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    1.0.7 • Public • Published

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    U2F API for browsers


    • 1.0.0
      • Support for custom promise libraries removed
      • Promises no longer cancellable



    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 58+, although not proper support for facets
      • multi-domain registrations will work differently from Chrome

    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.


    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.

    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


    import { register } from 'u2f-api'
      signRequestsSignRequest[], // optional
      timeoutnumber // 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.


    import { sign } from 'u2f-api'
      timeoutnumber // optional
    )Promise< SignResponse >

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


    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
    TIMEOUT = 5


    Loading the library

    The library is promisified and will use the built-in native promises of the browser, unless another promise library is injected (deprecated since 1.0).

    var u2fApi = require( 'u2f-api' ); // CommonJS
    import u2fApi from 'u2f-api' // ES modules

    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 );
            ... // Other authentication method
    } )
    .catch( ... );

    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


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