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OpenDrop: an Open Source AirDrop Implementation

OpenDrop is a command-line tool that allows sharing files between devices directly over Wi-Fi. Its unique feature is that it is protocol-compatible with Apple AirDrop which allows to share files with Apple devices running iOS and macOS. Currently (and probably also for the foreseeable future), OpenDrop only supports sending to Apple devices that are discoverable by everybody as the default contacts only mode requires Apple-signed certificates.

Disclaimer

OpenDrop is experimental software and is the result of reverse engineering efforts by the Open Wireless Link project. Therefore, it does not support all features of AirDrop or might be incompatible with future AirDrop versions. OpenDrop is not affiliated with or endorsed by Apple Inc. Use this code at your own risk.

Requirements

To achieve compatibility with Apple AirDrop, OpenDrop requires the target platform to support a specific Wi-Fi link layer. In addition, it requires Python >=3.6 as well as several libraries.

Apple Wireless Direct Link. As AirDrop exclusively runs over Apple Wireless Direct Link (AWDL), OpenDrop is only supported on macOS or on Linux systems running an open re-implementation of AWDL such as OWL.

Libraries. OpenDrop relies on current versions of OpenSSL and libarchive. macOS ships with rather old versions of the two, so you will need to install newer version, for example, via Homebrew. In any case, you will need to set the two environmental variables LIBARCHIVE and LIBCRYPTO accordingly. For example, use brew to install the libraries:

brew install libarchive [email protected]

Then set environmental variables:

export LIBARCHIVE=/usr/local/opt/libarchive/lib/libarchive.dylib
export LIBCRYPTO=/usr/local/opt/[email protected]/lib/libcrypto.dylib

Linux distributions should ship with more up-to-date versions, so this won't be necessary.

Installation

Installation of the python package is straight forward. After cloning this repository to <PATH>, install via pip3:

pip3 install <PATH>

Usage

We briefly explain how to send and receive files using opendrop. To see all command line options, run opendrop -h.

Sending a File

Sending a file is typically a two-step procedure. You first discover devices in proximity using the find command. Stop the process once you have found the receiver.

$ opendrop find
Looking for receivers. Press enter to stop ...
Found  index 0  ID eccb2f2dcfe7  name John’s iPhone
Found  index 1  ID e63138ac6ba8  name Jane’s MacBook Pro

You can then send a file using

$ opendrop send -r 0 -f /path/to/some/file
Asking receiver to accept ...
Receiver accepted
Uploading file ...
Uploading has been successful

Instead of the index, you can also use ID or name. OpenDrop will try to interpret the input in the order (1) index, (2) ID, and (3) name and fail if no match was found.

Receiving Files

Receiving is much easier. Simply use the receive command. OpenDrop will accept all incoming files automatically and put received files in the current directory.

$ opendrop receive

Current Limitations/TODOs

OpenDrop is the result of a research project and, thus, has several limitations (non-exhaustive list below). I do not have the capacity to work on them myself but am happy to provide assistance if somebody else want to take them on.

  • Triggering macOS/iOS receivers via Bluetooth Low Energy. Apple devices start their AWDL interface and AirDrop server only after receiving a custom advertisement via Bluetooth LE (see USENIX paper for details). This means, that Apple AirDrop receivers may not be discovered even if they are discoverable by everyone.

  • Sender/Receiver authentication and connection state. Currently, there is no peer authentication as in Apple's AirDrop, in particular, (1) OpenDrop does not verify that the TLS certificate is signed by Apple's root and (2) that the Apple ID validation record is correct (see USENIX paper for details). In addition, OpenDrop automatically accepts any file that it receives due to a missing connection state.

  • Sending multiple files. Apple AirDrop supports sending multiple files at once, OpenDrop does not (would require adding more files to the archive, modify HTTP /Ask request, etc.).

Related Papers

  • Milan Stute, Sashank Narain, Alex Mariotto, Alexander Heinrich, David Kreitschmann, Guevara Noubir, and Matthias Hollick. A Billion Open Interfaces for Eve and Mallory: MitM, DoS, and Tracking Attacks on iOS and macOS Through Apple Wireless Direct Link. 28th USENIX Security Symposium (USENIX Security ’19), August 14–16, 2019, Santa Clara, CA, USA. Link

Authors

  • Milan Stute (email, web)
  • Alexander Heinrich

License

OpenDrop is licensed under the GNU General Public License v3.0. We use a modified version of the python-zeroconf package (essentially adding rudimentary IPv6 and AWDL support) which is licensed under the GNU Lesser General Public License v2.1. Both licenses are found in the COPYING file.