Web Authentication: A Web API for accessing scoped credentials

1. Use Cases

This section is not normative.

This specification defines an API for web pages to access scoped credentials through JavaScript, for the purpose of strongly authenticating a user. Scoped credentials are always scoped to a single WebAuthn Relying Party. This scoping is enforced jointly by the User Agent implementing the Web Authentication API and the authenticator that holds the credential, by constraining the availabilty and usage of credentials. Scoped credentials created by a WebAuthn Relying Party can only be accessed by web origins belonging to that WebAuthn Relying Party. Additionally, privacy across WebAuthn Relying Parties must be maintained; scripts must not be able to detect any properties, or even the existence, of scoped credentials belonging to other WebAuthn Relying Parties.

Scoped credentials are located on authenticators, which can use them to perform operations subject to user consent. Broadly, authenticators are of two types:

1. Embedded authenticators#embedded-authenticatorsReferenced in:1.1. Registration (embedded authenticator mode)9. Terminology have their operation managed by the same computing device (e.g., smart phone, tablet, desktop PC) as the user agent is running on. For instance, such an authenticator might consist of a Trusted Platform Module (TPM) or Secure Element (SE) integrated into the computing device, along with appropriate platform software to mediate access to this device’s functionality.

2. External authenticators#external-authenticatorsReferenced in:1.2. Authentication (external authenticator mode)8.2. Authentication9. Terminology operate autonomously from the device running the user agent, and accessed over a transport such as Universal Serial Bus (USB), Bluetooth Low Energy (BLE) or Near Field Communications (NFC).

Note that an external authenticator may itself contain an embedded authenticator. For example, consider a smart phone that contains a scoped credential. The credential may be accessed by a web browser running on the phone itself. In this case the module containing the credential is functioning as an embedded authenticator. However, the credential may also be accessed over BLE by a user agent on a nearby laptop. In this latter case, the phone is functioning as an external authenticator. These modes may even be used in a single end-to-end user scenario. One such scenario is described in the remainder of this section.

1.1. Registration (embedded authenticator mode)

• On the phone:

  • User goes to example.com in the browser, and signs in using whatever method they have been using (possibly a legacy method such as a password).

  • The phone prompts, "Do you want to register this device with example.com?"

  • User agrees.

  • The phone prompts the user for a previously configured authorization gesture (PIN, biometric, etc.); the user provides this.

  • Website shows message, "Registration complete."

1.2. Authentication (external authenticator mode)

• On the laptop:

  • User goes to example.com in browser, sees an option "Sign in with your phone."

  • User chooses this option and gets a message from the browser, "Please complete this action on your phone."

• Next, on the phone:

  • User sees a discreet prompt or notification, "Sign in to example.com."

  • User selects this prompt / notification.

  • User is shown a list of their example.com identities, e.g., "Sign in as Alice / Sign in as Bob."

  • User picks an identity, is prompted for an authorization gesture (PIN, biometric, etc.) and provides this.

• Now, on the laptop:

  • Web page shows that the selected user is signed in, and navigates to the signed-in page.

1.3. Other configurations

A variety of additional use cases and configurations are also possible, including (but not limited to):

• User goes to example.com on their laptop, is guided through a flow to create and register a credential on their phone.

• User employs a scoped credential as described above to authorize a single transaction, such as a payment or other financial transaction.

2. Conformance

This specification defines criteria for a Conforming User Agent. A User Agent MUST behave as described in this specification in order to be considered conformant. User Agents MAY implement algorithms given in this specification in any way desired, so long as the end result is indistinguishable from the result that would be obtained by the specification’s algorithms. A conforming Web Authentication API User Agent MUST also be a conforming implementation of the IDL fragments of this specification, as described in the “Web IDL” specification. [WebIDL-1]

This specification also defines a model of a Web Authentication compliant authenticator. This is a set of functional and security requirements for an authenticator to be usable by a User Agent that implements the Web Authentication API. As described in §1 Use Cases, the authenticator itself may be implemented in the operating system underlying the User Agent, or in external hardware, or a combination of both.

2.1. Dependencies

This specification relies on several other underlying specifications.

HTML5

The concept of origin#originReferenced in:3.1.1. Create a new credential (makeCredential() method)3.1.2. Use an existing credential (getAssertion() method) and the Window interface are defined in [HTML5].

Web IDL

Many of the interface definitions and all of the IDL in this specification depend on [WebIDL-1]. This updated version of the Web IDL standard adds support for Promises#promisesReferenced in:3.1.1. Create a new credential (makeCredential() method)3.1.2. Use an existing credential (getAssertion() method), which are now the preferred mechanism for asynchronous interaction in all new web APIs.

DOM

DOMException#domexceptionReferenced in:3.1.1. Create a new credential (makeCredential() method)3.1.2. Use an existing credential (getAssertion() method) and the DOMException values used in this specification are defined in [DOM4].

Web Cryptography API

The AlgorithmIdentifier#dictdef-algorithmidentifierReferenced in:3. Web Authentication API3.8.3. Cryptographic Algorithm Identifier (type AlgorithmIdentifier)4.3. Client data used in WebAuthn signatures (dictionary ClientData) type and the method for normalizing an algorithm are defined in Web Cryptography API §algorithm-dictionary.

The JsonWebKey#jsonwebkeyReferenced in:4.3. Client data used in WebAuthn signatures (dictionary ClientData)4.4.2.3.4. AndroidAttestationClientData interface for representing cryptographic keys is defined in Web Cryptography API §JsonWebKey-dictionary.

Base64url encoding

The term Base64url Encoding refers to the base64 encoding using the URL- and filename-safe character set defined in Section 5 of [RFC4648], with all trailing '=' characters omitted (as permitted by Section 3.2) and without the inclusion of any line breaks, whitespace, or other additional characters. This is the same encoding as used by JSON Web Signature (JWS) [RFC7515].

3. Web Authentication API

This section normatively specifies the API for creating and using scoped credentials. Support for deleting credentials is deliberately omitted; this is expected to be done through platform-specific user interfaces rather than from a script. The basic idea is that the credentials belong to the user and are managed by the browser and underlying platform. Scripts can (with the user’s consent) request the browser to create a new credential for future use by the script’s origin. Scripts can also request the user’s permission to perform authentication operations with an existing credential held by the platform. However, all such operations are mediated by the browser and/or platform on the user’s behalf. At no point does the script get access to the credentials themselves; it only gets information about the credentials in the form of objects.

User agents SHOULD only expose this API to callers in secure contexts, as defined in [powerful-features].

The API is defined by the following Web IDL fragment.

partial interface Window {
    readonly attribute WebAuthentication webauthn;
};

interface WebAuthentication {
    Promise < ScopedCredentialInfo > makeCredential (
        Account                                 accountInformation,
        sequence < ScopedCredentialParameters > cryptoParameters,
        BufferSource                            attestationChallenge,
        optional unsigned long                  credentialTimeoutSeconds,
        optional sequence < Credential >        blacklist,
        optional WebAuthnExtensions             credentialExtensions
    );

    Promise < WebAuthnAssertion > getAssertion (
        BufferSource                     assertionChallenge,
        optional unsigned long           assertionTimeoutSeconds,
        optional sequence < Credential > whitelist,
        optional WebAuthnExtensions      assertionExtensions
    );
};

interface ScopedCredentialInfo {
    readonly attribute Credential           credential;
    readonly attribute any                  publicKey;
    readonly attribute WebAuthnAttestation  attestation;
};

dictionary Account {
    required DOMString rpDisplayName;
    required DOMString displayName;
    DOMString          name;
    DOMString          id;
    DOMString          imageURL;
};

dictionary ScopedCredentialParameters {
    required CredentialType        type;
    required AlgorithmIdentifier   algorithm;
};

interface WebAuthnAssertion {
    readonly attribute Credential  credential;
    readonly attribute ArrayBuffer clientData;
    readonly attribute ArrayBuffer authenticatorData;
    readonly attribute ArrayBuffer signature;
};

dictionary WebAuthnExtensions {
};

interface WebAuthnAttestation {
    readonly    attribute DOMString     type;
    readonly    attribute ArrayBuffer   clientData;
    readonly    attribute any           statement;
};

enum CredentialType {
    "ScopedCred"
};

interface Credential {
    readonly attribute CredentialType type;
    readonly attribute BufferSource   id;
};

3.1. WebAuthentication#webauthenticationReferenced in:3. Web Authentication API (2) Interface

This interface has two methods, which are described in the following subsections.

3.1.1. Create a new credential (makeCredential()#dom-webauthentication-makecredentialReferenced in:3. Web Authentication API4.1.1. The authenticatorMakeCredential operation4.4.2.3.5. Verifying AndroidClientData specific contextual bindings5. WebAuthn Extensions (2)5.2. Defining extensions5.3. Extending request parameters (2)6.2. Authenticator Selection Extension method)

With this method, a script can request the User Agent to create a new credential of a given type and persist it to the underlying platform, which may involve data storage managed by the browser or the OS. The user agent will prompt the user to approve this operation. On success, the promise will be resolved with a ScopedCredentialInfo object describing the newly created credential.

