Web Authentication: An API for accessing Scoped Credentials

4.10.6. Cryptographic Algorithm Identifier (type AlgorithmIdentifier)

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

5. WebAuthn Authenticator model

The API defined in this specification implies a specific abstract functional model for an authenticator. This section describes the authenticator model. Client platforms may implement and expose this abstract model in any way desired. For instance, 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. The overall requirement is that the behavior of the client’s Web Authentication API implementation, when operating on the authenticators supported by that platform, MUST be indistinguishable from the behavior specified in §4 Web Authentication API.

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

Each authenticator has an AAGUID, which is a 128-bit identifier that indicates 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 probability 1-2^-128 or greater) from the AAGUIDs of all other types of authenticators. The RP 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.

5.1. Authenticator operations

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

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

5.1.1. The authenticatorMakeCredential operation

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

• The SHA-256 hash of the caller’s RP ID, as determined by the user agent and the client.

• The clientDataHash, which is the hash of the serialized ClientData and is provided by the client.

• The Account information provided by the Relying Party.

• The ScopedCredentialType and cryptographic parameters requested by the Relying Party, with the cryptographic algorithms normalized as per the procedure in Web Cryptography API §algorithm-normalization-normalize-an-algorithm.

• A list of ScopedCredential 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.

• Extension data created by the client based on the extensions requested by the Relying Party.

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

• 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.

• Check if at least one of the specified combinations of ScopedCredentialType and cryptographic parameters is supported. If not, return an error code equivalent to NotSupportedError and terminate the operation.

• Check if a credential matching any of the supplied ScopedCredential identifiers is present on this authenticator. If so, return an error code equivalent to NotAllowedError and terminate the operation.

• Prompt the user for consent to create 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. If the user denies consent, return an error code equivalent to NotAllowedError and terminate the operation.

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

  • Generate a set of cryptographic keys using the most preferred combination of ScopedCredentialType and cryptographic parameters supported by this authenticator.

  • Generate an identifier for this credential, such that this identifier is globally unique with high probability across all credentials with the same type across all authenticators.

  • Associate the credential with the specified RP ID hash and the user’s account identifier id.

  • Delete any older credentials with the same RP ID hash and id that are stored locally in the authenticator.

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

• Process all the supported extensions requested by the client, and generate an attestation statement. If no authority key is available to sign such an attestation statement, then the authenticator performs self attestation of the credential with its own private key. For more details on attestation, see §5.3 Credential Attestation Statements.

On successful completion of this operation, the authenticator must return the following to the client:

• The type and unique identifier of the new credential.

• The public key associated with the new credential.

• The fields of the attestation structure WebAuthnAttestation, including information about the attestation format used.

5.1.2. The authenticatorGetAssertion operation

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

• The SHA-256 hash of the caller’s RP ID, as determined by the user agent and the client.

• The clientDataHash, which is the hash of the serialized ClientData and is provided by the client.

• A list of credentials acceptable to the Relying Party (possibly filtered by the client).

• Extension data created by the client based on the extensions requested by the Relying Party.

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

• 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.

• If a list of credentials was supplied by the client, filter it by removing those credentials that are not present on this authenticator. If no list was supplied, create a list with all credentials stored for the caller’s RP ID (as determined by an exact match of the RP ID hash).

• If the previous step resulted in an empty list, return an error code equivalent to NotAllowedError and terminate the operation.

• Prompt the user to select a credential from among the above list. Obtain user consent for using this credential. The prompt for obtaining this consent may be shown by the authenticator if it has its own output capability, or by the user agent otherwise.

• Process all the supported extensions requested by the client, then generate a cryptographic signature using the private key of the selected credential (as specified in §5.2 Signature Format), and use it to construct an assertion.

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

On successful completion, the authenticator must return to the user agent:

• The identifier of the credential used to generate the signature.

• The authenticatorData used to generate the signature.

• The signature itself.

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.

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

5.1.3. 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.

5.2. Signature Format

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

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

1. The Relying Party (RP), which uses the WebAuthn services. The RP consists of a server component and a web-application running in a browser.

2. The WebAuthn Client platform, which consists of the User Agent and the OS and device on which it executes.

3. The Authenticator itself, which provides key management and cryptographic signatures. This may be embedded in the WebAuthn client, or housed in a separate device entirely. In the latter case, the interface between the WebAuthn client and the authenticator is a separately-defined protocol. The authenticator may itself 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.

This specification defines the common signature format shared by all the above layers. This includes how the different contextual bindings are encoded, signed over, and delivered to the RP.

The goals of this design can be summarized as follows.

