Security Issues and Solutions in Peer-to-peer Systems for Realtime
Communications
Columbia University
1214 Amsterdam Avenue
New York, NY
10027
USA
hgs@cs.columbia.edu
Telecom Italia
Via G. Reiss Romoli, 274
Turin
10148
Italy
enrico.marocco@telecomitalia.it
SIP Communicator
4 rue Blaise Pascal
Strasbourg Cedex
F-67070
France
emcho@sip-communicator.org
Peer-to-peer (P2P) networks offer higher robustness against
failure, easier configuration and are generally more economical
than their client-server counterparts. It has therefore become
reasonable for resource consuming and typically centralized
applications like Voice over IP (VoIP) and, in general, realtime
communication to adapt and exploit the benefits of P2P. Such a
migration needs to address a new set of P2P specific security
problems. This document describes some of the known issues found
in common P2P networks, analyzing the relevance of such issues
and the applicability of existing solutions when using P2P
architectures for realtime communication.
Peer to Peer (P2P) overlays have become quite popular with the
advent of file-sharing applications such as Napster, KaZaa and BitTorrent. After their success in
file-sharing and content
distribution, P2P networks are now also being used for
applications such as Voice over IP (VoIP)
and television
. However most of these systems are
not purely P2P and have centralized components like the login
server in Skype or moderators and
trackers in BitTorrent . Securing pure
P2P networks is therefore still a field of very active research
. P2P overlays can be broadly classified
as structured and unstructured ,
depending on their routing model. Unstructured overlays are
often relatively simple but search operations in them, usually
based on flooding, tend to be inefficient. Structured P2P
overlays use distributed hash tables (DHT) to perform directed searches which make
lookups more efficient in locating data. This document will
mostly focus on DHT-based P2P overlays.
When analyzing the various attacks that are possible on P2P
systems, it is important to first understand the motivation of
the attackers as well as the resources (i.e. computation power,
access to different IP subnets) that they would have at their
disposal.
Once the threat has been identified, admission control is the
first step towards security . Most solutions
rely on the assumption that malicious nodes represent a small
fraction of all peers. It is therefore important to restrict
their number in the overlay.
Other P2P specific security problems discussed here include
attacks on the routing of queries, targeted denial of service
attacks and attacks on data integrity.
This document, after discussing some of the main security issues
and proposed solutions for P2P systems in general, focuses on
one particular application -- realtime communication. The idea
behind P2P realtime communication is using the DHTs employed by
file-sharing applications, in order to implement services such
as registration, user location lookup, and assistance with NAT
and firewall traversal. Even if, from a technical point of view,
P2P communication services may seem similar to file-sharing,
shows that some important
differences, mostly related to privacy and availability,
significantly increase security requirements.
File-sharing
Realtime communication
Distributed database
Shared file locations are indexed in a table distributed
among peers; often hundreds or thousands per user.
User locations are indexed in a table distributed among peers;
rarely more than one per user.
Availability
Same files are usually available at multiple locations and
failures involving single istances are overcame by abundancy
of resources; attacks targeting single files need to be
addressed to the distributed index.
Users are unique; attacks targeting single users may be
addressed both to the distributed index and to the user's
device directly.
Integrity
Attackers may want to share corrupted files in place of
popular content, e.g. to discourage users from acquiring
copyrighted material; constitute a threat for the service, but
not for the users.
Attackers may want to impersonate different users in order to
handle calls directed to them; constitute a particular threat
for the user as, in case of success, the attacker acquires
full control on the victim's personal communications.
Confidentiality
Shared files are, by definition, readable by all users; in
some cases encryption is used to avoid elements not involved
in the service to detect traffic.
Communications are usually meant to be private and need to be
encrypted; evesdropping may reveal sensitive data and is a
serious threat for users.
Main differences between P2P applications used for
file-sharing and for realtime communication.
The rest of the document is organized as follows. In , we discuss P2P security attackers. We try
to elaborate on their motivation, the resources that would
generally be available to them, their victims and the timing of
their attacks. In , we discuss
admission control problems. In , we
identify the problem of where a node joins in the overlay. In
, we describe problems related to
identification of malicious nodes and the dissemination of this
information. In , we describe the
issues of routing and data integrity in P2P networks. Finally,
in we discuss how issues and
solutions previously presented apply in P2P overlays for
realtime communication.
