The Relying Party

aka the application you are interacting with
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
);
Even if it is optional, we highly recommend setting the application ID
The rp ID shall be the domain of the application without the scheme, userinfo, port, path, user…. IP addresses are not allowed either.
Allowed: www.sub.domain.com, sub.domain.com, domain.com
Not allowed:
www.sub.domain.com:1337, https://domain.com:443, sub.domain.com/index, https://user:[email protected].
12.65.76.43 or [2001:db8:85a3:8d3:1319:8a2e:370:7348]
The domain localhost can be used if the browser considers the context is safe (especially the IP address corresponds to a local address)
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.
Discussion is ongoing for associating authenticators to multiple RP IDs such as my-company.com, meine-firma.de et ma-compagnie.fr
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,iVBORw0KGgoAAAANSUhEUgAAABQAAAAUCAMAAAC6V+0/AAAAwFBMVEXm7NK41k3w8fDv7+q01Tyy0zqv0DeqyjOszDWnxjClxC6iwCu11z6y1DvA2WbY4rCAmSXO3JZDTxOiwC3q7tyryzTs7uSqyi6tzTCmxSukwi9aaxkWGga+3FLv8Ozh6MTT36MrMwywyVBziSC01TbT5ZW9z3Xi6Mq2y2Xu8Oioxy7f572qxzvI33Tb6KvR35ilwTmvykiwzzvV36/G2IPw8O++02+btyepyDKvzzifvSmw0TmtzTbw8PAAAADx8fEC59dUAAAA50lEQVQYV13RaXPCIBAG4FiVqlhyX5o23vfVqUq6mvD//1XZJY5T9xPzzLuwgKXKslQvZSG+6UXgCnFePtBE7e/ivXP/nRvUUl7UqNclvO3rpLqofPDAD8xiu2pOntjamqRy/RqZxs81oeVzwpCwfyA8A+8mLKFku9XfI0YnSKXnSYZ7ahSII+AwrqoMmEFKriAeVrqGM4O4Z+ADZIhjg3R6LtMpWuW0ERs5zunKVHdnnnMLNQqaUS0kyKkjE1aE98b8y9x9JYHH8aZXFMKO6JFMEvhucj3Wj0kY2D92HlHbE/9Vk77mD6srRZqmVEAZAAAAAElFTkSuQmCC'
);
The Webauthn specification does not set any limit for the length of the third argument.
The icon may be ignored by browsers, especially if its length is greater than 128 bytes.
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Extensions
Next - Prerequisites
Credential Source