our back-channel call to OP, notice the "client_secret" // in the above params. var response = await httpClient.PostAsync(tokenUrl, postContent); if (response.StatusCode != HttpStatusCode.OK) { throw new ExternalAuthenticationException($@"Error retrieving id_token. Status code:
{response.StatusCode},
Reason: {response.ReasonPhrase}")
{
Detail = JsonConvert.SerializeObject(response, Formatting.Indented)
};
}

if(result.IsNullOrEmpty())
throw new ExternalAuthenticationException($"Empty token response!"); return JsonConvert.DeserializeObject<OpenIdConnectTokenResponse>(result); } private static JwtSecurityToken ValidateToken( string token, string issuer, string audience, IEnumerable<SecurityKey> signingKeys) { if (token.IsNullOrEmpty()) throw new ArgumentNullException(nameof(token)); if (issuer.IsNullOrEmpty()) throw new ArgumentNullException(nameof(issuer)); if (signingKeys == null) throw new ArgumentNullException(nameof(signingKeys)); if (!signingKeys.Any()) throw new ArgumentException(@"Error: At least one signing key must be available to perform token validation", nameof(signingKeys)); var validationParams = new TokenValidationParameters { RequireExpirationTime = true, RequireSignedTokens = true, ValidateIssuer = true, ValidIssuer = issuer, ValidateIssuerSigningKey = true, IssuerSigningKeys = signingKeys, ValidateLifetime = true, ClockSkew = TimeSpan.FromSeconds(10) }; validationParams.ValidateAudience = !audience.IsNullOrEmpty(); if (validationParams.ValidateAudience) { validationParams.ValidAudience = audience; } try { var result = new JwtSecurityTokenHandler() .ValidateToken(token, validationParams, out var validatedToken); return (JwtSecurityToken)validatedToken; } catch (SecurityTokenValidationException stve) { throw new ExternalAuthenticationException($"Error validating JWT", stve);
}
}

// Type for handling token response
private class OpenIdConnectTokenResponse
{
[JsonProperty("access_token")]
public string AccessToken { get; set; }

[JsonProperty("token_type")]
public string TokenType { get; set; }

[JsonProperty("expires_in")]
public int ExpiresIn { get; set; }

[JsonProperty("scope")]
public string Scope { get; set; }

[JsonProperty("refresh_token")]
public string RefreshToken { get; set; }

[JsonProperty("id_token")]
public string IdToken { get; set; }
}


Summary of what we are doing in the code above:

1. Callback action is called via the OP with code as a GET param
2. Based on the code received, we request an OIDC token from Token endpoint of OP
3. We validate the id_token, see on okta for more details/explaination
4. Then we read user identifier from id_token, this can be the sub claim or any other specified by OP or if OP allows customization.