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

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

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

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

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

5.2.1. Authenticator data

The authenticator data encodes contextual bindings made by the authenticator itself. These bindings are controlled by the authenticator itself, and derive their trust from the Relying Party’s assessment of the security 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 ClientData. 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 components.

The encoding of authenticator data is a byte array of 37 bytes or more, as follows.

Length (in bytes)	Description
32	SHA-256 hash of the RP ID associated with the credential.
1	Flags (bit 0 is the least significant bit): Bit 0: Test of User Presence (TUP) result. Bits 1-5: Reserved for future use (RFU). Bit 6: Attestation data included (AT). Indicates whether the authenticator added attestation data. Bit 7: Extension data included (ED). Indicates if the authenticator data has extensions.
4	Signature counter (signCount), 32-bit unsigned big-endian integer.
variable (if present)	Attestation data (if present). See §5.3.3 Generating an Attestation Statement for details. Its length n depends on the length of the public key and credential ID of the credential being attested.
variable (if present)	Extension-defined authenticator data. This is a CBOR [RFC7049] map with extension identifiers as keys, and extension authenticator data values as values. See §7 WebAuthn Extensions for details.

The RP ID hash 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 making sure that the RP ID hash associated with the requested credential exactly matches the RP ID hash supplied by the client. These differences also explain why the RP ID hash is always a SHA-256 hash instead of being crypto-agile like the clientDataHash; for a given RP ID, we need the hash to be computed the same way by all clients for all operations so that authenticators can roam among clients without losing interoperability.

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

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

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

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

Note that the authenticatorData describes its own length: If the AT and ED flags are not set, it is always 37 bytes long. The attestation data (which is only present if the AT flag is set) describes its own length. If the ED flag is set, then the total length is 37 bytes plus the length of the attestation data, plus the length of the CBOR map that follows.

5.2.2. Generating a signature

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

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

The authenticator MUST return both the authenticatorData and the raw signature back to the client. The client, in turn, MUST return clientDataJSON, authenticatorData and the signature to the RP. The first two are returned in the clientData and authenticatorData members respectively of the WebAuthnAssertion and WebAuthnAttestation structures.

5.3. Credential Attestation Statements

An attestation statement is a specific type of signed data object, containing statements about a credential itself and the authenticator that created it. It is created using the process described in §5.2 Signature Format, with the important difference that the signature is generated not using the private key associated with the credential but using the key of the attesting authority. In order to correctly interpret an attestation statement, a Relying Party needs to understand two aspects of the attestation:

1. The attestation 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 devices and platforms (such as TPMs and the Android OS) have previously defined attestation formats. This specification supports a variety of such formats in an extensible way, as defined in §5.3.1 Attestation Formats.

2. The attestation type defines the semantics of the attestation statement and its underlying trust model. It defines how a Relying Party establishes trust in a particular attestation statement, after verifying that it is cryptographically valid.

In general, there is no simple mapping between attestation formats and attestation types. For example the "packed" attestation format defined in §6.2 Packed Attestation 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 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 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.

5.3.1. Attestation Formats

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

• The name of the format, used to identify it in a WebAuthnAttestation structure. This MUST be an ASCII string, and MUST NOT be an ASCII case-insensitive match for the name of any other attestation format.

• The set of attestation types supported by the format.

• The syntax of an attestation statement produced in this format.

• The procedure for computing an attestation statement in this format given the attToBeSigned for the attestation, created as per §5.3.3 Generating an Attestation Statement.

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

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

  • The clientDataHash of the client’s contextual bindings

  • A trust anchor (a root certificate, a DAA root key or the public key of the credential itself)

  and returns a Boolean value indicating whether the attestation is cryptographically valid, and if so the attestation type.

The initial list of supported formats is in §6 Defined Attestation Formats.

5.3.2. Attestation Types

WebAuthn supports multiple attestation types:

Basic Attestation

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

Self Attestation

In the case of self attestation, also known as surrogate basic attestation [UAFProtocol], the Authenticator doesn’t have any specific attestation key. Instead it uses the authentication key itself to sign the attestation statement. Authenticators without meaningful protection measures for an attestation private key typically use this attestation type.

Privacy CA

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

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

Direct Anonymous Attestation (DAA)

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

5.3.3. Generating an Attestation Statement

This section specifies the algorithm for generating an attestation statement, independent of attestation format.

