[Dev Tip] JSON Web Token in ASP.NET Web API 2 using Owin


In the previous post Decouple OWIN Authorization Server from Resource Server we saw how we can separate the Authorization Server and the Resource Server by unifying the “decryptionKey” and “validationKey” key values in machineKey node in the web.config file for the Authorization and the Resource server. So once the user request an access token from the Authorization server, the Authorization server will use this unified key to encrypt the access token, and at the other end when the token is sent to the Resource server, it will use the same key to decrypt this access token and extract the authentication ticket from it.

The source code for this tutorial is available on GitHub.

This way works well if you have control on your Resource servers (Audience) which will rely on your Authorization server (Token Issuer) to obtain access tokens from, in other words you are fully trusting those Resource servers so you are sharing with them the same “decryptionKey” and “validationKey” values.

But in some situations you might have big number of Resource servers rely on your Authorization server, so sharing the same “decryptionKey” and “validationKey” keys with all those parties become inefficient process as well insecure, you are using the same keys for multiple Resource servers, so if a key is compromised all the other Resource servers will be affected.

To overcome this issue we need to configure the Authorization server to issue access tokens using JSON Web Tokens format (JWT) instead of the default access token format, as well on the Resource server side we need to configure it to consume this new JWT access tokens, as well you will see through out this post that there is no need to unify the “decryptionKey” and “validationKey” key values anymore if we used JWT.

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What is JSON Web Token (JWT)?

JSON Web Token is a security token which acts as a container for claims about the user, it can be transmitted easily between the Authorization server (Token Issuer), and the Resource server (Audience), the claims in JWT are encoded using JSON which make it easier to use especially in applications built using JavaScript.

JSON Web Tokens can be signed following the JSON Web Signature (JWS) specifications, as well it can be encrypted following the JSON Web Encryption (JWE) specifications, in our case we will not transmit any sensitive data in the JWT payload, so we’ll only sign this JWT to protect it from tampering during the transmission between parties.

JSON Web Token (JWT) Format

Basically the JWT is a string which consists of three parts separated by a dot (.) The JWT parts are: <header>.<payload>.<signature>.

The header part is JSON object which contains 2 nodes always and looks as the following:  { “typ”: “JWT”,“alg”: “HS256” } The “type” node has always “JWT” value, and the node “alg” contains the algorithm used to sign the token, in our case we’ll use “HMAC-SHA256″ for signing.

The payload part is JSON object as well which contains all the claims inside this token, check the example shown in the snippet below:

All those claims are not mandatory  in order to build JWT, you can read more about JWT claims here. In our case we’ll always use those set of claims in the JWT we are going to issue, those claims represent the below:

  • The “sub” (subject) and “unique_claim” claims represent the user name this token issued for.
  • The “role” claim represents the roles for the user.
  • The “iss” (issuer) claim represents the Authorization server (Token Issuer) party.
  • The “aud” (audience) claim represents the recipients that the JWT is intended for (Relying Party – Resource Server). More on this unique string later in this post.
  • The “exp” (expiration time) claim represents the expiration time of the JWT, this claim contains UNIX time value.
  • The “nbf” (not before) claim represent the time which this JWT must not be used before, this claim contains UNIX time vale.

Lastly the signature part of the JWT is created by taking the header and payload parts, base 64 URL encode them, then concatenate them with “.”, then use the “alg” defined in the <header> part to generate the signature, in our case “HMAC-SHA256″. The resulting part of this signing process is byte array which should be base 64 URL encoded then concatenated with the <header>.<payload> to produce a complete JWT.

JSON Web Tokens support in ASP.NET Web API and Owin middleware.

There is no direct support for issuing JWT in ASP.NET Web API or ready made Owin middleware responsible for doing this, so in order to start issuing JWTs we need to implement this manually by implementing the interface “ISecureDataFormat” and implement the method “Protect”. More in this later. But for consuming the JWT in a Resource server there is ready middleware named “Microsoft.Owin.Security.Jwt” which understands validates, and and de-serialize JWT tokens with minimal number of line of codes.

So most of the heavy lifting we’ll do now will be in implementing the Authorization Server.

What we’ll build in this tutorial?

We’ll build a single Authorization server which issues JWT using ASP.NET Web API 2 on top of Owin middleware, the Authorization server is hosted on Azure (http://JwtAuthZSrv.azurewebsites.net) so you can test it out by adding new Resource servers. Then we’ll build a single Resource server (audience) which will process JWTs issued by our Authorization server only.

I’ll split this post into two sections, the first section will be for creating the Authorization server, and the second section will cover creating Resource server.

The source code for this tutorial is available on GitHub.

Section 1: Building the Authorization Server (Token Issuer)

Step 1.1: Create the Authorization Server Web API Project

Create an empty solution and name it “JsonWebTokensWebApi” then add a new ASP.NET Web application named “AuthorizationServer.Api”, the selected template for the project will be “Empty” template with no core dependencies. Notice that the authentication is set to “No Authentication”.

