Web Authentication:
An API for accessing Public Key Credentials
Level 2

1. Introduction

This section is not normative.

This specification defines an API enabling the creation and use of strong, attested, scoped, public key-based credentials by web applications, for the purpose of strongly authenticating users. A public key credential is created and stored by a WebAuthn Authenticator at the behest of a WebAuthn Relying Party, subject to user consent. Subsequently, the public key credential can only be accessed by origins belonging to that Relying Party. This scoping is enforced jointly by conforming User Agents and authenticators. Additionally, privacy across Relying Parties is maintained; Relying Parties are not able to detect any properties, or even the existence, of credentials scoped to other Relying Parties.

Relying Parties employ the Web Authentication API during two distinct, but related, ceremonies involving a user. The first is Registration, where a public key credential is created on an authenticator, and scoped to a Relying Party with the present user’s account (the account might already exist or might be created at this time). The second is Authentication, where the Relying Party is presented with an Authentication Assertion proving the presence and consent of the user who registered the public key credential. Functionally, the Web Authentication API comprises a PublicKeyCredential which extends the Credential Management API [CREDENTIAL-MANAGEMENT-1], and infrastructure which allows those credentials to be used with navigator.credentials.create() and navigator.credentials.get(). The former is used during Registration, and the latter during Authentication.

Broadly, compliant authenticators protect public key credentials, and interact with user agents to implement the Web Authentication API. Implementing compliant authenticators is possible in software executing (a) on a general-purpose computing device, (b) on an on-device Secure Execution Environment, Trusted Platform Module (TPM), or a Secure Element (SE), or (c) off device. Authenticators being implemented on device are called platform authenticators. Authenticators being implemented off device (roaming authenticators) can be accessed over a transport such as Universal Serial Bus (USB), Bluetooth Low Energy (BLE), or Near Field Communications (NFC).

1.1. Specification Roadmap

While many W3C specifications are directed primarily to user agent developers and also to web application developers (i.e., "Web authors"), the nature of Web Authentication requires that this specification be correctly used by multiple audiences, as described below. All audiences ought to begin with §1.2 Use Cases, §12 Sample Scenarios, and §4 Terminology, and should also refer to [WebAuthnAPIGuide] for an overall tutorial.

• Relying Party web application developers, expecially those responsible for Relying Party web application login flows, account recovery flows, user account database content, etc.

• Web framework developers

  • The above two audiences should in particular refer to §7 WebAuthn Relying Party Operations. The introduction to §5 Web Authentication API may be helpful, though readers should realize that the §5 Web Authentication API section is targeted specifically at user agent developers, not web application developers. Additionally, if they intend to verify authenticator attestations, then §6.5 Attestation and §8 Defined Attestation Statement Formats will also be relevant. §9 WebAuthn Extensions, and §10 Defined Extensions will be of interest if they wish to make use of extensions.
    
    

• User agent developers

• OS platform developers, responsible for OS platform API design and implementation in regards to platform-specific authenticator APIs, platform WebAuthn Client instantiation, etc.

  • The above two audiences should read §5 Web Authentication API very carefully, along with §9 WebAuthn Extensions if they intend to support extensions.
    
    

• Authenticator developers. These readers will want to pay particular attention to §6 WebAuthn Authenticator Model, §8 Defined Attestation Statement Formats, §9 WebAuthn Extensions, and §10 Defined Extensions.
  
  

1.2. Use Cases

The below use case scenarios illustrate use of two very different types of authenticators, as well as outline further scenarios. Additional scenarios, including sample code, are given later in §12 Sample Scenarios.

1.2.1. Registration

• On a phone:

  • User navigates to example.com in a browser and signs in to an existing account using whatever method they have been using (possibly a legacy method such as a password), or creates a new account.

  • 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."

1.2.2. Authentication

• On a laptop or desktop:

  • User pairs their phone with the laptop or desktop via Bluetooth.

  • User navigates to example.com in a browser and initiates signing in.

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

• Next, on their phone:

  • User sees a discrete 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, back on the laptop:

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

1.2.3. New Device Registration

This use case scenario illustrates how a Relying Party can leverage a combination of a roaming authenticator (e.g., a USB security key fob) and a platform authenticator (e.g., a built-in fingerprint sensor) such that the user has:

• a "primary" roaming authenticator that they use to authenticate on new-to-them client devices (e.g., laptops, desktops) or on such client devices that lack a platform authenticator, and

• a low-friction means to strongly re-authenticate on client devices having platform authenticators.

Note: This approach of registering multiple authenticators for an account is also useful in account recovery use cases.

• First, on a desktop computer (lacking a platform authenticator):

  • User navigates to example.com in a browser and signs in to an existing account using whatever method they have been using (possibly a legacy method such as a password), or creates a new account.

  • User navigates to account security settings and selects "Register security key".

  • Website prompts the user to plug in a USB security key fob; the user does.

  • The USB security key blinks to indicate the user should press the button on it; the user does.

  • Website shows message, "Registration complete."

  Note: Since this computer lacks a platform authenticator, the website may require the user to present their USB security key from time to time or each time the user interacts with the website. This is at the website’s discretion.

• Later, on their laptop (which features a platform authenticator):

  • User navigates to example.com in a browser and initiates signing in.

  • Website prompts the user to plug in their USB security key.

  • User plugs in the previously registered USB security key and presses the button.

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

  • Website prompts, "Do you want to register this computer with example.com?"

  • User agrees.

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

  • Website shows message, "Registration complete."

  • User signs out.

• Later, again on their laptop:

  • User navigates to example.com in a browser and initiates signing in.

  • Website shows message, "Please follow your computer’s prompts to complete sign in."

  • Laptop prompts the user for an authorization gesture (PIN, biometric, etc.); the user provides this.

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

1.2.4. Other Use Cases and Configurations

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

• A user navigates to example.com on their laptop, is guided through a flow to create and register a credential on their phone.

• A user obtains a discrete, roaming authenticator, such as a "fob" with USB or USB+NFC/BLE connectivity options, loads example.com in their browser on a laptop or phone, and is guided though a flow to create and register a credential on the fob.

• A Relying Party prompts the user for their authorization gesture in order to authorize a single transaction, such as a payment or other financial transaction.

1.3. Platform-Specific Implementation Guidance

This specification defines how to use Web Authentication in the general case. When using Web Authentication in connection with specific platform support (e.g. apps), it is recommended to see platform-specific documentation and guides for additional guidance and limitations.

2. Conformance

This specification defines three conformance classes. Each of these classes is specified so that conforming members of the class are secure against non-conforming or hostile members of the other classes.

2.1. User Agents

A User Agent MUST behave as described by §5 Web Authentication API in order to be considered conformant. Conforming 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 User Agent MUST also be a conforming implementation of the IDL fragments of this specification, as described in the “Web IDL” specification. [WebIDL]

2.2. Authenticators

A WebAuthn Authenticator MUST provide the operations defined by §6 WebAuthn Authenticator Model, and those operations MUST behave as described there. This is a set of functional and security requirements for an authenticator to be usable by a Conforming User Agent.

As described in §1.2 Use Cases, an authenticator may be implemented in the operating system underlying the User Agent, or in external hardware, or a combination of both.

2.2.1. Backwards Compatibility with FIDO U2F

Authenticators that only support the §8.6 FIDO U2F Attestation Statement Format have no mechanism to store a user handle, so the returned userHandle will always be null.

2.3. WebAuthn Relying Parties

A WebAuthn Relying Party MUST behave as described in §7 WebAuthn Relying Party Operations to obtain all the security benefits offered by this specification. See §13.3 Security Benefits for WebAuthn Relying Parties for further discussion of this.

2.4. All Conformance Classes

All CBOR encoding performed by the members of the above conformance classes MUST be done using the CTAP2 canonical CBOR encoding form. All decoders of the above conformance classes SHOULD reject CBOR that is not validly encoded in the CTAP2 canonical CBOR encoding form and SHOULD reject messages with duplicate map keys.

3. Dependencies

This specification relies on several other underlying specifications, listed below and in Terms defined by reference.

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.

CBOR

A number of structures in this specification, including attestation statements and extensions, are encoded using the CTAP2 canonical CBOR encoding form of the Compact Binary Object Representation (CBOR) [RFC7049], as defined in [FIDO-CTAP].

CDDL

This specification describes the syntax of all CBOR-encoded data using the CBOR Data Definition Language (CDDL) [CDDL].

COSE

CBOR Object Signing and Encryption (COSE) [RFC8152]. The IANA COSE Algorithms registry established by this specification is also used.

Credential Management

The API described in this document is an extension of the Credential concept defined in [CREDENTIAL-MANAGEMENT-1].

DOM

DOMException and the DOMException values used in this specification are defined in [DOM4].

ECMAScript

%ArrayBuffer% is defined in [ECMAScript].

HTML

The concepts of relevant settings object, origin, opaque origin, and is a registrable domain suffix of or is equal to are defined in [HTML52].

URL

The concept of same site is defined in [URL].

Web IDL

Many of the interface definitions and all of the IDL in this specification depend on [WebIDL]. This updated version of the Web IDL standard adds support for Promises, which are now the preferred mechanism for asynchronous interaction in all new web APIs.

FIDO AppID

The algorithms for determining the FacetID of a calling application and determining if a caller’s FacetID is authorized for an AppID (used only in the AppID extension) are defined by [FIDO-APPID].

The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this document are to be interpreted as described in [RFC2119].

4. Terminology

Assertion

See Authentication Assertion.

Attestation

Generally, attestation is a statement serving to bear witness, confirm, or authenticate. In the WebAuthn context, attestation is employed to attest to the provenance of an authenticator and the data it emits; including, for example: credential IDs, credential key pairs, signature counters, etc. An attestation statement is conveyed in an attestation object during registration. See also §6.5 Attestation and Figure 5. Whether or how the client conveys the attestation statement and AAGUID portions of the attestation object to the Relying Party is described by attestation conveyance.

Attestation Certificate

A X.509 Certificate for the attestation key pair used by an authenticator to attest to its manufacture and capabilities. At registration time, the authenticator uses the attestation private key to sign the Relying Party-specific credential public key (and additional data) that it generates and returns via the authenticatorMakeCredential operation. Relying Parties use the attestation public key conveyed in the attestation certificate to verify the attestation signature. Note that in the case of self attestation, the authenticator has no distinct attestation key pair nor attestation certificate, see self attestation for details.

Authentication

Authentication Ceremony

The ceremony where a user, and the user’s client (containing at least one authenticator) work in concert to cryptographically prove to a Relying Party that the user controls the credential private key associated with a previously-registered public key credential (see Registration). Note that this includes a test of user presence or user verification.

Authentication Assertion

The cryptographically signed AuthenticatorAssertionResponse object returned by an authenticator as the result of an authenticatorGetAssertion operation.

This corresponds to the [CREDENTIAL-MANAGEMENT-1] specification’s single-use credentials.

Authenticator

WebAuthn Authenticator

A cryptographic entity, existing in hardware or software, that can register a user with a given Relying Party and later assert possession of the registered public key credential, and optionally verify the user, when requested by the Relying Party. Authenticators can report information regarding their type and security characteristics via attestation during registration.

A WebAuthn Authenticator could be a roaming authenticator, a dedicated hardware subsystem integrated into the client device, or a software component of the client or client device.

Authorization Gesture

An authorization gesture is a physical interaction performed by a user with an authenticator as part of a ceremony, such as registration or authentication. By making such an authorization gesture, a user provides consent for (i.e., authorizes) a ceremony to proceed. This MAY involve user verification if the employed authenticator is capable, or it MAY involve a simple test of user presence.

Biometric Recognition

The automated recognition of individuals based on their biological and behavioral characteristics [ISOBiometricVocabulary].

Biometric Authenticator

Any authenticator that implements biometric recognition.

Bound credential

A public key credential source or public key credential is said to be bound to its managing authenticator. This means that only the managing authenticator can generate assertions for the public key credential sources bound to it.

Ceremony

The concept of a ceremony [Ceremony] is an extension of the concept of a network protocol, with human nodes alongside computer nodes and with communication links that include user interface(s), human-to-human communication, and transfers of physical objects that carry data. What is out-of-band to a protocol is in-band to a ceremony. In this specification, Registration and Authentication are ceremonies, and an authorization gesture is often a component of those ceremonies.

Client

WebAuthn Client

Also referred to herein as simply a client. See also Conforming User Agent. A WebAuthn Client is an intermediary entity typically implemented in the user agent (in whole, or in part). Conceptually, it underlies the Web Authentication API and embodies the implementation of the [[Create]](origin, options, sameOriginWithAncestors) and [[DiscoverFromExternalSource]](origin, options, sameOriginWithAncestors) internal methods. It is responsible for both marshalling the inputs for the underlying authenticator operations, and for returning the results of the latter operations to the Web Authentication API's callers.

The WebAuthn Client runs on, and is distinct from, a WebAuthn Client Device.

Client Device

WebAuthn Client Device

The hardware device on which the WebAuthn Client runs, for example a smartphone, a laptop computer or a desktop computer, and the operating system running on that hardware.

The distinctions between a WebAuthn Client device and a client are:

• a single client device MAY support running multiple clients, i.e., browser implementations, which all have access to the same authenticators available on that client device, and

• platform authenticators are bound to a client device rather than a WebAuthn Client.

A client device and a client together constitute a client platform.

Client Platform

A client device and a client together make up a client platform. A single hardware device MAY be part of multiple distinct client platforms at different times by running different operating systems and/or clients.

Client-Side

This refers in general to the combination of the user’s client platform, authenticators, and everything gluing it all together.

Client-side-resident Public Key Credential Source

Resident Credential

A Client-side-resident Public Key Credential Source, or Resident Credential for short, is a public key credential source whose credential private key is stored in the authenticator, client or client device. Such client-side storage requires a resident credential capable authenticator and has the property that the authenticator is able to select the credential private key given only an RP ID, possibly with user assistance (e.g., by providing the user a pick list of credentials scoped to the RP ID). By definition, the credential private key is always exclusively controlled by the authenticator. In the case of a resident credential, the authenticator might offload storage of wrapped key material to the client device, but the client device is not expected to offload the key storage to remote entities (e.g., WebAuthn Relying Party Server). See also: client-side credential storage modality.

