An Authenticator is a cryptographic entity used to generate a public key credential and registered by a Relying Party (i.e. an application). This public key is used to authenticate by potentially verifying a user in the form of an authentication assertion and other data.

Authenticators may have additional features such as PIN code or biometric sensors (fingerprint, facial recognition…) that offer user verification.

Roaming Authenticators

The roaming authenticator may have different forms. The most common form is a USB device the user plugs into its computer. It can be a paired Bluetooth device or a card with NFC capabilities.

Authenticators of this class are removable from, and can "roam" among, client devices.

Platform Authenticators

A platform authenticator is usually not removable from the client device. For example an Android smartphone or a Windows 10 computer with the associated security chips can act as an authenticator.

This framework contains several sub-packages that you don’t necessarily need. It is highly recommended to install what you need and not the whole framework.

The preferred way to install the library you need is to use composer:

composer require web-auth/webauthn-lib

Hereafter the dependency tree:

• web-auth/webauthn-lib: this is the core library. This package can be used in any PHP project or within any popular framework (Laravel, CakePHP…)

• web-auth/webauthn-symfony-bundle: this is a Symfony bundle that ease the integration of this authentication mechanism in your Symfony project.

The core library also depends on web-auth/cose-lib and web-auth/metadata-service. What are these dependencies?

web-auth/cose-lib contains several cipher algorithms and COSE key support to verify the digital signatures sent by the authenticators during the creation and authentication ceremonies. These algorithms are compliant with the RFC8152. This library can be used by any other PHP projects. At the moment only signature algorithms are available, but it is planned to add encryption algorithms.

web-auth/metadata-service provides classes to support the Fido Alliance Metadata Service. If you plan to use Attestation Statements during the creation ceremony, this service is mandatory. Please note that Attestation Statements decreases the user privacy as they may leak data that allow to identify a specific user. The use of Attestation Statements and this service are generally not recommended unless you REALLY need this information. This library can also be used by any other PHP projects.

Framework and Dependency Sizes

The total size of the core package is approximately 760ko. Hereafter the detail for each component:

• web-auth/cose-lib: 85ko

• web-auth/metadata-service: 81ko

• web-auth/webauthn-lib: 207ko

• web-auth/webauthn-symfony-bundle: 385ko

The total size of the core package + the direct dependencies is approximately 1.7Mo.

Attestation Statement

During the Attestation Ceremony (i.e. the registration of the authenticator), you can ask for the Attestation Statement of the authenticator. The Attestation Statements have one of the following types:

• None (none): no Attestation Statement is provided

• Basic Attestation (basic): Authenticator’s attestation key pair is specific to an authenticator model.

• Surrogate Basic Attestation (or Self Attestation - self): Authenticators that have no specific attestation key use the credential private key to create the attestation signature

• Attestation CA (AttCA): Authenticators are based on a Trusted Platform Module (TPM). They can generate multiple attestation identity key pairs (AIK) and requests an Attestation CA to issue an AIK certificate for each.

• Anonymization CA (AnonCA): Authenticators use an Anonymization CA, which dynamically generates per-credential attestation certificates such that the attestation statements presented to Relying Parties do not provide uniquely identifiable information.

• Elliptic Curve based Direct Anonymous Attestation (ECDAA): Authenticator receives direct anonymous attestation (DAA) credentials from a single DAA-Issuer. These DAA credentials are used along with blinding to sign the attested credential data.

Metadata Statement

The Metadata Statements are issued by the manufacturers of the authenticators. These statements contain details about the authenticators (supported algorithms, biometric capabilities...) and all the necessary information to verify the Attestation Statements generated during the attestation ceremony.

There are several possible sources to get these Metadata Statements. The main source is the FIDO Alliance Metadata Service that allows fetching statements on-demand, but some of them may be provided by other means.

Webauthn is now supported by all main web browsers:

• Mozilla Firefox 60+ and Firefox for Android 68+

• Google Chrome 67+

• Microsoft EDGE 18+ and Microsoft EDGE Chromium 79+

• Opera 54+

• Safari 13+ and iOS Safari 13.3+

• Android Browser 76+

For more information and limitation on these browsers, please have a look at the following page:

User verification may be instigated through various authorization gesture modalities: a touch plus PIN code, password entry, or biometric recognition (presenting a fingerprint). The intent is to be able to distinguish individual users.

Eligible authenticators are filtered and only capable of satisfying this requirement will interact with the user.

Possible user verification values are:

• required: this value indicates that the application requires user verification for the operation and will fail the operation if the response does not have the UV flag set.

• preferred: this value indicates that the application prefers user verification for the operation if possible, but will not fail the operation if the response does not have the UV flag set.

• discouraged: this value indicates that the application does not want user verification employed during the operation (e.g.,in the interest of minimizing disruption to the user interaction flow).

Public constants are provided by AuthenticatorSelectionCriteria.

• AuthenticatorSelectionCriteria::USER_VERIFICATION_REQUIREMENT_REQUIRED

• AuthenticatorSelectionCriteria::USER_VERIFICATION_REQUIREMENT_PREFERRED

• AuthenticatorSelectionCriteria::USER_VERIFICATION_REQUIREMENT_DISCOURAGED

The Relying Party (or rp) corresponds to the application that will ask for the user to interact with the authenticator.

The library provides a simple class to handle the rp information: Webauthn\PublicKeyCredentialRpEntity.

<?php

use Webauthn\PublicKeyCredentialRpEntity;

$rpEntity = PublicKeyCredentialRpEntity::create(
    'ACME Webauthn Server' // The application name
);

This $rpEntity object will be useful for the next steps.

Relying Party ID

In the example above, we created a simple relying party object with it’s name. The relying party may also have an ID that corresponds to the domain applicable for that rp. By default, the relying party ID is null i.e. the current domain will be used.

It may be useful to specify the rp ID, especially if your application has several sub-domains. The rp ID can be set during the creation of the object as 2nd constructor parameter.

<?php

use Webauthn\PublicKeyCredentialRpEntity;

$rpEntity = PublicKeyCredentialRpEntity::create(
    'ACME Webauthn Server', // The application name
    'acme.com'              // The application ID = the domain
);

The rp ID shall be the domain of the application without the scheme, userinfo, port, path, user…. IP addresses are not allowed either.

How to determine the Relying Party ID?

The Relying Party ID should be determined depending on the common URLs for your web application.

If you have a web application that can be reached at https://m.my-app.com (for mobiles) and https://my-app.com or https://www.my-app.com (for other devices), your Relying Party ID should be my-app.com.

If the domain is shared between sub-projects, the rp ID should be limited to that sub-projects.

For example, a web site is located at https://(www.)site1.host.com and another at https://(www.)site2.host.com, then the Relying Party IDs should be site1.host.com and site2.host.com respectively. If you set host.com, there is a risk that users from site1.host.com can log in at site2.host.com.

Relying Party Icon

Your application may also have a logo. You can indicate this logo as third argument. Please note that for safety reason this icon is a priori authenticated URL i.e. an image that uses the data scheme.

<?php

use Webauthn\PublicKeyCredentialRpEntity;

$rpEntity = PublicKeyCredentialRpEntity::create(
    'ACME Webauthn Server',
    'acme.com',
    'data:image/png;base64,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'
);