When requested to generate an attestation statement for a given credential using a particular attestation format, the authenticator MUST first generate an authenticatorData structure, with the attestation data field populated as follows:

Length (in bytes)	Description
16	The AAGUID of the authenticator.
2	Byte length l of Credential ID
(length)	Credential ID (l bytes)
2	Public key algorithm and encoding (16-bit big-endian value). Allowed values are: 0x0100. This is raw ANSI X9.62 formatted Elliptic Curve public key [SEC1], i.e., [0x04, X (n bytes), Y (n bytes)], where the byte 0x04 denotes the uncompressed point compression method and n denotes the key length in bytes. 0x0102. Raw encoded RSA PKCS1 or RSASSA-PSS public key [RFC3447]. In the case of RSASSA-PSS, the default parameters according to [RFC4055] MUST be assumed, i.e., Mask Generation Algorithm MGF1 with SHA256 Salt Length of 32 bytes, i.e., the length of a SHA256 hash value. Trailer Field value of 1, which represents the trailer field with hexadecimal value 0xBC. That is, [modulus (256 bytes), e (m-n bytes)], where m is the total length of the field. This total length should be taken from the object containing this key
2	Byte length m of following public key bytes (16 bit value with most significant byte first).
(length)	The public key (m bytes) according to the encoding denoted before.

The authenticator MUST then concatenate this authenticatorData and the client-supplied clientDataHash as specified in §5.2.2 Generating a signature to form attToBeSigned. It must then run the signing procedure for the desired attestation format, with attToBeSigned as input.

5.3.4. Verifying an Attestation Statement

This section specifies the algorithm for verifying an attestation statement, independent of attestation format.

Upon receiving an attestation statement in the form of a WebAuthnAttestation structure, the Relying Party shall:

1. Perform JSON decoding to extract the ClientData used for the attestation from the clientData.

2. Verify that the challenge in the ClientData matches the challenge that was sent to the authenticator.

3. Verify that the origin in the ClientData matches the Relying Party’s origin.

4. Verify that the tokenBinding in the ClientData matches the token binding public key for the TLS connection over which the attestation was obtained.

5. Verify that the extensions in the ClientData is a proper subset of the extensions requested by the RP.

6. Verify that the RP ID hash in the authenticatorData is indeed the SHA-256 hash of the RP ID expected by the RP.

7. Perform an ASCII case-insensitive match on format to determine the attestation format.

8. Look up the attestation root certificate or DAA root key from a trusted source. For example, the FIDO Metadata Service [FIDOMetadataService] provides one way to access such information. The AAGUID in the authenticatorData can be used for this lookup.

9. Using the verification process for the above attestation format, validate that the attestation attestation is valid for the given authenticatorData, clientData and the above trust anchor.

The Relying Party MAY take any of the below actions when verification of an attestation statement fails, according to its policy:

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

• Accept the request associated with the attestation statement but treat the attested Scoped Credential as one with self attestation (see §5.3.2 Attestation Types). If doing so, the Relying Party is asserting there is no cryptographic proof that the Scoped Credential has been generated by a particular Authenticator model. See [FIDOSecRef] and [UAFProtocol] for a more detailed discussion.

Verification of attestation statements requires that the Relying Party has a trusted method of determining the trust anchor in Step 8 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 didn’t provide this chain in the attestation information.

5.3.5. Security Considerations

5.3.5.1. Privacy

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

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

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

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

5.3.5.2. Attestation Certificate and Attestation Certificate CA Compromise

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

A WebAuthn Authenticator attestation certificate must be revoked by the issuing CA if its key has been compromised. A WebAuthn Authenticator manufacturer may need to ship a firmware update and inject new attestation keys and certificates into already manufactured WebAuthn Authenticators, if the exposure was due to a firmware flaw. (The process by which this happens is out of scope for this specification.) If the WebAuthn Authenticator manufacturer does not have this capability, then it may not be possible for Relying Parties to trust any further valid attestation statements from the affected WebAuthn Authenticators.

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

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

5.3.5.3. Attestation Certificate Hierarchy

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

If the attestation root certificate is not dedicated to a single WebAuthn Authenticator device line (i.e., AAGUID), the AAGUID should be specified in the attestation certificate itself, so that it can be verified against the authenticatorData.

6. Defined Attestation Formats

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

6.1. Attestation Format Identifiers

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

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

Unregistered attestation format identifiers SHOULD use reverse domain-name naming, using a domain name registered by the attestation type developer, in order to assure uniqueness of the identifier. All attestation format identifiers MUST be a maximum of 32 octets in length and MUST consist only of printable USASCII characters, i.e., VCHAR as defined in [RFC5234] (note: this means attestation format identifiers based on domain names MUST incorporate only LDH Labels [RFC5890]). Implementations MUST match WebAuthn attestation format identifiers in a case-insensitive fashion.

Attestation formats that may exist in multiple versions SHOULD include a version in their identifier. In effect, different versions are thus treated as different extensions, e.g., packed2 as a new version of the packed attestation format.