Step 1.2: Install the needed NuGet Packages:

Open package manger console and install the below Nuget packages:

You can refer to the previous post if you want to know what each package is responsible for. The package named “System.IdentityModel.Tokens.Jwt” is responsible for validating, parsing and generating JWT tokens. As well we’ve used the package “Thinktecture.IdentityModel.Core” which contains class named “HmacSigningCredentials”  which will be used to facilitate creating signing keys.

Step 1.3: Add Owin “Startup” Class:

Right click on your project then add a new class named “Startup”. It will contain the code below:

Here we’ve created new instance from class “OAuthAuthorizationServerOptions” and set its option as the below:

  • The path for generating JWT will be as :”http://jwtauthzsrv.azurewebsites.net/oauth2/token”.
  • We’ve specified the expiry for token to be 30 minutes
  • We’ve specified the implementation on how to validate the client and Resource owner user credentials in a custom class named “CustomOAuthProvider”.
  • We’ve specified the implementation on how to generate the access token using JWT formats, this custom class named “CustomJwtFormat” will be responsible for generating JWT instead of default access token using DPAPI, note that both are using bearer scheme.

We’ll come to the implementation of both class later in the next steps.

Step 1.4: Resource Server (Audience) Registration:

Now we need to configure our Authorization server to allow registering Resource server(s) (Audience), this step is very important because we need a way to identify which Resource server (Audience) is requesting the JWT token, so the Authorization server will be able to issue JWT token for this audience only.

The minimal needed information to register a Recourse server into an Authorization server are a unique Client Id, and shared symmetric key. For the sake of keeping this tutorial simple I’m persisting those information into a volatile dictionary so the values for those Audiences will be removed from the memory if IIS reset toke place, for production scenario you need to store those values permanently on a database.

Now add new folder named “Entities” then add new class named “Audience” inside this class paste the code below:

Then add new folder named “Models” the add new class named “AudienceModel” inside this class paste the code below:

Now add new class named “AudiencesStore” and paste the code below:

Basically this class acts like a repository for the Resource servers (Audiences), basically it is responsible for two things, adding new audience and finding exiting one.

Now if you take look on method “AddAudience” you will notice that we’ve implemented the following:

  • Generating random string of 32 characters as an identifier for the audience (client id).
  • Generating 256 bit random key using the “RNGCryptoServiceProvider” class then base 64 URL encode it, this key will be shared between the Authorization server and the Resource server only.
  • Add the newly generated audience to the in-memory “AudiencesList”.
  • The “FindAudience” method is responsible for finding an audience based on the client id.
  • The constructor of the class contains fixed audience for the demo purpose.

Lastly we need to add an end point in our Authorization server which allow registering new Resource servers (Audiences), so add new folder named “Controllers” then add new class named “AudienceController” and paste the code below:

This end point can be accessed by issuing HTTP POST request to the URI http://jwtauthzsrv.azurewebsites.net/api/audience as the image below, notice that the Authorization server is responsible for generating the client id and the shared symmetric key. This symmetric key should not be shared with any party except the Resource server (Audience) requested it.

Note: In real world scenario the Resource server (Audience) registration process won’t be this trivial, you might go through workflow approval. Sharing the key will take place using a secure admin portal, as well you might need to provide the audience with the ability to regenerate the key in case it get compromised.

Register Audience

Step 1.5: Implement the “CustomOAuthProvider” Class

Now we need to implement the code responsible for issuing JSON Web Token when the requester issue HTTP POST request to the URI: http://jwtauthzsrv.azurewebsites.net/oauth2/token the request will look as the image below, notice how we are setting the client id for for the Registered resource (audience) from the previous step using key (client_id).

Issue JWT

To implement this we need to add new folder named “Providers” then add new class named “CustomOAuthProvider”, paste the code snippet below:

As you notice this class inherits from class “OAuthAuthorizationServerProvider”, we’ve overridden two methods “ValidateClientAuthentication” and “GrantResourceOwnerCredentials”

  • The first method “ValidateClientAuthentication” will be responsible for validating if the Resource server (audience) is already registered in our Authorization server by reading the client_id value from the request, notice that the request will contain only the client_id without the shared symmetric key. If we take the happy scenario and the audience is registered we’ll mark the context as a valid context which means that audience check has passed and the code flow can proceed to the next step which is validating that resource owner credentials (user who is requesting the token).
  • The second method “GrantResourceOwnerCredentials” will be responsible for validating the resource owner (user) credentials, for the sake of keeping this tutorial simple I’m considering that each identical username and password combination are valid, in read world scenario you will do your database checks against membership system such as ASP.NET Identity, you can check this in theprevious post.
  • Notice that we are setting the authentication type for those claims to “JWT”, as well we are passing the the audience client id as a property of the “AuthenticationProperties”, we’ll use the audience client id in the next step.
  • Now the JWT access token will be generated when we call “context.Validated(ticket), but we still need to implement the class “CustomJwtFormat” we talked about in step 1.3.

