Network investigation methodology for BitTorrent Sync: A Peer

Network investigation methodology for BitTorrent Sync: A Peer
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
Available online at www.sciencedirect.com
ScienceDirect
journal homepage: www.elsevier.com/locate/cose
Network investigation methodology for
BitTorrent Sync: A Peer-to-Peer based file
synchronisation service
Mark Scanlon*, Jason Farina, M-Tahar Kechadi
School of Computer Science and Informatics, University College Dublin, Belfield, Dublin 4, Ireland
article info
abstract
Article history:
High availability is no longer just a business continuity concern. Users are increasingly
Received 1 February 2015
dependant on devices that consume and produce data in ever increasing volumes. A
Received in revised form
popular solution is to have a central repository which each device accesses after centrally
2 May 2015
managed authentication. This model of use is facilitated by cloud based file synchronisa-
Accepted 13 May 2015
tion services such as Dropbox, OneDrive, Google Drive and Apple iCloud. Cloud architec-
Available online xxx
ture allows the provisioning of storage space with “always-on” access. Recent concerns
over unauthorised access to third party systems and large scale exposure of private data
Keywords:
have made an alternative solution desirable. These events have caused users to assess
BitTorrent Sync
their own security practices and the level of trust placed in third party storage services.
Distributed storage
One option is BitTorrent Sync, a cloudless synchronisation utility provides data availability
Peer-to-Peer
and redundancy. This utility replicates files stored in shares to remote peers with access
Network traffic analysis
controlled by keys and permissions. While lacking the economies brought about by scale,
Remote evidence acquisition
complete control over data access has made this a popular solution. The ability to replicate
data without oversight introduces risk of abuse by users as well as difficulties for forensic
investigators. This paper suggests a methodology for investigation and analysis of the
protocol to assist in the control of data flow across security perimeters.
© 2015 Elsevier Ltd. All rights reserved.
1.
Introduction
Applications such as Evernote and Dropbox leverage the
decreasing cost of hard disk storage seen in infrastructure as a
service providers, e.g., Amazon S3, to provide data storage on
the cloud to home users and businesses alike. The main
advantage of services such as Dropbox, Google Drive, Microsoft Skydive (now OneDrive) and Apple iCloud to the end user
is that their data is stored in a virtual extension of their local
machine with no direct user interaction required after
installation. It is also backed up by a fully distributed datacentre architecture that would be completely outside the
financial reach of the average consumer. Their data is available anywhere with Internet access and is usually machine
agnostic so the same data can be accessed on multiple devices
without any need to re-format partitions or wasting space by
creating multiple copies of the same file for each device. Some
services such as Dropbox, also have offline client applications
that allow for synchronisation of data to a local folder for
offline access.
* Corresponding author.
E-mail addresses: [email protected] (M. Scanlon), [email protected] (J. Farina), [email protected] (M.-T. Kechadi).
http://dx.doi.org/10.1016/j.cose.2015.05.003
0167-4048/© 2015 Elsevier Ltd. All rights reserved.
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
2
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
As Internet accessibility continues to become more
commonplace and allows for increasingly faster access, it is
not unexpected that many utilities that are intended for
general use will aid in the perpetration of some variety of
cybercrime. One attribute that is highly desirable by those
contemplating illegal activities is the notion of anonymity and
data security e especially the ability to keep data secure
transfer secure from inspection while in transit. BitTorrent
Sync (also referred to as BTSync, BitSync and bsync) is a file
replication utility that would seem to serve exactly this
function for the user. Designed to be server agnostic, the
protocol is built on already popular and widespread technologies that would not seem out of place in any network activity
log.
Each of the aforementioned consumer focused services
can be categorised as cloud synchronisation services. This
means that while the data is synchronised between user
machines, a copy of the data is also stored remotely in the
cloud. In recent headline news, much of this data is easily
available to governmental agencies without the need of a
warrant or just cause. BTSync provides the same synchronisation functionality (without the cloud storage aspect) and
provides a similar level of data availability. The service has
numerous desirable attributes for any Internet user
(BitTorrent Inc, 2013a):
Compatibility and availability e clients are built for most
common desktop and mobile operating systems, e.g.,
Windows, Mac OS, Linux, BSD, Android and iOS.
Synchronisation options e users can choose whether to
sync their content over a local network or over the Internet
to remote machines with no requirement for scripting or
schedule management making this an accessible technology compared to existing options such as RSYNC.
No limitations or cost e most cloud synchronisation services provide a free tier offering a small amount of storage
and subsequently charge when the user outgrows the
available space. BTSync eliminates these limitations and
costs. The only limitation to the volume of storage and
speed of the service is down to the limitations of the
synchronised users machines.
Automated backup e like most competing products, once
the initial install and configuration is complete, the data
contained within specified folders is automatically
synchronised between machines.
Decentralised technology e all data transmission and
synchronisation takes place solely in a Peer-to-Peer (P2P)
fashion, based on the BitTorrent file sharing protocol.
Encrypted data transmission e while synchronising data
between computers, the data is encrypted using RSA
encryption. Under the BTSync API, developers can also
enable remote file storage encryption (BitTorrent Inc,
2013b). e this could result in users storing their data on
untrusted remote locations for the purposes of data
redundancy and secure off site backup.
Proprietary technology e the precise protocol and operation of the technology is not documented by the developer.
There is debate over whether security through obscurity or
peer code evaluation, i.e., open source, is better. Some
enterprise security policies prohibit the use of open source
applications as a result of the source code being open to
inspection by those looking for flaws in the implementation. From the point of view of the consumer, BitTorrent
Inc. have stated that they will not give access to traffic to
any LEA without due process and the bespoke protocol
makes casual eavesdropping or crawling less likely.
As a result of these attributes, BTSync has grown to
become a popular alternative to cloud based synchronisation
services. Less than a year after its release, the active user base
had grown to over one million by November 2013, doubling to
two million by December 2013 (BitTorrent Inc, 2013c), and to
over ten million users by August 2014 (BitTorrent Inc, 2014).
Due to this rapid growth and popularity the service will undoubtedly be of interest to both law enforcement officers and
digital forensics investigators in future investigations. Like
many other file distribution technologies, this interest may be
centred around recovery of the data itself, proof of the modification of data or evidence of data distribution and enumeration of the recipients.
While BTSync is based on the same technology as BitTorrent for the transfer of files, the intention of the application is
quite different. This results in a change of users' behaviours,
as well as a necessary change in the assumptions an investigator should make. BitTorrent is designed to be a one-to-many
data dissemination utility. The uploader usually does not care
about the identity of the downloader and a single seeder can
deliver data to a large number of unique peers over the life of
the torrent file. Data integrity and transfer speed take precedence over privacy of data in transit.
