Key Words

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]].

Low-level MessagePort API

RP web pages communicate with the FIDO client over an instance of the HTML5 MessagePort interface. Client implementations may choose how this instance is made available to web pages.

Messages sent to the FIDO client SHOULD be Request dictionaries:

DOMString type

The type of request, either "u2f_register_request" or "u2f_sign_request".

SignRequest[] signRequests

A list of SignRequest dictionaries, one for each token already registered with this RP.

RegisterRequest[]? registerRequests

A list of RegisterRequest dictionaries, one for each protocol version that the RP is willing to register.

int? timeoutSeconds

A timeout for the FIDO Client's processing, in seconds.

optional int? requestId

An integer identifying this request from concurrent requests.

SignRequest and RegisterRequest are defined below. If timeoutSeconds is omitted, timeout behavior is unspecified. If requestId is present, the FIDO client MUST include its value the corresponding Response dictionary under the same key.

Responses from the FIDO client to the RP webpage SHOULD be Response dictionaries:

DOMString type

The response type, either "u2f_register_response" or "u2f_sign_response"

(Error or RegisterResponse or SignResponse) responseData

The response data, see

int? requestId

The requestId value of the corresponding request, if present. Otherwise omitted.

Errors are indicated by an Error dictionary sent as the response data. An error dictionary can be identified by checking for its non-zero integer errorCode key. RegisterResponse and SignResponse do not define this key. An error object may optionally contain a string errorMessage with further description of the error.

ErrorCode errorCode

An error code from the ErrorCode enumeration.

DOMString? errorMessage

A description of the error.

High-level Javascript API

A FIDO client MAY provide a JavaScript convenience API that abstracts the lower-level MessagePort API. Implementations may choose how to make such an API available to RP web pages. If such an API is provided, it SHOULD provide a namespace object u2f of the following interface.

void register(...)

RegisterRequest[] registerRequests

Register requests, one for each U2F protocol version accepted by RP

SignRequest[] signRequests

Sign requests for already registered tokens

function(RegisterResponse or Error) callback

Response handler

optional int? opt_timeoutSeconds

Timeout in seconds, for the FIDO client's handling of the request.

void sign(...)

SignRequest[] signRequests

Sign requests, one for each registered token

function(SignResponse or Error) callback

Response handler

optional int? opt_timeoutSeconds

Timeout in seconds, for the FIDO client's handling of the request.

The JavaScript API MUST invoke the provided callbacks with either response objects, or an error object. An error can be detected by testing for a non-zero errorCode key.

  u2f.sign(reqs, function(response) {
                  if (response.errorCode) {
                    // response is an Error
                    ...
                  } else {
                    // response is a SignResponse
                    ...
                  }
                });
          

Registration

To register a U2F token for a user account at the RP, the RP MUST:

• decide which U2F protocol version(s) of device it wants to register,
• pick an appropriate application id for the registration request,
• generate a random challenge, and
• store all private information associated with the registration (expiration times, user ids, etc.)

For each version it is willing to register, it then prepares a RegisterRequest dictionary as follows:

DOMString version

The version of the protocol that the to-be-registered token must speak. E.g. "U2F_V2".

DOMString challenge

The websafe-base64-encoded challenge.

DOMString appId

The application id that the RP asserts. The new key pair that the U2F token generates will be associated with this application id. (For application id details see [[!FIDOAppIDAndFacets]] in bibliography).

The RP delivers a registration request to the FIDO client either via the low-level MessagePort API, or by invoking the high-level JavaScript API.

// Low-level API
var port = <obtain U2F MessagePort in a browser specific manner>;
port.addEventListener(‘message’, responseHandler);
port.postMessage({
 ‘type’: ‘u2f_register_request’,
 ‘registerRequests’: [<RegisterRequest instance>, ...],
 ‘signRequests’: [<SignRequest for known token 1>, ...],
 ‘timeoutSeconds’: 30,
 ‘requestId’: <unique integer>  // optional
});
          

// High-level API
u2f.register([<RegisterRequest instance>, ...], 
             [<SignRequest for known token 1>, ...],
             registerResponseHandler);
          

The FIDO client SHOULD treat the order of RegisterRequest dictionaries in the first parameter as a prioritized list. That is, if multiple tokens are present that support more than one version provided by the RP, the version that appears first should be selected. Note that this means multiple RegisterRequests with the same version are redundant, since the first one will always be selected.

Note also that the responseHandler in the low-level API receives a Response object, while the registerResponseHandler in the high-level API receives the Error or RegisterResponse objects directly.

The FIDO client will create the raw registration and sign request messages from this data (see [[!U2FRawMsgs]] in bibliography), and attempt to perform a registration operation with a U2F token. The sign request messages will have the (internal) checkOnly boolean of the control state set to true, and are used to identity such U2F tokens that are already registered with the relying party. The registration request message is then used to register a U2F token that is not already registered (if such a token is present).

Note that as part of creating the registration request message, the FIDO client will create a Client Data object (see [[!U2FRawMsgs]]). This Client Data object will be returned to the caller as part of the registration response (see below).

If the registration is successful, the FIDO client returns (via the message port, or the JS API callback) a RegisterResponse dictionary as follows.

DOMString registrationData

The raw registration response websafe-base64

DOMString clientData

The client data created by the FIDO client, websafe-base64 encoded.

Generating signed identity assertions

To obtain an identity assertion from a locally-attached U2F token, the RP must

• prepare a SignRequest object for each U2F token that the user has currently registered with the RP.

DOMString version

Version of the protocol that the to-be-registered U2F token must speak. E.g. "U2F_V2"

DOMString challenge

The websafe-base64-encoded challenge.

DOMString keyHandle

The registered keyHandle to use for signing, as returned by the U2F token during registration.

DOMString appId

The application id that the RP would like to assert.

In response to a sign request, the FIDO client should perform the following steps:

• Verify the application identity of the caller.
• Using the provided challenge, create a client data object.
• Using the client data, the application id, and the key handle, create a raw authentication request message (see [[!U2FRawMsgs]] in bibliography) and send it to the U2F token.

Eventually the FIDO client must respond (via the MessageChannel or the provided callback). In the case of an error, an Error dictionary is returned. In case of success, a SignResponse is returned.

DOMString keyHandle

The keyHandle of the SignRequest that was processed.

DOMString signatureData

The raw response from U2F device, websafe-base64 encoded.

DOMString clientData

The client data created by the FIDO client, websafe-base64 encoded.

If there are multiple U2F tokens that responded to the authentication request, the FIDO client will pick one of the responses and pass it to the caller.

Error codes

When an Error object is returned, its errorCode field is set to a non-negative integer indicating the general error that occurred, from the following enumeration.

const int OK = 0

Success. Not used in errors but reserved

const int OTHER_ERROR = 1

An error otherwise not enumerated here

const int BAD_REQUEST = 2

The request cannot be processed

const int CONFIGURATION_UNSUPPORTED = 3

Client configuration is not supported

const int DEVICE_INELIGIBLE = 4

The presented device is not eligible for this request. For a registration request this may mean that the token is already registered, and for a sign request it may mean the token does not know the presented key handle.

const int TIMEOUT = 5

Timeout reached before request could be satisfied