How to get Identity User from his authentication JWT token in .Net Core API?

Question

I'm using .Net Core for my API, so no views or whatsoever. I'm also using ASP.net Core Identity framework to authorize users in my database. For logging in users, I use this code:

private string GenerateAuthenticationResult(ApplicationUser user)
        {
            var tokenHandler = new JwtSecurityTokenHandler();
            var key = Encoding.ASCII.GetBytes(_jwtSettings.Secret);
            var tokenDescriptor = new SecurityTokenDescriptor
            {
                // Things to be included and encoded in the token
                Subject = new ClaimsIdentity(new[]
                {
                    new Claim(JwtRegisteredClaimNames.Sub, user.Email),
                    new Claim(JwtRegisteredClaimNames.Jti, Guid.NewGuid().ToString()),
                    new Claim(JwtRegisteredClaimNames.Email, user.Email),
                    new Claim("id", user.Id)
                }),
                // Token will expire 2 hours from which it was created
                Expires = DateTime.UtcNow.AddHours(2),
                //
                SigningCredentials = new SigningCredentials(new SymmetricSecurityKey(key), SecurityAlgorithms.HmacSha256Signature)
            };

            var token = tokenHandler.CreateToken(tokenDescriptor);

            return tokenHandler.WriteToken(token);
        }

This works like a charm for authenticating user actions, but how can I know whom my server is talking to provided that the user used the token I provided earlier for logging in in his request header (Bearer).

TL;dr

I want to extract user ID or user Email from the token provided in the request header.

Thanks.

Nan Yu Nan Yu · Accepted Answer · 2020-02-28T05:14:43

You can use AddJwtBearer validating JWT tokens :

var sharedKey = new SymmetricSecurityKey(
            Encoding.UTF8.GetBytes("yourkey"));
services.AddAuthentication(x =>
{
    x.DefaultAuthenticateScheme = JwtBearerDefaults.AuthenticationScheme;
    x.DefaultChallengeScheme = JwtBearerDefaults.AuthenticationScheme;
})
.AddJwtBearer(x =>
{
    x.RequireHttpsMetadata = false;
    x.SaveToken = true;

    x.TokenValidationParameters = new TokenValidationParameters
    {
        ValidateIssuerSigningKey = true,
        IssuerSigningKey = sharedKey,

        ValidateIssuer = false,
        ValidateAudience = false,
        ValidateLifetime = false,            
    };
});

And enable asp.net core authentication middleware via adding app.UseAuthentication(); in Configure method . After that , you can add [Authorize] attribute on protected actions/controllers .

To get the email and user id after authentication in action :

var  email= User.Claims.Where(x => x.Type == ClaimTypes.Email).FirstOrDefault()?.Value;
var  userid= User.Claims.Where(x => x.Type == "id").FirstOrDefault()?.Value;

Here ClaimTypes.Email is used since JwtRegisteredClaimNames.Email will map to ClaimTypes.Email by middleware automatically . See source code .

Here are some useful articles for JWT Authentication :

https://jasonwatmore.com/post/2018/08/14/aspnet-core-21-jwt-authentication-tutorial-with-example-api

https://jasonwatmore.com/post/2019/10/11/aspnet-core-3-jwt-authentication-tutorial-with-example-api

How to get Identity User from his authentication JWT token in .Net Core API?

1 Answers