BTSync on the other hand, is designed to be a secure data
replication protocol for making a faithful replica of a data set
on a remote machine. Data integrity is still highly prised but
data privacy is now the top priority and speed-throughdispersion is sacrificed as a result. The files can only be read
by users specifically given access to the repository. The
advertisement of data availability is completely scalable by
the owner with options ranging from restricting access to
known IP addresses through to registration with a centralised
tracker. Given the nature of the application, users are much
more likely to know the operator of the remote site (this does
not apply to secrets advertised online though that could be a
point of commonality that would not necessarily have existed
for pure BitTorrent clients).
1.1.
Aim and contribution of this work
The aim of this work is to provide a reference for digital investigators discovering the use of BitTorrent Sync in an active
investigation. However, it is hoped that the analysis presented
may be of use to security personnel looking to detect and
control the use of this protocol within their perimeter.
To accommodate these goals this work presents an analysis of the protocol and its network interaction. Activities
undertaken to perform a synchronisation are presented and
described at the packet level in order to facilitate both post
mortem traffic analysis and to enable the development of
feature based detection rules and deep packet inspection for
Network Intrusion Detection Systems (NIDS) or firewall
appliances.
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
The contribution of this work presents a suggested a
network investigation methodology for BitTorrent Sync, outlined in Section 5. This methodology includes recommendations for the investigation of a number of hypothetical
scenarios where BTSync could be used to aid in criminal or
illicit activities. Legitimate usage of the system, e.g., backup
and synchronisation, group modification, data transfer between systems, etc., may itself be of interest to an investigation. However, the technology may also be suitable in the aid
of a number of potential scenarios of interest such as industrial espionage, copyright infringement, sharing of illicit images of children, etc., outlined in greater detail in Section 2.3.
This work also documents each of the observed packets sent
and received during regular operation of BTSync. Finally, the
results from two digital forensic investigations of the service
are outlined in Section 6 and Section 7 respectively.
2.
Background
In order to gain an understanding of how BTSync functions,
one must first understand the technologies upon which it is
built. The application is a product built by BitTorrent Inc. (the
creators and maintainers of the eponymous file-sharing protocol). As a result, the technologies used by the regular BitTorrent protocol and BTSync are developed using a similar
premise. This section provides a brief overview of the required
background information and outlined the key differences
between the two applications.
2.1.
BitTorrent file sharing protocol
The BitTorrent protocol is designed to easily facilitate the
distribution of files to a large number of downloaders with
minimal load on the original file source (Cohen, 2008). This is
achieved through the downloaders uploading their completed
parts of the entire file to other downloaders. A BitTorrent
swarm is made up of both seeders (peers with complete copies
of the content shared in the swarm), and leechers (peers who
are downloading the content and may have none or some of
the content). Due to BitTorrent's ease of use and minimal
bandwidth requirements, it lends itself as an ideal platform
for the unauthorised distribution of copyrighted material. The
unauthorised distribution of copyrighted material typically
commences with a single original source sharing large sized
files to many downloaders.
2.1.1.
Bencoding
Bencoding is a method of notation for storing data in an array
list. The main advantage of bencoding is that it avoids the
pitfalls of system-byte order requirements (such as big-endian
or little-endian), which can cause issues for cross platform
communication between applications. The datagram packet
can easily be converted to a human readable UTF-8 encoded
sequence of key: value pairs. Indicative key: value pairings are
presented in Table 1.
The value for any pair is stored as a sequence of-bytes with
the exception of integer values. Associated with the integer
indicating keys, bencoding uses the lowercase “i” to indicate
3
the start of an integer value, which is also terminated with a
lowercase “e”.
2.1.2.
Active peer discovery
Each BitTorrent client must be able to identify a list of active
peers in the same swarm who have at least one piece of the
content and is willing to share it, i.e., identify a peer that has
an available open connection and has the bandwidth available
to upload. By the nature of the implementation of the protocol, any peer that wishes to partake in a swarm must be able to
communicate and share files with other active peers. BitTorrent provides a number of methods available for peer discovery. There are a number of methods that a BitTorrent client
can use in an attempt to discover new peers who are in the
swarm outlined below.
1. Tracker communication e BitTorrent trackers maintain a
list of seeders and leechers for each BitTorrent swarm they
are currently supporting (Cohen, 2003). Each BitTorrent
client will contact the tracker intermittently throughout
the download of a particular piece of content to report that
they are still alive on the network and to download a short
list of new peers on the network.
2. Peer exchange (PEX) e as set out in the standard BitTorrent
specification, there is no intercommunication between
peers of different BitTorrent swarms besides data transmission. Peer Exchange is a BitTorrent Enhancement Proposal (BEP) whereby when two peers are communicating
(sharing the data referenced by a torrent file), a subset of
their respective peer lists are shared during the
communication.
3. Distributed hash tables (DHT) e many BitTorrent clients,
such as Vuze and mTorrent contain implementations of a
common distributed hash table as part of the standard
client features. The common DHT maintains a list of each
active peer using the corresponding clients and enables
cross-swarm communication between peers. Each known
peer active in swarms with DHT contributors is added to
the DHT. The mainline BitTorrent DHT protocol (also used
by BTSync), is based on the Kademlia protocol. Regular
BitTorrent file-sharing users and BTSync users contribute
to the update and maintenance of the DHT. The DHT provides an entirely decentralised approach aiding in the
discovery of new peers sharing particular pieces of content.
The Kademlia DHT structures its ID space as a tree (Li et al.,
2005). The distance between two keys in the ID space is
their “exclusive or” (xor). Each user in the DHT generates a
unique key that is used for identification when connecting
to the DHT. The piece of the DHT that each peer stores is
related to this xor calculation. Those peerIDs that are
closest to the key, e.g., a torrent's info_hash, are responsible for facilitating lookups for those keys. The same DHT
responsible for regular BitTorrent file-sharing is also
responsible for maintaining a lookup for BTSync shared
content. In this scenario, the key used is based on the
public read-only key generated for each shared folder in
BTSync.
While a DHTs decentralised nature results in a much more
resilient service compared to server based tracker, it also
results in it be vulnerable to certain attacks, as outlined in
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
4
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
Table 1 e BTSync packet bencoding fields.