This method takes the following parameters:

• The accountInformation#accountinformationReferenced in:3.1.1. Create a new credential (makeCredential() method) parameter specifies information about the user account for which the credential is being created. This is meant for later use by the authenticator when it needs to prompt the user to select a credential.

• The cryptoParameters#cryptoparametersReferenced in:3.1.1. Create a new credential (makeCredential() method) (2) (3) (4) parameter supplies information about the desired properties of the credential to be created. The sequence is ordered from most preferred to least preferred. The platform makes a best effort to create the most preferred credential that it can.

• The attestationChallenge#attestationchallengeReferenced in:3.1.1. Create a new credential (makeCredential() method) parameter contains a challenge intended to be used for generating the attestation statement of the newly created credential.

• The optional credentialTimeoutSeconds#credentialtimeoutsecondsReferenced in:3.1.1. Create a new credential (makeCredential() method) (2) parameter specifies a time, in seconds, that the caller is willing to wait for the call to complete. This is treated as a hint, and may be overridden by the platform.

• The optional blacklist#blacklistReferenced in:3.1.1. Create a new credential (makeCredential() method) (2) parameter is intended for use by WebAuthn Relying Parties that wish to limit the creation of multiple credentials for the same account on a single authenticator. The platform is requested to return an error if the new credential would be created on an authenticator that also contains one of the credentials enumerated in this parameter.

• The optional credentialExtensions#credentialextensionsReferenced in:3.1.1. Create a new credential (makeCredential() method)5.3. Extending request parameters parameter contains additional parameters requesting additional processing by the client and authenticator. For example, the caller may request that only authenticators with certain capabilities be used to create the credential, or that additional information be returned in the attestation statement. Alternatively, the caller may specify an additional message that they would like the authenticator to display to the user. Extensions are defined in §5 WebAuthn Extensions.

When this method is invoked, the user agent MUST execute the following algorithm:

1. If credentialTimeoutSeconds was specified, check if its value lies within a reasonable range as defined by the platform and if not, correct it to the closest value lying within that range. Set adjustedTimeout to this adjusted value. If credentialTimeoutSeconds was not specified then set adjustedTimeout to a platform-specific default.

2. Let promise be a new Promise. Return promise and start a timer for adjustedTimeout seconds. Then asynchronously continue executing the following steps.

3. Set callerOrigin to the origin of the caller. Derive the RP ID from callerOrigin by computing the "public suffix + 1" or "PS+1" (which is also referred to as the "Effective Top-Level Domain plus One" or "eTLD+1") part of callerOrigin [PSL]. Set rpId to the RP ID.

4. Initialize issuedRequests to an empty list.

5. Process each element of cryptoParameters using the following steps:

   • Let current be the currently selected element of cryptoParameters.

   • If current.type does not contain a CredentialType supported by this implementation, then stop processing current and move on to the next element in cryptoParameters.

   • Let normalizedAlgorithm be the result of normalizing an algorithm using the procedure defined in [WebCryptoAPI], with alg set to current.algorithm and op set to 'generateKey'. If an error occurs during this procedure, then stop processing current and move on to the next element in cryptoParameters.

6. If blacklist is undefined, set it to the empty list.

7. If credentialExtensions was specified, process any extensions supported by this client platform, to produce the extension data that needs to be sent to the authenticator. Call this data clientExtensions.

8. For each embedded or external authenticator currently available on this platform: asynchronously invoke the authenticatorMakeCredential operation on that authenticator with callerOrigin, rpId, accountInformation, current.type, normalizedAlgorithm, blacklist, attestationChallenge and clientExtensions as parameters. Add a corresponding entry to issuedRequests.

9. While issuedRequests is not empty, perform the following actions depending upon the adjustedTimeout timer and responses from the authenticators:

   • If the adjustedTimeout timer expires, then for each entry in issuedRequests invoke the authenticatorCancel operation on that authenticator and remove its entry from the list.

   • If any authenticator returns a status indicating that the user cancelled the operation, delete that authenticator’s entry from issuedRequests. For each remaining entry in issuedRequests invoke the authenticatorCancel operation on that authenticator and remove its entry from the list.

   • If any authenticator returns an error status, delete the corresponding entry from issuedRequests.

   • If any authenticator indicates success, create a new ScopedCredentialInfo object named value and populate its fields with the values returned from the authenticator. Resolve promise with value and terminate this algorithm.

10. Resolve promise with a DOMException whose name is "NotFoundError", and terminate this algorithm.

During the above process, the user agent SHOULD show some UI to the user to guide them in the process of selecting and authorizing an authenticator.

3.1.2. Use an existing credential (getAssertion()#dom-webauthentication-getassertionReferenced in:3. Web Authentication API3.6. WebAuthn Assertion Extensions (dictionary WebAuthnExtensions)4.1.2. The authenticatorGetAssertion operation5. WebAuthn Extensions (2)5.2. Defining extensions5.3. Extending request parameters (2) method)

This method is used to discover and use an existing scoped credential, with the user’s consent. The script optionally specifies some criteria to indicate what credentials are acceptable to it. The user agent and/or platform locates credentials matching the specified criteria, and guides the user to pick one that the script should be allowed to use. The user may choose not to provide a credential even if one is present, for example to maintain privacy.

This method takes the following parameters:

• The assertionChallenge#assertionchallengeReferenced in:3.1.2. Use an existing credential (getAssertion() method) parameter contains a challenge that the selected authenticator is expected to sign to produce the assertion.

• The optional assertionTimeoutSeconds#assertiontimeoutsecondsReferenced in:3.1.2. Use an existing credential (getAssertion() method) (2) parameter specifies a time, in seconds, that the caller is willing to wait for the call to complete. This is treated as a hint, and may be overridden by the platform.

• The optional whitelist#whitelistReferenced in:3.1.2. Use an existing credential (getAssertion() method) (2) member contains a list of credentials acceptable to the caller, in order of the caller’s preference.

• The optional assertionExtensions#assertionextensionsReferenced in:3.1.2. Use an existing credential (getAssertion() method)5.3. Extending request parameters parameter contains additional parameters requesting additional processing by the client and authenticator. For example, if transaction confirmation is sought from the user, then the prompt string would be included in an extension. Extensions are defined in a companion specification.

When this method is invoked, the user agent MUST execute the following algorithm:

1. If assertionTimeoutSeconds was specified, check if its value lies within a reasonable range as defined by the platform and if not, correct it to the closest value lying within that range. Set adjustedTimeout to this adjusted value. If assertionTimeoutSeconds was not specified then set adjustedTimeout to a platform-specific default.

2. Let promise be a new Promise. Return promise and start a timer for adjustedTimeout seconds. Then asynchronously continue executing the following steps.

3. Set callerOrigin to the origin of the caller. Derive the RP ID from callerOrigin by computing the "public suffix + 1" or "PS+1" (which is also referred to as the "Effective Top-Level Domain plus One" or "eTLD+1") part of callerOrigin [PSL]. Set rpId to the RP ID.

4. Initialize issuedRequests to an empty list.

5. If assertionExtensions was specified, process any extensions supported by this client platform, to produce the extension data that needs to be sent to the authenticator. Call this data clientExtensions.

6. For each embedded or external authenticator currently available on this platform, perform the following steps:

   • If whitelist is undefined or empty, let credentialList be a list containing a single wildcard entry.

   • If whitelist is defined and non-empty, optionally execute a platform-specific procedure to determine which of these credentials can possibly be present on this authenticator. Set credentialList to this filtered list. If credentialList is empty, ignore this authenticator and do not perform any of the following per-authenticator steps.

   • Asynchronously invoke the authenticatorGetAssertion operation on this authenticator with callerOrigin, rpId, assertionChallenge, credentialList, and clientExtensions as parameters.

   • Add an entry to issuedRequests, corresponding to this request.

7. While issuedRequests is not empty, perform the following actions depending upon the adjustedTimeout timer and responses from the authenticators:

   • If the timer for adjustedTimeout expires, then for each entry in issuedRequests invoke the authenticatorCancel operation on that authenticator and remove its entry from the list.

   • If any authenticator returns a status indicating that the user cancelled the operation, delete that authenticator’s entry from issuedRequests. For each remaining entry in issuedRequests invoke the authenticatorCancel operation on that authenticator, and remove its entry from the list.

   • If any authenticator returns an error status, delete the corresponding entry from issuedRequests.

   • If any authenticator returns success, create a new WebAuthnAssertion object named value and populate its fields with the values returned from the authenticator. Resolve promise with value and terminate this algorithm.

8. Resolve promise with a DOMException whose name is "NotFoundError", and terminate this algorithm.

During the above process, the user agent SHOULD show some UI to the user to guide them in the process of selecting and authorizing an authenticator with which to complete the operation.