Conforming User Agent

A user agent implementing, in cooperation with the underlying client device, the Web Authentication API and algorithms given in this specification, and handling communication between authenticators and Relying Parties.

Credential ID

A probabilistically-unique byte sequence identifying a public key credential source and its authentication assertions.

Credential IDs are generated by authenticators in two forms:

1. At least 16 bytes that include at least 100 bits of entropy, or

2. The public key credential source, without its Credential ID, encrypted so only its managing authenticator can decrypt it. This form allows the authenticator to be nearly stateless, by having the Relying Party store any necessary state.

   Note: [FIDO-UAF-AUTHNR-CMDS] includes guidance on encryption techniques under "Security Guidelines".

Relying Parties do not need to distinguish these two Credential ID forms.

Credential Properties

A credential property is some characteristic property of a public key credential source, such as whether it is a resident credential or a non-resident credential.

Credential Public Key

User Public Key

The public key portion of a Relying Party-specific credential key pair, generated by an authenticator and returned to a Relying Party at registration time (see also public key credential). The private key portion of the credential key pair is known as the credential private key. Note that in the case of self attestation, the credential key pair is also used as the attestation key pair, see self attestation for details.

Note: The credential public key is referred to as the user public key in FIDO UAF [UAFProtocol], and in FIDO U2F [FIDO-U2F-Message-Formats] and some parts of this specification that relate to it.

Human Palatability

An identifier that is human-palatable is intended to be rememberable and reproducible by typical human users, in contrast to identifiers that are, for example, randomly generated sequences of bits [EduPersonObjectClassSpec].

Public Key Credential Source

A credential source ([CREDENTIAL-MANAGEMENT-1]) used by an authenticator to generate authentication assertions. A public key credential source consists of a struct with the following items:

type

whose value is of PublicKeyCredentialType, defaulting to public-key.

id

A Credential ID.

privateKey

The credential private key.

rpId

The Relying Party Identifier, for the Relying Party this public key credential source is scoped to.

userHandle

The user handle associated when this public key credential source was created. This item is nullable.

otherUI

OPTIONAL other information used by the authenticator to inform its UI. For example, this might include the user’s displayName.

The authenticatorMakeCredential operation creates a public key credential source bound to a managing authenticator and returns the credential public key associated with its credential private key. The Relying Party can use this credential public key to verify the authentication assertions created by this public key credential source.

Public Key Credential

Generically, a credential is data one entity presents to another in order to authenticate the former to the latter [RFC4949]. The term public key credential refers to one of: a public key credential source, the possibly-attested credential public key corresponding to a public key credential source, or an authentication assertion. Which one is generally determined by context.

Note: This is a willful violation of [RFC4949]. In English, a "credential" is both a) the thing presented to prove a statement and b) intended to be used multiple times. It’s impossible to achieve both criteria securely with a single piece of data in a public key system. [RFC4949] chooses to define a credential as the thing that can be used multiple times (the public key), while this specification gives "credential" the English term’s flexibility. This specification uses more specific terms to identify the data related to an [RFC4949] credential:

"Authentication information" (possibly including a private key)

Public key credential source

"Signed value"

Authentication assertion

[RFC4949] "credential"

Credential public key or attestation object

At registration time, the authenticator creates an asymmetric key pair, and stores its private key portion and information from the Relying Party into a public key credential source. The public key portion is returned to the Relying Party, who then stores it in conjunction with the present user’s account. Subsequently, only that Relying Party, as identified by its RP ID, is able to employ the public key credential in authentication ceremonies, via the get() method. The Relying Party uses its stored copy of the credential public key to verify the resultant authentication assertion.

Rate Limiting

The process (also known as throttling) by which an authenticator implements controls against brute force attacks by limiting the number of consecutive failed authentication attempts within a given period of time. If the limit is reached, the authenticator should impose a delay that increases exponentially with each successive attempt, or disable the current authentication modality and offer a different authentication factor if available. Rate limiting is often implemented as an aspect of user verification.

Registration

Registration Ceremony

The ceremony where a user, a Relying Party, and the user’s client (containing at least one authenticator) work in concert to create a public key credential and associate it with the user’s Relying Party account. Note that this includes employing a test of user presence or user verification.

Relying Party

See WebAuthn Relying Party.

Relying Party Identifier

RP ID

A valid domain string that identifies the WebAuthn Relying Party on whose behalf a given registration or authentication ceremony is being performed. A public key credential can only be used for authentication with the same entity (as identified by RP ID) it was registered with.

By default, the RP ID for a WebAuthn operation is set to the caller’s origin's effective domain. This default MAY be overridden by the caller, as long as the caller-specified RP ID value is a registrable domain suffix of or is equal to the caller’s origin's effective domain. See also §5.1.3 Create a New Credential - PublicKeyCredential’s [[Create]](origin, options, sameOriginWithAncestors) Method and §5.1.4 Use an Existing Credential to Make an Assertion - PublicKeyCredential’s [[Get]](options) Method.

Note: An RP ID is based on a host's domain name. It does not itself include a scheme or port, as an origin does. The RP ID of a public key credential determines its scope. I.e., it determines the set of origins on which the public key credential may be exercised, as follows:

• The RP ID must be equal to the origin's effective domain, or a registrable domain suffix of the origin's effective domain.

• The origin's scheme must be https.

• The origin's port is unrestricted.

For example, given a Relying Party whose origin is https://login.example.com:1337, then the following RP IDs are valid: login.example.com (default) and example.com, but not m.login.example.com and not com.

This is done in order to match the behavior of pervasively deployed ambient credentials (e.g., cookies, [RFC6265]). Please note that this is a greater relaxation of "same-origin" restrictions than what document.domain's setter provides.

Server-side-resident Public Key Credential Source

Non-Resident Credential

A Server-side-resident Public Key Credential Source, or Non-Resident Credential for short, is a public key credential source whose credential private key is not stored in the authenticator, client or client device, but is instead encrypted (i.e. wrapped) and returned to the server as the credential ID. See also: server-side credential storage modality.

Test of User Presence

A test of user presence is a simple form of authorization gesture and technical process where a user interacts with an authenticator by (typically) simply touching it (other modalities may also exist), yielding a Boolean result. Note that this does not constitute user verification because a user presence test, by definition, is not capable of biometric recognition, nor does it involve the presentation of a shared secret such as a password or PIN.

User Consent

User consent means the user agrees with what they are being asked, i.e., it encompasses reading and understanding prompts. An authorization gesture is a ceremony component often employed to indicate user consent.

User Handle

The user handle is specified by a Relying Party, as the value of user.id, and used to map a specific public key credential to a specific user account with the Relying Party. Authenticators in turn map RP IDs and user handle pairs to public key credential sources.

A user handle is an opaque byte sequence with a maximum size of 64 bytes. User handles are not meant to be displayed to users. The user handle MUST NOT contain personally identifying information about the user, such as a username or e-mail address; see §14.9 User Handle Contents for details.

User Verification

The technical process by which an authenticator locally authorizes the invocation of the authenticatorMakeCredential and authenticatorGetAssertion operations. User verification MAY be instigated through various authorization gesture modalities; for example, through a touch plus pin code, password entry, or biometric recognition (e.g., presenting a fingerprint) [ISOBiometricVocabulary]. The intent is to be able to distinguish individual users. Note that invocation of the authenticatorMakeCredential and authenticatorGetAssertion operations implies use of key material managed by the authenticator. Note that for security, user verification and use of credential private keys must occur within a single logical security boundary defining the authenticator.

User verification procedures MAY implement rate limiting as a protection against brute force attacks.

User Present

UP

Upon successful completion of a user presence test, the user is said to be "present".

User Verified

UV

Upon successful completion of a user verification process, the user is said to be "verified".

WebAuthn Relying Party

The entity whose web application utilizes the Web Authentication API to register and authenticate users.

Note: While the term Relying Party is also often used in other contexts (e.g., X.509 and OAuth), an entity acting as a Relying Party in one context is not necessarily a Relying Party in other contexts. In this specification, the term WebAuthn Relying Party is often shortened to be just Relying Party, and explicitly refers to a Relying Party in the WebAuthn context. Note that in any concrete instantiation a WebAuthn context may be embedded in a broader overall context, e.g., one based on OAuth.

5. Web Authentication API

This section normatively specifies the API for creating and using public key credentials. The basic idea is that the credentials belong to the user and are managed by a WebAuthn Authenticator, with which the WebAuthn Relying Party interacts through the client platform. Relying Party scripts can (with the user’s consent) request the browser to create a new credential for future use by the Relying Party. See Figure 1, below.

Scripts can also request the user’s permission to perform authentication operations with an existing credential. See Figure 2, below.

All such operations are performed in the authenticator and are mediated by the client 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.

In addition to the above script interface, the authenticator MAY implement (or come with client software that implements) a user interface for management. Such an interface MAY be used, for example, to reset the authenticator to a clean state or to inspect the current state of the authenticator. In other words, such an interface is similar to the user interfaces provided by browsers for managing user state such as history, saved passwords, and cookies. Authenticator management actions such as credential deletion are considered to be the responsibility of such a user interface and are deliberately omitted from the API exposed to scripts.

The security properties of this API are provided by the client and the authenticator working together. The authenticator, which holds and manages credentials, ensures that all operations are scoped to a particular origin, and cannot be replayed against a different origin, by incorporating the origin in its responses. Specifically, as defined in §6.3 Authenticator Operations, the full origin of the requester is included, and signed over, in the attestation object produced when a new credential is created as well as in all assertions produced by WebAuthn credentials.

Additionally, to maintain user privacy and prevent malicious Relying Parties from probing for the presence of public key credentials belonging to other Relying Parties, each credential is also scoped to a Relying Party Identifier, or RP ID. This RP ID is provided by the client to the authenticator for all operations, and the authenticator ensures that credentials created by a Relying Party can only be used in operations requested by the same RP ID. Separating the origin from the RP ID in this way allows the API to be used in cases where a single Relying Party maintains multiple origins.

The client facilitates these security measures by providing the Relying Party's origin and RP ID to the authenticator for each operation. Since this is an integral part of the WebAuthn security model, user agents only expose this API to callers in secure contexts.

The Web Authentication API is defined by the union of the Web IDL fragments presented in the following sections. A combined IDL listing is given in the IDL Index.

5.1. PublicKeyCredential Interface

The PublicKeyCredential interface inherits from Credential [CREDENTIAL-MANAGEMENT-1], and contains the attributes that are returned to the caller when a new credential is created, or a new assertion is requested.

[SecureContext, Exposed=Window]
interface PublicKeyCredential : Credential {
    [SameObject] readonly attribute ArrayBuffer              rawId;
    [SameObject] readonly attribute AuthenticatorResponse    response;
    AuthenticationExtensionsClientOutputs getClientExtensionResults();
};

id

This attribute is inherited from Credential, though PublicKeyCredential overrides Credential's getter, instead returning the base64url encoding of the data contained in the object’s [[identifier]] internal slot.

rawId

This attribute returns the ArrayBuffer contained in the [[identifier]] internal slot.

response, of type AuthenticatorResponse, readonly

This attribute contains the authenticator's response to the client’s request to either create a public key credential, or generate an authentication assertion. If the PublicKeyCredential is created in response to create(), this attribute’s value will be an AuthenticatorAttestationResponse, otherwise, the PublicKeyCredential was created in response to get(), and this attribute’s value will be an AuthenticatorAssertionResponse.

getClientExtensionResults()

This operation returns the value of [[clientExtensionsResults]], which is a map containing extension identifier → client extension output entries produced by the extension’s client extension processing.

[[type]]

The PublicKeyCredential interface object's [[type]] internal slot's value is the string "public-key".

Note: This is reflected via the type attribute getter inherited from Credential.

[[discovery]]

The PublicKeyCredential interface object's [[discovery]] internal slot's value is "remote".

[[identifier]]

This internal slot contains the credential ID, chosen by the authenticator. The credential ID is used to look up credentials for use, and is therefore expected to be globally unique with high probability across all credentials of the same type, across all authenticators.

Note: This API does not constrain the format or length of this identifier, except that it MUST be sufficient for the authenticator to uniquely select a key. For example, an authenticator without on-board storage may create identifiers containing a credential private key wrapped with a symmetric key that is burned into the authenticator.

[[clientExtensionsResults]]

This internal slot contains the results of processing client extensions requested by the Relying Party upon the Relying Party's invocation of either navigator.credentials.create() or navigator.credentials.get().

PublicKeyCredential's interface object inherits Credential's implementation of [[CollectFromCredentialStore]](origin, options, sameOriginWithAncestors), and defines its own implementation of [[Create]](origin, options, sameOriginWithAncestors), [[DiscoverFromExternalSource]](origin, options, sameOriginWithAncestors), and [[Store]](credential, sameOriginWithAncestors).

5.1.1. CredentialCreationOptions Dictionary Extension

To support registration via navigator.credentials.create(), this document extends the CredentialCreationOptions dictionary as follows:

partial dictionary CredentialCreationOptions {
    PublicKeyCredentialCreationOptions      publicKey;
};

5.1.2. CredentialRequestOptions Dictionary Extension

To support obtaining assertions via navigator.credentials.get(), this document extends the CredentialRequestOptions dictionary as follows:

partial dictionary CredentialRequestOptions {
    PublicKeyCredentialRequestOptions      publicKey;
};

5.1.3. Create a New Credential - PublicKeyCredential’s [[Create]](origin, options, sameOriginWithAncestors) Method

5.1.4. Use an Existing Credential to Make an Assertion - PublicKeyCredential’s [[Get]](options) Method

WebAuthn Relying Parties call navigator.credentials.get({publicKey:..., ...}) to discover and use an existing public key credential, with the user’s consent. Relying Party script optionally specifies some criteria to indicate what credential sources are acceptable to it. The client platform locates credential sources matching the specified criteria, and guides the user to pick one that the script will be allowed to use. The user may choose to decline the entire interaction even if a credential source is present, for example to maintain privacy. If the user picks a credential source, the user agent then uses §6.3.3 The authenticatorGetAssertion Operation to sign a Relying Party-provided challenge and other collected data into an assertion, which is used as a credential.

