GET /foo
Authorization: Bearer [token]

// secretKey contains a secret passphrase only your server knows
var secretKey = "mysupersecret_secretkey!123";
var signingKey = new SymmetricSecurityKey(Encoding.ASCII.GetBytes(secretKey));

var tokenValidationParameters = new TokenValidationParameters
{
  // The signing key must match!
  ValidateIssuerSigningKey = true,
  IssuerSigningKey = signingKey,

  // Validate the JWT Issuer (iss) claim
  ValidateIssuer = true,
  ValidIssuer = "ExampleIssuer",

  // Validate the JWT Audience (aud) claim
  ValidateAudience = true,
  ValidAudience = "ExampleAudience",

  // Validate the token expiry
  ValidateLifetime = true,

  // If you want to allow a certain amount of clock drift, set that here:
  ClockSkew = TimeSpan.Zero
};

app.UseJwtBearerAuthentication(new JwtBearerOptions
{
  AutomaticAuthenticate = true,
  AutomaticChallenge = true,
  TokenValidationParameters = tokenValidationParameters
});

using System;
using System.IdentityModel.Tokens.Jwt;
using System.Security.Claims;
using Microsoft.AspNetCore.Authentication;
using Microsoft.AspNetCore.Http.Authentication;
using Microsoft.IdentityModel.Tokens;
 
namespace SimpleTokenProvider
{
  public class CustomJwtDataFormat : ISecureDataFormat<AuthenticationTicket>
  {
    private readonly string algorithm;
    private readonly TokenValidationParameters validationParameters;
 
    public CustomJwtDataFormat(string algorithm, TokenValidationParameters validationParameters)
    {
      this.algorithm = algorithm;
      this.validationParameters = validationParameters;
    }
 
    public AuthenticationTicket Unprotect(string protectedText)
      => Unprotect(protectedText, null);
 
    public AuthenticationTicket Unprotect(string protectedText, string purpose)
    {
      var handler = new JwtSecurityTokenHandler();
      ClaimsPrincipal principal = null;
      SecurityToken validToken = null;
 
      try
      {
        principal = handler.ValidateToken(protectedText, this.validationParameters, out validToken);
 
        var validJwt = validToken as JwtSecurityToken;
 
        if (validJwt == null)
        {
          throw new ArgumentException("Invalid JWT");
        }
 
        if (!validJwt.Header.Alg.Equals(algorithm, StringComparison.Ordinal))
        {
          throw new ArgumentException($"Algorithm must be '{algorithm}'");
        }
 
        // Additional custom validation of JWT claims here (if any)
      }
      catch (SecurityTokenValidationException)
      {
        return null;
      }
      catch (ArgumentException)
      {
        return null;
      }
 
      // Validation passed. Return a valid AuthenticationTicket:
      return new AuthenticationTicket(principal, new AuthenticationProperties(), "Cookie");
    }
 
    // This ISecureDataFormat implementation is decode-only
    public string Protect(AuthenticationTicket data)
    {
      throw new NotImplementedException();
    }
 
    public string Protect(AuthenticationTicket data, string purpose)
    {
      throw new NotImplementedException();
    }
  }
}

var tokenValidationParameters = new TokenValidationParameters
{
  // The signing key must match!
  ValidateIssuerSigningKey = true,
  IssuerSigningKey = signingKey,
 
  // Validate the JWT Issuer (iss) claim
  ValidateIssuer = true,
  ValidIssuer = "ExampleIssuer",
 
  // Validate the JWT Audience (aud) claim
  ValidateAudience = true,
  ValidAudience = "ExampleAudience",
 
  // Validate the token expiry
  ValidateLifetime = true,
 
  // If you want to allow a certain amount of clock drift, set that here:
  ClockSkew = TimeSpan.Zero
};
 
app.UseCookieAuthentication(new CookieAuthenticationOptions
{
  AutomaticAuthenticate = true,
  AutomaticChallenge = true,
  AuthenticationScheme = "Cookie",
  CookieName = "access_token",
  TicketDataFormat = new CustomJwtDataFormat(
    SecurityAlgorithms.HmacSha256,
    tokenValidationParameters)
});

using System;
using Microsoft.IdentityModel.Tokens;
 
namespace SimpleTokenProvider
{
  public class TokenProviderOptions
  {
    public string Path { get; set; } = "/token";
 
    public string Issuer { get; set; }
 
    public string Audience { get; set; }
 
    public TimeSpan Expiration { get; set; } = TimeSpan.FromMinutes(5);
 
    public SigningCredentials SigningCredentials { get; set; }
  }
}

using System.IdentityModel.Tokens.Jwt;
using System.Security.Claims;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;
using Microsoft.Extensions.Options;
using Newtonsoft.Json;

namespace SimpleTokenProvider
{
  public class TokenProviderMiddleware
  {
    private readonly RequestDelegate _next;
    private readonly TokenProviderOptions _options;

    public TokenProviderMiddleware(
      RequestDelegate next,
      IOptions<TokenProviderOptions> options)
    {
      _next = next;
      _options = options.Value;
    }

    public Task Invoke(HttpContext context)
    {
      // If the request path doesn't match, skip
      if (!context.Request.Path.Equals(_options.Path, StringComparison.Ordinal))
      {
        return _next(context);
      }