3.2. ScopedCredentialInfo#scopedcredentialinfoReferenced in:3. Web Authentication API (2)3.1.1. Create a new credential (makeCredential() method) (2) Interface

3.3. User Account Information (dictionary Account#dictdef-accountReferenced in:3. Web Authentication API (2))

3.4. Parameters for Credential Generation (dictionary ScopedCredentialParameters#dictdef-scopedcredentialparametersReferenced in:3. Web Authentication API (2))

3.5. WebAuthn Assertion (interface WebAuthnAssertion#webauthnassertionReferenced in:3. Web Authentication API (2)3.1.2. Use an existing credential (getAssertion() method)4.3.2. Generating a signature)

Scoped credentials produce a cryptographic signature that provides proof of possession of a private key as well as evidence of user consent to a specific transaction. The structure of these signatures is defined as follows.

3.6. WebAuthn Assertion Extensions (dictionary WebAuthnExtensions#dictdef-webauthnextensionsReferenced in:3. Web Authentication API (2) (3)4.3. Client data used in WebAuthn signatures (dictionary ClientData))

This is a dictionary containing zero or more extensions as defined in §5 WebAuthn Extensions. An extension is an additional parameter that can be passed to the getAssertion() method and triggers some additional processing by the client platform and/or the authenticator.

If the caller wants to pass extensions to the platform, it SHOULD do so by adding one entry per extension to this dictionary with the extension identifier as the key, and the extension’s value as the value (see §4.2 Signature Format for details).

3.7. Credential Attestation Statement (interface WebAuthnAttestation#webauthnattestationReferenced in:3. Web Authentication API (2))

Authenticators also provide some form of attestation. The basic requirement is that the authenticator can produce, for each credential public key, attestation information that can be verified by a WebAuthn Relying Party. Typically this information contains a signature by an attesting key over the attested public key and a challenge, as well as a certificate or similar information providing provenance information for the attesting key, enabling a trust decision to be made.

This attestation statement is delivered to the WebAuthn Relying Party by the script, using methods outside the scope of this specification. It contains all the information that the WebAuthn Relying Party’s server requires to validate the statement, as well as to decode and validate the bindings of both the client and authenticator data.

3.8. Supporting Data Structures

The scoped credential type uses certain data structures that are specified in supporting specifications. These are as follows.

3.8.1. Credential Type enumeration (enum CredentialType#enumdef-credentialtypeReferenced in:3. Web Authentication API (2) (3)3.1.1. Create a new credential (makeCredential() method))

3.8.2. Unique Identifier for Credential (interface Credential#credentialReferenced in:3. Web Authentication API (2) (3) (4) (5))

This interface contains the attributes that are returned to the caller when a new credential is created, and can be used later by the caller to select a credential for use.

3.8.3. Cryptographic Algorithm Identifier (type AlgorithmIdentifier)

A string or dictionary identifying a cryptographic algorithm and optionally a set of parameters for that algorithm. This type is defined in [WebCryptoAPI].

4. WebAuthn Authenticator model

The API defined in this specification implies a specific abstract functional model for an authenticator. This section describes the authenticator model. Client platforms may implement and expose this abstract model in any way desired. However, the behavior of the client’s Web Authentication API implementation, when operating on the embedded and external authenticators supported by that platform, MUST be indistinguishable from the behavior specified in §3 Web Authentication API.

In this abstract model, each authenticator stores some number of scoped credentials. Each scoped credential has an identifier which is unique (or extremely unlikely to be duplicated) among all scoped credentials. Each credential is also associated with a WebAuthn Relying Party, whose identity is represented by a Relying Party Identifier (RP ID).

4.1. Authenticator operations

A client must connect to an authenticator in order to invoke any of the operations of that authenticator. This connection defines an authenticator session. An authenticator must maintain isolation between sessions. It may do this by only allowing one session to exist at any particular time, or by providing more complicated session management.

The following operations can be invoked by the client in an authenticator session.

4.1.1. The authenticatorMakeCredential#authenticatormakecredentialReferenced in:3.1.1. Create a new credential (makeCredential() method)4.1.3. The authenticatorCancel operation (2) operation

This operation must be invoked in an authenticator session which has no other operations in progress. It takes the following input parameters:

• The web origin of the script on whose behalf the operation is being initiated, as determined by the user agent and the client.

• The RP ID corresponding to the above web origin, as determined by the user agent and the client.

• All input parameters accepted by the makeCredential() method.

When this operation is invoked, the authenticator obtains user consent for creating a new credential. The prompt for obtaining this consent is shown by the authenticator if it has its own output capability, or by the user agent otherwise. Once user consent is obtained, the authenticator generates the appropriate cryptographic keys and creates a new credential. It then associates the credential with the specified RP ID such that it will be able to retrieve the RP ID later, given the credential ID.

On successful completion of this operation, the authenticator returns the type and unique identifier of this new credential to the user agent.

If the user refuses consent, the authenticator returns an appropriate error status to the client.

4.1.2. The authenticatorGetAssertion#authenticatorgetassertionReferenced in:3.1.2. Use an existing credential (getAssertion() method)4.1.3. The authenticatorCancel operation (2) operation

This operation must be invoked in an authenticator session which has no other operations in progress. It takes the following input parameters:

• The web origin of the script on whose behalf the operation is being initiated, as determined by the user agent and the client.

• The RP ID corresponding to the above web origin, as determined by the user agent and the client.

• All input parameters accepted by the getAssertion() method, specified below.

When this method is invoked, the authenticator allows the user to select a credential from among the credentials associated with that WebAuthn Relying Party and matching the specified criteria, then obtains user consent for using that credential. The prompt for obtaining this consent may be shown by the authenticator if it has its own output capability, or by the user agent otherwise. Once a credential is selected and user consent is obtained, the authenticator computes a cryptographic signature using the credential’s private key and constructs an assertion as specified in §4.2 Signature Format. It then returns this assertion to the user agent.

If the authenticator cannot find any credential corresponding to the specified WebAuthn Relying Party that matches the specified criteria, it terminates the operation and returns an error.

If the user refuses consent, the authenticator returns an appropriate error status to the client.

4.1.3. The authenticatorCancel#authenticatorcancelReferenced in:3.1.1. Create a new credential (makeCredential() method) (2)3.1.2. Use an existing credential (getAssertion() method) (2) operation

This operation takes no input parameters and returns no result.

When this operation is invoked by the client in an authenticator session, it has the effect of terminating any authenticatorMakeCredential or authenticatorGetAssertion operation currently in progress in that authenticator session. The authenticator stops prompting for, or accepting, any user input related to authorizing the canceled operation. The client ignores any further responses from the authenticator for the canceled operation.

This operation is ignored if it is invoked in an authenticator session which does not have an authenticatorMakeCredential or authenticatorGetAssertion operation currently in progress.

4.2. Signature Format

WebAuthn signatures are bound to various contextual data. These data are observed, and added at different levels of the stack as a signature request passes from the server to the authenticator. In verifying a signature, the server checks these bindings against expected values.

The components of a system using WebAuthn can be divided into three layers:

1. The WebAuthn Relying Party (RP), which uses the WebAuthn services. The RP may, for example, be a web-application running in a browser, or a native application that runs directly on the OS platform.

2. The WebAuthn Client platform, which consists of the user’s OS and device used to host the RP’s client-side app. For web-applications, the browser also belongs to this layer.

3. The Authenticator itself, which provides key management and cryptographic signatures.

When the WebAuthn Relying Party client-side application is a web-application, the interface between 1 and 2 is the §3 Web Authentication API, but is platform specific for native applications. In cases where the authenticator is not tightly integrated with the platform, the interface between 2 and 3 is a separately-defined protocol. This specification defines the common signature format shared by all layers. This includes how the different contextual bindings are encoded, signed over, and delivered to the RP.

The goals of this design can be summarized as follows.

• The scheme for generating signatures should accommodate cases where the link between the client platform and authenticator is very limited, in bandwidth and/or latency. Examples include Bluetooth Low Energy and Near-Field Communication.

• The data processed by the authenticator should be small and easy to interpret in low-level code. In particular, authenticators should not have to parse high-level encodings such as JSON.

• Both the client platform and the authenticator should have the flexibility to add contextual bindings as needed.

• The design aims to reuse as much as possible of existing encoding formats in order to aid adoption and implementation.

The contextual bindings are divided in two: Those added by the RP or the client platform, referred to as client data; and those added by the authenticator, referred to as the authenticator data. The client data must be signed over, but an authenticator is otherwise not interested in its contents. To save bandwidth and processing requirements on the authenticator, the client platform hashes the client data and sends only the result to the authenticator. The authenticator signs over the combination of this hash, and its own authenticator data.