The get() implementation [CREDENTIAL-MANAGEMENT-1] calls PublicKeyCredential.[[CollectFromCredentialStore]]() to collect any credentials that should be available without user mediation (roughly, this specification’s authorization gesture), and if it does not find exactly one of those, it then calls PublicKeyCredential.[[DiscoverFromExternalSource]]() to have the user select a credential source.

Since this specification requires an authorization gesture to create any credentials, the PublicKeyCredential.[[CollectFromCredentialStore]](origin, options, sameOriginWithAncestors) internal method inherits the default behavior of Credential.[[CollectFromCredentialStore]](), of returning an empty set.

This navigator.credentials.get() operation can be aborted by leveraging the AbortController; see DOM §3.3 Using AbortController and AbortSignal objects in APIs for detailed instructions.

5.1.4.1. PublicKeyCredential’s [[DiscoverFromExternalSource]](origin, options, sameOriginWithAncestors) Method

5.1.5. Store an Existing Credential - PublicKeyCredential’s [[Store]](credential, sameOriginWithAncestors) Method

5.1.6. Preventing Silent Access to an Existing Credential - PublicKeyCredential’s [[preventSilentAccess]](credential, sameOriginWithAncestors) Method

5.1.7. Availability of User-Verifying Platform Authenticator - PublicKeyCredential’s isUserVerifyingPlatformAuthenticatorAvailable() Method

partial interface PublicKeyCredential {
    static Promise<boolean> isUserVerifyingPlatformAuthenticatorAvailable();
};

5.2. Authenticator Responses (interface AuthenticatorResponse)

Authenticators respond to Relying Party requests by returning an object derived from the AuthenticatorResponse interface:

[SecureContext, Exposed=Window]
interface AuthenticatorResponse {
    [SameObject] readonly attribute ArrayBuffer      clientDataJSON;
};

5.2.1. Information About Public Key Credential (interface AuthenticatorAttestationResponse)

The AuthenticatorAttestationResponse interface represents the authenticator's response to a client’s request for the creation of a new public key credential. It contains information about the new credential that can be used to identify it for later use, and metadata that can be used by the WebAuthn Relying Party to assess the characteristics of the credential during registration.

[SecureContext, Exposed=Window]
interface AuthenticatorAttestationResponse : AuthenticatorResponse {
    [SameObject] readonly attribute ArrayBuffer      attestationObject;
    sequence<AuthenticatorTransport>                 getTransports();
};

5.2.2. Web Authentication Assertion (interface AuthenticatorAssertionResponse)

The AuthenticatorAssertionResponse interface represents an authenticator's response to a client’s request for generation of a new authentication assertion given the WebAuthn Relying Party's challenge and OPTIONAL list of credentials it is aware of. This response contains a cryptographic signature proving possession of the credential private key, and optionally evidence of user consent to a specific transaction.

[SecureContext, Exposed=Window]
interface AuthenticatorAssertionResponse : AuthenticatorResponse {
    [SameObject] readonly attribute ArrayBuffer      authenticatorData;
    [SameObject] readonly attribute ArrayBuffer      signature;
    [SameObject] readonly attribute ArrayBuffer?     userHandle;
};

5.3. Parameters for Credential Generation (dictionary PublicKeyCredentialParameters)

dictionary PublicKeyCredentialParameters {
    required PublicKeyCredentialType      type;
    required COSEAlgorithmIdentifier      alg;
};

5.4. Options for Credential Creation (dictionary PublicKeyCredentialCreationOptions)

dictionary PublicKeyCredentialCreationOptions {
    required PublicKeyCredentialRpEntity         rp;
    required PublicKeyCredentialUserEntity       user;

    required BufferSource                             challenge;
    required sequence<PublicKeyCredentialParameters>  pubKeyCredParams;

    unsigned long                                timeout;
    sequence<PublicKeyCredentialDescriptor>      excludeCredentials = [];
    AuthenticatorSelectionCriteria               authenticatorSelection;
    AttestationConveyancePreference              attestation = "none";
    AuthenticationExtensionsClientInputs         extensions;
};

5.4.1. Public Key Entity Description (dictionary PublicKeyCredentialEntity)

The PublicKeyCredentialEntity dictionary describes a user account, or a WebAuthn Relying Party, which a public key credential is associated with or scoped to, respectively.

dictionary PublicKeyCredentialEntity {
    required DOMString    name;
    USVString             icon;
};

5.4.2. Relying Party Parameters for Credential Generation (dictionary PublicKeyCredentialRpEntity)

The PublicKeyCredentialRpEntity dictionary is used to supply additional Relying Party attributes when creating a new credential.

dictionary PublicKeyCredentialRpEntity : PublicKeyCredentialEntity {
    DOMString      id;
};

5.4.3. User Account Parameters for Credential Generation (dictionary PublicKeyCredentialUserEntity)

The PublicKeyCredentialUserEntity dictionary is used to supply additional user account attributes when creating a new credential.

dictionary PublicKeyCredentialUserEntity : PublicKeyCredentialEntity {
    required BufferSource   id;
    required DOMString      displayName;
};

5.4.4. Authenticator Selection Criteria (dictionary AuthenticatorSelectionCriteria)

WebAuthn Relying Parties may use the AuthenticatorSelectionCriteria dictionary to specify their requirements regarding authenticator attributes.

dictionary AuthenticatorSelectionCriteria {
    AuthenticatorAttachment      authenticatorAttachment;
    boolean                      requireResidentKey = false;
    ResidentKeyRequirement       residentKey;
    UserVerificationRequirement  userVerification = "preferred";
};

5.4.5. Authenticator Attachment Enumeration (enum AuthenticatorAttachment)

This enumeration’s values describe authenticators' attachment modalities. Relying Parties use this to express a preferred authenticator attachment modality when calling navigator.credentials.create() to create a credential.

enum AuthenticatorAttachment {
    "platform",
    "cross-platform"
};

Note: An authenticator attachment modality selection option is available only in the [[Create]](origin, options, sameOriginWithAncestors) operation. The Relying Party may use it to, for example, ensure the user has a roaming credential for authenticating on another client device; or to specifically register a platform credential for easier reauthentication using a particular client device. The [[DiscoverFromExternalSource]](origin, options, sameOriginWithAncestors) operation has no authenticator attachment modality selection option, so the Relying Party SHOULD accept any of the user’s registered credentials. The client and user will then use whichever is available and convenient at the time.

5.4.6. Resident Key Requirement Enumeration (enum ResidentKeyRequirement)

enum ResidentKeyRequirement {
    "discouraged",
    "preferred",
    "required"
};

This enumeration’s values describe the Relying Party's requirements for resident credentials:

Note: The Relying Party can use a combination of the value provided for options.authenticatorSelection.residentKey and the Credential Properties Extension's return value to determine whether or not the authenticator created a resident credential. This is useful when values of discouraged or preferred are used for options.authenticatorSelection.residentKey, because in those cases it is possible for an authenticator to create either a resident credential or a non-resident credential.

5.4.7. Attestation Conveyance Preference Enumeration (enum AttestationConveyancePreference)

WebAuthn Relying Parties may use AttestationConveyancePreference to specify their preference regarding attestation conveyance during credential generation.

enum AttestationConveyancePreference {
    "none",
    "indirect",
    "direct"
};

5.5. Options for Assertion Generation (dictionary PublicKeyCredentialRequestOptions)

The PublicKeyCredentialRequestOptions dictionary supplies get() with the data it needs to generate an assertion. Its challenge member MUST be present, while its other members are OPTIONAL.

dictionary PublicKeyCredentialRequestOptions {
    required BufferSource                challenge;
    unsigned long                        timeout;
    USVString                            rpId;
    sequence<PublicKeyCredentialDescriptor> allowCredentials = [];
    UserVerificationRequirement          userVerification = "preferred";
    AuthenticationExtensionsClientInputs extensions;
};

challenge, of type BufferSource

This member represents a challenge that the selected authenticator signs, along with other data, when producing an authentication assertion. See the §13.1 Cryptographic Challenges security consideration.

timeout, of type unsigned long

This OPTIONAL member specifies a time, in milliseconds, that the caller is willing to wait for the call to complete. The value is treated as a hint, and MAY be overridden by the client.

rpId, of type USVString

This OPTIONAL member specifies the relying party identifier claimed by the caller. If omitted, its value will be the CredentialsContainer object’s relevant settings object's origin's effective domain.

allowCredentials, of type sequence<PublicKeyCredentialDescriptor>, defaulting to None

This OPTIONAL member contains a list of PublicKeyCredentialDescriptor objects representing public key credentials acceptable to the caller, in descending order of the caller’s preference (the first item in the list is the most preferred credential, and so on down the list).

userVerification, of type UserVerificationRequirement, defaulting to "preferred"

This OPTIONAL member describes the Relying Party's requirements regarding user verification for the get() operation. Eligible authenticators are filtered to only those capable of satisfying this requirement.

extensions, of type AuthenticationExtensionsClientInputs

This OPTIONAL member 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 might be included as an extension.

5.6. Abort Operations with AbortSignal

Developers are encouraged to leverage the AbortController to manage the [[Create]](origin, options, sameOriginWithAncestors) and [[DiscoverFromExternalSource]](origin, options, sameOriginWithAncestors) operations. See DOM §3.3 Using AbortController and AbortSignal objects in APIs section for detailed instructions.

Note: DOM §3.3 Using AbortController and AbortSignal objects in APIs section specifies that web platform APIs integrating with the AbortController must reject the promise immediately once the aborted flag is set. Given the complex inheritance and parallelization structure of the [[Create]](origin, options, sameOriginWithAncestors) and [[DiscoverFromExternalSource]](origin, options, sameOriginWithAncestors) methods, the algorithms for the two APIs fulfills this requirement by checking the aborted flag in three places. In the case of [[Create]](origin, options, sameOriginWithAncestors), the aborted flag is checked first in Credential Management 1 §2.5.4 Create a Credential immediately before calling [[Create]](origin, options, sameOriginWithAncestors), then in §5.1.3 Create a New Credential - PublicKeyCredential’s [[Create]](origin, options, sameOriginWithAncestors) Method right before authenticator sessions start, and finally during authenticator sessions. The same goes for [[DiscoverFromExternalSource]](origin, options, sameOriginWithAncestors).

The visibility and focus state of the Window object determines whether the [[Create]](origin, options, sameOriginWithAncestors) and [[DiscoverFromExternalSource]](origin, options, sameOriginWithAncestors) operations should continue. When the Window object associated with the [Document loses focus, [[Create]](origin, options, sameOriginWithAncestors) and [[DiscoverFromExternalSource]](origin, options, sameOriginWithAncestors) operations SHOULD be aborted.

The WHATWG HTML WG is discussing whether to provide a hook when a browsing context gains or loses focuses. If a hook is provided, the above paragraph will be updated to include the hook. See WHATWG HTML WG Issue #2711 for more details.

5.7. WebAuthn Extensions Inputs and Outputs

The subsections below define the data types used for conveying WebAuthn extension inputs and outputs.

Note: Authenticator extension outputs are conveyed as a part of Authenticator data (see Table 1).

Note: The types defined below—AuthenticationExtensionsClientInputs, AuthenticationExtensionsClientOutputs, and AuthenticationExtensionsAuthenticatorInputs—are applicable to both registration extensions and authentication extensions. The "Authentication..." portion of their names should be regarded as meaning "WebAuthentication..."

5.7.1. Authentication Extensions Client Inputs (dictionary AuthenticationExtensionsClientInputs) ## {#iface-authentication-extensions-client-inputs}

dictionary AuthenticationExtensionsClientInputs {
};

This is a dictionary containing the client extension input values for zero or more WebAuthn Extensions.

5.7.2. Authentication Extensions Client Outputs (dictionary AuthenticationExtensionsClientOutputs) ## {#iface-authentication-extensions-client-outputs}

dictionary AuthenticationExtensionsClientOutputs {
};

This is a dictionary containing the client extension output values for zero or more WebAuthn Extensions.

5.7.3. Authentication Extensions Authenticator Inputs (typedef AuthenticationExtensionsAuthenticatorInputs) ## {#iface-authentication-extensions-authenticator-inputs}

typedef record<DOMString, DOMString> AuthenticationExtensionsAuthenticatorInputs;

This is a dictionary containing the authenticator extension input values for zero or more WebAuthn Extensions.

5.8. Supporting Data Structures

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

5.8.1. Client Data Used in WebAuthn Signatures (dictionary CollectedClientData)

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

Note: The CollectedClientData may be extended in the future. Therefore it’s critical when parsing to be tolerant of unknown keys and of any reordering of the keys.

dictionary CollectedClientData {
    required DOMString           type;
    required DOMString           challenge;
    required DOMString           origin;
    TokenBinding                 tokenBinding;
};

dictionary TokenBinding {
    required TokenBindingStatus status;
    DOMString id;
};

enum TokenBindingStatus { "present", "supported" };

5.8.2. Credential Type Enumeration (enum PublicKeyCredentialType)

enum PublicKeyCredentialType {
    "public-key"
};

5.8.3. Credential Descriptor (dictionary PublicKeyCredentialDescriptor)

dictionary PublicKeyCredentialDescriptor {
    required PublicKeyCredentialType      type;
    required BufferSource                 id;
    sequence<AuthenticatorTransport>      transports;
};

This dictionary contains the attributes that are specified by a caller when referring to a public key credential as an input parameter to the create() or get() methods. It mirrors the fields of the PublicKeyCredential object returned by the latter methods.

5.8.4. Authenticator Transport Enumeration (enum AuthenticatorTransport)

enum AuthenticatorTransport {
    "usb",
    "nfc",
    "ble",
    "internal"
};

5.8.5. Cryptographic Algorithm Identifier (typedef COSEAlgorithmIdentifier)

typedef long COSEAlgorithmIdentifier;

5.8.6. User Verification Requirement Enumeration (enum UserVerificationRequirement)

enum UserVerificationRequirement {
    "required",
    "preferred",
    "discouraged"
};

A WebAuthn Relying Party may require user verification for some of its operations but not for others, and may use this type to express its needs.