      // Request must be POST with Content-Type: application/x-www-form-urlencoded
      if (!context.Request.Method.Equals("POST")
        || !context.Request.HasFormContentType)
      {
        context.Response.StatusCode = 400;
        return context.Response.WriteAsync("Bad request.");
      }

      return GenerateToken(context);
    }
  }
}

private async Task GenerateToken(HttpContext context)
{
  var username = context.Request.Form["username"];
  var password = context.Request.Form["password"];
 
  var identity = await GetIdentity(username, password);
  if (identity == null)
  {
    context.Response.StatusCode = 400;
    await context.Response.WriteAsync("Invalid username or password.");
    return;
  }
 
  var now = DateTime.UtcNow;
 
  // Specifically add the jti (random nonce), iat (issued timestamp), and sub (subject/user) claims.
  // You can add other claims here, if you want:
  var claims = new Claim[]
  {
    new Claim(JwtRegisteredClaimNames.Sub, username),
    new Claim(JwtRegisteredClaimNames.Jti, Guid.NewGuid().ToString()),
    new Claim(JwtRegisteredClaimNames.Iat, ToUnixEpochDate(now).ToString(), ClaimValueTypes.Integer64)
  };
 
  // Create the JWT and write it to a string
  var jwt = new JwtSecurityToken(
    issuer: _options.Issuer,
    audience: _options.Audience,
    claims: claims,
    notBefore: now,
    expires: now.Add(_options.Expiration),
    signingCredentials: _options.SigningCredentials);
  var encodedJwt = new JwtSecurityTokenHandler().WriteToken(jwt);
 
  var response = new
  {
    access_token = encodedJwt,
    expires_in = (int)_options.Expiration.TotalSeconds
  };
 
  // Serialize and return the response
  context.Response.ContentType = "application/json";
  await context.Response.WriteAsync(JsonConvert.SerializeObject(response, new JsonSerializerSettings { Formatting = Formatting.Indented }));
}

private Task<ClaimsIdentity> GetIdentity(string username, string password)
{
  // DON'T do this in production, obviously!
  if (username == "TEST" && password == "TEST123")
  {
    return Task.FromResult(new ClaimsIdentity(new System.Security.Principal.GenericIdentity(username, "Token"), new Claim[] { }));
  }
 
  // Credentials are invalid, or account doesn't exist
  return Task.FromResult<ClaimsIdentity>(null);
}

public static long ToUnixEpochDate(DateTime date)
  => (long)Math.Round((date.ToUniversalTime() - new DateTimeOffset(1970, 1, 1, 0, 0, 0, TimeSpan.Zero)).TotalSeconds);

using System.Text;
using Microsoft.AspNetCore.Builder;
using Microsoft.AspNetCore.Hosting;
using Microsoft.Extensions.Configuration;
using Microsoft.Extensions.DependencyInjection;
using Microsoft.Extensions.Logging;
using Microsoft.Extensions.Options;
using Microsoft.IdentityModel.Tokens;
 
namespace SimpleTokenProvider
{
  public partial class Startup
  {
    public Startup(IHostingEnvironment env)
    {
      var builder = new ConfigurationBuilder()
        .AddJsonFile("appsettings.json", optional: true);
      Configuration = builder.Build();
    }
 
    public IConfigurationRoot Configuration { get; set; }
 
    public void ConfigureServices(IServiceCollection services)
    {
      services.AddMvc();
    }
 
    // The secret key every token will be signed with.
    // In production, you should store this securely in environment variables
    // or a key management tool. Don't hardcode this into your application!
    private static readonly string secretKey = "mysupersecret_secretkey!123";
 
    public void Configure(IApplicationBuilder app, IHostingEnvironment env, ILoggerFactory loggerFactory)
    {
      loggerFactory.AddConsole(LogLevel.Debug);
      loggerFactory.AddDebug();
 
      app.UseStaticFiles();
 
      // Add JWT generation endpoint:
      var signingKey = new SymmetricSecurityKey(Encoding.ASCII.GetBytes(secretKey));
      var options = new TokenProviderOptions
      {
        Audience = "ExampleAudience",
        Issuer = "ExampleIssuer",
        SigningCredentials = new SigningCredentials(signingKey, SecurityAlgorithms.HmacSha256),
      };
 
      app.UseMiddleware<TokenProviderMiddleware>(Options.Create(options));
 
      app.UseMvc();
    }
  }
}

POST /token
Content-Type: application/x-www-form-urlencoded
username=TEST&password=TEST123