Proof of Concept Chrome extension for RSA signing using an OpenPGP-compatible smart card
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README.rst

Smart Card Connector App for Chrome OS

Overview

The project aim is to bring the support of smart cards (see <https://en.wikipedia.org/wiki/Smart_card>) into Chrome OS (see <http://www.chromium.org/chromium-os>) by utilizing the already existing Chrome APIs, mainly:

Using these APIs allows developers to implement a smart card stack without the need to modify the Chrome OS and/or Chrome. Instead, all the new code is implemented in the form of Chrome Apps (see <https://developer.chrome.com/apps/about_apps>).

The high-level overview of the architecture is:

  1. The App named "Smart Card Connector" (available on Chrome Web Store at <https://chrome.google.com/webstore/detail/khpfeaanjngmcnplbdlpegiifgpfgdco>).

    The App is basically an implementation of the PC/SC API (see <https://en.wikipedia.org/wiki/PC/SC>). The API allows middleware Apps to operate smart card readers through a unified interface.

    The API is exposed to other Apps through a message-exchange protocol (see <https://developer.chrome.com/apps/messaging>). For the details of the API, see the Smart Card Connector App API section below.

    The App is bundled with the free USB CCID driver (see <https://pcsclite.alioth.debian.org/ccid.html>). This driver is patched appropriately in order to be able to work through the chrome.usb Chrome API.

    Note: no other smart card reader drivers except the previously mentioned free USB CCID driver are supplied. Support of pluggable drivers is also not implemented yet, though it's possible technically (each driver can be created as a separate App talking to the Connector App through some defined interface). A list of readers supported by CCID can be found on their website <https://pcsclite.alioth.debian.org/ccid/section.html>.

  2. A number of third-party middleware Apps containing the smart card drivers.

    The middleware Apps are written based on the PC/SC API provided by the Connector App.

    The actual features implemented by the Apps may vary, but the typical example would be to read the certificates stored on the smart card, inject them into the browser's network stack (using the chrome.certificateProvider Chrome API) and proxy digest signing operations occurring during TLS handshake phase to the smart card.

Smart Card Connector App

As was mentioned before, the Connector App acts as the provider of the PC/SC API to third-party Apps.

The App plays basically the same role as the PC/SC-Lite Daemon does on Linux (see <http://linux.die.net/man/8/pcscd>). And, actually, the App sources are heavily based on the PC/SC-Lite middleware sources (see <https://pcsclite.alioth.debian.org/pcsclite.html>).

For the details of the internal structure and the implementation of the Smart Card Connector App, please refer to its own README file (see the smart_card_connector_app/ directory).

Smart Card Connector App API Permissions

Due to various privacy and security concerns, the following decisions were made:

  1. When a middleware App tries to talk to the Connector App, a user prompt dialog is generally shown, asking whether to allow or to block this middleware App.

    If the user decides to allow the middleware App, the decision is remembered in the Connector App's local storage, and the requests made by the middleware App start being actually executed by the Connector App. Otherwise, all the requests made by the middleware App are refused.

    Note: this system has nothing to do with the Chrome App permissions model (see <https://developer.chrome.com/apps/declare_permissions>), as Chrome allows to use the messaging API without any additional permissions. This intent is to close the hole left open by cross-app messaging between Apps with different permissions.

  2. The Connector App is bundled with a whitelist of known middleware App identifiers (and a mapping to their display names). For all middleware Apps not from this list, the user prompt will contain a big scary warning message.

    (Note: This behavior was introduced in the Smart Card Connector app version 1.2.7.0. Before this version, all requests from unknown apps were silently ignored.)

  3. For the enterprise cases, it's possible to configure the Connector App through an admin policy (see <https://www.chromium.org/administrators/configuring-policy-for-extensions>).

    The policy can specify which middleware Apps are force allowed to talk to the Connector App without checking against whitelist or prompting the user. The policy-configured permission always has the higher priority than the user's selections that could have been already made.