4.3. Client data used in WebAuthn signatures (dictionary ClientData#dictdef-clientdataReferenced in:4.3. Client data used in WebAuthn signatures (dictionary ClientData)4.3.2. Generating a signature (2)4.4.2.3.4. AndroidAttestationClientData5. WebAuthn Extensions (2)5.2. Defining extensions (2)5.3. Extending request parameters5.4. Extending client processing (2))

The client data represents the contextual bindings of both the WebAuthn Relying Party and the client platform. It is a key-value mapping with string-valued keys. Values may be any type that has a valid encoding in JSON. Its structure is defined by the following Web IDL.

dictionary ClientData {
    required DOMString           challenge;
    required DOMString           facet;
    required AlgorithmIdentifier hashAlg;
    JsonWebKey                   tokenBinding;
    WebAuthnExtensions           extensions;
};

4.3.1. Authenticator data

The authenticator data encodes contextual bindings made by the authenticator itself. The authenticator data has a compact but extensible encoding. This is desired since authenticators can be devices with limited capabilities and low power requirements, with much simpler software stacks than the client platform components.

The encoding of authenticator data is a byte array Web Cryptography API §JsonWebKey-dictionary of 5 bytes or more, as follows.

The TUP flag SHALL be set if and only if the authenticator detected a user through an authenticator specific gesture. The RFU bits in the flags byte SHALL be set to zero.

If the authenticator does not include any extension data, it MUST set the ED flag in the first byte to zero, and to one if extension data is included.

The figure below shows a visual representation of the authenticator data structure.

Note: The signatureData describes its own length: If the ED flag is not set, it is always 5 bytes long. If the ED flag is set, it is 5 bytes plus the CBOR map that follows.

4.3.2. Generating a signature

Before making a request to an authenticator, the client platform layer SHALL perform the following steps.

1. Represent the parameters passed in by the RP in the form of a ClientData structure.

2. Let clientDataJSON#clientdatajsonReferenced in:3.7. Credential Attestation Statement (interface WebAuthnAttestation)4.3.2. Generating a signature (2) (3) be the UTF-8 encoded JSON serialization [RFC7159] of this ClientData dictionary.

3. Let clientDataHash#clientdatahashReferenced in:4.3.2. Generating a signature (2) be the hash (computed using hashAlg) of clientDataJSON, as an array.

The clientDataHash value is delivered to the authenticator.

The hash algorithm hashAlg used to compute clientDataHash is included in the ClientData object. This way it is available to the WebAuthn Relying Party and it is also hashed over when computing clientDataHash and hence anchored in the signature itself.

A raw cryptographic signature must assert the integrity of both the client data and the authenticator data. Thus, an authenticator SHALL compute a signature over the concatenation of the authenticatorData and the clientDataHash.

Note: A simple, undelimited concatenation, is safe to use here because the authenticatorData describes its own length. The clientDataHash (which potentially has a variable length) is always the last element.

The authenticator MUST return both the authenticatorData and the raw signature back to the client. The client, in turn, MUST return clientDataJSON, authenticatorData and the signature to the RP. The clientDataJSON is returned in the clientData member of the WebAuthnAssertion and AttestationStatement structures.

4.4. Credential Attestation Statements

An attestation statement is a specific type of signature, which contains statements about a credential itself and the authenticator that holds it. Therefore, the format of attestation statements and the procedures for generating them closely parallel those for generating WebAuthn assertions as described in §4.2 Signature Format, though the semantics of the contextual bindings are quite different.

This specification defines a number of types of attestation statements. All of these types share the following format:

interface AttestationStatement#attestationstatementReferenced in:4.3.2. Generating a signature {
    readonly    attribute unsigned long version;
    readonly    attribute ArrayBuffer   claimedAAGUID;
    readonly    attribute DOMString[]   x5c;
    readonly    attribute DOMString     alg;
    readonly    attribute ArrayBuffer   rawData;
    readonly    attribute ArrayBuffer   signature;
};

This specification defines a number of attestation types, i.e., ways to serialize the data being attested to by the Authenticator. The reason is to be able to support existing devices like TPMs and other platform-specific formats. Each attestation type provides the ability to cryptographically attest to a public key, the authenticator model, and contextual data to a remote party. They differ in the details of how the rawData element of the statement is laid out, and how the signature is computed from the rawData. The different attestation types are defined in §4.4.2 Defined Attestation Types.

Attestation types are orthogonal to attestation models, i.e. attestation types in general are not restricted to a single attestation model.

4.4.1. Attestation Models

WebAuthn supports multiple attestation models:

Full Basic Attestation

In the case of full basic attestation [UAFProtocol], the Authenticator’s attestation private key is specific to an Authenticator model. That means that an Authenticator of the same model typically shares the same attestation private key. This model is also used for FIDO UAF 1.0 and FIDO U2F 1.0.

Surrogate Basic Attestation

In the case of surrogate basic attestation [UAFProtocol], the Authenticator doesn’t have any specific attestation key. Instead it uses the authentication key to (self-)sign the (surrogate) attestation message. Authenticators without meaningful protection measures for an attestation private key typically use this attestation model.

Privacy CA

In this case, the Authenticator owns an authenticator-specific (endorsement) key. This key is used to securely communicate with a trusted third party, the Privacy CA. The Authenticator can generate multiple attestation key pairs and asks the Privacy CA to issue an attestation certificate for it. Using this approach, the Authenticator can limit the exposure of the endorsement key (which is a global correlation handle) to Privacy CA(s). Attestation keys can be requested for each scoped credential individually.

Note: This concept typically leads to multiple attestation certificates. The attestation certificate requested most recently is called "active".

Direct Anonymous Attestation (DAA)

In this case, the Authenticator receives DAA credentials from a single DAA-Issuer. These DAA credentials are used along with blinding to sign the attestation data. The concept of blinding avoids the DAA credentials being misused as global correlation handle. WebAuthn supports DAA using elliptic curve cryptography and bilinear pairings, called ECDAA (see [FIDOEcdaaAlgorithm]) in this specification.

Compliant servers MUST support all attestation models. Authenticators can choose what attestation model to implement.

Note: WebAuthn Relying Parties can always decide what attestation models are acceptable to them by policy.

4.4.2. Defined Attestation Types

WebAuthn supports pluggable attestation data types. This allows support of TPM generated attestation data as well as support of other WebAuthn authenticators. As mentioned in §4.4 Credential Attestation Statements, these differ in how the rawData and signature fields are populated. This section defines the behavior of these fields for the attestation types defined by this specification.

The contents of the attestation data must be controlled (i.e., generated or at least verified) by the authenticator itself.

4.4.2.1. Packed Attestation (type="packed")

Packed attestation is a WebAuthn optimized format of attestation data. It uses a very compact but still extensible encoding method. Encoding this format can even be implemented by authenticators with very limited resources (e.g., secure elements).

4.4.2.1.1. Attestation rawData

The attestation data encodes contextual bindings made by the authenticator itself. Unlike client data, the authenticator data has a compact but extensible encoding. This is desired since authenticators can be devices with limited capabilities and low power requirements, with much simpler software stacks than the client platform components.

For this type, only version="1" is defined at this time.

The field rawData for this type is a byte array of 45 bytes + length of public key + length of KeyHandle + potentially more extensions. The first bytes before the extensions have a fixed layout as follows:

The TUP flag SHALL be set if and only if the authenticator detected a user through an authenticator-specific gesture. The RFU bits in the flags byte SHALL be cleared (i.e., zeroed).

If the authenticator does not wish to add extensions, it MUST clear the ED flag in the third byte.

4.4.2.1.2. Signature

The signature is computed over the rawData field. The following algorithms must be implemented by servers:

1. "ES256" [RFC7518]

2. "RS256" [RFC7518]

3. "PS256" [RFC7518]

4. "ED256" [FIDOEcdaaAlgorithm]

Authenticators can choose which algorithm(s) to implement. WebAuthn Relying Parties must implement all the algorithms implemented by the authenticators that they support.

4.4.2.1.3. Packed attestation statement certificate requirements

Note: In the case of DAA attestation [FIDOEcdaaAlgorithm] no attestation certificate is used.

The attestation certificate MUST have the following fields/extensions:

• Version must be set to 3.

• Subject field MUST be set to:

  Subject-C

  Country where the Authenticator vendor is incorporated

  Subject-O

  Legal name of the Authenticator vendor

  Subject-OU

  Authenticator Attestation

  Subject-CN

  No stipulation.

• If the related attestation root certificate is used for multiple authenticator models, the Extension OID 1 3 6 1 4 1 45724 1 1 4 (id-fido-gen-ce-aaguid) MUST be present, containing the AAGUID as value.

• The Basic Constraints extension MUST have the cA component set to false

• An Authority Information Access (AIA) extension with entry id-ad-ocsp and a CRL Distribution Point extension [RFC5280] are both optional as the status of attestation certificates is available through the FIDO Metadata Service [FIDOMetadataService].

4.4.2.2. TPM Attestation (type="tpm")

4.4.2.2.1. Attestation rawData

The value of rawData is either a TPM_CERTIFY_INFO or a TPM_CERTIFY_INFO2 structure [TPMv1-2-Part2] sections 11.1 and 11.2, if version equals 1. Else, if version equals 2, it MUST be a TPMS_ATTEST structure as defined in [TPMv2-Part2] section 10.12.9.

