This article explains two-factor authentication in detail and how to It can be implemented in .NET 5 using Identity Frameworks like IdentityServer4 and an SMS Provider (Twilio). 

It contains the following sections:

First off, Let’s understand What 2FA is and why it needs to be an integral part of every modern web application. 

What is Two-Factor Authentication?

A factor in this context implies a way you can convince an app or service that you are the owner of the account. Username/password is widely used as the most common authentication factor. However, Due to many security issues associated with it and recent widespread data breaches – single-factor authentication has become less secure.  

Two-factor authentication is an additional layer of security, which comes into play before you can access your account. In-addition to the standard login process –  It introduces an extra step to verify user identity by sending a code (to your email or as a text message). In this way, your identity is proved and only then access is granted. 

How does 2FA work? 

In 2FA, password is still your first authentication factor – so when you login to your account It will redirect you to another page where you are required to verify the account ownership. This can be done using various ways: 

  1. Application sends a verification code often called OTP ( one-time passwords) to your Email address. 
  2. Code is delivered as a text message on your phone. 
  3. You install an authenticator app on mobile through which you can authorize login requests. 

Note: These verification codes are generated randomly and are expired once used. Also,they are short lived – so there’s a short window before you can use the code (This prevents the hacker to brute force the verification codes). 

Can we call 2FA as Multi-Factor Authentication? 

Authentication methods are not limited to two-factor. Many apps and services are moving their users beyond 2FA and using Multi-Factor Authentication.

2FA and MFA (multi-factor authentication) are often used interchangeably. However there is a difference. In multi-factor authentication two or more factors are used.

It can check a user’s IP address, geo-location and device information in-addition to other factors such as a password and OTP to verify the user’s identity. 

Therefore, we can say the 2FA is a subset of MFA. In 2FA there will only be two factors whereas multi-factor can utilize two or more factors. MFA makes it difficult for hackers as it adds multiple layers of security in the traditional authentication process. 

Are there any drawbacks using 2FA?

Similar to many ‘security and privacy’ solutions that exist in modern apps. It also comes with a  price – Inconvenience,  as there is an additional step  involved which may cause friction in user experience.

However, It’s being adopted by many apps and services so this trade-off is becoming acceptable. 

How can I implement 2FA Authentication?

In this section, we will learn how to implement 2FA in .NET5 using IdentityServer4 and Twilio. 

So Let’s create a trial account on Twilio:

  1. Signup on
  2. Verify your account by providing a valid phone number. 

 Once your account is verified you can then use their console dashboard.

Twilio Dashboard
  1. Copy your Account SID and Auth Token from the Dashboard. 

  2. Navigate to to Buy a Number That you will use for sending SMS/ 

Note: Twilio gives 15$ credit in all trial accounts which you will use to buy the phone number. 

Twilio - Phone Number Search

You may choose any number as long as it supports ‘SMS Capabilities’. (Note down your phone number which we will use to configure later in our project). 

  1. Using trial account, Twilio only allows you to send sms to a verified caller IDs which you can manage using below link: 

That’s all you need to configure using Twilio Dashboard. 

Let’s create a ASP.NET Core Web App (Select Project template as below and target .NET5.

Visual Studio - New Project

We will now integrate IdentityServer4,Twilio packages from NuGet. 

  1. Install-Package IdentityServer4 -Version 4.1.1
  2. Install-Package Twilio -Version 5.55.0

IdentityServer4 Quick UI is available at:

You can include it in your project by running the below command in developer shell:

iex ((New-Object System.Net.WebClient).DownloadString('<a href=""></a>'))

If it was installed correctly – You would see the QuickStart folder in your project:

Project Hierarchy

Let’s build and run this project. You should be able to see the welcome page as shown below:

This Template uses in-memory data store so you may add your test users in Config.cs file

Config.cs Class

Open Startup.cs and add following IdentityServer4 Services in ConfigServices method:


Now add IdentityServer in request pipeline :


Run the project and Navigate to _/account/login _

IdentityServer4 - Login

Login with the credentials as added in **config.cs **

Once IdentityServer is configured and running. We can then add support for 2FA using Twilio. 

Let’s add following class:

  public class TwilioSettings
        public string Sid { get; set; }
        public string Token { get; set; }
        public string From { get; set; }

This configuration class will be used to inject settings using dependency injection. 

  1. RightClick on the project and add the user secrets with Twilio’s configuration (Sid, Token, PhoneNumber)
  2. Inject settings in ConfigureService method of Startup.cs
 var twilioSettings = Configuration.GetSection("TwilioSettings");

Startup.cs has been configured. We can now move on to adding a new service called AuthMessageSender which will be responsible for sending SMS using Twilio.

Let’s create an interface which AuthMessageSender will implement.

public interface ISmsSender
        Task SendSmsAsync(string number, string message);

We will inject this in Startup.cs.

services.AddTransient <ISmsSender, AuthMessageSender>();

The SMS sending code is pretty simple. We simply create a Twilio client and invoke MessageResource.CreateAsync method to send SMS.

public Task SendSmsAsync(string number, string message)
              var sid = _twilioSettings.Value.Sid;
            var token = _twilioSettings.Value.Token;
            var from = _twilioSettings.Value.From;
            TwilioClient.Init(sid, token);
            MessageResource.CreateAsync(new PhoneNumber(number),
                from: new PhoneNumber(from),
                body: message);
            return Task.FromResult(0);

We will modify the Login method to check if the user has enabled 2FA so we can redirect them for code verification.

Add VerifyCode.cshtml in QuickStart/Views:

2FA In Action

Modify Account/Login method to generate and send verification code after verifying credentials. We don’t want to complete the sign in request at this moment.

Therefore, HttpContext.SignInAsync method call will be removed, and instead we will simply generate a random verification code and store it (Currently we are using in-memory store for demonstration purpose).


public async Task<IActionResult> Login(LoginInputModel model, string button)
Code Screenshot

And then we will redirect the user to the “VerifyCode” page where code verification will be performed and the sign-in workflow will be completed.

In VerifyCode action we are simply fetching user and code information from memory and matching it with the provided code.

Code Screenshot

If the code is matched then we simply complete the authentication request by invoking SignInAsync method and redirect the user to the authorized page.

Let’s run the project

Step -1 : User provides credentials. 

2FADemo - Login

Step-2: Credentials are verified and a verification code is sent:

2FADemo - SMS

Step-3: User verifies the correct code.

2FADemo - VerifyCode

Step-4 Code is verified and the 2FA authentication has been completed.

2FADemo - Authorized Page


In this article, we have learned about 2FA and Its implementation in .NET5 using IdentityServer4 and Twilio. You can download the sample code used in this article from this repo.

Using SMS for 2FA surely strengthens your security but It’s still vulnerable to SIM Swap Attacks. Therefore, Security researchers are encouraging 2FA to use other approaches like Authenticator apps and Security Keys (YubiKey) which can’t be intercepted on the phone network. We will learn more on that in an upcoming article – Stay Tuned!