Personal Cloud using Bittorrent Sync
Flavio Martins Marcos Vit
Faculdade de Engenharia Elétrica e de Computação – Universidade Estadual de
Av. James Maxwell 30, Cidade Universitária Zeferino Vaz, Distrito Barão Geraldo
CEP 13083-852 - Campinas-SP Brasil
Abstract. The BitTorrent peer-to-peer protocol has proved to be very efficient
for sharing files in a network without the coordination of central servers.
There are currently several cloud storage solutions such as Dropbox but
recent events involving USA government, private data leakage and spying
motivated the development of Bittorrent Sync for offering a personal cloud for
Bittorrent Sync by Bittorrent, Inc is a proprietary solution for sharing files among
other computers or mobile devices based on the Bittorrent peer-to-peer protocol, which
was adapted for this purpose. It is currently in Beta phase and it is available for
Windows, Mac, Linux, BSD, iOS and Android.
The files are shared only with the devices specified by the user. It requires no cloud and
it is fully independent of any servers.
Figure 1. BitTorrent Sync – Data sharing only among user’s devices
There is no data size limitation or subscription fees that come with traditional cloud
based solutions such as Dropbox.
This work will depict this new application checking its performance, security, usability
and possible new applications.
Bram Cohen developed the Bittorrent protocol and released the first version in 2001. It
proved along the last years to be a successful solution for sharing large volumes of files
using peer-to-peer networks.
There are available in the market many solutions for storing data remotely but all of
them relies on central sever for storing the data.
There was a gap for an application able to automatically sync files without the
dependency of a central coordination and not storing the files outside the user domain.
Bittorrent Sync was conceived to fulfilling this gap ensuring the user could have total
control of what is being shared.
2.1 P2P Protocol
BitTorrent Sync synchronizes your files using a peer-to-peer (P2P) protocol. This
protocol is very effective for transferring large files across multiple devices, and is very
similar to the powerful protocol used by applications like µTorrent and BitTorrent.
The data is transferred in pieces from each of the syncing devices, and BitTorrent Sync
chooses the optimal algorithm to make sure you have a maximum download and upload
speed during the process.
The devices chosen to be synched are connected directly using UDP, NAT traversal and
UPnP port mapping. It is also provided additional methods of ensuring connectivity as
relay and tracker servers. If the devices are on the same local network, BitTorrent Sync
will use the client LAN for faster synchronization.
BitTorrent Sync was designed with focus on privacy and security. The system uses SRP
 for mutual authentication and for generating session keys that ensure Perfect
Forward Secrecy. All traffic between devices is encrypted with AES-128 in counter
mode, using a unique session key. Modification requests are all verified using Ed25519
 signatures and only systems with full access keys can generate valid modification
The secret is a randomly generated 20-byte key. It is Base32-encoded in order to be
readable by humans. BitTorrent Sync uses /dev/random (Mac, Linux) and the Crypto
API (Windows) in order to produce a completely random string. This authentication
approach is significantly stronger than a login/password combination used by other
services. That's why using a secret generated by BitTorrent Sync is very safe and
The secret can be shared using copy and paste or using QR code and a camera from the
Figure 2. Using a QR code for connecting a mobile device to a sync folder
2.4 Peer Discovery
In order to find proper peers that have the same secret, Sync uses:
• Local peer discovery. Sending broadcast packets discovers all peers inside local
network. If there are peers with the same secret they respond to the broadcast
message and connect.
• Peer exchange (PEX). When two peers are connected, they exchange
information about other peers they know.
• Known hosts (folder settings). If there is a known host with a static ip:port, it
is possible to specify this in Sync client, so that it connects to the peer using this
• DHT. Sync uses DHT to distribute information about itself and obtain the
information about other peers with this secret. Sync sends
SHA1(Secret):ip:port to DHT to announce itself and will get a list of peers by
asking DHT for the following key SHA1(Secret)
• BitTorrent tracker. BitTorrent Sync can use a specific tracker server to
facilitate peer discovery. The tracker server sees the combination of
SHA1(secret):ip:port and helps peers connect directly. The BitTorrent Sync
tracker also acts like a STUN server  and can help do a NAT traversal for
peers so that they can establish a direct connection even behind a NAT.
2.5 Traffic Relay
There are rare cases when peers can’t talk directly. This usually happens when devices
are in an office behind strong firewalls. In such a case BitTorrent provides a relay server
to route traffic between peers. All traffic is AES encrypted with your secret, so there is
no chance for us to see any of your data.
3 Developer API
It is available an API for developing new applications based on BitTorrent Sync. It
offers to developers a wide range of possibilities. It is possible to integrate Sync into
new apps, build, improve, and change the way Sync works on the devices, or use the
API to automate workflows.
