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:
Authentication becomes faster and simpler.
Strong Device-Based Authentication
Passkeys can leverage:
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:
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:
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.