The following sections present a set of currently-defined and registered attestation formats and their identifiers. See the WebAuthn Attestation Format Identifier Registry defined in [WebAuthn-Registries] for an up-to-date list of registered WebAuthn Attestation Formats.

6.2. Packed Attestation Format

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

Attestation format identifier

packed

Attestation types supported

All

Syntax

A Packed Attestation statement has the following format:

interface PackedAttestation {
    readonly    attribute ArrayBuffer   x5c;
    readonly    attribute ArrayBuffer   daaKey;
    readonly    attribute DOMString     alg;
    readonly    attribute ArrayBuffer   signature;
};

The x5c attribute contains the attestation certificate and its certificate chain as described in [RFC7515] section 4.1.6.

The alg element contains the name of the algorithm used to generate the attestation signature according to [RFC7518] section 3.1. The following algorithms are supported:

1. "ES256" [RFC7518]

2. "RS256" [RFC7518]

3. "PS256" [RFC7518]

4. "ED256" [FIDOEcdaaAlgorithm]

5. "ED512" [FIDOEcdaaAlgorithm]

The signature element contains the attestation signature.

Signing procedure

The authenticator signs the attToBeSigned using the attestation private key.

Verification procedure

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

• Verify the signature using the public key in x5c with the algorithm specified in alg.

• Verify that x5c correctly chains to the trust anchor provided.

• Verify that x5c meets the requirements in §6.2.1 Packed attestation statement certificate requirements.

If daaKey is present, then the attestation type is DAA. In this case:

• Verify that alg is "ED256" or "ED512".

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

• If x5c 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 the authenticatorData.

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

• Verify the signature using the public key of the credential.

• Validate that alg matches the algorithm in authenticatorData.

6.2.1. Packed attestation statement certificate requirements

The attestation certificate MUST have the following fields/extensions:

• Version must be set to 3.

• Subject field MUST be set to:

  Subject-C

  Country where the Authenticator vendor is incorporated

  Subject-O

  Legal name of the Authenticator vendor

  Subject-OU

  Authenticator Attestation

  Subject-CN

  No stipulation.

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

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

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

6.3. TPM Attestation Format

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

Attestation format identifier

tpm

Attestation types supported

Privacy CA, DAA

Syntax

A TPM Attestation statement has the following format:

interface TpmAttestation {
    readonly    attribute DOMString     tpmVersion;
    readonly    attribute ArrayBuffer   x5c;
    readonly    attribute ArrayBuffer   daaKey;
    readonly    attribute ArrayBuffer   certifyInfo;
    readonly    attribute DOMString     alg;
    readonly    attribute ArrayBuffer   signature;
};

The tpmVersion field contains the version of the TPM specification to which the signature conforms. Currently supported versions are "1.2" and "2.0".

The x5c attribute contains the attestation certificate and its certificate chain as described in [RFC7515] section 4.1.6. This will be an AIK certificate.

The alg element contains the name of the algorithm used to generate the attestation signature according to [RFC7518] section 3.1. The following algorithms are supported:

1. "RSA1_5" [RFC7518] (TPM v1.2 only)

2. "ES256" [RFC7518] (TPM v2.0 only)

3. "RS256" [RFC7518] (TPM v2.0 only)

4. "PS256" [RFC7518] (TPM v2.0 only)

5. "ED256" [FIDOEcdaaAlgorithm] (TPM v2.0 only)

6. "ED512" [FIDOEcdaaAlgorithm] (TPM v2.0 only)

The signature element contains the attestation signature.

Signing procedure

If using TPM version 1.2, generate a signature using the procedure specified in [TPMv1-2-Part3] Section 13.8 or Section 13.9, using the attestation private key and setting the antiReplay parameter to the SHA-1 hash of attToBeSigned.

If using TPM version 2.0, generate a signature using the procedure specified in [TPMv2-Part3] Section 18.2, using the attestation private key and setting the qualifyingData parameter to attToBeSigned.

In both the above cases, return the certifyInfo output parameter along with the signature.

Verification procedure

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

• Verify the signature is over the certifyInfo using the public key in x5c with the algorithm specified in alg.

• If tpmVersion is "1.2", verify that the certifyInfo contains a TPM_CERTIFY_INFO or TPM_CERTIFY_INFO2 structure with the data field set to the SHA-1 hash of the concatenation of authenticatorData and clientData.

• If tpmVersion is "2.0", verify that certifyInfo is a TPMS_ATTEST structure with the extraData field set to the concatenation of authenticatorData and clientData.

• Verify that x5c correctly chains to the trust anchor provided.

• Verify that x5c meets the requirements in §6.3.1 TPM attestation statement certificate requirements.