Key
Explanation
d:
l:
Marks the start of a dictionary
List start, the start of a list of field:
value pairs in an array. Lists are
terminated with an “e”
Local address IP: Port in networkbyte order
External address IP: Port in networkbyte order
Message type header, e.g., ping
[PeerID]
[ShareID]
16-byte nonce for key exchange
between peers negotiating data
exchange
Marks the end of a dictionary or list
la:
ea:
m:
Peer:
Share:
Nonce:
e:
greater details in Sit et al.,'s 2002 paper (Sit and Morris,
2002).
4. Local peer discovery (LPD) e this is enabled by checking the
“Search LAN” option in most BitTorrent client's application
preferences. When enabled the application will announce
its availability to potential local peers using multicast
packets. Once a client on the network receives a multicast
packet, that client will check its current list of shares to see
if a match is found. Is a match it found, that peer will
respond to the origin of the request offering to synchronise
the content.
2.1.3.
Downloading of content through BitTorrent
To commence the download of the content in a particular
BitTorrent swarm, a metadata.torrent file or a corresponding
magnet universal resource identifier (URI) must be acquired
from a BitTorrent indexing website. This file/URI is then
opened using a BitTorrent client, which proceeds to identify
other active peers sharing the specific content required. The
client application then attempts to connect to several active
members and downloads the content piece by piece. Each
BitTorrent swarm is built around a single piece of content
which is determined through a unique identifier based on an
SHA-1 hash of the file information contained in this UTF-8
encoded metadata file/URI, e.g., name, piece length, piece
hash values, length and path.
2.2.
BitTorrent Sync
BTSync is a file replication utility created by BitTorrent Inc.
and released as a private alpha in April 2013 (BitTorrent Inc,
2013a). It is not a cloud backup solution, nor necessarily
intended as any form of off-site storage. Any data transferred
using BTSync resides in whole files on at least one of the
synchronised devices. This makes the detection of data much
simpler for digital forensic purposes as there is no distributed
file system, redundant data block algorithms or need to contact a cloud storage provider to get a list of all traffic to or from
a container using discovered credentials. The investigation
remains an examination of the local suspect machine. However, because BTSync uses DHT to transfer data there is also
no central authority to manage authentication or log data
access attempts. A suspect file found on a system may have
been downloaded from one or many sources and may have
been uploaded to one or more recipients. Additionally while
the paid services offer up to 1 TB of storage (Amazon S3 paid
storage plan), the free versions which are much more popular
with home users cap at approximately 10 GB. BTSync is
limited only by the size of the folder being set as a share.
Another concern for any investigation into BTSync folders is
that unless the system being examined is the owner/originator of the folder being shared, it is quite possible that any
files present were downloaded without prior knowledge of
their content or nature. Before v2.0, BTSync had no built in
content preview facility in its protocol, it merely blindly synchronises from host to target without any selection process
available to the user. In v2.0, an option was added to the
preferences for each folder that allows the user to only synchronise file titles as a zero byte place holder file. If the file is
selected the content of the file is downloaded. An update to
the link descriptor in v1.4 allows users to get an approximation of the share size at the time of joining.
2.2.1.
Keys
The “secrets” used as part of the original release of BTSync
were renamed as “keys” in v1.4. The structure has not been
changed however and still consists of a 33 character human
readable string consisting of a Base32 encoded string generated when the folder was first provisioned. This Base32
pattern is then prepended with a single letter indicating its
nature. Keys are the unique identifiers used by the BTSync
service to differentiate between shared folders. In order for
the 20-byte keys to be human readable, they are displayed
using Base32 encoding (BitTorrent Inc, 2013a). BTSync facilitates the generation of three categories of secrets for the
sharing of data contained within specific folders, as can be
seen in Fig. 1.
The initial Read & Write (RW) key is still generated using
CryptoApi on Windows based systems (this is downloaded as
part of the installation process if it is not installed already).
This RW key is the equivalent of the original “master secret” in
that, if it is shared then the receiving party has an equal level
of access to the share as the original owner including the
ability to delete content and add new content that will be
replicated to any synchronising peer whether downstream or
of equal rank.
From this initial RW key, a read only key (RO) is generated
automatically for sharing. As can be seen in Fig. 1, these are
the only two keys readily available to the user. However, these
are not the only keys available for use. BTSync defines six
Fig. 1 e Keys (formerly secrets) are generated at share
provision. The ability to view the keys is not available in
v2.0.
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
standard keys of which three can be generated using the
default installation of the desktop client. These keys are
identified by their prepended letter as follows:
[A] This is the RW key generated at the time the share is
provisioned. This key gives the user full control over the
share contents.
[B] This identifies the read only key and can be used to
create a child, or downstream, peer that can only replicate
share contents from another peer. Any changes made to
share contents, including deletion, will invalidate the file
changed and prevent any further replication actions for
that particular file in the future, or until the share is reprovisioned on that client (or the share's *.db file is
altered but this may cause the entire share to be deemed
invalid).
[C] The C type key is a read only one-use key that is discarded after its first use. This key can be generated from
either type A or type B keys and is used primarily in the
distribution of other keys.
[D] Generated through the use of the Sync API, this type of
key allows read & write access to encrypted shares.
[E] A read only key capable of replicating data from type D
encrypted shares and decrypting the contents. This key is
calculated form the type D key and so is not possible using
that standard BTSync v1.4 or v2.0 installation.
[F] Encrypted read only key capable of replicating data from
an encrypted share but unable to decrypt the share contents. This type of key can be used to store data in an
encrypted state on a remote, untrusted, system and still
provide authenticity and availability.
Older versions of these, such as the ‘R’ prepended read
only key of v0.x are still usable but are no longer generated
by the application. As with the earlier BTSync versions, a
user may also generate his or her own key that has been
Base64 encoded. As a result, these default prepended identification letters cannot always be taken as a definite indicator of the access level granted by a key before it has been
applied.
The Keys outlined above need never necessarily be shared
publicly, i.e., any user can create a number of keys solely for
his personal use across his different machines. Depending on
the level of access the user wishes to give to a third-party, he
can give the corresponding key to any other user through
regular one-to-one communication methods (e-mail, instant
messaging, social networking, SMS, etc.). If public distribution
is desirable, there are a number of public online avenues for
BTSync users to share secrets with each other (e.g. www.
btsynckeys.com, http://www.reddit.com/r/btsecrets, among
others). Version 1.4 presents a change to the method of
sharing a link with a peer that has been modified further in
v2.0. In v1.4, a user can still view the RW and RO key of a share
and can copy this key and send it via any medium to the
remote device. Using this method, the remote device user
adds a new share and inputs the key causing the share to
automatically query a tracker (if this option is left enabled) for
the location of remote peers hosting a share matching the
applied key. An alternative to this method was added to the
client and works as follows:
5
In the application the user that currently has access to the
share (the owner) can select the option to provision the share
to another user (a peer which can be a different person or a
remote system under the control of the owner), as depicted in
Fig. 2, and is presented with a choice of restrictions and
methods presented as options.
