.Net core JWT authentication using AWS Cognito User Pool

While working with .net core I needed to create API. For this what I aimed to have was proper authentication. Therefore I decided to use JSON Web Token (JWT) authentication.

However I wanted to avoid creating any of this logic by myself or spending too much time on it. That’s why I decided to use AWS Cognito User Pools to provide me with user management and to generate JWT I need.

It took me some time to gather information how to wire it all together so I will try to outline the most important.

AWS setup

1. Create user pool in AWS Cognito
2. Get the newly created user pool ID and run the following command

   curl https://cognito-idp.<region>.amazonaws.com/<user-pool-id>/.well-known/jwks.json > result.json

   * if you want to you can also just navigate to the URL (https://cognito-idp.<region>.amazonaws.com/<user-pool-id>/.well-known/jwks.json ) . Just replace region and user pool ID with correct information.

3. The information you receive will be used by us to validate the tokens given by AWS.
4. Save the results for later use.

.Net core API project

1. Create new .net core webapi project

   dotnet new webapi

2. Install additional packages

   dotnet add package Microsoft.AspNetCore.Authentication.JwtBearer
   dotnet add package Microsoft.IdentityModel.Tokens
   dotnet add package Microsoft.AspNetCore.Identity

    

3. add JWT authentication policy ( we will decorate our controllers with it )

               services.AddAuthorization(auth =>
               {
                   auth.AddPolicy("Bearer", new AuthorizationPolicyBuilder()   
                       .AddAuthenticationSchemes(JwtBearerDefaults.AuthenticationScheme‌​)
                       .RequireAuthenticatedUser()
                       .Build());
               });

    

4. Before making further modifications we will add 2 methods used which will be used to validate the signature and issuer ( this has potential to be made much better 🙂 )Key is the “n” value and Expo is the “e” value in the keys you got form the url in AWS setup

           public RsaSecurityKey SigningKey(string Key, string Expo)
           {
                   RSA rrr = RSA.Create();
   
                   rrr.ImportParameters(
                       new RSAParameters()
                       {
                           Modulus =  Base64UrlEncoder.DecodeBytes(Key),
                           Exponent = Base64UrlEncoder.DecodeBytes(Expo)
                       }
                   );
       
                   return new RsaSecurityKey(rrr);  
           }
   
           public TokenValidationParameters TokenValidationParameters(string issuer)
           {
                   // Basic settings - signing key to validate with, audience and issuer.
                   return new TokenValidationParameters
                   {
                       // Basic settings - signing key to validate with, IssuerSigningKey and issuer.
                       IssuerSigningKey = this.SigningKey(<key-comes-here>,<expo-comes-here>),
                       ValidIssuer      = issuer,
                           
                       // when receiving a token, check that the signing key
                       ValidateIssuerSigningKey = true,
       
                       // When receiving a token, check that we've signed it.
                       ValidateIssuer = true,
       
                       // When receiving a token, check that it is still valid.
                       ValidateLifetime = true,
                           
                       // Do not validate Audience on the "access" token since Cognito does not supply it but it is      on the "id"
                       ValidateAudience = false,
       
                       // This defines the maximum allowable clock skew - i.e. provides a tolerance on the token expiry time 
                       // when validating the lifetime. As we're creating the tokens locally and validating them on the same 
                       // machines which should have synchronised time, this can be set to zero. Where external tokens are
                       // used, some leeway here could be useful.
                       ClockSkew = TimeSpan.FromMinutes(0)
                   };
               
           }

    

5. Modify Configure method to enable JWT

               app.UseJwtBearerAuthentication(new JwtBearerOptions()
               { 
                   
                   TokenValidationParameters = this.TokenValidationParameters(<issuer-comes-here>)
               });

   The issuer format has the following format : https://cognito-idp.<region>.amazonaws.com/<user-pool-id>

    

6. Modify controller and enable the authentication by using the following decorator

       [Authorize(Policy = "Bearer")]

    

Testing the solution

With the authentication enabled we get the following while requesting controller

> http http://localhost:5000/api/values

HTTP/1.1 401 Unauthorized
Content-Length: 0
Date: Sun, 30 Jul 2017 11:41:33 GMT
Server: Kestrel
WWW-Authenticate: Bearer

 

And if we pass the JWT 🙂

http --auth-type=jwt -v http://localhost:5000/api/values

GET /api/values HTTP/1.1
Accept: */*
Accept-Encoding: gzip, deflate
Authorization: Bearer ey.....
Host: localhost:5000
User-Agent: HTTPie/0.9.9



HTTP/1.1 200 OK
Content-Type: application/json; charset=utf-8
Date: Sun, 30 Jul 2017 11:45:41 GMT
Server: Kestrel
Transfer-Encoding: chunked

 

 

Code

Full gist below 🙂