5.9. Feature Policy integration

This specification defines a policy-controlled feature identified by the feature policy token "webauthn". Its default allowlist is 'self'. [Feature-Policy]

A Document's feature policy determines whether any content in that document is allowed to successfully invoke the Web Authentication API, i.e., via navigator.credentials.create({publicKey:..., ...}) and navigator.credentials.get({publicKey:..., ...}). If disabled in any document, no content in the document will be allowed to use the foregoing methods: attempting to do so will return an error.

Note: Algorithms specified in [CREDENTIAL-MANAGEMENT-1] perform the actual feature policy evaluation. This is because such policy evaluation needs to occur when there is access to the current settings object. The [[Create]](origin, options, sameOriginWithAncestors) and [[DiscoverFromExternalSource]](origin, options, sameOriginWithAncestors) internal methods do not have such access since they are invoked in parallel (by algorithms specified in [CREDENTIAL-MANAGEMENT-1]).

5.10. Using Web Authentication within iframe elements

The Web Authentication API is disabled by default in cross-origin iframes. To override this default policy and indicate that a cross-origin iframe is allowed to invoke the Web Authentication API, specify the allow attribute on the iframe element and include the webauthn feature policy token in the allow attribute’s value.

6. WebAuthn Authenticator Model

The Web Authentication API implies a specific abstract functional model for a WebAuthn Authenticator. This section describes that 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 authenticators supported by that client platform, MUST be indistinguishable from the behavior specified in §5 Web Authentication API.

Note: [FIDO-CTAP] is an example of a concrete instantiation of this model, but it is one in which there are differences in the data it returns and those expected by the WebAuthn API's algorithms. The CTAP2 response messages are CBOR maps constructed using integer keys rather than the string keys defined in this specification for the same objects. The client is expected to perform any needed transformations on such data. The [FIDO-CTAP] specification details the mapping between CTAP2 integer keys and WebAuthn string keys, in section §6.2. Responses.

For authenticators, this model defines the logical operations that they MUST support, and the data formats that they expose to the client and the WebAuthn Relying Party. However, it does not define the details of how authenticators communicate with the client device, unless they are necessary for interoperability with Relying Parties. For instance, this abstract model does not define protocols for connecting authenticators to clients over transports such as USB or NFC. Similarly, this abstract model does not define specific error codes or methods of returning them; however, it does define error behavior in terms of the needs of the client. Therefore, specific error codes are mentioned as a means of showing which error conditions MUST be distinguishable (or not) from each other in order to enable a compliant and secure client implementation.

Relying Parties may influence authenticator selection, if they deem necessary, by stipulating various authenticator characteristics when creating credentials and/or when generating assertions, through use of credential creation options or assertion generation options, respectively. The algorithms underlying the WebAuthn API marshal these options and pass them to the applicable authenticator operations defined below.

In this abstract model, the authenticator provides key management and cryptographic signatures. It can be embedded in the WebAuthn client or housed in a separate device entirely. The authenticator itself can contain a cryptographic module which operates at a higher security level than the rest of the authenticator. This is particularly important for authenticators that are embedded in the WebAuthn client, as in those cases this cryptographic module (which may, for example, be a TPM) could be considered more trustworthy than the rest of the authenticator.

Each authenticator stores a credentials map, a map from (rpId, [userHandle]) to public key credential source.

Additionally, each authenticator has an AAGUID, which is a 128-bit identifier indicating the type (e.g. make and model) of the authenticator. The AAGUID MUST be chosen by the manufacturer to be identical across all substantially identical authenticators made by that manufacturer, and different (with high probability) from the AAGUIDs of all other types of authenticators. The AAGUID for a given type of authenticator SHOULD be randomly generated to ensure this. The Relying Party MAY use the AAGUID to infer certain properties of the authenticator, such as certification level and strength of key protection, using information from other sources.

The primary function of the authenticator is to provide WebAuthn signatures, which 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. These contextual bindings are divided in two: Those added by the Relying Party or the client, referred to as client data; and those added by the authenticator, referred to as the authenticator data. The authenticator signs over the client data, but is otherwise not interested in its contents. To save bandwidth and processing requirements on the authenticator, the client hashes the client data and sends only the result to the authenticator. The authenticator signs over the combination of the hash of the serialized client data, and its own authenticator data.

The goals of this design can be summarized as follows.

• The scheme for generating signatures should accommodate cases where the link between the client device 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 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.

Authenticators produce cryptographic signatures for two distinct purposes:

1. An attestation signature is produced when a new public key credential is created via an authenticatorMakeCredential operation. An attestation signature provides cryptographic proof of certain properties of the authenticator and the credential. For instance, an attestation signature asserts the authenticator type (as denoted by its AAGUID) and the credential public key. The attestation signature is signed by an attestation private key, which is chosen depending on the type of attestation desired. For more details on attestation, see §6.5 Attestation.

2. An assertion signature is produced when the authenticatorGetAssertion method is invoked. It represents an assertion by the authenticator that the user has consented to a specific transaction, such as logging in, or completing a purchase. Thus, an assertion signature asserts that the authenticator possessing a particular credential private key has established, to the best of its ability, that the user requesting this transaction is the same user who consented to creating that particular public key credential. It also asserts additional information, termed client data, that may be useful to the caller, such as the means by which user consent was provided, and the prompt shown to the user by the authenticator. The assertion signature format is illustrated in Figure 4, below.

The formats of these signatures, as well as the procedures for generating them, are specified below.

6.1. Authenticator Data

The authenticator data structure encodes contextual bindings made by the authenticator. These bindings are controlled by the authenticator itself, and derive their trust from the WebAuthn Relying Party's assessment of the security properties of the authenticator. In one extreme case, the authenticator may be embedded in the client, and its bindings may be no more trustworthy than the client data. At the other extreme, the authenticator may be a discrete entity with high-security hardware and software, connected to the client over a secure channel. In both cases, the Relying Party receives the authenticator data in the same format, and uses its knowledge of the authenticator to make trust decisions.

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.

The authenticator data structure is a byte array of 37 bytes or more, laid out as shown in Table 1.

The RP ID is originally received from the client when the credential is created, and again when an assertion is generated. However, it differs from other client data in some important ways. First, unlike the client data, the RP ID of a credential does not change between operations but instead remains the same for the lifetime of that credential. Secondly, it is validated by the authenticator during the authenticatorGetAssertion operation, by verifying that the RP ID that the requested credential is scoped to exactly matches the RP ID supplied by the client.

Authenticators perform the following steps to generate an authenticator data structure:

• Hash RP ID using SHA-256 to generate the rpIdHash.

• The UP flag SHALL be set if and only if the authenticator performed a test of user presence. The UV flag SHALL be set if and only if the authenticator performed user verification. The RFU bits SHALL be set to zero.

  Note: If the authenticator performed both a test of user presence and user verification, possibly combined in a single authorization gesture, then the authenticator will set both the UP flag and the UV flag.

• For attestation signatures, the authenticator MUST set the AT flag and include the attestedCredentialData. For authentication signatures, the AT flag MUST NOT be set and the attestedCredentialData MUST NOT be included.

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

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

6.1.1. Signature Counter Considerations

Authenticators SHOULD implement a signature counter feature. The signature counter is incremented for each successful authenticatorGetAssertion operation by some positive value, and its value is returned to the WebAuthn Relying Party within the authenticator data. The signature counter's purpose is to aid Relying Parties in detecting cloned authenticators. Clone detection is more important for authenticators with limited protection measures.

A Relying Party stores the signature counter of the most recent authenticatorGetAssertion operation. Upon a new authenticatorGetAssertion operation, the Relying Party compares the stored signature counter value with the new signCount value returned in the assertion’s authenticator data. If this new signCount value is less than or equal to the stored value, a cloned authenticator may exist, or the authenticator may be malfunctioning.

Detecting a signature counter mismatch does not indicate whether the current operation was performed by a cloned authenticator or the original authenticator. Relying Parties should address this situation appropriately relative to their individual situations, i.e., their risk tolerance.

Authenticators:

• SHOULD implement per credential signature counters. This prevents the signature counter value from being shared between Relying Parties and being possibly employed as a correlation handle for the user. Authenticators may implement a global signature counter, i.e., on a per-authenticator basis, but this is less privacy-friendly for users.

• SHOULD ensure that the signature counter value does not accidentally decrease (e.g., due to hardware failures).

6.1.2. FIDO U2F Signature Format Compatibility

The format for assertion signatures, which sign over the concatenation of an authenticator data structure and the hash of the serialized client data, are compatible with the FIDO U2F authentication signature format (see Section 5.4 of [FIDO-U2F-Message-Formats]).

This is because the first 37 bytes of the signed data in a FIDO U2F authentication response message constitute a valid authenticator data structure, and the remaining 32 bytes are the hash of the serialized client data. In this authenticator data structure, the rpIdHash is the FIDO U2F application parameter, all flags except UP are always zero, and the attestedCredentialData and extensions are never present. FIDO U2F authentication signatures can therefore be verified by the same procedure as other assertion signatures generated by the authenticatorMakeCredential operation.

6.2. Authenticator Taxonomy

Authenticator types vary along several different dimensions: authenticator attachment modality, employed transport(s), credential storage modality, and authentication factor capability. The combination of these dimensions defines an authenticator's authenticator type, which in turn determines the broad use cases the authenticator supports. Table 2 defines names for some authenticator types.

These types can be further broken down into subtypes, such as which transport(s) a roaming authenticator supports. The following sections define the terms authenticator attachment modality, credential storage modality and authentication factor capability.

6.2.1. Authenticator Attachment Modality

Clients can communicate with authenticators using a variety of mechanisms. For example, a client MAY use a client device-specific API to communicate with an authenticator which is physically bound to a client device. On the other hand, a client can use a variety of standardized cross-platform transport protocols such as Bluetooth (see §5.8.4 Authenticator Transport Enumeration (enum AuthenticatorTransport)) to discover and communicate with cross-platform attached authenticators. We refer to authenticators that are part of the client device as platform authenticators, while those that are reachable via cross-platform transport protocols are referred to as roaming authenticators.

• A platform authenticator is attached using a client device-specific transport, called platform attachment, and is usually not removable from the client device. A public key credential bound to a platform authenticator is called a platform credential.

• A roaming authenticator is attached using cross-platform transports, called cross-platform attachment. Authenticators of this class are removable from, and can "roam" among, client devices. A public key credential bound to a roaming authenticator is called a roaming credential.

Some platform authenticators could possibly also act as roaming authenticators depending on context. For example, a platform authenticator integrated into a mobile device could make itself available as a roaming authenticator via Bluetooth. In this case the client would recognize it only as a roaming authenticator, and not as a platform authenticator.

A primary use case for platform authenticators is to register a particular client device as a "trusted device" available as a something you have authentication factor for future authentication. This gives the user the convenience benefit of not needing a roaming authenticator for future authentication ceremonies, e.g., the user will not have to dig around in their pocket for their key fob or phone.

A primary use case for roaming authenticators is for initial authentication on a new client device, or on client devices that are rarely used or do not include a platform authenticator; or when policy or preference dictates that the authenticator be kept separate from the clients it is used with. A roaming authenticator can also be used to hold backup credentials in case another authenticator is lost.

6.2.2. Credential Storage Modality

An authenticator can store a public key credential source in one of two ways:

1. In persistent storage embedded in the authenticator, client or client device, e.g., in a secure element. A public key credential source stored in this way is a resident credential.

2. By encrypting (i.e., wrapping) the credential private key such that only this authenticator can decrypt (i.e., unwrap) it and letting the resulting ciphertext be the credential ID for the public key credential source. The credential ID is stored by the Relying Party and returned to the authenticator via the allowCredentials option of get(), which allows the authenticator to decrypt and use the credential private key.

   This enables the authenticator to have unlimited storage capacity for credential private keys, since the encrypted credential private keys are stored by the Relying Party instead of by the authenticator - but it means that a credential stored in this way must be retrieved from the Relying Party before the authenticator can use it.

Which of these storage strategies an authenticator supports defines the authenticator's credential storage modality as follows:

• An authenticator has the client-side credential storage modality if it supports client-side-resident public key credential sources. An authenticator with client-side credential storage modality is also called resident credential capable.

• An authenticator has the server-side credential storage modality if it does not have the client-side credential storage modality, i.e., it only supports storing credential private keys as a ciphertext in the credential ID.

Note that a resident credential capable authenticator MAY support both storage strategies. In this case, the authenticator MAY at its discretion use different storage strategies for different credentials, though subject to the residentKey or requireResidentKey options of create().

6.2.3. Authentication Factor Capability

There are three broad classes of authentication factors that can be used to prove an identity during an authentication ceremony: something you have, something you know and something you are. Examples include a physical key, a password, and a fingerprint, respectively.

All WebAuthn Authenticators belong to the something you have class, but an authenticator that supports user verification can also act as one or two additional kinds of authentication factor. For example, if the authenticator can verify a PIN, the PIN is something you know, and a biometric authenticator can verify something you are. Therefore, an authenticator that supports user verification is multi-factor capable. Conversely, an authenticator that is not multi-factor capable is single-factor capable. Note that a single multi-factor capable authenticator could support several modes of user verification, meaning it could act as all three kinds of authentication factor.

Although user verification is performed locally on the authenticator and not by the Relying Party, the authenticator indicates if user verification was performed by setting the UV flag in the signed response returned to the Relying Party. The Relying Party can therefore use the UV flag to verify that additional authentication factors were used in a registration or authentication ceremony. The authenticity of the UV flag can in turn be assessed by inspecting the authenticator's attestation statement.

6.3. Authenticator Operations

A WebAuthn 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.