2.2.1.1. Permissions
Read only (default)
Read & write
2.2.1.2. Security options.
Invited participants must be approved e the owner will
receive notification in the application that a peer wishes to
share the resource. The device ID of the remote peer will be
presented and the owner can accept or reject their membership. This option is enabled by default.
Expiration date e the link to the share will only remain
active for a set number of days from the time it is generated. This option is enabled by default and the time limit is
set to three days, but can be changed to any number of days
the owner inputs.
Number of uses e this option allows the owner to limit the
number of times a link can be used to join a share. This is
set to off by default.
The link generated by this process is presented as https://
link.getsync.com/[URLoptions], where the URL options are
each separated by an ampersand. For example a link shared
from v1.4 for a folder called winhex with no expiry or usage
limitation would present as https://link.getsync.com/
#f¼winhex&sz¼35E5&s¼XIQSFD2MCDPS2QKITWKJROJ2VU
SV2YNA&i¼CKKR3V2BBM7MXIOTPU3XWK55JBUFWG3EY
&p¼CALSNMDGCZZAUQXBXEIR6Q57UMTVOSFI&e¼143127
7452 where:
#f ¼ (folder name of the share in plain text)
sz ¼ (approximate size of the share contents)
s ¼ (the shareID of the folder encoded in Base32)
i ¼ (a one time key used to provide access to the real key,
this changes every time the link for the folder is generated)
p ¼ (PeerID of the peer performing the server role in the
upcoming key exchange)
e ¼ (the expiry timestamp of the link if it is set, if it is not set
this item will not be present in the link)
v ¼ (the version of the client. This is only present in the v2.0
client and is not optional)
This URL can be copied to the system clipboard, sent via
email (the email option will open the default mail application
on the system) or converted to a QR code for scanning by a
mobile device.
At a minimum the link must contain the folder, shareID
and one time key fields to resolve to a share if entered directly
into a browser however removing the version may cause the
actual replication to fail if the remote version is incompatible
with the version adding the share. An example how this
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
6
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
Fig. 2 e Key sharing is now managed from within the application with optional restrictions.
stripped down link resolves is shown in Fig. 3. Once an option
is selected, the share link is converted into a URL that can be
opened by the locally installed client if the client satisfies all of
the requirements such as version number.
An alternative to opening the link in a web browser is to
enter the link in the client itself as if it was a share key, as
shown in Fig. 4. However, if the version is not correct the
replication will fail and, if authorisation is required, the
request will never be sent to the owner.
The process of joining a share has also been changed in
v1.4 and v2.0. Using the .509 security certificates and public
private key pairs stored in the sync.dat file in the BitTorrent
Sync folder. Once a host address is retrieved a connection is
made and a request for the RO or RW key is sent using the
One-Time-Key (i in the optional data) along with the peer's
public key generated the first time a link is received or
generated. The user and device name set at this time will be
the user and device name that the owner will see if they check
the identity of the peer requesting access. The device name
will also be present in the device list available for each share
as can be seen in Fig. 5. Once authorised, the requesting peer
receives a copy of the required key encrypted with their public
key which they then decrypt and apply to the share on their
end of the connection. Once complete the process of synchronisation can begin and the new peer will be registered on
the tracker if that option is left enabled.
2.3.
Potential scenarios pertinent to digital forensic
investigation
2.3.1.
Fig. 3 e A received link can be shortened and still be
resolved to a share by the server.
Industrial espionage
Many companies are aware of the dangers of allowing BitTorrent traffic on their networks. However, quite often
corporate IT departments enforce a blocking of the technology
through protocol blocking rules on their perimeter firewalls.
This has the effect of cutting off any BitTorrent clients
installed on the LAN from the outside world. In addition to
deep packet inspection (DPI) to investigate the data portion of
a network packet passing the inspection point, basic blocking
of known torrent tracker sites using firewall rule sets can be
used. BTSync does use BitTorrent as the protocol for file
transfer but once the transfer session is established using the
BTSync protocol all traffic is encrypted using AES and may not
be open to inspection by a firewall. It also does not follow the
current known patterns that would identify an encrypted
BitTorrent stream as the targetesource profile is different.
Blocking t.usyncapp.com and r.usyncapp.com will stop the
tracker and relay options from being used but BTSync can
operate quite well without those services. Local peer discovery
can use multicast or direct “known peer” configuration where
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
7
Fig. 4 e A received link can also be added in the section to manually add a share.
a known IP: Port combination is used to identify a specific
machine allowed to participate in the share. This specificity
would negate the issue of multicast packets usually not being
routed beyond the current network segment. A scenario
where BTSync can be used to transfer files within a LAN would
be to transfer data to a machine with lower security protocols
in place such as the capability to write to a USB device or
perhaps even unmonitored access to the Internet (and the
BitTorrent protocol) through a designated guest LAN.
The BitTorrent Sync API (BitTorrent Inc, 2013b) adds the
functionality to generate an “encryption secret”. Through the
use of encryption secrets, a BTSync user has the ability to
remotely store encrypted data, e.g., personal, sensitive or
illegal, on one or more remote machines. These remote machines do not have the ability to decrypt the information
stored. The data could then be securely wiped off the original
machine and easily recovered at a later stage.
2.3.2.
2.3.4.
Cloudless backup
By synchronising between two or more machines accessible
to the user, data can be stored in multiple locations as a form
of backup. The secondary copies of a file would be stored using
a read only key so that only changes on the primary system
will ever replicated. A feature of BTSync that is enabled by
default but can be disabled in the configuration file, is the use
of the SyncArchive folder that stores a copy of any file deleted
or changed for a preset period of time allowing for a form of
file recovery or versioning.
2.3.3.
Encrypted remote P2P backup
Dead drop
Due to BTSync's intended use as a file replication utility, it is
assumed that a person receiving a copy of a shared directory is
aware of the contents of the folder. As a result, no method was
included to gather details of the contents of a share before
synchronisation. The API (BitTorrent Inc, 2013b) introduced
this function but only a node configured correctly with an API
key will return a folder or file listing when queried.
Fig. 5 e Requests for access can be verified (redacted) and share members can be reviewed.
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
8
2.3.5.
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
Secure P2P messaging
For example, the proof of concept found at http://missiv.es/.
