Firefox OS/Remote Control

Introduction

Remote control is a feature for Firefox OS on TV. After Send tab to TV, user can easily extend browsing experience on TV via Firefox on Android, using virtual touchpad to control TV.

Meta Bug: bug 1205939

Prototype Demo Video: http://bit.ly/1Fa2ST1

UX & Visual Spec

UX Spec: http://bit.ly/1jrWqgn

Visual Spec: http://bit.ly/1jrWQU3

Architecture Designs

Remote Control Service: Entry point of remote control feature. Control service related logic and data in runtime.
TLS server: Serves connections from user.

User interaction of remote control is divided into two parts:

Peer authentication
Control event processing

Peer authentication

Remote control is designed to protect data transmitted between client and server, for example: user input string may contains privacy data. The connection is based on self-signed TLS server. To prevent man in the middle(MITM) attack, we adopt J-PAKE to exchange symmetric key for peer authentication. Following describes how it works:

The user sends the tab to TV on fennec.
Client sends request to handshake without ID.
Server replies with the handshake type is 1st time handshake.
Client computes J-PAKE round 1 and sends the result to server.
Server generates PIN code and show on screen, computes J-PAKE round 1 and sends the result to client.
Server computes J-PAKE round 2 with PIN code attached first 12 characters of server's TLS certificate fingerprint as weak secret.
User enter PIN code, client computs J-PAKE round 2 with PIN code attached first 12 characters of server's TLS certificate fingerprint as weak secret.
Client sends round 2 result to server.
Server dismisses PIN code notification on screen, sends round 2 result to client.
Both client and server compute J-PAKE final round, gets AES and HMAC key.
Server use two keys to get a signature of H(H(AES key)), then send the signature to client.
Client verifies the signature matches it has.
Client use two keys to get a signature of H(AES key), then send the signature to server.
Server verifies the signature matches it has.
Server replies with the handshake finish with client's ID for connection in the future.

Re-authentication

User is only required to input PIN code when first time connect to TV. In the second time, client and server leverage previous AES key value and derive new AES and HMAC key. This can reduce number of user input and make the procedure from send tab to TV to remote control more smoothly.

The user sends the tab to TV on fennec.
Client sends request to handshake with ID.
Server looks up ID/AES key mapping and replies with the handshake type is 2nd time handshake.
Client computes J-PAKE round 1 and sends the result to server.
Server computes J-PAKE round 1 and sends the result to client.
Client and server computes J-PAKE round 2 with previous AES key value attached first 12 characters of server's TLS certificate fingerprint as weak secret.
Client sends round 2 result to server.
Server sends round 2 result to client.
Both client and server compute J-PAKE final round, gets new AES and HMAC key.
Server use two keys to get a signature of H(H(AES key)), then send the signature to client.
Client verifies the signature matches it has.
Client use two keys to get a signature of H(AES key), then send the signature to server.
Server verifies the signature matches it has.
Server replies with the handshake finish.

After authentication, client can sends control event to server.

Control event processing

Control page provides: 1) a virtual touchpad, 2) a virtual vertical scrollbar, 3) 3 function keys. Every touch, swipe or click will be generated to an event in JSON format then send to TV. Following describes how control event is sent and processed:

User operates control page.
Client generates and sends the event in JSON format.
Server parses the event to JSON object, dispatch to client.sjs.
Client.sjs parses event and dispatch to Gecko or Gaia system app.

Data encryption/decryption

There are three kinds of data are encrypted while tranmission:

Client polls UUID in establish secure connection stage
Client sends PIN code in pin code pairing
Client sends event in control event processing

As data are encrypted by symmetric key, AES-GCM, there are two things need noticed:

AES-GCM need initialization vector(IV) to encrypt/decrypt. Currently, IV is random value for each data, length is 12 bytes. Encrypted data is appended after IV. Receiver needs to slice first 12 bytes as IV to decrypt.
Each message type is string, use TextEncoder.encode() to encode as an UInt8Array, then encrypt to ArrayBuffer. Vice versa.
Data encryption/decryption is asynchronous but HTTP request need response immediately. For UUID and PIN code case, remote control uses a ticket number to get status after decryption. But for event, we reduce polling result but use latest event's result instead.

