u2f-api

U2F API for browsers

API

Support

U2F has for a long time been supported in Chrome, although not with the standard window.u2f methods, but through a built-in extension. Nowadays, browsers seem to use window.u2f to expose the functionality.

Supported browsers are:

• Chrome, using Chrome-specific hacks
• Opera, using Chrome-specific hacks

Firefox, Safari and other browsers still lack U2F support.

Since 0.1.0, this library supports the standard window.u2f methods.

The library should be complemented with server-side functionality, e.g. using the u2f package.

Basics

u2f-api exports two main functions and an error "enum". The main functions are register() and sign(), although since U2F isn't widely supported, the functions isSupported() as well as ensureSupport() helps you build applications which can use U2F only when the client supports it.

The register() and sign() functions return cancellable promises, i.e. promises you can cancel manually. This helps you to ensure your code doesn't continue in success flow and by mistake accept a registration or authentification request. The returned promise has a function cancel() which will immediately reject the promise.

Check or ensure support

import { isSupported } from 'u2f-api'

isSupported(): Promise< Boolean > // Doesn't throw/reject

import { ensureSupport } from 'u2f-api'

ensureSupport(): Promise< void > // Throws/rejects if not supported

Register

import { register } from 'u2f-api'

register(
  registerRequests: RegisterRequest[],
  signRequests: SignRequest[], // optional
  timeout: number // optional
): Promise< RegisterResponse >

The registerRequests can be either a RegisterRequest or an array of such. The optional signRequests must be, unless ignored, an array of SignRequests. The optional timeout is in seconds, and will default to an implementation specific value, e.g. 30.

Sign

import { sign } from 'u2f-api'

sign(
  signRequests: SignRequest[],
  timeout: number // optional
): Promise< SignResponse >

The values and interpretation of the arguments are the same as with register( ).

Errors

register() and sign() can return rejected promises. The rejection error is an Error object with a metaData property containing code and type. The code is a numerical value describing the type of the error, and type is the name of the error, as defined by the ErrorCodes enum in the "FIDO U2F Javascript API" specification. They are:

OK = 0 // u2f-api will never throw errors with this code
OTHER_ERROR = 1
BAD_REQUEST = 2
CONFIGURATION_UNSUPPORTED = 3
DEVICE_INELIGIBLE = 4
TIMEOUT = 5
CANCELLED = -1 // Added by this library

Usage

Loading the library

The library is promisified and will use the built-in native promises of the browser, unless another promise library is injected.

The following are valid ways to load the library:

var u2fApi = require( 'u2f-api' ); // Will use the native Promise
// ... or
var u2fApi = require( 'u2f-api' )( require( 'bluebird' ) ); // Will use bluebird for promises

Registering a passkey

With registerRequestsFromServer somehow received from the server, the client code becomes:

u2fApi.register( registerRequestsFromServer )
.then( sendRegisterResponseToServer )
.catch( ... );

Signing a passkey

With signRequestsFromServer also received from the server somehow:

u2fApi.sign( signRequestsFromServer )
.then( sendSignResponseToServer )
.catch( ... );

Example with checks for client support

u2fApi.isSupported( )
.then( function( supported ) {
	if ( supported )
	{
		return u2fApi.sign( signRequestsFromServer )
		.then( sendSignResponseToServer );
	}
	else
	{
		... // Other authentication method
	}
} )
.catch( ... );

Canceling

As mentioned in the API section above, the returned promises from register() and sign() have a method cancel() which will cancel the request. This is nothing more than a helper function.

Example implementation

U2F is a challenge-response protocol. The server sends a challenge to the client, which responds with a response.

This library is intended to be used in the client (the browser). There is another package intended for server-side: https://www.npmjs.com/package/u2f

Common problems

If you get BAD_REQUEST, the most common situations are that you either don't use https (which you must), or that the AppID doesn't match the server URI. In fact, the AppID must be exactly the base URI to your server (such as https://your-server.com), including the port if it isn't 443.

For more information, please see https://developers.yubico.com/U2F/Libraries/Client_error_codes.html and https://developers.yubico.com/U2F/App_ID.html