The application currently operates by saving messages to an
“outbox” folder that has a read only key shared to the person
you want to receive the message. They in turn send you a read
only key to their outbox. One to many can be achieved by
sharing the read only key with more than one person but no
testing has been done with synchronisation timing issues yet
and key management may become an issue as a new outbox
would be needed for each private conversation required.
2.3.6.
Piracy
e BitTorrent, like any other P2P technology, was designed for
one-to-many distribution of large content and has become
almost synonymous with piracy. BTSync was not necessarily
intended to be a one-to-many distribution utility. However, it
does allow for a group of users to set one another as “known
peers” so that they can communicate directly through
encrypted channels. Websites such as http://btsynckeys.com/
have examples of users posting keys publicly and advertising
the content as being copyrighted material.
2.3.7.
Serverless website hosting
e This involves the creation of static websites served through
a BTSync shared folder. These websites could be directly
viewed on each user's local machine. The local copies of the
website could receive updates from the webmaster automatically through the synchronisation of the content associated
with a read only secret.
2.3.8.
Malicious software distribution
e Due to the lack of any trust level being associated with any
publicly shared secret, the synchronised files may contain
infected executables.
For each of the above scenarios, an added dimension can
be created by the BTSync user: time. Due to the ability to
create “throw away” or temporary secrets for any piece of
content, the timeframe where evidence may be recovered
from remote sharing peers might be very short.
3.
Related work
This paper is focused on the network communication protocol
employed by BTSync and the investigation thereof. The work
presented as part of this paper builds upon the work of Farina
et al. (Farina et al., 2014), which outlines the forensic analysis
of the BTSync client application on a host machine. This paper
outlines the procedures for identifying a current or previous
install of the BTSync application and the extraction of secrets
from gain physical access to a machines hard drive and performing a regular digital forensic investigation on its image. At
the time of publication, there are no other academic publications focusing on BTSync. However, seeing as BTSync shares a
number of attributes and functionalities with cloud synchronisation services, e.g., Dropbox, Google Drive, etc., and is
largely based on the BitTorrent protocol, this section outlines
a number of related case studies and investigative techniques
for these technologies.
3.1.
BitTorrent forensics
Numerous investigations have been made into identifying the
peer information of those involved in BitTorrent swarms.
Most of these publications focus on the investigation of the
unauthorised distributed of copyrighted material Layton and
Watters (2010); Scanlon et al. (2010) and Le Blond et al.
(2010). Depending on the focus of the investigation, peer information may be recorded for a particular piece of material
under investigation or a larger landscape view of the peer
activity across numerous pieces of content.
3.2.
Client-side synchronisation tool forensics
Forensics of cloud storage utilities can prove challenging, as
presented by Chung et al. in their 2012 paper (Chung et al.,
2012). The difficulty arises because, unless complete local
synchronisation has been performed, the data can be stored
across various distributed locations. For example, it may only
reside in temporary local files, volatile storage (such as the
system's RAM) or dispersed across multiple datacentres of the
service provider's cloud storage facility. Any digital forensic
examination of these systems must pay particular attention to
the method of access, usually the Internet browser connecting
to the service provider's storage access page (https://www.
dropbox.com/login for Dropbox for example). This temporary access serves to highlight the importance of live forensic
techniques when investigating a suspect machine as a “pull
out the plug” anti-forensic technique would not only lose access to any currently opened documents but may also lose any
currently stored sessions or other authentication tokens that
are stored in RAM.
In 2013, Martini and Choo published the results of a cloud
storage forensics investigation on the ownCloud service from
both the perspective of the client and the server elements of
the service (Martini and Choo, 2013). They found that artefacts
were found on both the client machine and on the server
facilitating the identification of files stored by different users.
The module client application was found to store authentication and file metadata relating to files stored on the device
itself and on files only stored on the server. Using the client
artefacts, the authors were able to decrypt the associated files
stored on the server instance.
3.3.
Extension of the digital evidence acquisition window
In 2014, Scanlon et al., outlined a case study on BTSync
whereby the remote recovery of evidence from a BTSync
shared folder can enable the recovery of evidence that is no
longer accessible on the local machine (Scanlon et al., 2014).
This evidence may have been securely deleted, corrupted or
overwritten on the local device or viewed (not stored) on a
mobile device using the BitTorrent Sync app. The paper outlines a number of entry points from the local machine into the
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
investigation and the remote recovery of such evidence
including local and network sources.
4.
BitTorrent Sync network protocol analysis
Starting with the beta release of v1.4, BTSync changed its protocol to more closely resemble that of the underlying BitTorrent protocol. In addition to changes to the directory structure
and the introduction of public/private key storage for shares,
the network traffic profile of the protocol changed dramatically
by utilising the Micro Transmission Protocol (mTP) as outlined
in the BitTorrent Extension to Protocol (BEP) 29, which is officially specified here: http://www.bittorrent.org/beps/bep_
0029.html. This protocol was already used by BitTorrent once
actual file transfer was initiated but now BTSync has adapted
its communications to use mTP signalling resulting in a smaller
overall usage of bandwidth but a more noticeable footprint.
Where the initial release of BTSync used custom packets
that all started with the header BSYNC[00] or BSync[80], this
purely cosmetic identifying header was replaced with the mTP
DATA version 1 (01) header for all request and transfer packets
and STATE (21) was used to perform the same functionality of
the original PING used to update peer availability and provide
connection details and data.
As with the original mTP protocol the connection management packets and headers used by BTSync v1.4 and onwards
are:
SYN: initiates the two-way mTP handshake to establish a
connection with the remote peer. This packet has its type
indicator set to 4.
STATE: the most common packet in mTP, this “ACK” replaces the BTSync response to PING and serves as both the
keep-alive and the response to the handshake initiation.
This packet is identified by the type value of 2.
DATA: this packet is used to carry messages such as the
peer request message sent to the tracker or the peer list
sent in response. This packet has a type value of 0.
RST: as with TCP the RST packet is used to reset the
connection in the event of an error in transmission. This
has a type identifier set to 3
FIN: Indicates the end of a connection and is denoted by
the type value of 1.
The mTP message headers have a similar layout that is
formatted as follows:
9
On provision of a new share several options are enabled
automatically by the application as shown in Fig. 6. These
options can be disabled or re-enabled by the user at any time
to customise the network behaviour of the local repository
being edited. These changes can also be managed through
direct editing of the application configuration files. The
default behaviour for BTSync is to utilise the tracker server at
t.usyncapp.com. The DNS request resolves to four IP addresses: 52.0.102.230, 52.0.104.40, 52.1.40.103 and 52.1.1.135.
