Register Authenticators

During this step, your application will send a challenge to the device. The device will resolve this challenge by adding information and digitally signing the data.

The application will check the response from the device and get its credential ID. This ID will be used for further authentication requests.

Creation Request

To associate a device to a user, you need to instantiate a Webauthn\PublicKeyCredentialCreationOptions object.

It will need:

• The Relying Party

• The User data

• A challenge (random binary string)

• A list of supported public key parameters i.e. an algorithm list (at least one)

Optionally, you can customize the following parameters:

• A timeout

• A list of public key credential to exclude from the registration process

• The Authenticator Selection Criteria

• Attestation conveyance preference

• Extensions

Let’s see an example of the PublicKeyCredentialCreationOptions object. The following example is a possible Public Key Creation page for a dummy user "@cypher-Angel-3000".

<?php

declare(strict_types=1);

use Webauthn\PublicKeyCredentialCreationOptions;
use Webauthn\PublicKeyCredentialRpEntity;
use Webauthn\PublicKeyCredentialUserEntity;

// RP Entity i.e. the application
$rpEntity = PublicKeyCredentialRpEntity::create(
    'My Super Secured Application', //Name
    'foo.example.com',              //ID
    null                            //Icon
);

// User Entity
$userEntity = PublicKeyCredentialUserEntity::create(
    '@cypher-Angel-3000',                   //Name
    '123e4567-e89b-12d3-a456-426655440000', //ID
    'Mighty Mike',                          //Display name
    null                                    //Icon
);

// Challenge
$challenge = random_bytes(16);

$publicKeyCredentialCreationOptions =
    PublicKeyCredentialCreationOptions::create(
        $rpEntity,
        $userEntity,
        $challenge
    )
;

The options object can be converted into JSON and sent to the authenticator using the API.

use Symfony\Component\Serializer\Encoder\JsonEncode;
use Symfony\Component\Serializer\Normalizer\AbstractObjectNormalizer;

// The serializer is the same as the one created in the previous pages
$jsonObject = $serializer->serialize(
    $publicKeyCredentialCreationOptions,
    'json',
    [ // Optional
        AbstractObjectNormalizer::SKIP_NULL_VALUES => true,
        JsonEncode::OPTIONS => JSON_THROW_ON_ERROR,
    ]
);

It is important to store the user entity and the options object (e.g. in the session) for the next step. The data will be needed to check the response from the device.

You can change the default values for each and all options

use Webauthn\AuthenticatorSelectionCriteria;

$publicKeyCredentialCreationOptions =
    PublicKeyCredentialCreationOptions::create(
        $rpEntity,
        $userEntity,
        $challenge
    )
;

Public Key Credential Parameters

The argument pubKeyCredParams contains a list of Webauthn\PublicKeyCredentialParameters objects that refer to COSE algorithms. The authenticators must use one of the algorithms in this list, respecting the order of preference on this list.

An empty list corresponds to the default algorithms that are ES256 and RS256 (in this order). Those two algorithms are required by the specification.

use Cose\Algorithms;
use Webauthn\PublicKeyCredentialParameters;

$publicKeyCredentialParametersList = [
    PublicKeyCredentialParameters::create('public-key', Algorithms::COSE_ALGORITHM_ES256K), // More interesting algorithm
    PublicKeyCredentialParameters::create('public-key', Algorithms::COSE_ALGORITHM_ES256),  //      ||
    PublicKeyCredentialParameters::create('public-key', Algorithms::COSE_ALGORITHM_RS256),  //      || 
    PublicKeyCredentialParameters::create('public-key', Algorithms::COSE_ALGORITHM_PS256),  //      \/
    PublicKeyCredentialParameters::create('public-key', Algorithms::COSE_ALGORITHM_ED256),  // Less interesting algorithm
];

Customizing this list may lead to unexpected behavior. Please use with caution.

Authenticator Selection

Please read detail on this page.

Attestation

Please read detail on this page.