    The corresponding policy name is force_allowed_client_app_ids. Its value should be an array of strings representing the App identifiers. This is an example of the policy JSON blob:

    {
      "force_allowed_client_app_ids": {
        "Value": [
          "this_is_middleware_client_app_id",
          "this_is_some_other_client_app_id"]
      }
    }
    

Smart Card Connector App API

The API exposed by the Connector App is basically a PC/SC-Lite API (see <https://pcsclite.alioth.debian.org/api/group__API.html>) adopted for the message-exchanging nature of the communication between Chrome Apps (see <https://developer.chrome.com/apps/messaging>).

The PC/SC-Lite API function call is represented by a message sent to the Connector App. The message should have the following format:

{
  "type": "pcsc_lite_function_call::request",
  "data": {
    "request_id": <requestId>,
    "payload": {
      "function_name": <functionName>,
      "arguments": <functionArguments>
    }
  }
}

where <requestId> should be a number (unique in the whole session of the middleware App communication to the Connector App), <functionName> should be a string containing the PC/SC-Lite API function name, <functionArguments> should be an array of the input arguments that have to be passed to the PC/SC-Lite API function.

The results returned from the PC/SC-Lite API function call are represented by a message sent back from the Connector App to the middleware App.

If the request was processed successfully (i.e. the PC/SC-Lite function was recognized and called), then the message will have the following format:

{
  "type": "pcsc_lite_function_call::response",
  "data": {
    "request_id": <requestId>,
    "payload": <results>
  }
}

where <requestId> is the number taken from the request message, and <results> is an array of the values containing the function return value followed by the contents of the function output arguments.

If the request failed with some error (note: this is not the case when the PC/SC-Lite function returns non-zero error code), then the message will have the following format:

{
  "type": "pcsc_lite_function_call::response",
  "data": {
    "request_id": <requestId>,
    "error": <errorMessage>
  }
}

where <requestId> is the number taken from the request message, and <errorMessage> is a string containing the error details.

Additionally, Apps on both sides of the communication channel can send ping messages to each other:

{
  "type": "ping",
  "data": {}
}

The other end should response with a pong message having the following format:

{
  "type": "pong",
  "data": {
    "channel_id": <channelId>
  }
}

where <channelId> should be the number generated randomly in the beginning of the communication.

Pinging allows to track whether the other end is still alive and functioning (Chrome's long-lived messaging connections, when they are used, are able to detect most of the cases - but the one-time messages passing API is also allowed to be used). The <channelId> field value allows the other end to track cases when the App died and restarted while a response from it was awaited.

For simplifying the middleware Apps development, the wrapper libraries for JavaScript and C are provided (the latter one is basically an implementation of the functions defined in the original PC/SC-Lite headers). See the corresponding example Apps for the details (the example_js_smart_card_client_app/ and the example_cpp_smart_card_client_app/ directories), and the standalone JavaScript library (see the example_js_standalone_smart_card_client_library/ directory).

Common building prerequisites

Following are the common prerequisites required for building of the Apps:

  • OS: Linux.

    Building under different *nix system, Mac OS or Windows should be possible too, though most probably will require more efforts.

  • The following tools should be present in the system: bash, make, curl, sed, mktemp, realpath, xxd.

  • Python 2.7, including the dev package.

    Python 3.x is not supported yet.

  • git (version 2.2.1+ is recommended).

  • OpenSSL (version 1.0+ is recommended).

  • (for 64-bit Linux) 32-bit version of libstdc++.

    For example, on Ubuntu it's provided by the libstdc++6:i386 package.

  • Java Runtime Environment 7.

In order to run the built Apps, you will need either of these:

  • a Chromebook with Chrome OS >= 48.

    This will provide the closest environment to the real world's one.

    However, the disadvantage of this option is the inconvenient way of doing short development iterations: each time the built Apps will have to be somehow transferred to the Chromebook and installed onto it.

  • a locally installed Chrome browser with version >= 48.

    This option will save time during development, allowing to install and run the Apps easily on the local machine.

    For convenience, each App's Makefile provides a special run target that creates a temporary local Chrome profile and runs the browser with having the App installed and run into it. This allows to test the Apps locally, without interfering with the real Chrome profile.

