FIDO2 .NET library for FIDO2 / WebAuthn Attestation and Assertion using .NET
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FIDO2 .NET library (WebAuthn)

A working implementation library + demo for fido2 and WebAuthn using .NET


Provide a developer friendly and well tested .NET server side library for easy validation (attestation & assertion) of WebAuthn/FIDO2 credentials to increase the adoption of the technology, ultimately defeating phishing attacks.

Install-Package Fido2 -Version 1.0.0-preview


What is FIDO2?

The passwordless web is coming.
FIDO2 / WebAuthn is a new open authentication standard, supported by browsers and many large tech companies such as Microsoft, Google etc. The main driver is to allow a user to login without passwords, creating passwordless flows or strong MFA for user signup/login on websites. The standard is not limited to web applications with support coming to Active Directory and native apps. The technology builds on public/private keys, allowing authentication to happen without sharing a secret between the user & platform. This brings many benefits, such as easier and safer logins and makes phishing attempts extremely hard.

Read more about FIDO2: Why it's exciting, Medium, FIDO2 Alliance and Yubico.

Supported features

  • Attestation API & verification (Register and verify credentials/authenticators)
  • Assertion API & verification (Authenticate users)
  • 100% success rate in conformance testing (results)
  • Fido 2 Security Keys
  • Backwards compatibility with Fido-u2f.
  • Windows Hello support
  • ES256 Public Key format
  • "none", "fido-u2f", "android-key", "android-safetynet", "tpm" & "packed" attestation formats
  • Examples & demo's
  • Intellisense documentation
  • 💤 Formal documentation
  • 💤 Recommended usage patterns
  • Extensions


Only some options are mention here, see the Configuration class for all options

  • fido2:MDSAccessKey - App Secret / environment variable that holds the FIDO2 MDS AccessKey. Required when using the default MetadataService provider.
  • fido2:MDSCacheDirPath - App Secret / environment variable that sets the cache path for the MDS. Required when using the default MetadataService provider.


Please see the demo controller for full examples of both Attestation & Assertion.

Create attestation Options

To add fido2 credentials to an existing user account, we we perform a attestation process. It starts with returning options to the client.

// file: Controller.cs
// 1. Get user from DB by username (in our example, auto create missing users)
var user = DemoStorage.GetOrAddUser(username, () => new User
    DisplayName = "Display " + username,
    Name = username,
    Id = Encoding.UTF8.GetBytes(username) // byte representation of userID is required

// 2. Get user existing keys by username
List<PublicKeyCredentialDescriptor> existingKeys = DemoStorage.GetCredentialsByUser(user).Select(c => c.Descriptor).ToList();

// 3. Create options
var options = _lib.RequestNewCredential(user, existingKeys, AuthenticatorSelection.Default, AttestationConveyancePreference.Parse(attType));

// 4. Temporarily store options, session/in-memory cache/redis/db
HttpContext.Session.SetString("fido2.attestationOptions", options.ToJson());

// 5. return options to client
return Json(options);

Register credentials

When the client returns a response, we verify and register the credentials.

// file: Controller.cs
// 1. get the options we sent the client
var jsonOptions = HttpContext.Session.GetString("fido2.attestationOptions");
var options = CredentialCreateOptions.FromJson(jsonOptions);

// 2. Create callback so that lib can verify credential id is unique to this user
IsCredentialIdUniqueToUserAsyncDelegate callback = async (IsCredentialIdUniqueToUserParams args) =>
    List<User> users = await DemoStorage.GetUsersByCredentialIdAsync(args.CredentialId);
    if (users.Count > 0) return false;

    return true;

// 2. Verify and make the credentials
var success = await _lib.MakeNewCredentialAsync(attestationResponse, options, callback);

// 3. Store the credentials in db
DemoStorage.AddCredentialToUser(options.User, new StoredCredential
    Descriptor = new PublicKeyCredentialDescriptor(success.Result.CredentialId),
    PublicKey = success.Result.PublicKey,
    UserHandle = success.Result.User.Id

// 4. return "ok" to the client
return Json(success);

Create Assertion options

When a user wants to log a user in, we do an assertion based on the registered credentials.

First we create the assertion options and return to the client.

// file: Controller.cs
// 1. Get user from DB
var user = DemoStorage.GetUser(username);
if (user == null) return NotFound("username was not registered");

// 2. Get registered credentials from database
List<PublicKeyCredentialDescriptor> existingCredentials = DemoStorage.GetCredentialsByUser(user).Select(c => c.Descriptor).ToList();

// 3. Create options
var options = _lib.GetAssertionOptions(

// 4. Temporarily store options, session/in-memory cache/redis/db
HttpContext.Session.SetString("fido2.assertionOptions", options.ToJson());

// 5. Return options to client
return Json(options);

Verify the assertion response

When the client returns a response, we verify it and accepts the login.

// 1. Get the assertion options we sent the client
var jsonOptions = HttpContext.Session.GetString("fido2.assertionOptions");
var options = AssertionOptions.FromJson(jsonOptions);

// 2. Get registered credential from database
StoredCredential creds = DemoStorage.GetCredentialById(clientResponse.Id);

// 3. Get credential counter from database
var storedCounter = creds.SignatureCounter;

// 4. Create callback to check if userhandle owns the credentialId
IsUserHandleOwnerOfCredentialIdAsync callback = async (args) =>
    List<StoredCredential> storedCreds = await DemoStorage.GetCredentialsByUserHandleAsync(args.UserHandle);
    return storedCreds.Exists(c => c.Descriptor.Id.SequenceEqual(args.CredentialId));

// 5. Make the assertion
var res = await _lib.MakeAssertionAsync(clientResponse, options, creds.PublicKey, storedCounter, callback);

// 6. Store the updated counter
DemoStorage.UpdateCounter(res.CredentialId, res.Counter);

// 7. return OK to client
return Json(res);

Nuget package

Install-Package Fido2 -Version 1.0.0-preview

To run the project locally

Start Fido2Demo (preferably https, expected url https://localhost:44329) and open https://localhost:44329/ in the browser. You also need to either set the MetadataService to null or add the applicationSettings as described below.

In order to utilize the metadata provided from FIDO Alliance Metadata Service you must register for an access token ( Set the fido2:MDSAccessKey app secret value to your access token, and the fido2:MDSCacheDirPath app secret value to a local directory to cache metadata. See for more information on app secret usage.

The HTML and javascript is copied (and then updated) from

Feedback, issues and pull requests are VERY welcome.

Conformance testing tool

To run a suit of test of different verifications and attestation formats, register and download the FIDO Test tools


A complimentary blog post with some lessons learned since starting this library