Exclude Credentials

When the user already registered authenticators, you can pass a list of Webauthn\PublicKeyCredentialDescriptor objects as argument to avoid registering multiple times the same authenticator.

use Webauthn\PublicKeyCredentialCreationOptions;
use Webauthn\PublicKeyCredentialDescriptor;

$excludedPublicKeyDescriptors = [
    PublicKeyCredentialDescriptor::create('public-key', 'CREDENTIAL ID1'),
    PublicKeyCredentialDescriptor::create('public-key', 'CREDENTIAL ID2'),
    ...
];

$publicKeyCredentialCreationOptions =
    PublicKeyCredentialCreationOptions::create(
        $rpEntity,
        $userEntity,
        $challenge,
        excludeCredentials: $excludedPublicKeyDescriptors,
    )
;

To avoid username enumeration, you should not set any excluded credential when the authenticator registration is performed for anonymous users. If this value is present, it indicates that the username exists and has registered credentials.

On the contrary when adding new authenticators to already authenticated users, this value should be set.

Creation Response

What you receive must be a JSON object that looks like as follow:

{
    "id":"KVb8CnwDjpgAo[…]op61BTLaa0tczXvz4JrQ23usxVHA8QJZi3L9GZLsAtkcVvWObA",
    "type":"public-key",
    "rawId":"KVb8CnwDjpgAo[…]rQ23usxVHA8QJZi3L9GZLsAtkcVvWObA==",
    "response":{
        "clientDataJSON":"eyJjaGFsbGVuZ2UiOiJQbk1hVjBVTS[…]1iUkdHLUc4Y3BDSdGUifQ==",
        "attestationObject":"o2NmbXRmcGFja2VkZ2F0dFN0bXSj[…]YcGhf"
    }
}

There are two steps to perform with this object:

• Load the data

• Verify it with the creation options set above

Data Loading

Now that all components are set, we can load the data we receive using the Serializer (variable $serializer).

<?php

declare(strict_types=1);

use Webauthn\PublicKeyCredential;

$data = '
{
    "id":"KVb8CnwDjpgAo[…]op61BTLaa0tczXvz4JrQ23usxVHA8QJZi3L9GZLsAtkcVvWObA",
    "type":"public-key",
    "rawId":"KVb8CnwDjpgAo[…]rQ23usxVHA8QJZi3L9GZLsAtkcVvWObA==",
    "response":{
        "clientDataJSON":"eyJjaGFsbGVuZ2UiOiJQbk1hVjBVTS[…]1iUkdHLUc4Y3BDSdGUifQ==",
        "attestationObject":"o2NmbXRmcGFja2VkZ2F0dFN0bXSj[…]YcGhf"
    }
}';

$publicKeyCredential = $serializer->deserialize(
    $data,
    PublicKeyCredential::class,
    'json'
);

If no exception is thrown, you can go to the next step: the verification.

Response Verification

Now we have a fully loaded Public Key Credential object, but we need now to make sure that:

1. The authenticator response is of type AuthenticatorAttestationResponse

2. This response is valid.

The first step is easy to perform:

<?php

declare(strict_types=1);

use Webauthn\AuthenticatorAttestationResponse;

if (!$publicKeyCredential->response instanceof AuthenticatorAttestationResponse) {
    //e.g. process here with a redirection to the public key creation page. 
}

The second step is the verification against

• The Public Key Creation Options we created earlier,

• The URI host

The Authenticator Attestation Response Validator service (variable $authenticatorAttestationResponseValidator) will check everything for you: challenge, origin, attestation statement and much more.

<?php

declare(strict_types=1);

$publicKeyCredentialSource = $authenticatorAttestationResponseValidator->check(
    $authenticatorAttestationResponse,
    $publicKeyCredentialCreationOptions,
    'my-application.com'
);

If no exception is thrown, the response is valid. You can store the Public Key Credential Source ($publicKeyCredentialSource).

The way you store and associate these objects to the user is out of scope of this library.