If daaKey is present, then the attestation type is DAA.

• Verify that alg is "ED256" or "ED512".

• Perform DAA-Verify on signature to verify that it is over the certifyInfo (see [FIDOEcdaaAlgorithm]).

• Verify that certifyInfo is a TPMS_ATTEST structure with the extraData field set to the concatenation of authenticatorData and clientData.

• If x5c 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 the authenticatorData.

6.3.1. 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 if "version" equals 2 and [TPMv1-2-Credential-Profiles] section 3.2.9 if "version" equals 1.

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

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

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

6.4. Android Key Attestation 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 authenticatorData is produced outside this environment. The Relying Party is expected to check that the contents of authenticatorData are consistent with the fields of the attestation certificate’s extension data.

Attestation format identifier

android-key

Attestation types supported

Basic

Syntax

An Android key Attestation statement has the following format:

[SecureContext]
interface AndroidKeyAttestation {
    readonly    attribute ArrayBuffer   signature;
};

The signature field contains the Android attestation statement, which is a DER encoded X.509 certificate. See the Android developer documentation.

Signing procedure

Request a Android Key Attestation (i.e. calling "keyStore.getCertificateChain(myKeyUUID)") providing attToBeSigned as the challenge value (e.g. using "setAttestationChallenge").

Verification procedure

Verification is performed as follows:

• Verify that signature is a valid certificate chain, consisting of a time-valid X.509 certificate chaining up to a trusted attestation root key.

• Verify that the public key in the attestation certificate matches the credential public key in the attestation data field of the given authenticatorData.

• Verify that in the attestation certificate extension data:

  • The value of the attestationChallenge field is identical to attToBeSigned.

  • The AuthorizationList.allApplications field is not present, since ScopedCredentials must be bound to the RP ID.

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

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

6.5. Android SafetyNet Attestation Format

When the Authenticator in question is a platform-provided Authenticator on certain Android platforms, the attestation statement is 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 only provides some statements about the health of the platform and thehref identity of the calling application.

Attestation format identifier

android-safetynet

Attestation types supported

Basic

Syntax

An Android Attestation statement has the following format:

interface AndroidSafetyNetAttestation {
    readonly attribute unsigned long version;
    readonly attribute DOMString     safetyNetResponse;
};

The version element is set to the version number of Google Play Services responsible for providing the SafetyNet API.

The safetyNetResponse element contains the value returned by the above SafetyNet API. This value is a JWS [RFC7515] object (see SafetyNet online documentation) in Compact Serialization.

Signing procedure

Request a SafetyNet attestation, providing attToBeSigned as the nonce value.

Verification procedure

Verification is performed as follows:

• Verify that safetyNetResponse is a valid SafetyNet response of version version.

• Verify that the nonce in the safetyNetResponse is identical to attToBeSigned.

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

• Verify that the ctsProfileMatch attribute in the payload of the safetyNetResponse is true.

7. WebAuthn Extensions

The mechanism for generating scoped credentials, as well as requesting and generating WebAuthn assertions, as defined in §4 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. Extensions can define additions to the following steps and data:

• makeCredential() request parameters for registration extension.

• getAssertion() request parameters for authentication extensions.

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

• Authenticator processing, and the authenticatorData structure, for registration extensions and authentication extensions.

When requesting an assertion for a scoped credential, a Relying Party can list a set of extensions to be used, if they are supported by the client and/or the authenticator. It sends the client arguments for each extension in the getAssertion() call (for authentication extensions) or makeCredential() call (for registration extensions) to the client platform. The client platform performs additional processing for each extension that it supports, and augments ClientData as required by the extension. In addition, the client collects the authenticator arguments for the above extensions, and passes them to the authenticator in the authenticatorMakeCredential call (for registration extensions) or authenticatorGetAssertion call (for authentication extensions). These authenticator arguments are passed as name-value pairs, with the extension identifier as the name, and the corresponding authenticator argument as the value. The authenticator, in turn, performs additional processing for the extensions that it supports, and augments authenticatorData as specified by the extension.

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 argument by simply encoding the client argument 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 argument, 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.

7.1. Extension Identifiers

Extensions are identified by a string, called an extension identifier, chosen by the extension author.

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

Unregistered extension identifiers should aim to be globally unique, e.g., by including the defining entity such as myCompany_extension.

All extension identifiers MUST be a maximum of 32 octets in length and MUST consist only of printable USASCII characters, i.e., VCHAR as defined in [RFC5234]. Implementations MUST match WebAuthn extension identifiers in a case-insensitive fashion.