The field "extraData" (in the case of TPMS_ATTEST) or the field "data" (in the case of TPM_CERTIFY_INFO or TPM_CERTIFY_INFO2) MUST contain the clientDataHash (see [[#signature-format]).

4.4.2.2.2. Signature

If version equals 1, (i.e., for TPM 1.2), RSASSA-PKCS1-v1_5 signature algorithm (section 8.2 of [RFC3447]) can be used by WebAuthn Authenticators (i.e. alg="RS256").

If version equals 2, the following algorithms can be used by WebAuthn Authenticators:

1. TPM_ALG_RSASSA (0x14). This is the same algorithm RSASSA-PKCS1-v1_5 as for version 1 but for use with TPMv2. alg="RS256".

2. TPM_ALG_RSAPSS (0x16); alg="PS256".

3. TPM_ALG_ECDSA (0x18); alg="ES256".

4. TPM_ALG_ECDAA (0x1A); alg="ED256".

5. TPM_ALG_SM2 (0x1B); alg="SM256".

WebAuthn Relying Parties must implement all the algorithms implemented by the authenticators that they support.

The signature is computed over the rawData field.

4.4.2.2.3. TPM attestation statement certificate requirements

TPM attestation certificate MUST have the following fields/extensions:

• Version must be set to 3.

• Subject field MUST be set to empty.

• The Subject Alternative Name extension must be set as defined in [TPMv2-EK-Profile] section 3.2.9 if "version" equals 2 and [TPMv1-2-Credential-Profiles] section 3.2.9 if "version" equals 1.

• The Extended Key Usage extension MUST contain the "joint-iso-itu-t(2) internationalorganizations(23) 133 tcg-kp(8) tcg-kp-AIKCertificate(3)" OID.

• The Basic Constraints extension MUST have the CA component set to false

• An Authority Information Access (AIA) extension with entry id-ad-ocsp and a CRL Distribution Point extension [RFC5280] are both optional as the status of attestation certificates is available through the FIDO Metadata Service [FIDOMetadataService].

4.4.2.3. Android Attestation (type="android")

When the Authenticator in question is a platform-provided Authenticator on the Android platform, the attestation statement is based on the SafetyNet API.

Android attestation statement MUST always be used in conjunction with the more specific AndroidAttestationClientData (see §4.4.2.3.4 AndroidAttestationClientData) in order to let the WebAuthn Relying Party App store the public key in the attestation object.

4.4.2.3.1. Attestation rawData

Android SafetyNet returns a JWS [RFC7515] object (see SafetyNet online documentation) in Compact Serialization. A JWS in Compact Serialization consists of three segments (each a base64url-encoded string) separated by a dot ("."). The rawData object is the concatenation of:

1. the first segment (a base64url encoding of the UTF-8 encoded JWS Protected Header)

2. a dot "."

3. the second segment (a base64url encoding of the UTF-8 encoded JWS Payload).

In contrast to the "packed" and "tpm" attestation types, for the "android" attestation type, the rawData field and the rawData object are the same string. (In the "packed" and "tpm" attestation types the rawData field is the base64url-encoding of the rawData object.)

4.4.2.3.2. Signature

The signature is directly computed over the rawData field, as defined above (see [RFC7515] for more details). The third segment of the JWS returned by SafetyNet is the base64url encoding of this signature, and is decoded into the signature.

4.4.2.3.3. Converting SafetyNet response to AttestationStatement

The Authenticator and/or platform SHOULD use the steps outlined below to create an AttestationStatement object from an Android SafetyNet response. It MAY use a different algorithm, as long as the results are the same.

1. create clientDataJSON with type AndroidAttestationClientData (see below) and compute clientDataHash as the hash of clientDataJSON.

2. provide clientDataHash as Nonce value when requesting the SafetyNet attestation.

3. take SafetyNet response snr. This is a JWS object ([RFC7515]).

4. extract the base64url-encoded Protected Header hdr from snr (see [RFC7515])

5. extract the base64url-encoded payload p from snr

6. extract the base64url-encoded signature from snr. Let s be its base64url-decoded form.

7. set rawData = hdr | "." | p

8. set signature = s

9. base64url-decode hdr into hdr-d

10. set alg = hdr-d.alg

11. if hdr-d.x5c is present, then set x5c = hdr-d.x5c

12. if hdr-d.x5u is present, then resolve the URL and add the retrieved certificate chain to x5c

13. set version to the version number of Google Play Services responsible for providing the SafetyNet API

4.4.2.3.4. AndroidAttestationClientData

The ClientData dictionary is extended in the following way:

dictionary AndroidAttestationClientData : ClientData {
    JsonWebKey    publicKey;
    boolean       isInsideSecureHardware;
    DOMString     userAuthentication;
    unsigned long userAuthenticationValidityDurationSeconds; // optional
};

4.4.2.3.5. Verifying AndroidClientData specific contextual bindings

A WebAuthn Relying Party shall verify the clientData contextual bindings (see step 4 in §4.4.3 Verifying an Attestation Statement) as follows:

• Check that AndroidAttestationClientData.challenge equals the attestationChallenge that was passed into the makeCredential() call.

• Check that the facet and tokenBinding parameters in the AndroidAttestationClientData match the WebAuthn Relying Party App.

• Check that AndroidAttestationClientData.publicKey is the same key as the one returned in the ScopedCredentialInfo by the makeCredential call.

• Check that the hash of the clientDataJSON matches the nonce attribute in the payload of the safetynetResponse JWS.

• Check that the ctsProfileMatch attribute in the payload of the safetynetResponse is true.

• Check that the apkPackageName attribute in the payload of the safetynetResponse matches package name of the application calling SafetyNet API.

• Check that the apkDigestSha256 attribute in the payload of the safetynetResponse matches the package hash of the application calling SafetyNet API.

• Check that the apkCertificateDigestSha256 attribute in the payload of the safetynetResponse matches the hash of the signing certificate of the application calling SafetyNet API.

4.4.3. Verifying an Attestation Statement

This section outlines the recommended algorithm for verifying an attestation statement, independent of attestation type.

Upon receiving an attestation statement, the WebAuthn Relying Party shall:

1. Verify that the attestation statement is properly formatted.

2. If alg is not ECDAA (e.g., not "ED256" and not "ED512"):

   • Look up the attestation root certificate from a trusted source. The FIDO Metadata Service [FIDOMetadataService] provides an easy way to access such information. The claimedAAGUID can be used for this lookup.

   • Verify that the attestation certificate chain is valid and chains up to a trusted root (following [RFC5280]).

   • Verify that the attestation certificate has the right Extended Key Usage (EKU) based on the WebAuthn Authenticator type (as denoted by the type member). In case of a type="tpm", this EKU shall be OID "2.23.133.8.3".

   • If the attestation type is "android", verify that the attestation certificate is issued to the hostname "attest.android.com" (see SafetyNet online documentation).

   • Verify that all issuing CA certificates in the chain are valid and not revoked.

   • Verify the signature on rawData using the attestation certificate public key and algorithm as identified by alg.

   • Verify rawData syntax and that it doesn’t contradict the signing algorithm specified in alg.

   • If the attestation certificate contains an extension with OID 1 3 6 1 4 1 45724 1 1 4 (id-fido-gen-ce-aaguid) verify that the value of this extension matches claimedAAGUID. This identifies the Authenticator model.

   • If such extension doesn’t exist, the attestation root certificate is dedicated to a single Authenticator model.

3. If alg is ECDAA (e.g., "ED256", "ED512"):

   • Look up the DAA root key from a trusted source. The FIDO Metadata Service [FIDOMetadataService] provides an easy way to access such information. The claimedAAGUID can be used for this lookup.

   • Perform DAA-Verify on signature for rawData (see [FIDOEcdaaAlgorithm]).

   • Verify rawData syntax and that it doesn’t contradict the signing algorithm specified in alg.

   • The DAA root key is dedicated to a single Authenticator model.

4. Verify the contextual bindings (e.g., channel binding) from the clientData (see §4.3.2 Generating a signature).

5. Verify that user verification method and other authenticator characteristics related to this authenticator model, match the WebAuthn Relying Party policy. The FIDO Metadata Service [FIDOMetadataService] provides an easy way to access the authenticator characteristics.

The WebAuthn Relying Party MAY take any of the below actions when verification of an attestation statement fails:

• Reject the request, such as a registration request, associated with the attestation statement.

• Accept the request associated with the attestation statement but treat the attested Scoped Credential as one with surrogate basic attestation (see §4.4.1 Attestation Models), if policy allows it. If doing so, there is no cryptographic proof that the Scoped Credential has been generated by a particular Authenticator model. See [FIDOSecRef] and [UAFProtocol] for more details on the relevance on attestation.

