Implementing Passkey Authentication in ASP.NET Core Applications

Introduction

Passwords have been the standard method of authentication for decades, but they come with significant challenges. Users often create weak passwords, reuse them across multiple services, or become victims of phishing attacks. Organizations spend considerable resources managing password resets, enforcing password policies, and securing credential databases.

Passkeys are emerging as a modern authentication solution that eliminates many of these problems. Built on FIDO2 and WebAuthn standards, passkeys provide a passwordless authentication experience that is both more secure and more user-friendly.

Major platforms, including Windows, Android, iOS, Google, Microsoft, and Apple, support passkeys, making them increasingly practical for modern web applications.

In this article, you'll learn how passkey authentication works, its benefits, how it differs from traditional authentication, and how to implement passkey authentication in ASP.NET Core applications.

What Are Passkeys?

A passkey is a passwordless authentication credential based on public-key cryptography.

Instead of storing passwords, passkeys use:

A public key stored on the server
A private key stored securely on the user's device

When a user authenticates:

The server sends a challenge.
The device signs the challenge using the private key.
The server verifies the signature using the public key.

Since the private key never leaves the device, attackers cannot steal it through database breaches or phishing attacks.

Why Passkeys Are More Secure Than Passwords

Traditional password-based authentication introduces several security risks:

Weak passwords
Credential stuffing
Phishing attacks
Password reuse
Database breaches

Passkeys eliminate many of these threats.

Protection Against Phishing

Passkeys are bound to specific domains.

Even if users visit a fake website, authentication will fail because the domain does not match the original registration.

No Shared Secrets

Passwords are shared secrets stored on both the client and server.

Passkeys use asymmetric cryptography, eliminating shared secrets entirely.

Reduced Attack Surface

Attackers cannot:

Guess passkeys
Reuse stolen credentials
Capture passwords through phishing

This significantly improves authentication security.

Understanding WebAuthn and FIDO2

Passkeys are built on two key standards.

WebAuthn

Web Authentication (WebAuthn) is a web standard that allows browsers to communicate with authentication devices.

Supported browsers include:

Chrome
Edge
Firefox
Safari

FIDO2

FIDO2 defines secure authentication mechanisms using public-key cryptography.

Together, WebAuthn and FIDO2 enable passwordless authentication across devices and platforms.

How Passkey Authentication Works

A passkey implementation consists of two primary workflows:

Registration

The user creates a passkey.

Process:

User registers.
Server generates a challenge.
Browser creates a key pair.
Public key is sent to the server.
Private key remains on the device.

Authentication

The user signs in.

Process:

User enters username or email.
Server generates a challenge.
Device signs challenge using private key.
Server verifies signature.
Authentication succeeds.

No password is required.

Setting Up an ASP.NET Core Project

Create a new ASP.NET Core Web API project.

dotnet new webapi -n PasskeyDemo

Navigate to the project:

cd PasskeyDemo

Add a FIDO2 library.

dotnet add package Fido2

This package simplifies WebAuthn and passkey implementation in ASP.NET Core applications.

Configure FIDO2 Services

builder.Services.AddSingleton(
    new Fido2NetLib.Fido2(
        new Fido2Configuration
        {
            ServerDomain = "localhost",
            ServerName = "Passkey Demo",
            Origins = new HashSet<string>
            {
                "https://localhost:5001"
            }
        }));

This configuration defines the application's WebAuthn settings.

Creating a Registration Challenge

When a user begins registration, the server generates a challenge.

Example:

var options =
    fido.RequestNewCredential(
        user,
        new List<PublicKeyCredentialDescriptor>(),
        authenticatorSelection,
        AttestationConveyancePreference.None,
        null);

The generated challenge is sent to the browser.

The browser uses it to create a new passkey.

Handling Registration Responses

After the browser creates the credential, the public key is returned to the server.

Example:

var success =
    await fido.MakeNewCredentialAsync(
        credentialResponse,
        options,
        callback);

The server stores:

Credential ID
Public Key
User information

These records will be used for future authentication requests.

Creating an Authentication Challenge

When a user attempts to sign in, the server generates another challenge.

Example:

var options =
    fido.GetAssertionOptions(
        allowedCredentials,
        UserVerificationRequirement.Required);

The challenge is sent to the client device.

The private key signs the challenge securely.

Verifying Authentication

The signed assertion is returned to the server.

Verification example:

var result =
    await fido.MakeAssertionAsync(
        assertionResponse,
        options,
        storedPublicKey,
        storedSignatureCounter,
        callback);

If validation succeeds, the user is authenticated.

The application can then create an ASP.NET Core authentication session or issue a JWT token.

Combining Passkeys with ASP.NET Core Identity

Many applications already use ASP.NET Core Identity.

Passkeys can be added alongside existing authentication methods.

Example workflow:

User registers with email.
User creates a passkey.
Passkey becomes the preferred login method.
Password acts as a fallback option.

This approach simplifies migration to passwordless authentication.

Issuing JWT Tokens After Authentication

API-based applications often use JWT authentication.

After successful passkey verification:

var token =
    GenerateJwtToken(user);

The token is returned to the client.

return Ok(new
{
    AccessToken = token
});

This allows secure integration with mobile apps, SPAs, and APIs.

Practical Use Cases

Passkey authentication is particularly valuable for:

Banking Applications

Protect sensitive financial accounts.

Enterprise Portals

Reduce password management overhead.

Healthcare Platforms

Secure patient and provider access.

E-Commerce Systems

Improve account security while simplifying login.

SaaS Platforms

Enhance user experience and reduce support tickets related to password resets.

Benefits of Passkeys

Organizations adopting passkeys gain several advantages.

Improved Security

Public-key cryptography provides stronger protection than passwords.

Better User Experience

Users can authenticate using:

Fingerprint
Face recognition
Device PIN
Security key

Lower Support Costs

Password reset requests decrease significantly.

Cross-Platform Compatibility

Passkeys work across:

Windows
macOS
iOS
Android

Reduced Phishing Risk

Authentication only works on trusted domains.

Best Practices

When implementing passkeys in ASP.NET Core applications:

Use HTTPS in all environments.
Validate origins carefully.
Store credential metadata securely.
Enable multi-device passkey support.
Support account recovery mechanisms.
Combine passkeys with risk-based authentication where appropriate.
Monitor authentication logs.
Regularly update FIDO2 libraries.

These practices improve both security and reliability.

Common Challenges

Developers may encounter several challenges during implementation.

Device Compatibility

Older devices may not support passkeys.

User Education

Some users may be unfamiliar with passwordless authentication.

Recovery Workflows

Lost devices require secure account recovery procedures.

Migration Strategies

Organizations must decide how to transition from passwords to passkeys.

Planning for these scenarios ensures smoother adoption.

Conclusion

Passkeys represent a significant advancement in modern authentication. By leveraging WebAuthn and FIDO2 standards, organizations can eliminate many of the security risks associated with traditional passwords while delivering a faster and more convenient user experience.

ASP.NET Core provides an excellent foundation for implementing passkey authentication, whether through standalone solutions or integration with ASP.NET Core Identity. As passwordless authentication continues to gain adoption across major platforms and services, developers who understand passkeys will be well-positioned to build more secure and user-friendly applications.

For organizations seeking stronger security, reduced support costs, and protection against phishing attacks, passkeys offer a practical and future-ready authentication solution.