These four IP addresses are servers hosted on Amazon's EC2
cloud service. The BTSync tracker facilitates peer discovery
for clients looking to synchronise data. One peer request
message is sent for each share stored on the local machine
and the act of requesting a peer lookup also serves to register
the requesting client as a source for that share.
Packets sent from the client to the tracker server contain
registration details and get_peers message requests (when a
new share is created it registers the share with the tracker
using a get_peers packet). A get_peers packet takes the form
of:
(where the observed keys are defined in Table 2).
This packet is initially sent to the tracker server via TCP
and UDP to test connectivity. If both protocols succeed, UDP
is the preferred method of communication. Tracker updates
are performed at a rate of once every 600 s or if a change is
made to the share data, in which case the timer is reset. A
separate packet is sent for each share present on the local
machine. It is noteworthy that, even when a new share is
created, the first packet advertising that share to the server
uses a message type of get_peers. Depending on the bandwidth usage it is entirely possible for a single peer to
simultaneously contact and register with multiple tracker
server addresses. Each share will have its own Connection
ID value in the mTP header for that get_peers packet and
each request will prompt a separate type 2 (ACK) response
from the tracker server followed by a separate response to
the request itself.
The receiving tracker will respond to the requesting client
with the same protocol used in the get_peers message. This
has the consequence that if TCP and UDP are successful on the
first request, the first response will be a set of duplicate TCP
and UDP packet in the form:
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
10
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
Table 2 e Sample tracker packet.
mTP
000
d
2:la
2:lp
1:m
9:get_peers
4:peer
20:
5:share
20:
32:
where the observed keys are defined in Table 2The peer list
returns an entry for each peer currently in contact with the
tracker through get_peer requests. The current requesting
peer will be included in this list so the peers message will always have at least one entry in the peers list.
One unusual feature of the peers response is the inclusion
of a peer's local, non-routable, IP address and Port. This is so
that, if the local IP matches the local subnet of the requesting
peer, the requesting peer can attempt to communicate
directly over the LAN using the local address provided. If the
tracker server option is disabled then the local client will have
to use a different method to find peers local to it.
4.1.
The mTP data header that signifies
Null
Start of the dictionary of key: value
pairs
Local address label identifier which
consists of 6-bytes, the first 4 are IP,
the last two are port
local port in integer form
Message label identifier
message type value
Local peer label
Local PeerID
Local ShareID label
The 20 character ShareID a transform
of the secret used and can be found in
the.SyncID file.
A shareID based on some transform
of the 20 byte ShareID, the local IP
address and local port.
The format of these packets has not changed since the
original pre v1.4 BTSync. Once LAN discovery is enabled the
local neighbouring peers will respond to the multicast
broadcast with the “BSYNC[00]” TCP packet detailed below.
Once a peer receives a multicast message that contains a
ShareID that it possesses the peer responds with the content:
Local peer discovery
When the option to search LAN is enabled (the default
behaviour) the application will start sending out multicast
packets to port 3838 across the LAN. The multicast packets are
BTSync bencoded packets with the following format and the
keys are further explained in Table 3.
The keys have the same definitions as those shown in
Table 3 with the exception of the ShareID being the more
familiar 20 byte version.
Once the Ping has been sent the peers perform a BTSync
session negotiation involving the generation of a nonce value
as laid out in Table 1. The rest of the synchronisation takes
place over TCP IP and the mTP traffic runs alongside over UDP.
The synchronisation process is signed off with a mTP Type 1
(FIN) packet. After this there are regular mTP type 2 (STATE)
messages to check for changes.
Fig. 6 e A newly created share will have some preferences set by default that can be toggled by the user.
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
Table 3 e Multicast ping packet.
BSYNC
000
d
1:m
4:PING
4:peer
20:
5:share
32:
4.2.
The BTSync header
Null
Start of the dictionary of key: value
pairs
Message label identifier
The message type
Local peer label
PeerID of the multicasting peer
Local ShareID label
The Share32 ID that matches that
used in the v2.0 get_peers
11
5. The relay server confirms the SID status and supplies the
remote nonce to complete the bridge for encrypted data
transfer
6. The relay server contacts the local peer to deliver the
remote public key
7. The local peer delivers its public key to the relay server
8. Encrypted bidirectional traffic transfer commences with
the relay server acting as the router delivering packets to
each peer.
BTSync relay server
When BTSync finds that it needs to communicate directly
between two firewalled peers, the application may make use
of a relay server. The “Use Relay Server if required” option is
enabled by default on share creation. The relay server is
contacted by a DNS request sent out for r.usyncapp.com,
which resolves to the following IP addresses: 67.215.229.106
and 67.215.231.242.
These are the IP addresses of the relay servers contactable
on remote port 3000. Each peer contacts the relay server using
an outbound connection that should bypass any firewall rule
preventing unauthorised inbound connections. Once the
server handshake has taken place, the negotiation to set up a
secure connection between the two peers begins. The
following sequence of events is observable:
1. Peer contacts the relay server to initiate contact with the
remote peer.
2. The relay server responds to the peer using a standard TCP
ACK packet
3. The peer contacts the server to arrange transfer of the data
and to supply the nonce for encrypted traffic and provide a
status ID.
4. The relay contacts the peer to initiate the session counters
4.3.
BTSync data transfer
The transfer of data during a BTSync synchronisation operates in a similar fashion as a regular BitTorrent download as
described in Section 2.1.3 above. A unique magnet URI is
created for each file contained within the shared folder and
this is used for requesting chunks of the entire file from
known peers sharing this content.
4.4.
Differentiation from regular BitTorrent traffic
While much of the network topology of BTSync is shared with
regular BitTorrent, the request and response packets differ from
those employed by regular BitTorrent file-sharing traffic. The
most obvious addition is the BSYNC header attached to each
datagram transmitted on the network. In addition, the introduction of mTP causes increased volume of traffic recognisable
even though mTP results in lower overall bandwidth usage. Besides that addition, the active peer list that is returned also
contains additional information over the regular BitTorrent filesharing protocol: namely the inclusion of the local IP: port
address pairs for each peer. From an investigative perspective,
this extra information could prove useful in identifying the
particular machine involved in the BTSync network as opposed
to merely resolving the WAN IP address back to a router with
potentially hundreds of LAN users. The local DHCP records could
be used to resolve the MAC address (and often the hostname) of
the individual machine identified during the network
investigation.
In addition to the regular BitTorrent peer discovery
methods outline in Section 2.1.2 above, BTSync also allows the
user to manually add known IP addresses to the local cache of
peers. BTSync facilitates this through the option to add “Predefined Hosts” to the configuration or application options.
These are hardcoded IP address and port entries that are
saved in order of preference. BTSync will contact these peers
directly, without any requirement for a multicast (LPD) or
sending a get_peers request to an online tracker.
