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FIDO2 .NET Library (WebAuthn)

A working implementation library + demo for FIDO2 and WebAuthn using .NET
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NuGet Status Change log


Enable passwordless sign in for all .net apps (asp, core, native).

To provide a developer friendly and well tested .NET FIDO2 Server / WebAuthn relying party library for the easy validation of registration (attestation) and authentication (assertion) of FIDO2 / WebAuthn credentials, in order to increase the adoption of the technology, ultimately defeating phishing attacks.

Install-Package Fido2 -Version 1.1.0

To use the helpers, install the

Install-Package Fido2.AspNet -Version 1.1.0


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:

Supported features

  • Attestation API & verification (Register and verify credentials/authenticators)
  • Assertion API & verification (Authenticate users)
  • 100% pass rate in conformance testing (results)
  • FIDO2 security keys aka roaming authenticators (spec), like SoloKeys Solo, Yubico YubiKey, and Feitian BioPass FIDO2)
  • Device embedded authenticators aka platform authenticators (spec), like Android Key and TPM)
  • Backwards compatibility with FIDO U2F authenticators (spec)
  • Windows Hello
  • All currently referenced cryptographic algorithms for FIDO2 Server (spec)
  • All current attestation formats: "packed", "tpm", "android-key", "android-safetynet", "fido-u2f", and "none" (spec)
  • FIDO2 Server attestation validation via FIDO Metadata Service (spec)
  • WebAuthn extensions (spec)
  • Examples & demos
  • Intellisense documentation
  • 💤 Formal documentation
  • 💤 Recommended usage patterns


Only some options are mentioned 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. Defaults to "current user's temporary folder"/fido2mdscache. Optional when using the default MetadataService provider.


See the demo controller for full examples of both attestation and assertion.

See the test controller for examples of how to pass the conformance tests.

See the Active Directory Store information and example credential store for ideas on how to integrate this library with an on-premises Active Directory.

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 and remove it from storage
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 and remove from storage
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 and


See Contributing for informations about contributing to the project.


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