    One downside of this option is that the desktop Chrome does not provide all the APIs that are provided under Chrome OS. The most noticeable example is the chrome.certificateProvider API: it's only available under Chrome OS, so its usages in the Apps will have to be stubbed out when executing locally.

    Another downside is that the desktop OS may require additional setup in order to allow Chrome (and, consequently, the Apps being executed in it) to access the USB devices. Some instructions are given in the Troubleshooting Apps under desktop OSes below.

Building

Follow these steps for performing the initial build:

  1. Execute:

    env/initialize.sh
    

    It's enough to execute this command only once, after you have cloned the whole repository (unless you would like to update to the latest tools versions).

    This will download and install locally the following dependencies required for building the Apps:

  2. Execute:

    source env/activate
    

    This command sets the environment variables required for enabling the use of the tools downloaded at step 1.

  3. Execute:

    ./make-all.sh
    

    This builds the Connector App, the C++ Example App and the JS Example App and all the libraries shared between them.

After that, you can perform incremental building of either all of the Apps (by running the command from step 3.) or of the single App you work on (by following its build instructions).

You should only make sure, however, that the environment definitions are always here - and, if not, use the command from step 2 for setting them up back.

Debug and Release building modes

During the development process, it's useful to enable the extended levels of logging and (depending on the actual App) the more extensive debug assertions checks.

Switching to the Debug building mode can be performed by adjusting the CONFIG environment variable, i.e. by executing the following shell command before building the Apps:

export CONFIG=Debug

This triggers a number of things, basically (for some additional details regarding concrete Apps refer to their own README files):

  • For the compiled JavaScript code - enables the creation of the source map allowing to view the uncompiled code when debugging.
  • For the JavaScript code built using the Closure library logging subsystem - selects more verbose logging level by default and enables printing extended details in the log messages (e.g. dumps of all parameters for some functions).
  • For the C/C++ code - undefines the NDEBUG macro, which enables some extended debug assertion checks, more verbose logging level and enables printing extended details in the log messages (e.g. dumps of all parameters for some functions).

However, please ensure that the publicly released Apps are always using the Release mode. Otherwise, the user's privacy may be harmed as the debug log messages may contain sensitive data.

The Release mode is the default building mode; you can switch to it back from the Debug build by adjusting the CONFIG environment variable, for example:

export CONFIG=Release

or simply:

unset CONFIG

Troubleshooting Apps under desktop OSes

Despite that the target platform of the Apps is Chrome OS, most of their functions can work correctly when run under desktop OSes (i.e. Linux, Windows, etc.).

However, there may be some limitations and difficulties met when working under desktop OSes:

  • chrome.certificateProvider Chrome API is unavailable.

    This is working as intended. This Chrome API, along with several others, is provided only on Chrome OS (see the Chrome App APIs documentation at <https://developer.chrome.com/apps/api_index>).

    The usages of such APIs will have to be stubbed out when running under desktop OSes.

  • On *nix systems, a system-wide PCSCD daemon may prevent Chrome from accessing the USB devices.

    The simplest solution is to stop the system-wide daemon.

    For example, under Ubuntu this can be done with the following command:

    sudo service pcscd stop
    
  • On *nix systems, the USB device file permissions may prevent Chrome from accessing the device.

    The simplest solution, described below, is to give the writing permissions for the USB device file to all users; note that, however, this is unsafe on multi-user systems!

    So granting the write access for all users can be performed in two ways:

    • One quick option is to add the permissions manually:

      sudo chmod 666 /dev/bus/usb/<BUS>/<DEVICE>
      

      Where <BUS> and <DEVICE> numbers can be taken, for example, from the output of the lsusb tool:

      lsusb
      
    • Another, more robust, option is to add a udev rule (see, for example, the documentation at <https://www.kernel.org/pub/linux/utils/kernel/hotplug/udev/udev.html>).

  • On Windows, a generic USB driver may be required to make the smart card reader devices available to Chrome.

    For example, this can be done with the Zadig tool (Note: this is a third-party application that is not affiliated with Google in any way. Use at your own risk!): <http://zadig.akeo.ie>.