6.3.1. Lookup Credential Source by Credential ID Algorithm

The result of looking up a credential id credentialId in an authenticator authenticator is the result of the following algorithm:

1. If authenticator can decrypt credentialId into a public key credential source credSource:

   1. Set credSource.id to credentialId.

   2. Return credSource.

2. For each public key credential source credSource of authenticator’s credentials map:

   1. If credSource.id is credentialId, return credSource.

3. Return null.

6.3.2. The authenticatorMakeCredential Operation

It takes the following input parameters:

hash

The hash of the serialized client data, provided by the client.

rpEntity

The Relying Party's PublicKeyCredentialRpEntity.

userEntity

The user account’s PublicKeyCredentialUserEntity, containing the user handle given by the Relying Party.

requireResidentKey

The effective resident key requirement for credential creation, a Boolean value determined by the client.

requireUserPresence

The constant Boolean value true. It is included here as a pseudo-parameter to simplify applying this abstract authenticator model to implementations that may wish to make a test of user presence optional although WebAuthn does not.

requireUserVerification

The effective user verification requirement for credential creation, a Boolean value determined by the client.

credTypesAndPubKeyAlgs

A sequence of pairs of PublicKeyCredentialType and public key algorithms (COSEAlgorithmIdentifier) requested by the Relying Party. This sequence is ordered from most preferred to least preferred. The authenticator makes a best-effort to create the most preferred credential that it can.

excludeCredentialDescriptorList

An OPTIONAL list of PublicKeyCredentialDescriptor objects provided by the Relying Party with the intention that, if any of these are known to the authenticator, it SHOULD NOT create a new credential. excludeCredentialDescriptorList contains a list of known credentials.

extensions

A CBOR map from extension identifiers to their authenticator extension inputs, created by the client based on the extensions requested by the Relying Party, if any.

Note: Before performing this operation, all other operations in progress in the authenticator session MUST be aborted by running the authenticatorCancel operation.

When this operation is invoked, the authenticator MUST perform the following procedure:

1. Check if all the supplied parameters are syntactically well-formed and of the correct length. If not, return an error code equivalent to "UnknownError" and terminate the operation.

2. Check if at least one of the specified combinations of PublicKeyCredentialType and cryptographic parameters in credTypesAndPubKeyAlgs is supported. If not, return an error code equivalent to "NotSupportedError" and terminate the operation.

3. For each descriptor of excludeCredentialDescriptorList:

   1. If looking up descriptor.id in this authenticator returns non-null, and the returned item's RP ID and type match rpEntity.id and excludeCredentialDescriptorList.type respectively, then obtain user consent for creating a new credential. The method of obtaining user consent MUST include a test of user presence. If the user

      confirms consent to create a new credential

      return an error code equivalent to "InvalidStateError" and terminate the operation.

      does not consent to create a new credential

      return an error code equivalent to "NotAllowedError" and terminate the operation.

4. If requireResidentKey is true and the authenticator cannot store a client-side-resident public key credential source, return an error code equivalent to "ConstraintError" and terminate the operation.

5. If requireUserVerification is true and the authenticator cannot perform user verification, return an error code equivalent to "ConstraintError" and terminate the operation.

6. Obtain 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. The prompt SHOULD display rpEntity.id, rpEntity.name, userEntity.name and userEntity.displayName, if possible.

   If requireUserVerification is true, the method of obtaining user consent MUST include user verification.

   If requireUserPresence is true, the method of obtaining user consent MUST include a test of user presence.

   If the user does not consent or if user verification fails, return an error code equivalent to "NotAllowedError" and terminate the operation.

7. Once user consent has been obtained, generate a new credential object:

   1. Let (publicKey, privateKey) be a new pair of cryptographic keys using the combination of PublicKeyCredentialType and cryptographic parameters represented by the first item in credTypesAndPubKeyAlgs that is supported by this authenticator.

   2. Let userHandle be userEntity.id.

   3. Let credentialSource be a new public key credential source with the fields:

      type

      public-key.

      privateKey

      privateKey

      rpId

      rpEntity.id

      userHandle

      userHandle

      otherUI

      Any other information the authenticator chooses to include.

   4. If requireResidentKey is true or the authenticator chooses to create a client-side-resident public key credential source:

      1. Let credentialId be a new credential id.

      2. Set credentialSource.id to credentialId.

      3. Let credentials be this authenticator’s credentials map.

      4. Set credentials[(rpEntity.id, userHandle)] to credentialSource.

   5. Otherwise:

      1. Let credentialId be the result of serializing and encrypting credentialSource so that only this authenticator can decrypt it.

8. If any error occurred while creating the new credential object, return an error code equivalent to "UnknownError" and terminate the operation.

9. Let processedExtensions be the result of authenticator extension processing for each supported extension identifier → authenticator extension input in extensions.

10. If the authenticator supports:

    a global signature counter

    Use the global signature counter's actual value when generating authenticator data.

    a per credential signature counter

    allocate the counter, associate it with the new credential, and initialize the counter value as zero.

    no signature counter

    let the signature counter value for the new credential be constant at zero.

11. Let attestedCredentialData be the attested credential data byte array including the credentialId and publicKey.

12. Let authenticatorData be the byte array specified in §6.1 Authenticator Data, including attestedCredentialData as the attestedCredentialData and processedExtensions, if any, as the extensions.

13. Return the attestation object for the new credential created by the procedure specified in §6.5.4 Generating an Attestation Object using an authenticator-chosen attestation statement format, authenticatorData, and hash. For more details on attestation, see §6.5 Attestation.

On successful completion of this operation, the authenticator returns the attestation object to the client.

6.3.3. The authenticatorGetAssertion Operation

It takes the following input parameters:

rpId

The caller’s RP ID, as determined by the user agent and the client.

hash

The hash of the serialized client data, provided by the client.

allowCredentialDescriptorList

An OPTIONAL list of PublicKeyCredentialDescriptors describing credentials acceptable to the Relying Party (possibly filtered by the client), if any.

requireUserPresence

The constant Boolean value true. It is included here as a pseudo-parameter to simplify applying this abstract authenticator model to implementations that may wish to make a test of user presence optional although WebAuthn does not.

requireUserVerification

The effective user verification requirement for assertion, a Boolean value provided by the client.

extensions

A CBOR map from extension identifiers to their authenticator extension inputs, created by the client based on the extensions requested by the Relying Party, if any.

Note: Before performing this operation, all other operations in progress in the authenticator session MUST be aborted by running the authenticatorCancel operation.

When this method is invoked, the authenticator MUST perform the following procedure:

1. Check if all the supplied parameters are syntactically well-formed and of the correct length. If not, return an error code equivalent to "UnknownError" and terminate the operation.

2. Let credentialOptions be a new empty set of public key credential sources.

3. If allowCredentialDescriptorList was supplied, then for each descriptor of allowCredentialDescriptorList:

   1. Let credSource be the result of looking up descriptor.id in this authenticator.

   2. If credSource is not null, append it to credentialOptions.

4. Otherwise (allowCredentialDescriptorList was not supplied), for each key → credSource of this authenticator’s credentials map, append credSource to credentialOptions.

5. Remove any items from credentialOptions whose rpId is not equal to rpId.

6. If credentialOptions is now empty, return an error code equivalent to "NotAllowedError" and terminate the operation.

7. Prompt the user to select a public key credential source selectedCredential from credentialOptions. Obtain user consent for using selectedCredential. 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.

   If requireUserVerification is true, the method of obtaining user consent MUST include user verification.

   Note: For the overall WebAuthn security properties to hold, the user verified here must be the same user that was verified when this authenticator created this public key credential in Step 6 of §6.3.2 The authenticatorMakeCredential Operation.

   If requireUserPresence is true, the method of obtaining user consent MUST include a test of user presence.

   If the user does not consent, return an error code equivalent to "NotAllowedError" and terminate the operation.

8. Let processedExtensions be the result of authenticator extension processing for each supported extension identifier → authenticator extension input in extensions.

9. Increment the credential associated signature counter or the global signature counter value, depending on which approach is implemented by the authenticator, by some positive value. If the authenticator does not implement a signature counter, let the signature counter value remain constant at zero.

10. Let authenticatorData be the byte array specified in §6.1 Authenticator Data including processedExtensions, if any, as the extensions and excluding attestedCredentialData.

11. Let signature be the assertion signature of the concatenation authenticatorData || hash using the privateKey of selectedCredential as shown in Figure 4, below. A simple, undelimited concatenation is safe to use here because the authenticator data describes its own length. The hash of the serialized client data (which potentially has a variable length) is always the last element.

    

12. If any error occurred while generating the assertion signature, return an error code equivalent to "UnknownError" and terminate the operation.

13. Return to the user agent:

    • selectedCredential.id, if either a list of credentials (i.e., allowCredentialDescriptorList) of length 2 or greater was supplied by the client, or no such list was supplied.

      Note: If, within allowCredentialDescriptorList, the client supplied exactly one credential and it was successfully employed, then its credential ID is not returned since the client already knows it. This saves transmitting these bytes over what may be a constrained connection in what is likely a common case.

    • authenticatorData

    • signature

    • selectedCredential.userHandle

      Note: the returned userHandle value may be null, see: userHandleResult.

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

6.3.4. The authenticatorCancel 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.

6.4. String Handling

Authenticators may be required to store arbitrary strings chosen by a Relying Party, for example the name, displayName, and icon members in a PublicKeyCredentialUserEntity. Each will have some accommodation for the potentially limited resources available to an authenticator.

If string value truncation is the chosen accommodation, the authenticator SHOULD take care to truncate on character boundaries, although the authenticator MAY truncate without reference to any character encoding. For example, an authenticator could truncate a name and leave a partial UTF-8 sequence at the end. If the authenticator is using [FIDO-CTAP] then future messages from the authenticator may contain invalid CBOR since the value is typed as a CBOR string and thus is required to be valid UTF-8.

Additionally, a grapheme cluster may be truncated, altering the resulting character in the mind of the user.

User agents are tasked with handling this to avoid burdening authenticators with understanding character encodings and Unicode character properties. Thus, when dealing with authenticators, user agents SHOULD:

1. Ensure that any strings sent to authenticators are validly encoded.

2. Handle the case where strings have been truncated resulting in an invalid encoding. For example, any partial code point at the end may be dropped or replaced with U+FFFD.

6.5. Attestation

Authenticators SHOULD also provide some form of attestation, if possible. If an authenticator does, the basic requirement is that the authenticator can produce, for each credential public key, an attestation statement verifiable by the WebAuthn Relying Party. Typically, this attestation statement contains a signature by an attestation private key over the attested credential public key and a challenge, as well as a certificate or similar data providing provenance information for the attestation public key, enabling the Relying Party to make a trust decision. However, if an attestation key pair is not available, then the authenticator MAY either perform self attestation of the credential public key with the corresponding credential private key, or otherwise perform no attestation. All this information is returned by authenticators any time a new public key credential is generated, in the overall form of an attestation object. The relationship of the attestation object with authenticator data (containing attested credential data) and the attestation statement is illustrated in figure 5, below.

If an authenticator employs self attestation or no attestation, then no provenance information is provided for the Relying Party to base a trust decision on. In these cases, the authenticator provides no guarantees about its operation to the Relying Party.

An important component of the attestation object is the attestation statement. This is a specific type of signed data object, containing statements about a public key credential itself and the authenticator that created it. It contains an attestation signature created using the key of the attesting authority (except for the case of self attestation, when it is created using the credential private key). In order to correctly interpret an attestation statement, a Relying Party needs to understand these two aspects of attestation:

1. The attestation statement format is the manner in which the signature is represented and the various contextual bindings are incorporated into the attestation statement by the authenticator. In other words, this defines the syntax of the statement. Various existing components and OS platforms (such as TPMs and the Android OS) have previously defined attestation statement formats. This specification supports a variety of such formats in an extensible way, as defined in §6.5.2 Attestation Statement Formats. The formats themselves are identified by strings, as described in §8.1 Attestation Statement Format Identifiers.

2. The attestation type defines the semantics of attestation statements and their underlying trust models. Specifically, it defines how a Relying Party establishes trust in a particular attestation statement, after verifying that it is cryptographically valid. This specification supports a number of attestation types, as described in §6.5.3 Attestation Types.

In general, there is no simple mapping between attestation statement formats and attestation types. For example, the "packed" attestation statement format defined in §8.2 Packed Attestation Statement Format can be used in conjunction with all attestation types, while other formats and types have more limited applicability.

The privacy, security and operational characteristics of attestation depend on:

• The attestation type, which determines the trust model,

• The attestation statement format, which MAY constrain the strength of the attestation by limiting what can be expressed in an attestation statement, and

• The characteristics of the individual authenticator, such as its construction, whether part or all of it runs in a secure operating environment, and so on.

It is expected that most authenticators will support a small number of attestation types and attestation statement formats, while Relying Parties will decide what attestation types are acceptable to them by policy. Relying Parties will also need to understand the characteristics of the authenticators that they trust, based on information they have about these authenticators. For example, the FIDO Metadata Service [FIDOMetadataService] provides one way to access such information.

6.5.1. Attested Credential Data

Attested credential data is a variable-length byte array added to the authenticator data when generating an attestation object for a given credential. Its format is shown in Table 3.

6.5.1.1. Examples of credentialPublicKey Values Encoded in COSE_Key Format

This section provides examples of COSE_Key-encoded Elliptic Curve and RSA public keys for the ES256, PS256, and RS256 signature algorithms. These examples adhere to the rules defined above for the credentialPublicKey value, and are presented in [CDDL] for clarity.

[RFC8152] Section 7 defines the general framework for all COSE_Key-encoded keys. Specific key types for specific algorithms are defined in other sections of [RFC8152] as well as in other specifications, as noted below.