Verification of attestation statements requires that the relying party trusts the root of the attestation certificate chain. Also, the WebAuthn Relying Party must have access to certificate status information for the intermediate CA certificates. The relying party must also be able to build the attestation certificate chain if the client didn’t provide this chain in the attestation information.

4.4.4. Security Considerations

4.4.4.1. Privacy

Attestation keys may be used to track users or link various online identities of the same user together. This may be mitigated in several ways, including:

• A WebAuthn Authenticator manufacturer may choose to ship all of their devices with the same (or a fixed number of) attestation key(s) (called Full Basic Attestation). This will anonymize the user at the risk of not being able to revoke a particular attestation key should its WebAuthn Authenticator be compromised.

• A WebAuthn Authenticator may be capable of dynamically generating different attestation keys (and requesting related certificates) per origin (following the Privacy CA model). For example, a WebAuthn Authenticator can ship with a master attestation key (and certificate), and combined with a cloud operated privacy CA, can dynamically generate per origin attestation keys and attestation certificates.

• A WebAuthn Authenticator can implement direct anonymous attestation (see [FIDOEcdaaAlgorithm]). Using this scheme the authenticator generates a blinded attestation signature. This allows the WebAuthn Relying Party to verify the signature using the DAA root key, but the attestation signature doesn’t serve as a global correlation handle.

4.4.4.2. Attestation Certificate and Attestation Certificate CA Compromise

When an intermediate CA or a root CA used for issuing attestation certificates is compromised, WebAuthn Authenticator attestation keys are still safe although their certificates can no longer be trusted. A WebAuthn Authenticator manufacturer that has recorded the public attestation keys for their devices can issue new attestation certificates for these keys from a new intermediate CA or from a new root CA. If the root CA changes, the WebAuthn Relying Parties must update their trusted root certificates accordingly.

A WebAuthn Authenticator attestation certificate must be revoked by the issuing CA if its key has been compromised. A WebAuthn Authenticator manufacturer may need to ship a firmware update and inject new attestation keys and certificates into already manufactured WebAuthn Authenticators, if the exposure was due to a firmware flaw. (The process by which this happens is out of scope for this specification.) No further valid attestation statements can be made by the affected WebAuthn Authenticators unless the WebAuthn Authenticator manufacturer has this capability.

If attestation certificate validation fails due to a revoked intermediate attestation CA certificate, and the WebAuthn Relying Party’s policy requires rejecting the registration/authentication request in these situations, then it is recommended that the WebAuthn Relying Party also un-registers (or marks as "surrogate attestation" (see §4.4.1 Attestation Models), policy permitting) scoped credentials that were registered post the CA compromise date using an attestation certificate chaining up to the same intermediate CA. It is thus recommended that WebAuthn Relying Parties remember intermediate attestation CA certificates during Authenticator registration in order to un-register related Scoped Credentials if the registration was performed after revocation of such certificates.

If a DAA attestation key has been compromised, it can be added to the RogueList (i.e., the list of revoked authenticators) maintained by the related DAA-Issuer. The WebAuthn Relying Party should verify whether an authenticator belongs to the RogueList when performing DAA-Verify. The FIDO Metadata Service [FIDOMetadataService] provides an easy way to access such information.

4.4.4.3. Attestation Certificate Hierarchy

A 3 tier hierarchy for attestation certificates is recommended (i.e., Attestation Root, Attestation Issuing CA, Attestation Certificate). It is also recommended that for each WebAuthn Authenticator device line (i.e., model), a separate issuing CA is used to help facilitate isolating problems with a specific version of a device.

If the attestation root certificate is not dedicated to a single WebAuthn Authenticator device line (i.e., AAGUID), the AAGUID must be specified either in the attestation certificate itself or as an extension in the rawData.

5. WebAuthn Extensions

The mechanism for generating scoped credentials, as well as requesting and generating WebAuthn assertions, as defined in §3 Web Authentication API, can be extended to suit particular use cases. Each case is addressed by defining a registration extension and/or a signature extension. Extensions can define additions to the following steps and data:

• makeCredential() request parameters for registration extension.

• getAssertion() request parameters for signature extensions.

• Client processing, and the ClientData structure, for registration extensions and signature extensions.

• Authenticator processing, and the authenticatorData structure, for signature extensions.

When requesting an assertion for a scoped credential, a WebAuthn Relying Party can list a set of extensions to be used, if they are supported by the client and/or the authenticator. It sends the request parameters for each extension in the getAssertion() call (for signature extensions) or makeCredential() call (for registration extensions) to the client platform. The client platform performs additional processing for each extension that it supports, and augments ClientData as required by the extension. For extensions that the client platform does not support, it passes the request parameters on to the authenticator when possible (criteria defined below). This allows one to define extensions that affect the authenticator only.

Similarly, the authenticator performs additional processing for the extensions that it supports, and augments authenticatorData as specified by the extension.

Extensions that are not supported are ignored.

5.1. Extension identifiers

Extensions are identified by a string, chosen by the extension author. Extension identifiers should aim to be globally unique, e.g., by using reverse domain-name of the defining entity such as com.example.webauthn.myextension.

Extensions that may exist in multiple versions should take care to include a version in their identifier. In effect, different versions are thus treated as different extensions.

Extensions defined in this specification use a fixed prefix of webauthn for the extension identifiers. This prefix should not be used for extensions not defined by the W3C.

5.2. Defining extensions

A definition of an extension must specify, at minimum, an extension identifier and an extension client argument sent via the getAssertion() or makeCredential() call (see below). Additionally, extensions may specify additional values in ClientData, authenticatorData (in the case of signature extensions), or both.

Note: An extension that does not define additions to ClientData nor authenticatorData is possible, but should be avoided. In such cases, the WebAuthn Relying Party would have no indication if the extension was supported or processed by the client and/or authenticator.

5.3. Extending request parameters

An extension defines two request arguments. The client argument#client-argumentReferenced in:5.3. Extending request parameters is passed from the WebAuthn Relying Party to the client in the getAssertion() or makeCredential() call, while the authenticator argument is passed from the client to the authenticator during the processing of these calls.

Extension definitions MUST specify the valid values for their client argument. Clients are free to ignore extensions with an invalid client argument. Specifying an authenticator argument is optional, since some extensions may only affect client processing.

A WebAuthn Relying Party simultaneously requests the use of an extension and sets its client argument by including an entry in the credentialExtensions or assertionExtensions dictionary parameters to the makeCredential() or getAssertion() call. The entry key MUST be the extension identifier, and the value MUST be the client argument.

var assertionPromise = credentials.getAssertion(..., /* extensions */ {
    "com.example.webauthn.foobar": 42
});

Extensions that affect the behavior of the client platform can define their argument to be any set of values that can be encoded in JSON. Such an extension will in general (but not always) specify additional values to the ClientData structure (see below). It may also specify an authenticator argument that platforms implementing the extension are expected to send to the authenticator. The authenticator argument should be a byte string.

Note: Extensions should aim to define authenticator arguments that are as small as possible. Some authenticators communicate over low-bandwidth links such as Bluetooth Low-Energy or NFC.

Note: Extensions that do not need to pass any particular argument value, must still define a client argument. It is recommended that the argument be defined as the constant value true in this case.

For extensions that specify additional authenticator processing only, it is desirable that the platform need not know the extension. To support this, platforms SHOULD pass the client argument of unknown extension as the authenticator argument unchanged, under the same extension identifier. The authenticator argument should be the CBOR encoding of the client argument, as specified in Section 4.2 of [RFC7049]. Clients SHOULD silently drop unknown extensions whose client argument cannot be encoded as a CBOR structure.

5.4. Extending client processing

Extensions may define additional processing requirements on the client platform during the creation of credentials or the generation of an assertion. In order for the WebAuthn Relying Party to verify the processing took place, or if the processing has a result value that the WebAuthn Relying Party needs to be aware of, the extension should specify a client data value to be included in the ClientData structure.

The value may be any value that can be encoded as a JSON value. If any extension processed by a client defines such a value, the client SHOULD include a dictionary in ClientData with the key extensions. For each such extension, the client SHOULD add an entry to this dictionary with the extension identifier as the key, and the extension’s client data value.

5.5. Extending authenticator processing with signature extensions

Signature extensions that define additional authenticator processing should similarly define an authenticator data value. The value may be any data that can be encoded as a CBOR value. An authenticator that processes a signature extension that defines such a value must include it in the authenticatorData.

As specified in §4.3.1 Authenticator data, the authenticator data value of each processed extension is included in the extended data part of the authenticatorData. This part is a CBOR map, with extension identifiers as keys, and the authenticator data value of each extension as the value.

5.6. Example extension

This section is not normative.

To illustrate the requirements above, consider a hypothetical extension "Geo". This extension, if supported, lets both clients and authenticators embed their geolocation in signatures.

