JWT (JSON Web Token) becomes more and more popular as a standard for securing web sites, and REST services. I discuss how you can implement JWT security for both a REST service and a MVC web application all build with .Net Core. I divided the JWT security in 3 blogs
Oct 31, 2018 The method again uses the static SECRETKEY property to generate the signing key, and uses that to verify that the JWT has not been tampered with. The method will throw io.jsonwebtoken.SignatureException exception if the signature does not match the token. Private key or shared secret: Choose JWS signature algorithm and default value: Or specify signature. To generate signed JWT just pass 'Sign it!'
This is the first of the three blogs and I start with a small JWT explanation.
JWT (JSON Web Tokens) is open, security protocol for securely exchanging claims between 2 parties. A server generates or issues a token and is signed by a secret key. The client also knows the secret key and the key and can verify if the token is genuine. The token contains claims for authentication and authorization. Authentication is simply the verification if someone is really who he claims to be be. Authorization is when an user is granted to access a resource or execute a certain task. For example user A can view payments and user B can execute payments. JWT are self contained. Because JWT is a protocol and not a framework it works across different languages like .net , Java Python and many more. The JWT is usually transmitted by adding the JWT to the header of the request but can also be used as a parameter in an URL. This transmission makes the JWT stateless.
JWT has three parts:
The parts are separated with a dot.
aaaa.bbbb.cccc
The header and the payload has one or more key value pairs. The header contains the token type ('typ') and the hashing algorithm ('alg') SHA256.
The Header and the Payload parts are base64 encoded, this makes the Header part:
The payload part is the most interesting section because it contains all the claims. There are three claims types Registered, Public and Private claims.
The registered claims are part of the JWT standard and have the same purpose on all implementations. In order to keep the JWT size small the key is always 3 characters long. Here's the short list:
All registered claims dates are in the Unix Epoch date format and describe the seconds after UTC time 1 January 1970.
Public claims contain more general information for example 'name'. Public names are also registered to prevent collision with other claims.
A private claim is agreed between issuer and audience. Always check if a private claim does not collide with existing claims. The claim 'role' is private claim example we will use later on.
will result in
So far there was nothing secure about a JWT. All data is base64 encoded and although not human readable it's easy to decode it into a readable text. This where the signature comes in. With the signature we can verify if the JWT is genuine and has not been tampered. The signature is calculated from the Header, the Payload and a secret key.
The secret key is symmetric and is known to issuer and client. Needless to say, be care full where you store the secret key!
The screen dump below is constructed with help from https://jwt.io/ where you can test and debug JWT claims. The left pane holds the JWT and the other pane shows the extracted Header and Payload. If you add the secret key the page also verifies the signature.
The solution overview shows three separate servers, the Web application, the RESTful service and the JWT issuer server. They could be hosted in one server and in one project but I made three items for it. In this way it's much more clear how each server is configured. Because JWT is self contained there no need for some kind of connection between the JWT issuer and the REST service to validate the JWT claim.
The basic JWT flow is quite simple:
The main task is to deliver JWT claims based on user credentials. The project is a standard MVC application with Individual User Accounts as Authentication.
The Individual User Accounts Authentication is used to secure the website and having easy access to users and their roles and claims. I added the package Microsoft.AspNetCore.Authentication.JwtBearer for the actual JWT creation. Because JWT is not used to secure this web site caller there is no need to register JwtBearer services during start up. Only the JWT parameters are configured during start up.
The DI (Dependency Injection) pattern is applied for the configuration. The class JwtIssuerSettings maps to the config section JwtIssuerSettings in appsettings.json and the class JwtIssuerFactory creates and instance of IJwtIssuerOptions interface.
They are added to the service collection and are now available as parameters in controller constructor.
The function Login on controller JwtIssuerController creates the JWT claim. The process is pretty straight forward:
During startup an in-memory database is created. It contains three users and three roles and mimics an Human Resource department.
Roles:
Users:
Namespace Microsoft.AspNetCore.Identity contains RoleManager<IdentityRole> and is ready to use without explicit configuration. You don't read much about it in examples or documentation. It's a bit of a missed chance because the class is really useful for managing the roles in the system.
I added Swagger by adding package Swashbuckle.AspNetCore for testing. You can read here more how to configure swagger. In short it comes to this
Swagger can now be tested at http://localhost:49842/swagger/
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We can test the response at https://jwt.io/
and all looks fine and we can start securing the REST service.
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And repair the startup setting:
This blog demonstrates how you can setup a JWT (JSON Web Token) issuer. Stateless, self contained, scalable and other features makes JWT a smart design. With help from packages integrates JWT well with .Net Core and takes little effort to setup.
1.0 2017-08-31 Initial release
1.1 2017-09-05 Source Code upgraded for Dot Net Core 2.0