Below is an example of a COSE_Key-encoded Elliptic Curve public key in EC2 format (see [RFC8152] Section 13.1), on the P-256 curve, to be used with the ES256 signature algorithm (ECDSA w/ SHA-256, see [RFC8152] Section 8.1):

{
  1:   2,  ; kty: EC2 key type
  3:  -7,  ; alg: ES256 signature algorithm
 -1:   1,  ; crv: P-256 curve
 -2:   x,  ; x-coordinate as byte string 32 bytes in length
           ; e.g., in hex: 65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c08551d
 -3:   y   ; y-coordinate as byte string 32 bytes in length
           ; e.g., in hex: 1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd0084d19c
}

Below is the above Elliptic Curve public key encoded in the CTAP2 canonical CBOR encoding form, whitespace and line breaks are included here for clarity and to match the [CDDL] presentation above:

A5
   01  02

   03  26

   20  01

   21  58 20   65eda5a12577c2bae829437fe338701a10aaa375e1bb5b5de108de439c08551d

   22  58 20   1e52ed75701163f7f9e40ddf9f341b3dc9ba860af7e0ca7ca7e9eecd0084d19c

Below is an example of a COSE_Key-encoded 2048-bit RSA public key (see [RFC8230] Section 4), to be used with the PS256 signature algorithm (RSASSA-PSS with SHA-256, see [RFC8230] Section 2):

{
  1:   3,  ; kty: RSA key type
  3: -37,  ; alg: PS256
 -1:   n,  ; n:   RSA modulus n byte string 256 bytes in length
           ;      e.g., in hex (middle bytes elided for brevity): DB5F651550...6DC6548ACC3
 -2:   e   ; e:   RSA public exponent e byte string 3 bytes in length
           ;      e.g., in hex: 010001
}

Below is an example of the same COSE_Key-encoded RSA public key as above, to be used with the RS256 signature algorithm (RSASSA-PKCS1-v1_5 with SHA-256, see §11.3 COSE Algorithm Registrations):

{
  1:   3,  ; kty: RSA key type
  3:-257,  ; alg: RS256
 -1:   n,  ; n:   RSA modulus n byte string 256 bytes in length
           ;      e.g., in hex (middle bytes elided for brevity): DB5F651550...6DC6548ACC3
 -2:   e   ; e:   RSA public exponent e byte string 3 bytes in length
           ;      e.g., in hex: 010001
}

6.5.2. Attestation Statement Formats

As described above, an attestation statement format is a data format which represents a cryptographic signature by an authenticator over a set of contextual bindings. Each attestation statement format MUST be defined using the following template:

• Attestation statement format identifier:

• Supported attestation types:

• Syntax: The syntax of an attestation statement produced in this format, defined using [CDDL] for the extension point $attStmtFormat defined in §6.5.4 Generating an Attestation Object.

• Signing procedure: The signing procedure for computing an attestation statement in this format given the public key credential to be attested, the authenticator data structure containing the authenticator data for the attestation, and the hash of the serialized client data.

• Verification procedure: The procedure for verifying an attestation statement, which takes the following verification procedure inputs:

  • attStmt: The attestation statement structure

  • authenticatorData: The authenticator data claimed to have been used for the attestation

  • clientDataHash: The hash of the serialized client data

  The procedure returns either:

  • An error indicating that the attestation is invalid, or

  • An implementation-specific value representing the attestation type, and the trust path. This attestation trust path is either empty (in case of self attestation), an identifier of an ECDAA-Issuer public key (in the case of ECDAA), or a set of X.509 certificates.

The initial list of specified attestation statement formats is in §8 Defined Attestation Statement Formats.

6.5.3. Attestation Types

WebAuthn supports several attestation types, defining the semantics of attestation statements and their underlying trust models:

Note: This specification does not define any data structures explicitly expressing the attestation types employed by authenticators. Relying Parties engaging in attestation statement verification — i.e., when calling navigator.credentials.create() they select an attestation conveyance other than none and verify the received attestation statement — will determine the employed attestation type as a part of verification. See the "Verification procedure" subsections of §8 Defined Attestation Statement Formats. See also §14.4 Attestation Privacy.

Basic Attestation (Basic)

In the case of basic attestation [UAFProtocol], the authenticator’s attestation key pair is specific to an authenticator model. Thus, authenticators of the same model often share the same attestation key pair. See §14.4 Attestation Privacy for further information.

Self Attestation (Self)

In the case of self attestation, also known as surrogate basic attestation [UAFProtocol], the Authenticator does not have any specific attestation key. Instead it uses the credential private key to create the attestation signature. Authenticators without meaningful protection measures for an attestation private key typically use this attestation type.

Attestation CA (AttCA)

In this case, an authenticator is based on a Trusted Platform Module (TPM) and holds an authenticator-specific "endorsement key" (EK). This key is used to securely communicate with a trusted third party, the Attestation CA [TCG-CMCProfile-AIKCertEnroll] (formerly known as a "Privacy CA"). The authenticator can generate multiple attestation identity key pairs (AIK) and requests an Attestation CA to issue an AIK certificate for each. Using this approach, such an authenticator can limit the exposure of the EK (which is a global correlation handle) to Attestation CA(s). AIKs can be requested for each authenticator-generated public key credential individually, and conveyed to Relying Parties as attestation certificates.

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

Note: Attestation statements conveying attestations of type AttCA use the same data structure as attestation statements conveying attestations of type Basic, so the two attestation types are, in general, distinguishable only with externally provided knowledge regarding the contents of the attestation certificates conveyed in the attestation statement.

Elliptic Curve based Direct Anonymous Attestation (ECDAA)

In this case, the Authenticator receives direct anonymous attestation (DAA) credentials from a single DAA-Issuer. These DAA credentials are used along with blinding to sign the attested credential data. 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. Consequently we denote the DAA-Issuer as ECDAA-Issuer (see [FIDOEcdaaAlgorithm]).

No attestation statement (None)

In this case, no attestation information is available. See also §8.7 None Attestation Statement Format.

6.5.4. Generating an Attestation Object

To generate an attestation object (see: Figure 5) given:

attestationFormat

An attestation statement format.

authData

A byte array containing authenticator data.

hash

The hash of the serialized client data.

the authenticator MUST:

1. Let attStmt be the result of running attestationFormat’s signing procedure given authData and hash.

2. Let fmt be attestationFormat’s attestation statement format identifier

3. Return the attestation object as a CBOR map with the following syntax, filled in with variables initialized by this algorithm:

       attObj = {
                   authData: bytes,
                   $$attStmtType
                }
   
       attStmtTemplate = (
                             fmt: text,
                             attStmt: { * tstr => any } ; Map is filled in by each concrete attStmtType
                         )
   
       ; Every attestation statement format must have the above fields
       attStmtTemplate .within $$attStmtType
   

6.5.5. Signature Formats for Packed Attestation, FIDO U2F Attestation, and Assertion Signatures

• For COSEAlgorithmIdentifier -7 (ES256), and other ECDSA-based algorithms, a signature value is encoded as an ASN.1 DER Ecdsa-Sig-Value, as defined in [RFC3279] section 2.2.3.

          Example:
          30 44                                ; SEQUENCE (68 Bytes)
              02 20                            ; INTEGER (32 Bytes)
              |  3d 46 28 7b 8c 6e 8c 8c  26 1c 1b 88 f2 73 b0 9a
              |  32 a6 cf 28 09 fd 6e 30  d5 a7 9f 26 37 00 8f 54
              02 20                            ; INTEGER (32 Bytes)
              |  4e 72 23 6e a3 90 a9 a1  7b cf 5f 7a 09 d6 3a b2
              |  17 6c 92 bb 8e 36 c0 41  98 a2 7b 90 9b 6e 8f 13
  

  Note: As CTAP1/U2F authenticators are already producing signatures values in this format, CTAP2 authenticators will also produce signatures values in the same format, for consistency reasons. It is recommended that any new attestation formats defined not use ASN.1 encodings, but instead represent signatures as equivalent fixed-length byte arrays without internal structure, using the same representations as used by COSE signatures as defined in [RFC8152] and [RFC8230].

• For COSEAlgorithmIdentifier -257 (RS256), sig contains the signature generated using the RSASSA-PKCS1-v1_5 signature scheme defined in section 8.2.1 in [RFC8017] with SHA-256 as the hash function. The signature is not ASN.1 wrapped.

• For COSEAlgorithmIdentifier -37 (PS256), sig contains the signature generated using the RSASSA-PSS signature scheme defined in section 8.1.1 in [RFC8017] with SHA-256 as the hash function. The signature is not ASN.1 wrapped.

7. WebAuthn Relying Party Operations

A registration or authentication ceremony begins with the WebAuthn Relying Party creating a PublicKeyCredentialCreationOptions or PublicKeyCredentialRequestOptions object, respectively, which encodes the parameters for the ceremony. The Relying Party SHOULD take care to not leak sensitive information during this stage; see §14.10 Username Enumeration for details.

Upon successful execution of create() or get(), the Relying Party's script receives a PublicKeyCredential containing an AuthenticatorAttestationResponse or AuthenticatorAssertionResponse structure, respectively, from the client. It must then deliver the contents of this structure to the Relying Party server, using methods outside the scope of this specification. This section describes the operations that the Relying Party must perform upon receipt of these structures.

7.1. Registering a New Credential

When registering a new credential, represented by an AuthenticatorAttestationResponse structure response and an AuthenticationExtensionsClientOutputs structure clientExtensionResults, as part of a registration ceremony, a Relying Party MUST proceed as follows:

1. Let JSONtext be the result of running UTF-8 decode on the value of response.clientDataJSON.

   Note: Using any implementation of UTF-8 decode is acceptable as long as it yields the same result as that yielded by the UTF-8 decode algorithm. In particular, any leading byte order mark (BOM) MUST be stripped.

2. Let C, the client data claimed as collected during the credential creation, be the result of running an implementation-specific JSON parser on JSONtext.

   Note: C may be any implementation-specific data structure representation, as long as C’s components are referenceable, as required by this algorithm.

3. Verify that the value of C.type is webauthn.create.

4. Verify that the value of C.challenge matches the challenge that was sent to the authenticator in the create() call.

5. Verify that the value of C.origin matches the Relying Party's origin.

6. Verify that the value of C.tokenBinding.status matches the state of Token Binding for the TLS connection over which the assertion was obtained. If Token Binding was used on that TLS connection, also verify that C.tokenBinding.id matches the base64url encoding of the Token Binding ID for the connection.

7. Compute the hash of response.clientDataJSON using SHA-256.

8. Perform CBOR decoding on the attestationObject field of the AuthenticatorAttestationResponse structure to obtain the attestation statement format fmt, the authenticator data authData, and the attestation statement attStmt.

9. Verify that the rpIdHash in authData is the SHA-256 hash of the RP ID expected by the Relying Party.

10. Verify that the User Present bit of the flags in authData is set.

11. If user verification is required for this registration, verify that the User Verified bit of the flags in authData is set.

12. Verify that the values of the client extension outputs in clientExtensionResults and the authenticator extension outputs in the extensions in authData are as expected, considering the client extension input values that were given as the extensions option in the create() call. In particular, any extension identifier values in the clientExtensionResults and the extensions in authData MUST be also be present as extension identifier values in the extensions member of options, i.e., no extensions are present that were not requested. In the general case, the meaning of "are as expected" is specific to the Relying Party and which extensions are in use.

    Note: Since all extensions are OPTIONAL for both the client and the authenticator, the Relying Party MUST be prepared to handle cases where none or not all of the requested extensions were acted upon.

13. Determine the attestation statement format by performing a USASCII case-sensitive match on fmt against the set of supported WebAuthn Attestation Statement Format Identifier values. An up-to-date list of registered WebAuthn Attestation Statement Format Identifier values is maintained in the IANA registry of the same name [WebAuthn-Registries].

14. Verify that attStmt is a correct attestation statement, conveying a valid attestation signature, by using the attestation statement format fmt’s verification procedure given attStmt, authData and the hash of the serialized client data computed in step 7.

    Note: Each attestation statement format specifies its own verification procedure. See §8 Defined Attestation Statement Formats for the initially-defined formats, and [WebAuthn-Registries] for the up-to-date list.

15. If validation is successful, obtain a list of acceptable trust anchors (attestation root certificates or ECDAA-Issuer public keys) for that attestation type and attestation statement format fmt, from a trusted source or from policy. For example, the FIDO Metadata Service [FIDOMetadataService] provides one way to obtain such information, using the aaguid in the attestedCredentialData in authData.

16. Assess the attestation trustworthiness using the outputs of the verification procedure in step 14, as follows:

    • If no attestation was provided, verify that None attestation is acceptable under Relying Party policy.

    • If self attestation was used, verify that self attestation is acceptable under Relying Party policy.

    • If ECDAA was used, verify that the identifier of the ECDAA-Issuer public key, returned as the attestation trust path from the verification procedure, is included in the set of acceptable trust anchors obtained in step 15.

    • Otherwise, use the X.509 certificates returned as the attestation trust path from the verification procedure to verify that the attestation public key correctly chains up to an acceptable root certificate.

17. Check that the credentialId is not yet registered to any other user. If registration is requested for a credential that is already registered to a different user, the Relying Party SHOULD fail this registration ceremony, or it MAY decide to accept the registration, e.g. while deleting the older registration.

18. If the attestation statement attStmt verified successfully and is found to be trustworthy, then register the new credential with the account that was denoted in the options.user passed to create(), by associating it with the credentialId and credentialPublicKey in the attestedCredentialData in authData, as appropriate for the Relying Party's system.

19. If the attestation statement attStmt successfully verified but is not trustworthy per step 16 above, the Relying Party SHOULD fail the registration ceremony.

    NOTE: However, if permitted by policy, the Relying Party MAY register the credential ID and credential public key but treat the credential as one with self attestation (see §6.5.3 Attestation Types). If doing so, the Relying Party is asserting there is no cryptographic proof that the public key credential has been generated by a particular authenticator model. See [FIDOSecRef] and [UAFProtocol] for a more detailed discussion.

Verification of attestation objects requires that the Relying Party has a trusted method of determining acceptable trust anchors in step 15 above. Also, if certificates are being used, the 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 did not provide this chain in the attestation information.

7.2. Verifying an Authentication Assertion

When verifying a given PublicKeyCredential structure (credential) and an AuthenticationExtensionsClientOutputs structure clientExtensionResults, as part of an authentication ceremony, the Relying Party MUST proceed as follows:

1. If the allowCredentials option was given when this authentication ceremony was initiated, verify that credential.id identifies one of the public key credentials that were listed in allowCredentials.

2. Identify the user being authenticated and verify that this user is the owner of the public key credential source credentialSource identified by credential.id:

   If the user was identified before the authentication ceremony was initiated, e.g., via a username or cookie,

   verify that the identified user is the owner of credentialSource. If credential.response.userHandle is present, let userHandle be its value. Verify that userHandle also maps to the same user.

   If the user was not identified before the authentication ceremony was initiated,

   verify that credential.response.userHandle is present, and that the user identified by this value is the owner of credentialSource.