The extension identifier is chosen as com.example.webauthn.geo. The client argument is the constant value true, since the extension does not require the WebAuthn Relying Party to pass any particular information to the client, other than that it requests the use of the extension. The WebAuthn Relying Party sets this value in its request for an assertion:

var assertionPromise =
    credentials.getAssertion("SGFuIFNvbG8gc2hvdCBmaXJzdC4",
        {}, /* Empty filter */
        { 'com.example.webauthn.geo': true });

The extension defines the additional client data to be the client’s location, if known, as a GeoJSON [GeoJSON] point. The client constructs the following client data:

{
    ...,
    'extensions': {
        'com.example.webauthn.geo': {
            'type': 'Point',
            'coordinates': [65.059962, -13.993041]
        }
    }
}

The extension also requires the client to set the authenticator parameter to the fixed value 1.

Finally, the extension requires the authenticator to specify its geolocation in the authenticator data, if known. The extension e.g. specifies that the location shall be encoded as a two-element array of floating point numbers, encoded with CBOR. An authenticator does this by including it in the authenticatorData. As an example, authenticator data may be as follows (notation taken from [RFC7049]):

81 (hex)                                    -- Flags, ED and TUP both set.
20 05 58 1F                                 -- Signature counter
A1                                          -- CBOR map of one element
    6C                                      -- Key 1: CBOR text string of 12 bytes
        77 65 62 61 75 74 68 6E 2E 67 65 6F -- "webauthn.geo" UTF-8 string
    82                                      -- Value 1: CBOR array of two elements
        FA 42 82 1E B3                      -- Element 1: Latitude as CBOR encoded float
        FA C1 5F E3 7F                      -- Element 2: Longitude as CBOR encoded float

6. Standard extensions

This section defines standard extensions defined by the W3C.

6.1. Transaction authorization

This signature extension allows for a simple form of transaction authorization. A WebAuthn Relying Party can specify a prompt string, intended for display on a trusted device on the authenticator.

Extension identifier

webauthn.txauth.simple

Client argument

A single UTF-8 encoded string prompt.

Client processing

None, except default forwarding of client argument to authenticator argument.

Authenticator argument

The client argument encoded as a CBOR text string (major type 3).

Authenticator processing

The authenticator MUST display the prompt to the user before performing the user verification / test of user presence. The authenticator may insert line breaks if needed.

Authenticator data

A single UTF-8 string, representing the prompt as displayed (including any eventual line breaks).

The generic version of this extension allows images to be used as prompts as well. This allows authenticators without a font rendering engine to be used and also supports a richer visual appearance.

Extension identifier

webauthn.txauth.generic

Client argument

A CBOR map with one pair of data items (CBOR tagged as 0xa1). The pair of data items consists of

1. one UTF-8 encoded string contentType, containing the MIME-Type of the content, e.g. "image/png"

2. and the content#contentReferenced in:6.1. Transaction authorization (2) (3) (4) (5) itself, encoded as CBOR byte array.

Client processing

None, except default forwarding of client argument to authenticator argument.

Authenticator argument

The client argument encoded as a CBOR map.

Authenticator processing

The authenticator MUST display the content to the user before performing the user verification / test of user presence. The authenticator may add other information below the content. No changes are allowed to the content itself, i.e. inside content boundary box.

Authenticator data

The hash value of the content which was displayed. The authenticator MUST use the same hash algorithm as it uses for the signature itself.

6.2. Authenticator Selection Extension

This registration extension allows a WebAuthn Relying Party to guide the selection of the authenticator that will be leveraged when creating the credential. It is intended primarily for WebAuthn Relying Parties that wish to tightly control the experience around credential creation.

Extension identifier

webauthn.authn-sel

Client argument

A sequence of AAGUIDs:

typedef sequence < AAGUID > AuthenticatorSelectionList;

Each AAGUID corresponds to an authenticator attestation that is acceptable to the WebAuthn Relying Party for this credential creation. The list is ordered by decreasing preference.

An AAGUID is defined as a DOMString, and is the globally unique identifier of the authenticator attestation being sought.

typedef DOMString AAGUID#typedefdef-aaguidReferenced in:6.2. Authenticator Selection Extension;

Client processing

This extension can only used during makeCredential(). If the client supports the Authenticator Selection Extension, it MUST use the first available authenticator whose AAGUID is present in the AuthenticatorSelectionList. If none of the available authenticators match a provided AAGUID, the client MUST select an authenticator from among the available authenticators to generate the credential.

Authenticator argument

There is no authenticator argument.

Authenticator processing

None.

6.3. AAGUID Extension

Extension identifier

webauthn.aaguid

Client argument

N/A

Client processing

N/A

Authenticator argument

N/A

Authenticator processing

This extension is added automatically by the authenticator. This extension can be added to attestation statements and signatures.

Authenticator data

A 128-bit Authenticator Attestation GUID encoded as a CBOR text string (major type 3). This AAGUID is used to identify the Authenticator model (Authenticator Attestation GUID).

Note: The authenticator model (identified by the AAGUID) can be derived from

• here, or

• from the attestation certificate (if we have an authenticator specific or authenticator model specific attestation certificate), or

• from the claimed AAGUID in the client encoded attestation statement (if we have one attestation root certificate per authenticator model).

In the case of DAA there is no need for an X.509 attestation certificate hierarchy. Instead the trust anchor being known to the WebAuthn Relying Party is the DAA root key (i.e. ECPoint2 X, Y). This root key must be dedicated to a single authenticator model.

6.4. SupportedExtensions Extension

Extension identifier

webauthn.exts

Client argument

N/A

Client processing

N/A

Authenticator argument

N/A

Authenticator processing

This extension is added automatically by the authenticator. This extension can be added to attestation statements.

Authenticator data

The SupportedExtension extension is a list (CBOR array) of extension identifiers encoded as UTF-8 Strings.

6.5. User Verification Index (UVI) Extension

Extension identifier

webauthn.uvi

Client argument

N/A

Client processing

N/A

Authenticator argument

N/A

Authenticator processing

This extension is added automatically by the authenticator. This extension can be added to attestation statements and signatures.

Authenticator data

The user verification index (UVI) is a value uniquely identifying a user verification data record. The UVI is encoded as CBOR byte string (type 0x58). Each UVI value MUST be specific to the related key (in order to provide unlinkability). It also must contain sufficient entropy that makes guessing impractical. UVI values MUST NOT be reused by the Authenticator (for other biometric data or users).

The UVI data can be used by servers to understand whether an authentication was authorized by the exact same biometric data as the initial key generation. This allows the detection and prevention of "friendly fraud".

As an example, the UVI could be computed as SHA256(KeyID | SHA256(rawUVI)), where the rawUVI reflects (a) the biometric reference data, (b) the related OS level user ID and (c) an identifier which changes whenever a factory reset is performed for the device, e.g. rawUVI = biometricReferenceData | OSLevelUserID | FactoryResetCounter.

Servers supporting UVI extensions MUST support a length of up to 32 bytes for the UVI value.

Example for rawData containing one UVI extension

F1 D0                                       -- This is a WebAuthn packed rawData object
81                                          -- TUP and ED set
00 00 00 01                                 -- (initial) signature counter
...                                         -- all public key alg etc.
A1                                          -- extension: CBOR map of one element
    6C                                      -- Key 1: CBOR text string of 12 bytes
        77 65 62 61 75 74 68 6E 2E 75 76 69 -- "webauthn.uvi" UTF-8 string
    58 20                                   -- Value 1: CBOR byte string with 0x20 bytes
        00 43 B8 E3 BE 27 95 8C             -- the UVI value itself
        28 D5 74 BF 46 8A 85 CF
        46 9A 14 F0 E5 16 69 31
        DA 4B CF FF C1 BB 11 32
        82

7. IANA Considerations

This specification registers the algorithm names "S256", "S384", "S512", and "SM3" with the IANA JSON Web Algorithms registry as defined in section "Cryptographic Algorithms for Digital Signatures and MACs" in [RFC7518].

These names follow the naming strategy in draft-ietf-oauth-spop-15.

8. Sample scenarios

This section is not normative.

In this section, we walk through some events in the lifecycle of a scoped credential, along with the corresponding sample code for using this API. Note that this is an example flow, and does not limit the scope of how the API can be used.

As was the case in earlier sections, this flow focuses on a use case involving an external first-factor authenticator with its own display. One example of such an authenticator would be a smart phone. Other authenticator types are also supported by this API, subject to implementation by the platform. For instance, this flow also works without modification for the case of an authenticator that is embedded in the client platform. The flow also works for the case of an external authenticator without its own display (similar to a smart card) subject to specific implementation considerations. Specifically, the client platform needs to display any prompts that would otherwise be shown by the authenticator, and the authenticator needs to allow the client platform to enumerate all the authenticator’s credentials so that the client can have information to show appropriate prompts.

8.1. Registration

This is the first time flow, when a new credential is created and registered with the server.

1. The user visits example.com, which serves up a script. At this point, the user must already be logged in using a legacy username and password, or additional authenticator, or other means acceptable to the WebAuthn Relying Party.