Authentication and Event Protocol

Request handshake

Request

 {
   type: 'auth'
   action: 'request_handshake'
   detail: {
     id: <id assigned by server, optional>
   }
 }

Response

 {
   type: 'auth'
   action: 'response_handshake'
   detail: 1 or 2, 1 for 1st handshake, 2 for 2nd handshake
 }

J-PAKE key exchange

Client send round 1

 {
   type: 'auth'
   action: 'jpake_client_1',
   detail: {
     gx1: gx1.value,
     gx2: gx2.value,
     zkp_x1: { gr: gv1.value, b: r1.value, id: 'client' },
     zkp_x2: { gr: gv2.value, b: r2.value, id: 'client' }
   }
 }

Server reply round 1

 {
   type: 'auth'
   action: 'jpake_server_1',
   detail: {
     gx1: gx1.value,
     gx2: gx2.value,
     zkp_x1: { gr: gv1.value, b: r1.value, id: 'server' },
     zkp_x2: { gr: gv2.value, b: r2.value, id: 'server' }
   }
 }

Client send round 2

 {
   type: 'auth'
   action: 'jpake_client_2',
   detail: {
     A: A.value,
     zkp_A: { gr: gvA.value, b: rA.value, id: 'client' }
   }
 }

Server reply round 2

 {
   type: 'auth'
   action: 'jpake_server_2',
   detail: {
     A: A.value,
     zkp_A: { gr: gvA.value, b: rA.value, id: 'server' }
   }
 }

Key confirmation

Server key confirmation

 {
   type: 'auth'
   action: 'server_key_confirm'
   detail: {
     signature: <double signature of AES key by HMAC key, in base64>
   }
 }

Client key confirmation

 {
   type: 'auth'
   action: 'client_key_confirmation'
   detail: {
     signature: <signature of AES key by HMAC key, in base64>
   }
 }

Server finish handshake

 {
   type: 'auth'
   action: 'finish_handshake'
   detail: {
     id: <id assigned by server, optional>
   }
 }

Touch Events

 {
   type: 'command'
   action: 'touchstart',
   detail: {
     width: <touch panel width, integer, in pixels>,
     height: <touch panel height, integer, in pixels>,
     identifier: <A simple unique identifier for server to distinguish touch events between different clients, string>
   }
 }

 {
   type: 'command'
   action: 'touchmove',
   detail: {
     dx: <dx between current point and starting point, integer, in pixels>,
     dy: <dy between current point and starting point, integer, in pixels>,
     identifier: <should be identical with "touchstart">,
     duration: <duration from "touchstart", integer, in milliseconds>
   }
 }

 {
   type: 'command'
   action: 'touchend',
   detail: {
     dx: <same as "touchmove">,
     dy: <same as "touchmove">,
     identifier: <should be identical with "touchstart">,
     duration: <same as "touchmove">,
     swipe: <the gesture within this touch, string, should be "left", "right", "up" and "down">
   }
 }

Scroll Events

There are "scrollstart", "scrollmove" and "scrollend" events which details are all identical with corresponding touch events above.

Key Events

 {
   type: 'command'
   action: 'keypress',
   detail: <KeyEvent constant, string, sush as "DOM_VK_RETURN">
 }

Input Events

 {
   type: 'command'
   action: 'textinput',
   detail: {
     clear: <whether to clear the entire string in the current focused input field, boolean>,
     string: <new string to append, string>,
     keycode: <a specified key to be pressed after the string inputted, integer>
   }
 }

Custom Events

 {
   type: 'command'
   action: 'custom',
   detail: {
     action: <custom action name, string>,
     ...
   }
 }