3. Using credential’s id attribute (or the corresponding rawId, if base64url encoding is inappropriate for your use case), look up the corresponding credential public key.

4. Let cData, authData and sig denote the value of credential’s response's clientDataJSON, authenticatorData, and signature respectively.

5. Let JSONtext be the result of running UTF-8 decode on the value of cData.

   Note: Using any implementation of UTF-8 decode is acceptable as long as it yields the same result as that yielded by the UTF-8 decode algorithm. In particular, any leading byte order mark (BOM) MUST be stripped.

6. Let C, the client data claimed as used for the signature, be the result of running an implementation-specific JSON parser on JSONtext.

   Note: C may be any implementation-specific data structure representation, as long as C’s components are referenceable, as required by this algorithm.

7. Verify that the value of C.type is the string webauthn.get.

8. Verify that the value of C.challenge matches the challenge that was sent to the authenticator in the PublicKeyCredentialRequestOptions passed to the get() call.

9. Verify that the value of C.origin matches the Relying Party's origin.

10. Verify that the value of C.tokenBinding.status matches the state of Token Binding for the TLS connection over which the attestation was obtained. If Token Binding was used on that TLS connection, also verify that C.tokenBinding.id matches the base64url encoding of the Token Binding ID for the connection.

11. Verify that the rpIdHash in authData is the SHA-256 hash of the RP ID expected by the Relying Party.

12. Verify that the User Present bit of the flags in authData is set.

13. If user verification is required for this assertion, verify that the User Verified bit of the flags in authData is set.

14. Verify that the values of the client extension outputs in clientExtensionResults and the authenticator extension outputs in the extensions in authData are as expected, considering the client extension input values that were given as the extensions option in the get() call. In particular, any extension identifier values in the clientExtensionResults and the extensions in authData MUST be also be present as extension identifier values in the extensions member of options, i.e., no extensions are present that were not requested. In the general case, the meaning of "are as expected" is specific to the Relying Party and which extensions are in use.

    Note: Since all extensions are OPTIONAL for both the client and the authenticator, the Relying Party MUST be prepared to handle cases where none or not all of the requested extensions were acted upon.

15. Let hash be the result of computing a hash over the cData using SHA-256.

16. Using the credential public key looked up in step 3, verify that sig is a valid signature over the binary concatenation of authData and hash.

    Note: This verification step is compatible with signatures generated by FIDO U2F authenticators. See §6.1.2 FIDO U2F Signature Format Compatibility.

17. If the signature counter value authData.signCount is nonzero or the value stored in conjunction with credential’s id attribute is nonzero, then run the following sub-step:

    • If the signature counter value authData.signCount is

      greater than the signature counter value stored in conjunction with credential’s id attribute.

      Update the stored signature counter value, associated with credential’s id attribute, to be the value of authData.signCount.

      less than or equal to the signature counter value stored in conjunction with credential’s id attribute.

      This is a signal that the authenticator may be cloned, i.e. at least two copies of the credential private key may exist and are being used in parallel. Relying Parties should incorporate this information into their risk scoring. Whether the Relying Party updates the stored signature counter value in this case, or not, or fails the authentication ceremony or not, is Relying Party-specific.

18. If all the above steps are successful, continue with the authentication ceremony as appropriate. Otherwise, fail the authentication ceremony.

8. Defined Attestation Statement Formats

WebAuthn supports pluggable attestation statement formats. This section defines an initial set of such formats.

8.1. Attestation Statement Format Identifiers

Attestation statement formats are identified by a string, called an attestation statement format identifier, chosen by the author of the attestation statement format.

Attestation statement format identifiers SHOULD be registered per [WebAuthn-Registries] "Registries for Web Authentication (WebAuthn)". All registered attestation statement format identifiers are unique amongst themselves as a matter of course.

Unregistered attestation statement format identifiers SHOULD use lowercase reverse domain-name naming, using a domain name registered by the developer, in order to assure uniqueness of the identifier. All attestation statement format identifiers MUST be a maximum of 32 octets in length and MUST consist only of printable USASCII characters, excluding backslash and doublequote, i.e., VCHAR as defined in [RFC5234] but without %x22 and %x5c.

Note: This means attestation statement format identifiers based on domain names MUST incorporate only LDH Labels [RFC5890].

Implementations MUST match WebAuthn attestation statement format identifiers in a case-sensitive fashion.

Attestation statement formats that may exist in multiple versions SHOULD include a version in their identifier. In effect, different versions are thus treated as different formats, e.g., packed2 as a new version of the §8.2 Packed Attestation Statement Format.

The following sections present a set of currently-defined and registered attestation statement formats and their identifiers. The up-to-date list of registered WebAuthn Extensions is maintained in the IANA "WebAuthn Attestation Statement Format Identifier" registry established by [WebAuthn-Registries].

8.2. Packed Attestation Statement Format

This is a WebAuthn optimized attestation statement format. It uses a very compact but still extensible encoding method. It is implementable by authenticators with limited resources (e.g., secure elements).

Attestation statement format identifier

packed

Attestation types supported

Basic, Self, AttCA, ECDAA

Syntax

The syntax of a Packed Attestation statement is defined by the following CDDL:

    $$attStmtType //= (
                          fmt: "packed",
                          attStmt: packedStmtFormat
                      )

    packedStmtFormat = {
                           alg: COSEAlgorithmIdentifier,
                           sig: bytes,
                           x5c: [ attestnCert: bytes, * (caCert: bytes) ]
                       } //
                       {
                           alg: COSEAlgorithmIdentifier, (-260 for ED256 / -261 for ED512)
                           sig: bytes,
                           ecdaaKeyId: bytes
                       } //
                       {
                           alg: COSEAlgorithmIdentifier
                           sig: bytes,
                       }

The semantics of the fields are as follows:

alg

A COSEAlgorithmIdentifier containing the identifier of the algorithm used to generate the attestation signature.

sig

A byte string containing the attestation signature.

x5c

The elements of this array contain attestnCert and its certificate chain, each encoded in X.509 format. The attestation certificate attestnCert MUST be the first element in the array.

attestnCert

The attestation certificate, encoded in X.509 format.

ecdaaKeyId

The identifier of the ECDAA-Issuer public key. This is the BigNumberToB encoding of the component "c" of the ECDAA-Issuer public key as defined section 3.3, step 3.5 in [FIDOEcdaaAlgorithm].

Signing procedure

The signing procedure for this attestation statement format is similar to the procedure for generating assertion signatures.

1. Let authenticatorData denote the authenticator data for the attestation, and let clientDataHash denote the hash of the serialized client data.

2. If Basic or AttCA attestation is in use, the authenticator produces the sig by concatenating authenticatorData and clientDataHash, and signing the result using an attestation private key selected through an authenticator-specific mechanism. It sets x5c to the certificate chain of the attestation public key and alg to the algorithm of the attestation private key.

3. If ECDAA is in use, the authenticator produces sig by concatenating authenticatorData and clientDataHash, and signing the result using ECDAA-Sign (see section 3.5 of [FIDOEcdaaAlgorithm]) after selecting an ECDAA-Issuer public key related to the ECDAA signature private key through an authenticator-specific mechanism (see [FIDOEcdaaAlgorithm]). It sets alg to the algorithm of the selected ECDAA-Issuer public key and ecdaaKeyId to the identifier of the ECDAA-Issuer public key (see above).

4. If self attestation is in use, the authenticator produces sig by concatenating authenticatorData and clientDataHash, and signing the result using the credential private key. It sets alg to the algorithm of the credential private key and omits the other fields.

Verification procedure

Given the verification procedure inputs attStmt, authenticatorData and clientDataHash, the verification procedure is as follows:

1. Verify that attStmt is valid CBOR conforming to the syntax defined above and perform CBOR decoding on it to extract the contained fields.

2. If x5c is present, this indicates that the attestation type is not ECDAA. In this case:

   • Verify that sig is a valid signature over the concatenation of authenticatorData and clientDataHash using the attestation public key in attestnCert with the algorithm specified in alg.

   • Verify that attestnCert meets the requirements in §8.2.1 Packed Attestation Statement Certificate Requirements.

   • If attestnCert 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 the aaguid in authenticatorData.

   • Optionally, inspect x5c and consult externally provided knowledge to determine whether attStmt conveys a Basic or AttCA attestation.

   • If successful, return implementation-specific values representing attestation type Basic, AttCA or uncertainty, and attestation trust path x5c.

3. If ecdaaKeyId is present, then the attestation type is ECDAA. In this case:

   • Verify that sig is a valid signature over the concatenation of authenticatorData and clientDataHash using ECDAA-Verify with ECDAA-Issuer public key identified by ecdaaKeyId (see [FIDOEcdaaAlgorithm]).

   • If successful, return implementation-specific values representing attestation type ECDAA and attestation trust path ecdaaKeyId.

4. If neither x5c nor ecdaaKeyId is present, self attestation is in use.

   • Validate that alg matches the algorithm of the credentialPublicKey in authenticatorData.

   • Verify that sig is a valid signature over the concatenation of authenticatorData and clientDataHash using the credential public key with alg.

   • If successful, return implementation-specific values representing attestation type Self and an empty attestation trust path.

8.2.1. Packed Attestation Statement Certificate Requirements

The attestation certificate MUST have the following fields/extensions:

• Version MUST be set to 3 (which is indicated by an ASN.1 INTEGER with value 2).

• Subject field MUST be set to:

  Subject-C

  ISO 3166 code specifying the country where the Authenticator vendor is incorporated (PrintableString)

  Subject-O

  Legal name of the Authenticator vendor (UTF8String)

  Subject-OU

  Literal string “Authenticator Attestation” (UTF8String)

  Subject-CN

  A UTF8String of the vendor’s choosing

• 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 a 16-byte OCTET STRING. The extension MUST NOT be marked as critical.

  Note that an X.509 Extension encodes the DER-encoding of the value in an OCTET STRING. Thus, the AAGUID MUST be wrapped in two OCTET STRINGS to be valid. Here is a sample, encoded Extension structure:

  30 21                                     -- SEQUENCE
    06 0b 2b 06 01 04 01 82 e5 1c 01 01 04  -- 1.3.6.1.4.1.45724.1.1.4
    04 12                                   -- OCTET STRING
      04 10                                 -- OCTET STRING
        cd 8c 39 5c 26 ed ee de             -- AAGUID
        65 3b 00 79 7d 03 ca 3c
  

• 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 many attestation certificates is available through authenticator metadata services. See, for example, the FIDO Metadata Service [FIDOMetadataService].

8.3. TPM Attestation Statement Format

This attestation statement format is generally used by authenticators that use a Trusted Platform Module as their cryptographic engine.

Attestation statement format identifier

tpm

Attestation types supported

AttCA, ECDAA

Syntax

The syntax of a TPM Attestation statement is as follows:

    $$attStmtType // = (
                           fmt: "tpm",
                           attStmt: tpmStmtFormat
                       )

    tpmStmtFormat = {
                        ver: "2.0",
                        (
                            alg: COSEAlgorithmIdentifier,
                            x5c: [ aikCert: bytes, * (caCert: bytes) ]
                        ) //
                        (
                            alg:  COSEAlgorithmIdentifier, (-260 for ED256 / -261 for ED512)
                            ecdaaKeyId: bytes
                        ),
                        sig: bytes,
                        certInfo: bytes,
                        pubArea: bytes
                    }

The semantics of the above fields are as follows:

ver

The version of the TPM specification to which the signature conforms.

alg

A COSEAlgorithmIdentifier containing the identifier of the algorithm used to generate the attestation signature.

x5c

aikCert followed by its certificate chain, in X.509 encoding.

aikCert

The AIK certificate used for the attestation, in X.509 encoding.

ecdaaKeyId

The identifier of the ECDAA-Issuer public key. This is the BigNumberToB encoding of the component "c" as defined section 3.3, step 3.5 in [FIDOEcdaaAlgorithm].

sig

The attestation signature, in the form of a TPMT_SIGNATURE structure as specified in [TPMv2-Part2] section 11.3.4.

certInfo

The TPMS_ATTEST structure over which the above signature was computed, as specified in [TPMv2-Part2] section 10.12.8.

pubArea

The TPMT_PUBLIC structure (see [TPMv2-Part2] section 12.2.4) used by the TPM to represent the credential public key.

Signing procedure

Let authenticatorData denote the authenticator data for the attestation, and let clientDataHash denote the hash of the serialized client data.

Concatenate authenticatorData and clientDataHash to form attToBeSigned.

Generate a signature using the procedure specified in [TPMv2-Part3] Section 18.2, using the attestation private key and setting the extraData parameter to the digest of attToBeSigned using the hash algorithm corresponding to the "alg" signature algorithm. (For the "RS256" algorithm, this would be a SHA-256 digest.)

Set the pubArea field to the public area of the credential public key, the certInfo field to the output parameter of the same name, and the sig field to the signature obtained from the above procedure.

Verification procedure

Given the verification procedure inputs attStmt, authenticatorData and clientDataHash, the verification procedure is as follows:

Verify that attStmt is valid CBOR conforming to the syntax defined above and perform CBOR decoding on it to extract the contained fields.

Verify that the public key specified by the parameters and unique fields of pubArea is identical to the credentialPublicKey in the attestedCredentialData in authenticatorData.

Concatenate authenticatorData and clientDataHash to form attToBeSigned.

Validate that certInfo is valid:

• Verify that magic is set to TPM_GENERATED_VALUE.

• Verify that type is set to TPM_ST_ATTEST_CERTIFY.

• Verify that extraData is set to the hash of attToBeSigned using the hash algorithm employed in "alg".

• Verify that attested contains a TPMS_CERTIFY_INFO structure as specified in [TPMv2-Part2] section 10.12.3, whose name field contains a valid Name for pubArea, as computed using the algorithm in the nameAlg field of pubArea using the procedure specified in [TPMv2-Part1] section 16.

• Note that the remaining fields in the "Standard Attestation Structure" [TPMv2-Part1] section 31.2, i.e., qualifiedSigner, clockInfo and firmwareVersion are ignored. These fields MAY be used as an input to risk engines.

If x5c is present, this indicates that the attestation type is not ECDAA. In this case:

• Verify the sig is a valid signature over certInfo using the attestation public key in aikCert with the algorithm specified in alg.