5.
Investigation methodology
This section outlines a reproducible methodology for the
network investigation methodology. Depending on which of
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
12
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
the scenarios outlined above, the methodology may branch
according to what the desired outcome will be. Fig. 7 outlines
the five steps involved in the investigative process (each of
these steps are described in greater detail below).
5.1.
Identification of content
Depending on the scenario that motivates the BTSync
network investigation, there are a number of avenues that the
forensic investigator may find secrets (and corresponding
hash values) needed for investigation:
5.1.1.
Web discovery
e As soon as BTSync was released as a public alpha, publicly
accessible sharing secrets started to appear online. Two
“subreddits” appeared on Reddit (2014) and numerous websites and blogs were created to set up an online “dead drop”
secret share, for example http://www.12char.com and http://
www.btsynckeys.com. It is also feasible that an investigator
could come across an online community that shares secrets in
a private forum for the purposes of trading data and material
without 3rd party involvement. Keys to shares discovered in
this manner that possess a timestamp component will need to
be checked to determine if the link has expired or not.
5.1.2.
Local discovery
e An investigator could, in the course of an investigation find
evidence of BTSync having been used to transfer material to
the suspect machine. This could be that BTSync installed and
the folder listed in the list of shares stored in the configuration
file, webUI or the BTSync hidden.Sync folder. BTSync log files
(/.sync/sync.log), or, if BTSync is not present (uninstalled)
there could still be.SyncID files remaining in folders that were
synchronised from remote peers. A hexdump of the.SyncID
file or, more conveniently, the names of the *.db files found in
the.Sync folder will give the SHA1 encoded shareID that the
investigator needs to find other peers actively sharing that
content.
5.1.3.
5.2.
Identification of lookup hash
Requesting a list of peers through any of the peer discovery
methods outlined above requires a unique lookup hash. This
hash is used by the tracker, DHT, PEX and LPD in the association of know peers to a particular piece of content.
5.3.
Crawl the network to identify peer information
Each of the peer discovery methods outlined above should be
queried for a list of known active nodes sharing that content.
Due to the user configurable nature over which services are
enabled in the BTSync client, to ensure complete node
enumeration/identification, the results from each of the peer
discovery methods should be combined to form the final
result of collected information.
5.4.
Downloading and verification of content
Depending on the scenario being investigated, it may be
necessary to download a copy of the content stored remotely
for investigation or verification. In order to accomplish this, a
regular BitTorrent download can be started for each of the files
contained within the shared folder. If the investigation's goal
is to attempt to recreate content deliberately deleted off a
suspect's machine, the data can only be entirely recovered if
there is a complete copy of the data stored remotely. However,
this does not mean that any single node needs to have 100% of
the content. The original data can be recombined so long as a
complete copy exists split among the distributed nodes
actively sharing the content. An obstacle to this stage of the
investigation would be the use of limited use keys. The link
descriptor for a key has no component to indicate a restricted
number of uses. A further obstacle would be the option to
require authorisation before a peer can access a share. This is
unlikely to be the case for links discovered on a public
platform.
6.
Fig. 7 e Steps involved in performing a BTSync network
investigation.
LAN traffic
e Many companies configure their edge firewalls to block
torrent traffic for the general users. If the company uses
torrent for some other business purpose it will usually be
accounted for and allowed from or to a particular server or
subnet. However, BTSync allows for all external communicate
beyond the LAN to be turned off (in the configuration file or in
the settings dialogue the options for “Use DHT”, “Use Tracker”
and “Use Relay Server” can be disabled) leaving only the settings for LAN discovery or known peers. A security review of
the router logs may find active torrent traffic within the LAN or
system admins may discover evidence of torrent applications
run.
Proof of concept
In order to begin proof of concept testing for the investigation
methodology, a bespoke BTSync crawling application was first
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
designed and developed. This application was built to emulate
regular BTSync client usage, as outlined above, and recorded
the necessary results for analysis.
6.1.
Overview
To demonstrate the functionality of the application, an
investigation was conducted on a known publicly accessible
BTSync secret. One of the public BTSync online secret sharing
sites was used (http://www.btsynckeys.com/) to acquire a
secret likely to have active peers sharing the corresponding
content. The secret selected was advertised with the
description “45 GB Movie Collection [Movies] [R]” and the readonly secret BKV273YUFMWILMESLRDVLI5NHMWO3OCS7 was
supplied. It is important to note that there is no certainty that
the description accurately advertises the content within the
share. There is no method of verifying any of the containing
shared content until the syncing process begins and temporary files are created in the shared folder. Even at that point,
the user can merely see the file names of the content once the
download/synchronisation process has begun.
6.2.
13
Results
As part of the peer identification process a number of active
peers were returned to the investigative application. These
peers were recorded for later analysis. During the first snapshot taken for this investigation, 21 peers were identified as
sharing the specific content and 20 were identified on the
second. A snapshot accounts for all of the peers identified
sharing the specific content at the same instance in time.
Two peers (differentiated by PeerID) of particular interest
are listed as the second and third last peers in both tables in
Fig. 8 (highlighted in red (in the web version)). Comparing their
peer ID and local IP: Port address pairing, it is clear that these
two peers are referring to the same individual node. Between
the two snapshots taken of this shared content, their IP
address changed from one IP address range to another.
However, both of these IP address ranges are associated with
the ISP “Telefonica” in the same postal zip code in Berlin,
Germany (data gathered from Maxmind (Maxmind Inc, Jul.
2014)). This information indicates an ISP level IP address
reallocation sometime between the two snapshots as opposed
to the use of a VPN or other IP address masking system. The
Fig. 8 e Daily snapshot comparison for investigated secret (public IP addresses partially redacted).
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
14
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
Fig. 9 e Geolocation of discovered IP addresses.
two peers share the same external IP address but have
different external ports and local IP: port pairs indicating that
the BTSync install on these nodes are accessing the Internet
through a router employing Network Address Translation
(NAT).
6.3.
Churn rate
While the example investigation outlined as part of this paper
focuses on a single secret over a 24 h window, the low churn
rate of just 7% remains interesting. Most P2P networks
experience a high turnover of peers (Herrera and Znati, 2007);
following the assumption that most users are active on the
network while downloading some content and disconnect
upon completion. BTSync is designed to be a tool that functions in a similar manner to cloud file synchronisation services like Dropbox or Google Drive. These tools largely operate
on an “install and forget” approach whereby synchronisation
and updating between the cloud and potentially multiple
client machines does not require any direct user input.
