1.1 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].

3.1 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 U2fRequest dictionaries:

dictionary U2fRequest {
    DOMString      type;
    DOMString?     appId;
    unsigned long? timeoutSeconds;
    unsigned long? requestId;
};

Subtypes of U2fRequest for register and sign requests are defined below in their respective sections. 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 U2fResponse dictionaries:

dictionary U2fResponse {
    DOMString                                   type;
    (Error or RegisterResponse or SignResponse) responseData;
    unsigned long?                              requestId;
};

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.

dictionary Error {
    ErrorCode  errorCode;
    DOMString? errorMessage;
};

3.2 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.

interface u2f {
    void register (DOMString appId, sequence<RegisterRequest> registerRequests, sequence<RegisteredKey> registeredKeys, function(RegisterResponse or Error) callback, optional unsigned long? opt_timeoutSeconds);
    void sign (DOMString appId, DOMString challenge, sequence<RegisteredKey> registeredKeys, function(SignResponse or Error) callback, optional unsigned long? opt_timeoutSeconds);
};

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
                  ...
                }
              });

5.1 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.)

The RP may choose an application id for the registration request. If none is chosen, the RP's web origin is used as the application id. The new key pair that the U2F token generates will be associated with this application id. (For application id details see [FIDOAppIDAndFacets] in bibliography).

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

dictionary RegisterRequest {
    DOMString version;
    DOMString challenge;
};

Additionally, the RP SHOULD prepare a RegisteredKey for each U2F token that is already registered for the current user as follows:

dictionary RegisteredKey {
    DOMString   version;
    DOMString   keyHandle;
    Transports? transports;
    DOMString?  appId;
};

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. Using the low-level MessagePort API, the RP would construct a message of the U2fRegisterRequest type:

dictionary U2fRegisterRequest : U2fRequest {
    DOMString                 type = 'u2f_register_request';
    sequence<RegisterRequest> registerRequests;
    sequence<RegisteredKey>   registeredKeys;
};

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

// High-level API
u2f.register(<Application id>,
             [<RegisterRequest instance>, ...],
             [<RegisteredKey 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 messages from this data (see [U2FRawMsgs] in bibliography), and attempt to perform a registration operation with a U2F token. 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.

dictionary RegisterResponse {
    DOMString version;
    DOMString registrationData;
    DOMString clientData;
};

5.2 Generating signed identity assertions

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

• generate a random challenge, and
• prepare a RegisteredKey object for each U2F token that the user has currently registered with the RP.

The RP delivers a sign request to the FIDO client either via the low-level MessagePort API, or by invoking the high-level JavaScript API. Using the low-level MessagePort API, the RP would construct a message of the U2fSignRequest type:

dictionary U2fSignRequest : U2fRequest {
    DOMString               type = 'u2f_sign_request';
    DOMString               challenge;
    sequence<RegisteredKey> registeredKeys;
};

// Low-level API
var port = <obtain U2F MessagePort in a browser specific manner>;
port.addEventListener(‘message’, responseHandler);
port.postMessage({
 ‘type’: ‘u2f_sign_request’,
 ‘appId’: <Application id>,
 ‘challenge’: <random challenge>,
 ‘registeredKeys’: [<RegisteredKey for known token 1>, ...],
 ‘timeoutSeconds’: 30,
 ‘requestId’: <unique integer>  // optional
});

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.

dictionary SignResponse {
    DOMString keyHandle;
    DOMString signatureData;
    DOMString clientData;
};

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.

5.3 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.

interface ErrorCode {
    const short OK = 0;
    const short OTHER_ERROR = 1;
    const short BAD_REQUEST = 2;
    const short CONFIGURATION_UNSUPPORTED = 3;
    const short DEVICE_INELIGIBLE = 4;
    const short TIMEOUT = 5;
};

5.4 Backward compatibility with U2F 1.0 API

For backward compatibility with the U2F 1.0 API, the RP MAY prepare a SignRequest in lieu of a RegisteredKey for each U2F token that is already registered for the current user. See [U2FJSAPI] for the specification of SignRequest.

Similarly, U2F clients MAY implement backward compatibility with version 1.0 by accepting a signRequests key in lieu of registeredKeys.