What is a Relying Party in WebAuthn?

• A Relying Party, in the context of WebAuthn or passkeys, is the entity that seeks to authenticate a user. It typically refers to a web server or service that relies on an authenticator to verify the user's identity. This authentication process ensures secure user access while also providing a seamless user experience.
• The term "Relying Party" stems from its dependency on external authenticators (like hardware security keys, laptops or smartphones) to authenticate a user. The authentication process involves the use of a unique identifier known as the relying party ID (rpId) which aids in differentiating between various relying parties.

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The Relying Party is integral in the WebAuthn / passkey ecosystem. Here's a deeper look:

• The Relying Party's Objective: Its primary role is to initiate the authentication flow by challenging the user to prove their identity. This challenge-response mechanism ensures that unauthorized entities do not gain access.
• Interplay with Authenticators: Relying Party works hand-in-hand with authenticators. Once the user presents their credentials, the authenticator verifies it and sends back a signed response. The Relying Party then validates this response to complete the authentication process.
• Importance of Relying Party ID (rpId): The rpid is crucial as it provides a scope for the credentials. By ensuring the rpid matches the expected domain or origin, the Relying Party enhances security by preventing potential attacks, such as man-in-the-middle attacks.

Read more about the rpId and other aspects of the Relying Party in the respective blog article.

• Increased Security: With the reliance on external authenticators and the rpid's scope-binding, WebAuthn's Relying Party model provides an added layer of security.
• Improved User Experience: Users are not required to remember passwords, reducing password-related breaches and offering a smoother login process.
• Versatility: The model supports a broad range of authenticators, giving users the flexibility to choose their preferred method.

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The rpid is a unique identifier for the Relying Party, ensuring that credentials are scoped to the correct entity. It's pivotal for security, ensuring the authentication process is tied to the expected domain or origin. Thus, phishing attacks are prevented.

The Relying Party initiates the authentication by challenging the user, while the Authenticator is the device or method verifying the user's credentials and responding to the challenge.

WebAuthn's Relying Party model leverages external authenticators and the rpid mechanism, making it harder for attackers to impersonate users or intercept the authentication process.