Extensions that may exist in multiple versions should take care to include a version in their identifier. In effect, different versions are thus treated as different extensions, e.g., myCompany_extension_01

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

§8 Pre-defined extensions defines an initial set of currently-defined and registered extensions their identifiers. See the WebAuthn Extension Identifiers Registry defined in [WebAuthn-Registries] for an up-to-date list of registered WebAuthn Extension Identifiers.

7.2. Defining extensions

A definition of an extension must specify, at minimum, an extension identifier and an extension client argument sent via the getAssertion() or makeCredential() call. Additionally, extensions may specify additional values in ClientData, authenticatorData (in the case of authentication extensions), or both. Finally, if the extension requires any authenticator processing, it must also specify an authenticator argument to be sent via the authenticatorGetAssertion or authenticatorMakeCredential call.

Any extension that requires client processing MUST specify a method of augmenting ClientData that unambiguously lets the Relying Party know that the extension was honored by the client. Similarly, any extension that requires authenticator processing MUST specify a method of augmenting authenticatorData to let the Relying Party know that the extension was honored by the authenticator.

7.3. Extending request parameters

An extension defines up to two request arguments. The client argument is passed from the Relying Party to the client in the getAssertion() or makeCredential() call, while the authenticator argument is passed from the client to the authenticator during the processing of these calls.

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

var assertionPromise = credentials.getAssertion(..., /* extensions */ {
    "webauthnExample_foobar": 42
});

Extension definitions MUST specify the valid values for their client argument. Clients SHOULD ignore extensions with an invalid client argument. If an extension does not require any parameters from the Relying Party, it SHOULD be defined as taking a Boolean client argument, set to true to signify that the extension is requested by the Relying Party.

Extensions that only affect client processing need not specify an authenticator argument. Extensions that affect authenticator processing MUST specify a method of computing the authenticator argument from the client argument. For extensions that do not require additional parameters, and are defined as taking a Boolean client argument set to true, this method SHOULD consist of passing an authenticator argument of true (CBOR major type 7, value 21).

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

7.4. Extending client processing

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

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

Extensions that require authenticator processing MUST define the process by which the client argument can be used to determine the authenticator argument.

7.5. Extending authenticator processing

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

As specified in §5.2.1 Authenticator data, the authenticator data value of each processed extension is included in the extended data part of the authenticatorData. This part is a CBOR map, with extension identifiers as keys, and the authenticator data value of each extension as the value.

7.6. Example extension

This section is not normative.

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

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

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

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

{
    ...,
    'extensions': {
        'webauthnExample_geo': {
            'type': 'Point',
            'coordinates': [65.059962, -13.993041]
        }
    }
}

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

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

81 (hex)                                    -- Flags, ED and TUP both set.
20 05 58 1F                                 -- Signature counter
A1                                          -- CBOR map of one element
    73                                      -- Key 1: CBOR text string of 19 bytes
        77 65 62 61 75 74 68 6E 45 78 61
        6D 70 6C 65 5F 67 65 6F             -- "webauthnExample_geo" UTF-8 string
    82                                      -- Value 1: CBOR array of two elements
        FA 42 82 1E B3                      -- Element 1: Latitude as CBOR encoded float
        FA C1 5F E3 7F                      -- Element 2: Longitude as CBOR encoded float

8. Pre-defined extensions

This section defines an initial set of extensions. These are recommended for implementation by user agents targeting broad interoperability.

8.1. Transaction authorization

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

Extension identifier

webauthn_txAuthSimple

Client argument

A single UTF-8 encoded string prompt.

Client processing

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

Authenticator argument

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

Authenticator processing

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

Authenticator data

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

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

Extension identifier

webauthn_txAuthGeneric

Client argument

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

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

2. and the content itself, encoded as CBOR byte array.

Client processing

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

Authenticator argument

The client argument encoded as a CBOR map.

Authenticator processing

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

Authenticator data

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

8.2. Authenticator Selection Extension

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

Extension identifier

webauthn_authnSel

Client argument

A sequence of AAGUIDs:

typedef sequence < AAGUID > AuthenticatorSelectionList;

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

An AAGUID is defined as an array containing the globally unique identifier of the authenticator model being sought.

typedef BufferSource AAGUID;

Client processing

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

Authenticator argument

There is no authenticator argument.

Authenticator processing

None.

8.3. SupportedExtensions Extension

Extension identifier

webauthn_exts

Client argument

The Boolean value true to indicate that this extension is requested by the Relying Party.

Client processing

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

Authenticator argument

The Boolean value true, encoded in CBOR (major type 7, value 21).

Authenticator processing

The authenticator augments the authenticator data with a list of extensions that the authenticator supports, as defined below. This extension can be added to attestation statements.