BTSync uses a similar approach and as a result, low churn
rates would be expected.
Fig. 10 e Network-based entry point into investigation (ShareID highlighted in blue (For interpretation of the references to
colour in this figure legend, the reader is referred to the web version of this article)).
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
15
Fig. 11 e IP addresses discovered sharing the content.
Fig. 12 e Geolocation of discovered peers.
Fig. 13 e Evidence recovery from remote peers.
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
16
6.4.
c o m p u t e r s & s e c u r i t y x x x ( 2 0 1 5 ) 1 e1 7
Geolocation
Fig. 9 shows the geographic distribution of the peers identified
as part of the investigation. While the total number of peers
identified with this proof of concept investigation is quite low,
the data remains consistent with regular BitTorrent investigative results (Scanlon et al., 2010) with North America and
Europe being the most popular continents involved.
7.
Example investigation
In late August 2014, the iCloud accounts of numerous celebrities were hacked and compromising photos and videos were
posted online without their consent in what has gained notoriety in the media and among Internet users as “The Fappening” (Bora, Sep. 2014) or “Celebgate” (Muth, 2015). The
comprised photos spread across the globe with the help of
Internet forums, such as htpp://4chan.org and http://reddit.
com. At the time, there was concerns that iCloud itself had
been hacked and these leaks were merely a subset of the information stolen of Apple's servers, however an investigation
into the attack found that the passwords were cracked for
specific accounts (Bora, Sep. 2014).
7.1.
Entry point
The entry point to this investigation first involved verifying
that this content was being shared using BTSync. On the
public BTSync secret sharing “subreddit” http://reddit.com/r/
btsecrets, a number of public read-only secrets were shared
containing collections of the leaked content. For the purposes
of this investigation, one shared leaked content was investigated using the aforementioned BTSync investigative application. The secret investigated was bb63eb5b61969956e71273
026f00a1deca464413. The investigation took place one week
after the leak occurred.
A BTSync dissector for Wireshark was developed1 to
expedite the network analysis process. This dissector can
identify the various packets pertinent to the decentralised
service in the Wireshark traffic capture, as can be seen in
Fig. 10. Using the gathered ShareID from the network traffic,
the investigative application was launched and the ShareID
supplied.
7.2.
Peer discovery
Using the gathered ShareID, the application was able to gather
information about each of the peers sharing the content, as
can be seen in Fig. 11 using each of the peer discovery methods
outlined above.
7.3.
Geolocation
The IP addresses detected during the investigation were geolocated and found to be located in North America and Europe,
as can be seen in Fig. 12.
1
Wireshark Dissector is downloadable from http://www.
markscanlon.co/bittorrent-sync.
7.4.
Data recoverable from remote peers
Some of the evidence recoverable from remote peers in this
particular BTSync share can be seen in Fig. 13. The version of
the BTSync available at the time of the investigation (v1.4), did
not have selective sync functionality. As a result, each member of the secret must download all of the shared content. This
limitation of a lack of selective syncing means that each peer
identified will eventually have all of the content in the share.
This feature makes evidence recovery from such popular
shares more performant for digital investigators as each node
is a potential source of the pertinent evidence. With the
advent of v2.0 of the application, selective sync means that
each peer must be communicated with individually to identify
which active machines identified has what data.
8.
Conclusion
This paper documented the protocol used in BitTorrent Sync
during the discovery of peers and the synchronisation of data.
While BTSync is not necessarily intended to replace BitTorrent
as a file dissemination utility, it will likely be used for this
purpose. This is already facilitated though websites providing
shared secrets, e.g., Reddit (2014), etc., as a form of dead-drop.
The developers describe the tool as an end-to-end encrypted
method of transferring files without the use of a third party
staging area, which ensures that the content and personal
details remain hidden from unauthorised access. Analysis of
the network communication procedure produced unique
identifiable information on peers including their unique
PeerID, their external and local IP addresses and port
numbers. In combination with traditional digital forensic
methods, once a secret is identified, it is possible to discover
other nodes on the network who are also sharing this data.
Deleted data from a local shared folder could be downloaded
from the network and recombined for forensic investigation.
From an investigative perspective, the decentralised nature of
BTSync will always leave an avenue of gathering information
and identifying nodes sharing particular content open to the
forensic investigator.
Appendix A. Supplementary data
Supplementary data related to this article can be found at
http://dx.doi.org/10.1016/j.cose.2015.05.003.
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Dr. Mark Scanlon is a Lecturer in the School of Computer Science
and Informatics, University College Dublin, Ireland (UCD CSI).
Mark holds a BA (Hons) in Computer Science and Linguistics and
received his MSc in the area of Remote Digital Forensic Evidence
Gathering in 2009 and his PhD in the area of Peer-to-Peer Network
Based Botnet Investigation in 2013. Mark is an Associate Editor for
the International Journal on Digital Crime and Forensics and is on
the technical programme committees and organising committies
for several key conferences in the fields of digital forensics and
cybersecurity. His primary research interests include Digital Forensics & Cybercrime Investigation, Networks & Internet Systems,
Peer-to-Peer Applications and Web Technologies.
Mr. Jason Farina is a PhD Candidate in UCD CSI and is a teaching
assistant on the School's MSc in Forensic Computing and Cybercrime Investigation for law enfrocement officers. He is a systems
administrator with over a decade's experience and was awarded his
Master's in Digital Investigation in 2012. His dissertation focused on
live forensic acquisition of suspect virtualised guest systems on
untrusted hosts. His PhD research is centred around forensic investigations involving Peer-to-Peer and Cloud based data transfers.
Professor M-Tahar Kechadi holds both a PhD and a Master’s degree
in Computer Science. He joined UCD CSI in 1999 and is currently a
Professor of Computer Science and a Principal Investigator with
the Insight Centre for Data Analytics. His research interests span
the areas of Data Mining, Distributed Data Mining, Heterogeneous
Distributed Systems, Grid and Cloud Computing, and Digital Forensics & Cybercrime Investigation. Prof Kechadi has published
over 260 research articles in refereed journals and conferences. He
serves on the scientific committees for a number of international
conferences. He organised and hosted one of the leading conferences in his area and he is Chair of CKDD since 2009 and CISIS 2014
(Track #3). He is currently the editor in Chief of the Journal of
Computer Science of Science Publications, and an associate of the
Journal of Future Generation of Computer Systems of Elsevier.
Please cite this article in press as: Scanlon M, et al., Network investigation methodology for BitTorrent Sync: A Peer-to-Peer
based file synchronisation service, Computers & Security (2015), http://dx.doi.org/10.1016/j.cose.2015.05.003
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