Step 1.5: Implement the “CustomJwtFormat” Class

This class will be responsible for generating the JWT access token, so what we need to do is to add new folder named “Formats” then add new class named “CustomJwtFormat” and paste the code below:

What we’ve implemented in this class is the following:

  • The class “CustomJwtFormat” implements the interface “ISecureDataFormat<AuthenticationTicket>”, the JWT generation will take place inside method “Protect”.
  • The constructor of this class accepts the “Issuer” of this JWT which will be our Authorization server, this can be string or URI, in our case we’ll fix it to URI with the value “http://jwtauthzsrv.azurewebsites.net”
  • Inside “Protect” method we are doing the following:
    1. Reading the audience (client id) from the Authentication Ticket properties, then getting this audience from the In-memory store.
    2. Reading the Symmetric key for this audience and Base64 decode it to byte array which will be used to create a HMAC265 signing key.
    3. Preparing the raw data for the JSON Web Token which will be issued to the requester by providing the issuer, audience, user claims, issue date, expiry date, and the signing Key which will sign the JWT payload.
    4. Lastly we serialize the JSON Web Token to a string and return it to the requester.
  • By doing this requester for an access token from our Authorization server will receive a signed token which contains claims for a certain resource owner (user) and this token intended to certain Resource server (audience) as well.

So if we need to request a JWT from our Authorization server for a user named “SuperUser” that needs to access the Resource server (audience) “099153c2625149bc8ecb3e85e03f0022″, all we need to do is to issue HTTP POST to the token end point (http://jwtauthzsrv.azurewebsites.net/oauth2/token) as the image below:

Issue JWT2

The result for this will be the below JSON Web Token:

There is an online JWT debugger tool named jwt.io that allows you to paste the encoded JWT and decode it so you can interpret the claims inside it, so open the tool and paste the JWT above and you should receive response as the image below, notice that all the claims are set properly including the iss, aud, sub,role, etc…

One thing to notice here that there is red label which states that the signature is invalid, this is true because this tool doesn’t know about the shared symmetric key issued for the audience (099153c2625149bc8ecb3e85e03f0022).

So if we decided to share this symmetric key with the tool and paste the key in the secret text box; we should receive green label stating that signature is valid, and this is identical to the implementation we’ll see in the Resource server when it receives a request containing a JWT.

jwtio

Now the Authorization server (Token issuer) is able to register audiences and issue JWT tokens, so let’s move to adding a Resource server which will consume the JWT tokens.

Section 2: Building the Resource Server (Audience)

Step 2.1: Creating the Resource Server Web API Project

Add a new ASP.NET Web application named “ResourceServer.Api”, the selected template for the project will be “Empty” template with no core dependencies. Notice that the authentication is set to “No Authentication”.

Step 2.2: Installing the needed NuGet Packages:

Open package manger console and install the below Nuget packages:

The package “Microsoft.Owin.Security.Jwt” is responsible for protecting the Resource server resources using JWT, it only validate and de-serialize JWT tokens.

Step 2.3: Add Owin “Startup” Class:

Right click on your project then add a new class named “Startup”. It will contain the code below:

This is the most important step in configuring the Resource server to trust tokens issued by our Authorization server (http://jwtauthzsrv.azurewebsites.net), notice how we are providing the values for the issuer, audience (client id), and the shared symmetric secret we obtained once we registered the resource with the authorization server.

By providing those values to JwtBearerAuthentication middleware, this Resource server will be able to consume only JWT tokens issued by the trusted Authorization server and issued for this audience only.

Note: Always store keys in config files not directly in source code.

Step 2.4: Add new protected controller

Now we want to add a controller which will serve as our protected resource, this controller will return list of claims for the authorized user, those claims for sure are encoded within the JWT we’ve obtained from the Authorization Server. So add new controller named “ProtectedController” under “Controllers” folder and paste the code below:

Notice how we attribute the controller with [Authorize] attribute which will protect this resource and only will authentic HTTP GET requests containing a valid JWT access token, with valid I mean:

Conclusion

Using signed JSON Web Tokens facilitates and standardize the way of separating the Authorization server and the Resource server, no more need for unifying machineKey values nor having the risk of sharing the “decryptionKey” and “validationKey” key values among different Resource servers.

Keep in mind that the JSON Web Token we’ve created in this tutorial is signed only, so do not put any sensitive information in it :)

The source code for this tutorial is available on GitHub.

That’s it for now folks, hopefully this short walk through helped in understanding how we can configure the Authorization Server to issue JWT and how we can consume them in a Resource Server.

If you have any comment, question or enhancement please drop me a comment, I do not mind if you star theGitHub Repo too :)

REF: http://bitoftech.net/2014/10/27/json-web-token-asp-net-web-api-2-jwt-owin-authorization-server/

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