Authenticator data

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

8.4. User Verification Index (UVI) Extension

Extension identifier

webauthn_uvi

Client argument

The Boolean value true to indicate that this extension is requested by the Relying Party.

Client processing

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

Authenticator argument

The Boolean value true, encoded in CBOR (major type 7, value 21).

Authenticator processing

The authenticator augments the authenticator data with a user verification index indicating the method used by the user to authorize the operation, as defined below. This extension can be added to attestation statements and assertions.

Authenticator data

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

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

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

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

Example for authenticatorData containing one UVI extension

...                                         -- RP ID hash (32 bytes)
81                                          -- TUP and ED set
00 00 00 01                                 -- (initial) signature counter
...                                         -- all public key alg etc.
A1                                          -- extension: CBOR map of one element
    6C                                      -- Key 1: CBOR text string of 11 bytes
        77 65 62 61 75 74 68 6E 5F 75 76 69 -- "webauthn_uvi" UTF-8 string
    58 20                                   -- Value 1: CBOR byte string with 0x20 bytes
        00 43 B8 E3 BE 27 95 8C             -- the UVI value itself
        28 D5 74 BF 46 8A 85 CF
        46 9A 14 F0 E5 16 69 31
        DA 4B CF FF C1 BB 11 32
        82

8.5. Location Extension

Extension identifier

webauthn_loc

Client argument

The Boolean value true to indicate that this extension is requested by the Relying Party.

Client processing

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

Authenticator argument

The Boolean value true, encoded in CBOR (major type 7, value 21).

Authenticator processing

If the authenticator does not support the extension, then the authenticator MUST ignore the extension request. If the authenticator accepts the extension, then the authenticator SHOULD only add this extension data to a packed attestation or assertion.

Authenticator data

If the authenticator accepts the extension request, then authenticator data SHOULD provide location data in the form of a CBOR-encoded map, with the first value being the extension identifier and the second being an array of returned values. The array elements SHOULD be derived from (key,value) pairings for each location attribute that the authenticator supports. The following is an example of authenticatorData where the returned array is comprised of a {longitude, latitude, altitude} triplet, following the coordinate representation defined in The W3C Geolocation API Specification.

...                                         -- RP ID hash (32 bytes)
81                                          -- TUP and ED set
00 00 00 01                                 -- (initial) signature counter
...                                         -- all public key alg etc.
A1                                          -- extension: CBOR map of one element
    6C                                      -- Value 1: CBOR text string of 11 bytes
        77 65 62 61 75 74 68 6E 5F 6C 6F 63 -- "webauthn_loc" UTF-8 string
    86                                      -- Value 2: array of 6 elements
        68            -- Element 1:  CBOR text string of 8 bytes
           6C 61 74 69 74 75 64 65          -- “latitude” UTF-8 string
        FB ...            -- Element 2:  Latitude as CBOR encoded double-precision float
        69            -- Element 3:  CBOR text string of 9 bytes
           6C 6F 6E 67 69 74 75 64 65       -- “longitude” UTF-8 string
        FB ...            -- Element 4:  Longitude as CBOR encoded double-precision float
        68            -- Element 5:  CBOR text string of 8 bytes
          61 6C 74 69 74 75 64 65           -- “altitude” UTF-8 string
        FB ...            -- Element 6:  Altitude as CBOR encoded double-precision float

8.6. User Verification Mode (UVM) Extension

Extension identifier

webauthn_uvm

Client argument

The Boolean value true to indicate that this extension is requested by the WebAuthn Relying Party.

Client processing

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

Authenticator argument

The Boolean value true, encoded in CBOR (major type 7, value 21).

Authenticator processing

The authenticator augments the authenticator data with a user verification index indicating the method used by the user to authorize the operation, as defined below. This extension can be added to attestation statements and assertions.

Authenticator data

Authenticators can report up to 3 different user verification methods (factors) used in a single authentication instance. To accommodate this possibility the UVM is encoded as CBOR array (major type 4) with a maximum allowed length of 3 -

• Type 0x81 – only 1 factor was used for authentication.

• Type 0x82 – 2 factors were used.

• Type 0x83 – 3 or more factors were used.

Each data item is in turn a CBOR array of length 3 (type 0x83) with the following data items:

• Data Item 1 – User Verification Method. This is the authentication method/factor used by the authenticator to verify the user. Available values are defined in [FIDOReg], "User Verification Methods" section. It is encoded as a CBOR unsigned integer (Major type 0).

• Data Item 2 – Key Protection Type. This is the method used by the authenticator to protect the FIDO registration private key material. Available values are defined in [FIDOReg], "Key Protection Types" section. It is encoded as a CBOR 2 byte unsigned short (Major type 0).