This API is based on JSON and gives almost full flexibility to customize Sync
behaviors, as examples below:
• Get folders and the secrets if available
• Add folders to sync
• Remove folder from sync process
• Get files from a synced folder
• Set synced files preferences, allowing to selectively allow or not a file to be
• Get the folders peers which are currently connected to the P2P network
• Set folder hosts instead using the tracker and relay server form BitTorrent Inc
• Get download and upload speed
• Shutdown gracefully
4 BitTorrent Sync Setup and Tests
The following test environment was setup for verifying and measuring Bittorrent Sync
usability and performance:
• OSX MacBook Pro
• Android Sony Xperia L mobile phone
• Windows 7 notebook
• 802.11g WIFI D-LINK DIR-655 router connected to the Internet via DSL
The first 2 devices above were connected to same local network connected the WIFI
router. The Windows notebook was located in other network reachable through the
Bittorrent Sync was installed on each device. The user interfaces were slightly different
among each other but in general it was straightforward to install and setup.
The usage of Bittorrent Sync is also very similar in each distribution.
It will be described in the following sections the test scenarios, results and network
packets analysis using Wireshark 1.10.3.
4.1 Configuring the Bittorrent Sync
Basically the synching setup is very similar on all operating system. It is needed to
specify a synching folder and generate a shared secret key, or it is possible to get the
secret key from someone else who want to share data content.
It is also possible to define if the sync will allow full access or read only permissions to
For more details about Bittorrent Sync configuration and setup please refer to its online
4.2 Synching Devices Connected to LAN
The following test scenario was executed with devices connected to a LAN with no
1) An empty sync folder was defined in the MAC Book Pro (IP address
2) A newly shared secret was generated for this folder.
3) Wireshark was used for capturing network traffic for further analysis.
4) A new sync folder was setup on the Xperia L mobile phone (IP address
192.168.0.101) and configured the sharing secret from step 2) using the QR code
and the mobile phone camera.
5) A test file was created in the Mac Book Pro executing the command below in
the sync folder defined in the step 1)
dd if=/dev/zero of=test_file_512kB bs=500k count=1
6) After some seconds (folder scan interval set to 60 seconds by default) the test
file was synched with the mobile phone.
7) The observed transfer rate was closer to the limit of the WIFI connection used in
the test (average speed around 6~8Mbps).
4.3 Synching Devices Connected through Internet
This test scenario was similar to the previous one despite the fact that the Windows
notebook was located on the Internet instead the local area network.
The tests results indicated that Bittorrent Sync performed very well enabling the sharing
of files among several devices (PCs and mobile phone). The installation and
configuration is straightforward. The synchronization process was tested on a local
network and also with a PC in the Internet. On both scenarios the synchronization was
successful and it was noticed data transfer rates closer to the limit of the router and
Internet connection used in the tests.
The captured data from Wireshark was analyzed and the following results were
1. The first interesting package sent from the MacBook was a multicast UDP with
the message “ping4:peer”. BitTorrent Sync was trying to locate peers in the
local network that have folders configured with the same secret.
2. During the folder sync setup it was noticed communication with IP address
22.214.171.124, which it is under Amazon domain. This indicated that BitTorrent
Sync is using services in Amazon for hosting its tracker or relay servers for
enabling the peer discovery over the Internet. The protocol used was DIS and
it was possible to check that message “get_peers4:peer“ was used. A response
was received back with message “peers“ which seems to be the information
about the available peers for the configured folder.
3. The data transfer synchronization was done among the peers using UDP.
BitTorrent Sync demonstrated to offer a very innovative solution for sharing content
using an efficient P2P protocol and highly secure data transmission, allowing the user to
create a personal cloud for data storage, free of subscription fees, disk space limitation
and able to run even when not connected to the Internet.
This solution also offers a very flexible developer API that enables the development of
other applications on top of BitTorrent Sync.
However, it was noticed the following negative points:
1) It could be unsafe to share the folder secret key with full access permission with
someone else. There is a risk of compromising the user data if the secret is
forwarded to unreliable parties.
2) The user must be very careful when defining full access directories since an
accidental content change or deletion will be synched among all devices,
affecting the data permanently.
3) As noticed on Wireshark capture logs during the tests, there is some
communication being performed from BitTorrent Sync to some external IPs.
When a sync folder is created, it is enabled by default the relay and tracker
server offered by BitTorrent Inc. Those servers are used by the P2P protocol for
enabling peers discover and communication through the Internet. Case those
servers are down the sync process will not work over Internet. In other hands, it
is possible to disable the tracker and relay servers, so the user can define any
other server than those offered by BitTorrent Inc. for acting as tracker and relay
4) It was checked in the BitTorrent forum a comment from a developer who works
for BitTorrent Inc. explaining that the sync process sends some data to
BitTorrent servers but it is just for statistics purposes and no user content data is
exposed. Looking in the Wireshark capture data it was not possible to check
what information is being transferred since the data was encrypted.
1. BitTorrent Sync
4. Secure Remote Password Protocol
5. Ed25519: high-speed high-security signatures
Daniel J. Bernstein, Niels Duif, Tanja Lange, Peter Schwabe, Bo-Yin Yang.
High-speed high-security signatures. Journal of Cryptographic Engineering 2
(2012), 77–89. Document ID: a1a62a2f76d23f65d622484ddd09caf8.
6. Session Traversal Utilities for NAT
RFC 5389, J. Rosenberg, R. Mahy, P. Matthews, D. Wing
7. IEEE Standard for Distributed Interactive Simulation--Application Protocols
IEEE Std 1278.1-1995