How to Implement Passkey Authentication in ASP.NET Core Applications

Introduction

Passwords have long been the standard method for authenticating users, but they come with significant challenges. Weak passwords, password reuse, phishing attacks, and credential leaks continue to be major security concerns for organizations and developers.

Passkeys offer a modern alternative by allowing users to authenticate using biometrics, device PINs, or hardware security keys instead of traditional passwords. Built on the Web Authentication (WebAuthn) and FIDO2 standards, passkeys provide a more secure and user-friendly authentication experience.

For ASP.NET Core developers, implementing passkey authentication can enhance security while reducing user friction during login and registration.

In this article, you'll learn how passkey authentication works, how it differs from password-based authentication, and how to implement it in an ASP.NET Core application.

What Are Passkeys?

A passkey is a cryptographic credential stored securely on a user's device.

Instead of storing passwords, passkeys use public-key cryptography:

A public key is stored on the server.
A private key remains securely on the user's device.
Authentication occurs through a cryptographic challenge-response process.

This approach eliminates many common password vulnerabilities.

Traditional Password Authentication

User
  ↓
Username + Password
  ↓
Server Validation
  ↓
Authentication

Passkey Authentication

User
  ↓
Biometric / PIN
  ↓
Device Signs Challenge
  ↓
Server Verifies Signature
  ↓
Authentication

Because the private key never leaves the device, passkeys are highly resistant to phishing and credential theft.

Benefits of Passkeys

Passkeys offer several advantages over traditional authentication methods.

Improved Security

Passkeys help protect against:

Phishing attacks
Credential stuffing
Password reuse
Database password leaks

Better User Experience

Users no longer need to:

Remember complex passwords
Reset forgotten passwords
Manage password managers

Authentication becomes faster and simpler.

Strong Device-Based Authentication

Passkeys can leverage:

Fingerprint authentication
Facial recognition
Device PINs
Hardware security keys

This provides strong security without additional complexity.

Understanding WebAuthn and FIDO2

Passkeys rely on two key standards:

WebAuthn

WebAuthn is a browser API that enables web applications to perform secure authentication using cryptographic credentials.

Responsibilities include:

Credential registration
Authentication requests
Challenge handling

FIDO2

FIDO2 defines the authentication framework used by devices and servers.

It provides:

Passwordless authentication
Multi-device support
Strong cryptographic security

Together, WebAuthn and FIDO2 form the foundation of modern passkey authentication.

Setting Up ASP.NET Core for Passkeys

One popular approach is using the FIDO2 library for ASP.NET Core.

Install the package:

dotnet add package Fido2

This package provides server-side support for WebAuthn and passkey workflows.

Configure FIDO2 Services

In Program.cs, register the FIDO2 configuration.

builder.Services.AddFido2(options =>
{
    options.ServerDomain = "localhost";
    options.ServerName = "PasskeyDemo";
    options.Origins = new HashSet<string>
    {
        "https://localhost:5001"
    };
});

Configuration values should match your application's domain and deployment environment.

User Registration Flow

Passkey registration typically follows these steps:

User creates an account.
Server generates a registration challenge.
Browser invokes WebAuthn APIs.
Device creates a public/private key pair.
Public key is stored on the server.
Private key remains on the device.

Generate Registration Options

Example controller action:

[HttpPost]
public IActionResult Register()
{
    var options = _fido2.RequestNewCredential(
        user: user,
        excludeCredentials: new List<PublicKeyCredentialDescriptor>(),
        authenticatorSelection: null,
        attestationPreference: AttestationConveyancePreference.None);

    return Ok(options);
}

The generated options are sent to the browser for credential creation.

Client-Side Registration

The browser uses WebAuthn APIs to create a passkey.

const credential = await navigator.credentials.create({
    publicKey: options
});

The device may prompt the user for:

Fingerprint verification
Face recognition
Device PIN

Once verified, the passkey is created.

Saving the Credential

After successful registration, save the credential information.

Typical data stored includes:

Credential ID
Public key
User ID
Signature counter

Example model:

public class PasskeyCredential
{
    public string CredentialId { get; set; }
    public string PublicKey { get; set; }
    public string UserId { get; set; }
}

Only public information is stored on the server.

User Authentication Flow

Authentication works differently from password verification.

The process is:

User enters an identifier.
Server generates an authentication challenge.
Browser requests credential verification.
Device signs the challenge.
Server validates the signature.

If verification succeeds, the user is authenticated.

Generate Authentication Options

Example:

[HttpPost]
public IActionResult Login()
{
    var options = _fido2.GetAssertionOptions(
        allowedCredentials,
        UserVerificationRequirement.Required);

    return Ok(options);
}

The challenge is sent to the client.

Client-Side Authentication

The browser requests authentication.

const assertion = await navigator.credentials.get({
    publicKey: options
});

The user verifies identity using their device authentication method.

Verify the Assertion

The server validates the response.

var result = await _fido2.MakeAssertionAsync(
    clientResponse,
    options,
    storedPublicKey,
    storedCounter,
    callback);

If validation succeeds, authentication is complete.

Integrating with ASP.NET Core Identity

Many applications already use ASP.NET Core Identity.

Passkeys can be integrated alongside existing authentication methods.

Common approaches include:

Passwordless login
Passkey plus traditional login
Passkey-based multi-factor authentication
Gradual migration from passwords

This allows organizations to adopt passkeys without redesigning their entire authentication system.

Security Considerations

While passkeys are highly secure, several best practices should still be followed.

Always Use HTTPS

WebAuthn requires secure origins.

Ensure all authentication endpoints use HTTPS.

Validate Challenges Properly

Each challenge should:

Be unique
Expire quickly
Be verified server-side

Protect Credential Data

Store credential information securely and validate all authentication requests thoroughly.

Support Account Recovery

Users may lose access to devices.

Provide secure recovery mechanisms such as:

Verified email recovery
Secondary passkeys
Administrator-assisted recovery

Common Implementation Challenges

Browser Compatibility

Most modern browsers support WebAuthn, but testing across platforms is essential.

Multiple Devices

Users may authenticate from:

Mobile devices
Tablets
Laptops
Desktop computers

Design your authentication flow to support multiple registered credentials.

User Education

Many users are unfamiliar with passkeys.

Provide clear instructions during registration and login.

Best Practices

When implementing passkey authentication in ASP.NET Core:

Use established WebAuthn and FIDO2 libraries.
Always deploy authentication endpoints over HTTPS.
Store only public keys on the server.
Support multiple passkeys per user.
Implement secure account recovery workflows.
Monitor authentication failures and suspicious activity.
Consider gradual adoption alongside existing authentication methods.
Test across browsers and operating systems.
Keep authentication challenges short-lived.
Follow the latest WebAuthn and FIDO2 recommendations.

Conclusion

Passkey authentication represents a major advancement in application security by replacing vulnerable passwords with cryptographic credentials secured by user devices. Built on WebAuthn and FIDO2 standards, passkeys offer strong protection against phishing, credential theft, and password-related attacks while providing a smoother user experience.

ASP.NET Core developers can integrate passkeys into existing applications using FIDO2 libraries and modern browser APIs, enabling passwordless authentication with minimal disruption. As organizations continue to prioritize both security and usability, passkeys are becoming an increasingly valuable addition to modern authentication strategies.

By understanding the registration flow, authentication process, and implementation best practices, developers can build secure, user-friendly ASP.NET Core applications that are ready for the future of authentication.