• Data Item 3 – Matcher Protection Type. This is the method used by the authenticator to protect the matcher that performs user verification. Available values are defined in [FIDOReg], "Matcher Protection Types" section. It is encoded as a CBOR 2 byte unsigned short (Major type 0).

This is repeated for each factor used in the authentication instance.

If >3 factors can be used in an authentication instance the authenticator vendor must select the 3 factors it believes will be most relevant to the Server to include in the UVM.

Servers supporting the UVM extension MUST support a length up to 36 bytes for a 3 factor maximum UVM value.

Example for authenticatorData containing one UVM extension for a multi-factor authentication instance where 2 factors were used:

...                    -- RP ID hash (32 bytes)
81                     -- TUP and ED set
00 00 00 01            -- (initial) signature counter
...                    -- all public key alg etc.
A1                     -- extension: CBOR map of one element
    6C                 -- Key 1: CBOR text string of 12 bytes
        77 65 62 61 75 74 68 6E 2E 75 76 6d -- "webauthn_uvm" UTF-8 string
    82                 -- Value 1: CBOR array of length 2 indicating two factor usage
        83              -- Item 1: CBOR array of length 3
            02           -- Subitem 1: CBOR integer for User Verification Method Fingerprint
            04           -- Subitem 2: CBOR short for Key Protection Type TEE
            02           -- Subitem 3: CBOR short for Matcher Protection Type TEE
        83              -- Item 2: CBOR array of length 3
            04           -- Subitem 1: CBOR integer for User Verification Method Passcode
            01           -- Subitem 2: CBOR short for Key Protection Type Software
            01           -- Subitem 3: CBOR short for Matcher Protection Type Software

9. IANA Considerations

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

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

Algorithm Name	"S256"
Algorithm Description	The SHA256 hash algorithm.
Algorithm Usage Location(s)	"alg", i.e., used with JWS.
JOSE Implementation Requirements	Optional+
Change Controller	FIDO Alliance
Specification Documents	[FIPS-180-4]
Algorithm Analysis Document(s)	[SP800-107r1]

Algorithm Name	"S384"
Algorithm Description	The SHA384 hash algorithm.
Algorithm Usage Location(s)	"alg", i.e., used with JWS.
JOSE Implementation Requirements	Optional
Change Controller	FIDO Alliance
Specification Documents	[FIPS-180-4]
Algorithm Analysis Document(s)	[SP800-107r1]

Algorithm Name	"S512"
Algorithm Description	The SHA512 hash algorithm.
Algorithm Usage Location(s)	"alg", i.e., used with JWS.
JOSE Implementation Requirements	Optional+
Change Controller	FIDO Alliance
Specification Documents	[FIPS-180-4]
Algorithm Analysis Document(s)	[SP800-107r1]

Algorithm Name	"SM3"
Algorithm Description	The SM3 hash algorithm.
Algorithm Usage Location(s)	"alg", i.e., used with JWS.
JOSE Implementation Requirements	Optional
Change Controller	FIDO Alliance
Specification Documents	[OSCCA-SM3]
Algorithm Analysis Document(s)	N/A

10. Sample scenarios

This section is not normative.

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

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

10.1. Registration

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

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

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

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

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

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

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

7. If a new credential was created,

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

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

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

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

var webauthnAPI = navigator.authentication;

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

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

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

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

10.2. Authentication

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

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

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

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

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

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

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

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

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

9. If an assertion was successfully generated and returned,

   • The script sends the assertion to the server.

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

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

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

var webauthnAPI = navigator.authentication;

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

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

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

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

var webauthnAPI = navigator.authentication;

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

var challenge = "Y2xpbWIgYSBtb3VudGFpbg";
var acceptableCredential1 = {
    type: "ScopedCred",
    id: "ISEhISEhIWhpIHRoZXJlISEhISEhIQo="
};
var acceptableCredential2 = {
    type: "ScopedCred",
    id: "cm9zZXMgYXJlIHJlZCwgdmlvbGV0cyBhcmUgYmx1ZQo="
};

var options = {
                timeoutSeconds: 300,  // 5 minutes
                allowList: [acceptableCredential1, acceptableCredential2];
                extensions: { 'webauthn.txauth.simple':
                   "Wave your hands in the air like you just don’t care" };
              };

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

10.3. Decommissioning

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

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

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

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

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

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

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

  • Server deletes credential from its database during maintenance activity.

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

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

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

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

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

11. Acknowledgements

We thank the following for their contributions to, and thorough review of, this specification: Domenic Denicola, Rahul Ghosh, Brad Hill, Jing Jin, Anne van Kesteren, Giridhar Mandyam, Axel Nennker, Yaron Sheffer, Mike West.