2. The WebAuthn Relying Party script runs the code snippet below.

3. The client platform searches for and locates the external authenticator.

4. The client platform connects to the external authenticator, performing any pairing actions if necessary.

5. The external authenticator shows appropriate UI for the user to select the authenticator on which the new credential will be created, and obtains a biometric or other authorization gesture from the user.

6. The external authenticator returns a response to the client platform, which in turn returns a response to the WebAuthn Relying Party script. If the user declined to select an authenticator or provide authorization, an appropriate error is returned.

7. If a new credential was created,

   • The WebAuthn Relying Party script sends the newly generated public key to the server, along with additional information about public key such as attestation that it is held in trusted hardware.

   • The server stores the public key in its database and associates it with the user as well as with the strength of authentication indicated by attestation, also storing a friendly name for later use.

   • The script may store data such as the credential ID in local storage, to improve future UX by narrowing the choice of credential for the user.

The sample code for generating and registering a new key follows:

var webauthnAPI = window.webauthn;

if (!webauthnAPI) { /* Platform not capable. Handle error. */ }

var userAccountInformation = {
    rpDisplayName: "Acme",
    displayName: "John P. Smith",
    name: "johnpsmith@example.com",
    id: "1098237235409872",
    imageURL: "https://pics.acme.com/00/p/aBjjjpqPb.png"
};

// This Relying Party will accept either an ES256 or RS256 credential, but
// prefers an ES256 credential.
var cryptoParams = [
    {
        type: "ScopedCred",
        algorithm: "ES256"
    },
    {
        type: "ScopedCred",
        algorithm: "RS256"
    }
];
var challenge = "Y2xpbWIgYSBtb3VudGFpbg";
var timeoutSeconds = 300;  // 5 minutes
var blacklist = [];  // No blacklist
var extensions = {"webauthn.location": true};  // Include location information
                                               // in attestation

// Note: The following call will cause the authenticator to display UI.
webauthnAPI.makeCredential(userAccountInformation, cryptoParams, challenge,
                           timeoutSeconds, blacklist, extensions)
    .then(function (newCredentialInfo) {
    // Send new credential info to server for verification and registration.
}).catch(function (err) {
    // No acceptable authenticator or user refused consent. Handle appropriately.
});

8.2. Authentication

This is the flow when a user with an already registered credential visits a website and wants to authenticate using the credential.

1. The user visits example.com, which serves up a script.

2. The script asks the client platform for a WebAuthn identity assertion, providing as much information as possible to narrow the choice of acceptable credentials for the user. This may be obtained from the data that was stored locally after registration, or by other means such as prompting the user for a username.

3. The WebAuthn Relying Party script runs one of the code snippets below.

4. The client platform searches for and locates the external authenticator.

5. The client platform connects to the external authenticator, performing any pairing actions if necessary.

6. The external authenticator presents the user with a notification that their attention is required. On opening the notification, the user is shown a friendly selection menu of acceptable credentials using the account information provided when creating the credentials, along with some information on the origin that is requesting these keys.

7. The authenticator obtains a biometric or other authorization gesture from the user.

8. The external authenticator returns a response to the client platform, which in turn returns a response to the WebAuthn Relying Party script. If the user declined to select a credential or provide an authorization, an appropriate error is returned.

9. If an assertion was successfully generated and returned,

   • The script sends the assertion to the server.

   • The server examines the assertion and validates that it was correctly generated. If so, it looks up the identity associated with the associated public key; that identity is now authenticated. If the public key is not recognized by the server (e.g., deregistered by server due to inactivity) then the authentication has failed; each WebAuthn Relying Party will handle this in its own way.

   • The server now does whatever it would otherwise do upon successful authentication -- return a success page, set authentication cookies, etc.

If the WebAuthn Relying Party script does not have any hints available (e.g., from locally stored data) to help it narrow the list of credentials, then the sample code for performing such an authentication might look like this:

var webauthnAPI = window.webauthn;

if (!webauthnAPI) { /* Platform not capable. Handle error. */ }

var challenge = "Y2xpbWIgYSBtb3VudGFpbg";
var timeoutSeconds = 300;  // 5 minutes
var whitelist = [{ type: "ScopedCred" }];

webauthnAPI.getAssertion(challenge, timeoutSeconds, whitelist)
    .then(function (assertion) {
    // Send assertion to server for verification
}).catch(function (err) {
    // No acceptable credential or user refused consent. Handle appropriately.
});

On the other hand, if the WebAuthn Relying Party script has some hints to help it narrow the list of credentials, then the sample code for performing such an authentication might look like the following. Note that this sample also demonstrates how to use the extension for transaction authorization.

var webauthnAPI = window.webauthn;

if (!webauthnAPI) { /* Platform not capable. Handle error. */ }

var challenge = "Y2xpbWIgYSBtb3VudGFpbg";
var timeoutSeconds = 300;  // 5 minutes
var acceptableCredential1 = {
    type: "ScopedCred",
    id: "ISEhISEhIWhpIHRoZXJlISEhISEhIQo="
};
var acceptableCredential2 = {
    type: "ScopedCred",
    id: "cm9zZXMgYXJlIHJlZCwgdmlvbGV0cyBhcmUgYmx1ZQo="
};
var whitelist = [acceptableCredential1, acceptableCredential2];
var extensions = { 'webauthn.txauth.simple':
                   "Wave your hands in the air like you just don’t care" };

webauthnAPI.getAssertion(challenge, timeoutSeconds, whitelist, extensions)
    .then(function (assertion) {
    // Send assertion to server for verification
}).catch(function (err) {
    // No acceptable credential or user refused consent. Handle appropriately.
});

8.3. Decommissioning

The following are possible situations in which decommissioning a credential might be desired. Note that all of these are handled on the server side and do not need support from the API specified here.

• Possibility #1 -- user reports the credential as lost.

  • User goes to server.example.net, authenticates and follows a link to report a lost/stolen device.

  • Server returns a page showing the list of registered credentials with friendly names as configured during registration.

  • User selects a credential and the server deletes it from its database.

  • In future, the WebAuthn Relying Party script does not specify this credential in any list of acceptable credentials, and assertions signed by this credential are rejected.

• Possibility #2 -- server deregisters the credential due to inactivity.

  • Server deletes credential from its database during maintenance activity.

  • In the future, the WebAuthn Relying Party script does not specify this credential in any list of acceptable credentials, and assertions signed by this credential are rejected.

• Possibility #3 -- user deletes the credential from the device.

  • User employs a device-specific method (e.g., device settings UI) to delete a credential from their device.

  • From this point on, this credential will not appear in any selection prompts, and no assertions can be generated with it.

  • Sometime later, the server deregisters this credential due to inactivity.

9. Terminology

Attestation Certificate#attestation-certificateReferenced in:4.4.2.1.3. Packed attestation statement certificate requirements4.4.2.2.3. TPM attestation statement certificate requirements

A X.509 Certificate for a keypair used by an Authenticator to attest to its manufacture and capabilities.

Authenticator#authenticatorReferenced in:1. Use Cases4. WebAuthn Authenticator model4.2. Signature Format4.3.1. Authenticator data4.3.2. Generating a signature4.4. Credential Attestation Statements4.4.2. Defined Attestation Types4.4.2.1. Packed Attestation (type="packed")4.4.2.1.1. Attestation rawData (2)4.4.2.1.2. Signature4.4.2.3. Android Attestation (type="android")4.4.2.3.3. Converting SafetyNet response to AttestationStatement4.4.4.1. Privacy4.4.4.2. Attestation Certificate and Attestation Certificate CA Compromise6.3. AAGUID Extension8. Sample scenarios9. Terminology (2)

The device used by the user agent to authenticate with the WebAuthn Relying Party. These can be Embedded Authenticators or External Authenticators.

Client

WebAuthn Client#webauthn-clientReferenced in:4.2. Signature Format

See Conforming User Agent.

Conforming User Agent#conforming-user-agentReferenced in:2. Conformance9. Terminology

A user agent which implements algorithms given in this specification, and handles communication between the Authenticator and the WebAuthn Relying Party.

eTLD+1#etld1Referenced in:3.1.1. Create a new credential (makeCredential() method)3.1.2. Use an existing credential (getAssertion() method)

The effective top-level domain, plus the first label. Also known as a Registered Domain. See [PSL].

WebAuthn Relying Party#webauthn-relying-partyReferenced in:1. Use Cases3.1.1. Create a new credential (makeCredential() method)3.2. ScopedCredentialInfo Interface3.3. User Account Information (dictionary Account)3.7. Credential Attestation Statement (interface WebAuthnAttestation) (2)4. WebAuthn Authenticator model4.2. Signature Format4.4. Credential Attestation Statements5.3. Extending request parameters5.4. Extending client processing5.6. Example extension8.2. Authentication8.3. Decommissioning9. Terminology

The entity which needs to rely in the authentication provided by the WebAuthn specification. When registration concludes, the WebAuthn Relying Party has the public key that was created by the Authenticator.

10. Acknowledgements