U2F authentication library

This is a simple library to register and check signatures provided by U2F clients/devices.
It's intended to be used in Relying Parties - websites that want to add U2F 2-factor authentication
for their users.

To use U2F, it is recommended to familiarize yourself with FIDO Alliance Specifications,
although basic usage is shown below.

U2F Overview/properties

* U2F provides hardware-based 2-nd factor authentication system. Public/private key infrastructure is used
to ensure good security.
* Provides proof of posession of hardware key, plus user presence flag.
* Public/private key pairs are specific to website origin and 'application id'. Keys are useless if used from
other origins.
* Needs to be stored on server for each user: Key handle and public key (both strings).
* Cannot be used as main authentication system because server needs to provide
unique key handle to the user to get the signature.

Basic usage

$3

##### Server endpoints:

``javascript
const u2f = require('u2f');

// The app ID is a string used to uniquely identify your U2F app, for both registration requests and
// authentication requests. It is usually the fully qualified URL of your website. The website MUST
// be HTTPS, otherwise the registration will fail client-side.
const APP_ID = ...

function registrationChallengeHandler(req, res) {
// 1. Check that the user is logged in.

// 2. Generate a registration request and save it in the session.
const registrationRequest = u2f.request(APP_ID);
req.session.registrationRequest = registrationRequest;

// 3. Send the registration request to the client, who will use the Javascript U2F API to sign
// the registration request, and send it back to the server for verification. The registration
// request is a JSON object containing properties used by the client to sign the request.
return res.send(registrationRequest);
}

function registrationVerificationHandler(req, res) {
// 4. Verify the registration response from the client against the registration request saved
// in the server-side session.
const result = u2f.checkRegistration(req.session.registrationRequest, req.body.registrationResponse);

if (result.successful) {
// Success!
// Save result.publicKey and result.keyHandle to the server-side datastore, associated with
// this user.
return res.sendStatus(200);
}

// result.errorMessage is defined with an English-language description of the error.
return res.send({result});
}
`

##### Client logic:

Note that the window.u2f object is defined in the official Javascript U2F API, for which a polyfill is available as an npm module.

`javascript
const registrationRequest = ... // Retrieve this from hitting the registration challenge endpoint

window.u2f.register(registrationRequest.appId, [registrationRequest], [], (registrationResponse) => {
// Send this registration response to the registration verification server endpoint
});
`

$3

##### Server endpoints:

`javascript
const u2f = require('u2f');

function authenticationChallengeHandler(req, res) {
// 1. Check that the user is logged in using password authentication.

// 2. Fetch the user's key handle from the server-side datastore. This field should have been
// saved after the registration procedure.
const keyHandle = ...

// 3. Generate an authentication request and save it in the session. Use the same app ID that
// was used in registration!
const authRequest = u2f.request(APP_ID, keyHandle);
req.session.authRequest = authRequest;

// 4. Send the authentication request to the client, who will use the Javascript U2F API to sign
// the authentication request, and send it back to the server for verification.
return res.send(authRequest);
}

function authenticationVerificationHandler(req, res) {
// 5. Fetch the user's public key from the server-side datastore. This field should have been
// saved after the registration procedure.
const publicKey = ...

// 6. Verify the authentication response from the client against the authentication request saved
// in the server-side session.
const result = u2f.checkSignature(req.session.authRequest, req.body.authResponse, publicKey);

if (result.successful) {
// Success!
// User is authenticated.
return res.sendStatus(200);
}

// result.errorMessage is defined with an English-language description of the error.
return res.send({result});
}
`

##### Client logic:

`javascript
const authRequest = ...; // Retrieve this from hitting the authentication challenge endpoint

window.u2f.sign(authRequest.appId, challenge, [authRequest], (authResponse) => {
// Send this authentication response to the authentication verification server endpoint
});
``

Useful links

http://demo.yubico.com/u2f
https://github.com/Yubico/python-u2flib-server

TODO

* Provide instructions for client-side. How to get the 'u2f' namespace, what browsers are supported.
* Change API to enable multiple keyhandle/publickey pairs for a single user.
* Unpack registration certificate and check its own signature and time constraints.

License

MIT