• Verify that aikCert meets the requirements in §8.3.1 TPM Attestation Statement Certificate Requirements.

• If aikCert 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 the aaguid in authenticatorData.

• If successful, return implementation-specific values representing attestation type AttCA and attestation trust path x5c.

If ecdaaKeyId is present, then the attestation type is ECDAA.

• Perform ECDAA-Verify on sig to verify that it is a valid signature over certInfo (see [FIDOEcdaaAlgorithm]).

• If successful, return implementation-specific values representing attestation type ECDAA and attestation trust path ecdaaKeyId.

8.3.1. 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.

• 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 many attestation certificates is available through metadata services. See, for example, the FIDO Metadata Service [FIDOMetadataService].

8.4. Android Key Attestation Statement Format

When the authenticator in question is a platform-provided Authenticator on the Android "N" or later platform, the attestation statement is based on the Android key attestation. In these cases, the attestation statement is produced by a component running in a secure operating environment, but the authenticator data for the attestation is produced outside this environment. The WebAuthn Relying Party is expected to check that the authenticator data claimed to have been used for the attestation is consistent with the fields of the attestation certificate’s extension data.

Attestation statement format identifier

android-key

Attestation types supported

Basic

Syntax

An Android key attestation statement consists simply of the Android attestation statement, which is a series of DER encoded X.509 certificates. See the Android developer documentation. Its syntax is defined as follows:

    $$attStmtType //= (
                          fmt: "android-key",
                          attStmt: androidStmtFormat
                      )

    androidStmtFormat = {
                          alg: COSEAlgorithmIdentifier,
                          sig: bytes,
                          x5c: [ credCert: bytes, * (caCert: bytes) ]
                        }

Signing procedure

Let authenticatorData denote the authenticator data for the attestation, and let clientDataHash denote the hash of the serialized client data.

Request an Android Key Attestation by calling keyStore.getCertificateChain(myKeyUUID) providing clientDataHash as the challenge value (e.g., by using setAttestationChallenge). Set x5c to the returned value.

The authenticator produces sig by concatenating authenticatorData and clientDataHash, and signing the result using the credential private key. It sets alg to the algorithm of the signature format.

Verification procedure

Given the verification procedure inputs attStmt, authenticatorData and clientDataHash, the verification procedure is as follows:

• Verify that attStmt is valid CBOR conforming to the syntax defined above and perform CBOR decoding on it to extract the contained fields.

• Verify that sig is a valid signature over the concatenation of authenticatorData and clientDataHash using the public key in the first certificate in x5c with the algorithm specified in alg.

• Verify that the public key in the first certificate in x5c matches the credentialPublicKey in the attestedCredentialData in authenticatorData.

• Verify that the attestationChallenge field in the attestation certificate extension data is identical to clientDataHash.

• Verify the following using the appropriate authorization list from the attestation certificate extension data:

  • The AuthorizationList.allApplications field is not present on either authorization list (softwareEnforced nor teeEnforced), since PublicKeyCredential MUST be scoped to the RP ID.

  • For the following, use only the teeEnforced authorization list if the RP wants to accept only keys from a trusted execution environment, otherwise use the union of teeEnforced and softwareEnforced.

    • The value in the AuthorizationList.origin field is equal to KM_ORIGIN_GENERATED.

    • The value in the AuthorizationList.purpose field is equal to KM_PURPOSE_SIGN.

• If successful, return implementation-specific values representing attestation type Basic and attestation trust path x5c.

8.4.1. Android Key Attestation Statement Certificate Requirements

Android Key Attestation attestation certificate's android key attestation certificate extension data is identified by the OID "1.3.6.1.4.1.11129.2.1.17".

8.5. Android SafetyNet Attestation Statement Format

When the authenticator is platform-provided by certain Android platforms, the attestation statement may be based on the SafetyNet API. In this case the authenticator data is completely controlled by the caller of the SafetyNet API (typically an application running on the Android platform) and the attestation statement provides some statements about the health of the platform and the identity of the calling application (see SafetyNet Documentation for more details).

Attestation statement format identifier

android-safetynet

Attestation types supported

Basic

Syntax

The syntax of an Android Attestation statement is defined as follows:

    $$attStmtType //= (
                          fmt: "android-safetynet",
                          attStmt: safetynetStmtFormat
                      )

    safetynetStmtFormat = {
                              ver: text,
                              response: bytes
                          }

The semantics of the above fields are as follows:

ver

The version number of Google Play Services responsible for providing the SafetyNet API.

response

The UTF-8 encoded result of the getJwsResult() call of the SafetyNet API. This value is a JWS [RFC7515] object (see SafetyNet online documentation) in Compact Serialization.

Signing procedure

Let authenticatorData denote the authenticator data for the attestation, and let clientDataHash denote the hash of the serialized client data.

Concatenate authenticatorData and clientDataHash, perform SHA-256 hash of the concatenated string, and let the result of the hash form attToBeSigned.

Request a SafetyNet attestation, providing attToBeSigned as the nonce value. Set response to the result, and ver to the version of Google Play Services running in the authenticator.

Verification procedure

Given the verification procedure inputs attStmt, authenticatorData and clientDataHash, the verification procedure is as follows:

• Verify that attStmt is valid CBOR conforming to the syntax defined above and perform CBOR decoding on it to extract the contained fields.

• Verify that response is a valid SafetyNet response of version ver by following the steps indicated by the SafetyNet online documentation. As of this writing, there is only one format of the SafetyNet response and ver is reserved for future use.

• Verify that the nonce attribute in the payload of response is identical to the Base64 encoding of the SHA-256 hash of the concatenation of authenticatorData and clientDataHash.

• Verify that the SafetyNet response actually came from the SafetyNet service by following the steps in the SafetyNet online documentation.

• If successful, return implementation-specific values representing attestation type Basic and attestation trust path x5c.

8.6. FIDO U2F Attestation Statement Format

This attestation statement format is used with FIDO U2F authenticators using the formats defined in [FIDO-U2F-Message-Formats].

Attestation statement format identifier

fido-u2f

Attestation types supported

Basic, AttCA

Syntax

The syntax of a FIDO U2F attestation statement is defined as follows:

    $$attStmtType //= (
                          fmt: "fido-u2f",
                          attStmt: u2fStmtFormat
                      )

    u2fStmtFormat = {
                        x5c: [ attestnCert: bytes ],
                        sig: bytes
                    }

The semantics of the above fields are as follows:

x5c

A single element array containing the attestation certificate in X.509 format.

sig

The attestation signature. The signature was calculated over the (raw) U2F registration response message [FIDO-U2F-Message-Formats] received by the client from the authenticator.

Signing procedure

If the credential public key of the given credential is not of algorithm -7 ("ES256"), stop and return an error. Otherwise, let authenticatorData denote the authenticator data for the attestation, and let clientDataHash denote the hash of the serialized client data. (Since SHA-256 is used to hash the serialized client data, clientDataHash will be 32 bytes long.)

Generate a Registration Response Message as specified in [FIDO-U2F-Message-Formats] Section 4.3, with the application parameter set to the SHA-256 hash of the RP ID that the given credential is scoped to, the challenge parameter set to clientDataHash, and the key handle parameter set to the credential ID of the given credential. Set the raw signature part of this Registration Response Message (i.e., without the user public key, key handle, and attestation certificates) as sig and set the attestation certificates of the attestation public key as x5c.

Verification procedure

Given the verification procedure inputs attStmt, authenticatorData and clientDataHash, the verification procedure is as follows:

1. Verify that attStmt is valid CBOR conforming to the syntax defined above and perform CBOR decoding on it to extract the contained fields.

2. Check that x5c has exactly one element and let attCert be that element. Let certificate public key be the public key conveyed by attCert. If certificate public key is not an Elliptic Curve (EC) public key over the P-256 curve, terminate this algorithm and return an appropriate error.

3. Extract the claimed rpIdHash from authenticatorData, and the claimed credentialId and credentialPublicKey from authenticatorData.attestedCredentialData.

4. Convert the COSE_KEY formatted credentialPublicKey (see Section 7 of [RFC8152]) to Raw ANSI X9.62 public key format (see ALG_KEY_ECC_X962_RAW in Section 3.6.2 Public Key Representation Formats of [FIDO-Registry]).

   • Let x be the value corresponding to the "-2" key (representing x coordinate) in credentialPublicKey, and confirm its size to be of 32 bytes. If size differs or "-2" key is not found, terminate this algorithm and return an appropriate error.

   • Let y be the value corresponding to the "-3" key (representing y coordinate) in credentialPublicKey, and confirm its size to be of 32 bytes. If size differs or "-3" key is not found, terminate this algorithm and return an appropriate error.

   • Let publicKeyU2F be the concatenation 0x04 || x || y.

     Note: This signifies uncompressed ECC key format.

5. Let verificationData be the concatenation of (0x00 || rpIdHash || clientDataHash || credentialId || publicKeyU2F) (see Section 4.3 of [FIDO-U2F-Message-Formats]).

6. Verify the sig using verificationData and certificate public key per [SEC1].

7. Optionally, inspect x5c and consult externally provided knowledge to determine whether attStmt conveys a Basic or AttCA attestation.

8. If successful, return implementation-specific values representing attestation type Basic, AttCA or uncertainty, and attestation trust path x5c.

8.7. None Attestation Statement Format

The none attestation statement format is used to replace any authenticator-provided attestation statement when a WebAuthn Relying Party indicates it does not wish to receive attestation information, see §5.4.7 Attestation Conveyance Preference Enumeration (enum AttestationConveyancePreference).

The authenticator MAY also directly generate attestation statements of this format if the authenticator does not support attestation.

Attestation statement format identifier

none

Attestation types supported

None

Syntax

The syntax of a none attestation statement is defined as follows:

    $$attStmtType //= (
                          fmt: "none",
                          attStmt: emptyMap
                      )

    emptyMap = {}

Signing procedure

Return the fixed attestation statement defined above.

Verification procedure

Return implementation-specific values representing attestation type None and an empty attestation trust path.

9. WebAuthn Extensions

The mechanism for generating public key credentials, as well as requesting and generating Authentication assertions, as defined in §5 Web Authentication API, can be extended to suit particular use cases. Each case is addressed by defining a registration extension and/or an authentication extension.

Every extension is a client extension, meaning that the extension involves communication with and processing by the client. Client extensions define the following steps and data:

• navigator.credentials.create() extension request parameters and response values for registration extensions.

• navigator.credentials.get() extension request parameters and response values for authentication extensions.

• Client extension processing for registration extensions and authentication extensions.

When creating a public key credential or requesting an authentication assertion, a WebAuthn Relying Party can request the use of a set of extensions. These extensions will be invoked during the requested operation if they are supported by the client and/or the WebAuthn Authenticator. The Relying Party sends the client extension input for each extension in the get() call (for authentication extensions) or create() call (for registration extensions) to the client. The client performs client extension processing for each extension that the client platform supports, and augments the client data as specified by each extension, by including the extension identifier and client extension output values.

An extension can also be an authenticator extension, meaning that the extension involves communication with and processing by the authenticator. Authenticator extensions define the following steps and data:

• authenticatorMakeCredential extension request parameters and response values for registration extensions.

• authenticatorGetAssertion extension request parameters and response values for authentication extensions.

• Authenticator extension processing for registration extensions and authentication extensions.

For authenticator extensions, as part of the client extension processing, the client also creates the CBOR authenticator extension input value for each extension (often based on the corresponding client extension input value), and passes them to the authenticator in the create() call (for registration extensions) or the get() call (for authentication extensions). These authenticator extension input values are represented in CBOR and passed as name-value pairs, with the extension identifier as the name, and the corresponding authenticator extension input as the value. The authenticator, in turn, performs additional processing for the extensions that it supports, and returns the CBOR authenticator extension output for each as specified by the extension. Part of the client extension processing for authenticator extensions is to use the authenticator extension output as an input to creating the client extension output.

All WebAuthn Extensions are OPTIONAL for both clients and authenticators. Thus, any extensions requested by a Relying Party MAY be ignored by the client browser or OS and not passed to the authenticator at all, or they MAY be ignored by the authenticator. Ignoring an extension is never considered a failure in WebAuthn API processing, so when Relying Parties include extensions with any API calls, they MUST be prepared to handle cases where some or all of those extensions are ignored.

Clients wishing to support the widest possible range of extensions MAY choose to pass through any extensions that they do not recognize to authenticators, generating the authenticator extension input by simply encoding the client extension input in CBOR. All WebAuthn Extensions MUST be defined in such a way that this implementation choice does not endanger the user’s security or privacy. For instance, if an extension requires client processing, it could be defined in a manner that ensures such a naïve pass-through will produce a semantically invalid authenticator extension input value, resulting in the extension being ignored by the authenticator. Since all extensions are OPTIONAL, this will not cause a functional failure in the API operation. Likewise, clients can choose to produce a client extension output value for an extension that it does not understand by encoding the authenticator extension output value into JSON, provided that the CBOR output uses only types present in JSON.

When clients choose to pass through extensions they do not recognize, the JavaScript values in the client extension inputs are converted to CBOR values in the authenticator extension inputs. When the JavaScript value is an %ArrayBuffer%, it is converted to a CBOR byte array. When the JavaScript value is a non-integer number, it is converted to a 64-bit CBOR floating point number. Otherwise, when the JavaScript type corresponds to a JSON type, the conversion is done using the rules defined in Section 4.2 of [RFC7049] (Converting from JSON to CBOR), but operating on inputs of JavaScript type values rather than inputs of JSON type values. Once these conversions are done, canonicalization of the resulting CBOR MUST be performed using the CTAP2 canonical CBOR encoding form.

Likewise, when clients receive outputs from extensions they have passed through that they do not recognize, the CBOR values in the authenticator extension outputs are converted to JavaScript values in the client extension outputs. When the CBOR value is a byte string, it is converted to a JavaScript %ArrayBuffer% (rather than a base64url-encoded string). Otherwise, when the CBOR type corresponds to a JSON type, the conversion is done using the rules defined in Section 4.1 of [RFC7049] (Converting from CBOR to JSON), but producing outputs of JavaScript type values rather than outputs of JSON type values.