This document is partially derived from the article
"Peer-to-peer Overlays for Real-Time Communications: Issues
and Solutions," published in IEEE Surveys & Tutorials,
Vol. 11, No. 1 and originally authored by Dhruv Chopra,
Henning Schulzrinne, Enrico Marocco and Emil Ivov. Its goal is
to collect feedback from the IRTF community in order to
document the advances in the field of security of P2P systems
for realtime communications, for the benefit of related
standardization activities going on in IETF.
Attacks on networks happen for a variety of reasons such as
monetary gain, personal enmity or even for fame in the hacker
community. There are quite a few well known cases of denial of
service attacks for extortion in the client-server model . One of the salient points of the P2P model
is that the services it provides have higher robustness
against failure. However, such attacks are still possible
against individuals within the overlay if the attackers
possess sufficient resources. For instance, a network of
worm-affected malicious nodes spread across the Internet and
controlled by an attacker (often referred as botnet), could
simultaneously bombard lookup queries for a particular key in
the DHT. The peer responsible for this key would then come
under a lot of load and could crash . However with replication of key-value pairs at
multiple locations, such threats can be mitigated.
Attackers may also have other incentives, only indirectly
related to money. With the growth of illegal usage of sharing
files with copyrights, record companies have been known to
attempt polluting content in the overlays by putting up nodes
with corrupt chunks of data but with correct file names to
degrade the service and in hope that
users would get frustrated and stop using it. Similarly,
competition between different communications service
providers, either or both based on P2P technologies, and the
low level of traceability of attacks targeted to single users
could be considered as motivation for attemping service
disruption.
Attacks can also be launched by novice attackers who are there
attacking the overlay for fun or fame in a community. These
are perhaps less likely to be successful or cause damage,
since their resources tend to be relatively limited.
Resource constraints play an important role in determining the
nature of the attack. An attacker who controls a botnet can
use an Internet relay channel and launch distributed denial of
service attacks against another node. With respect to attacks
where a single node impersonates multiple identities, as in
the case of the sybil attack
described in , IP addresses are also
an important resource for the attacker since in DHTs such as
Chord, the position in the
overlay is determined by using a base hash function such as
SHA-1on the node's IP address. The
cryptographic puzzles that are
sometimes suggested as a way to deter sybil attacks by making
the join process harder are futile against an attacker with a
botnet and virtually unlimited computation power. Doucer proves that even with the assumption that
attackers only have minimum resources at their disposal, it is
not possible to defend against them in a pure P2P system.
The victim of an attack could be an individual node, a
particular content entry or the entire overlay service. If
malicious nodes are strategically placed in the overlay, they
can block a node from using its services. Attacks could also be
launched against specific content or even the
entire overlay service. For example, if the malicious nodes
are randomly placed in the overlay and drop packets or upload
malcontent, then the quality of the overlay would deteriorate.
A malicious node could start misbehaving as soon as it enters
the overlay or it could follow the rules of the overlay for a
finite amount of time and then attack. The latter could prove
to be more harmful if the overlay design suggests accumulating
trust in peers based on the amount of time they have been
present and/or not misbehaving. In Kademlia, for instance, the routing
tables are populated with nodes that have been up for a
certain amount of time. While this provides some robustness
from attacks in which the malicious nodes start dropping
routing requests from the moment they enter, it would take
time for the algorithm to adapt to nodes which start
misbehaving in a later stage (i.e., after they have been
recorded in routing tables). Similarly for reputation
management systems, it is important that they adapt to the
current behavior of a peer.
Admission control depends on who decides whether or not to admit
a node and how this permission is granted. Kim et. al answer these questions
independently of any particular environment or application.
They define two basic elements for admission in a peer group, a
group charter, which is an electronic document that specifies
the procedure of admission into the overlay, and a group
authority, which is an entity that can certify group
admission. A prospective member first gets a copy of the group
charter, satisfies the requirements and approaches the group
authority. The group authority then verifies the admission
request and grants a group membership certificate.
The group charter and authority verification can be provided by
a centralized certificate authority or a trusted third party, or
it could be provided by the peers themselves (by voting). The
former is more practical and tends to make the certification
process simpler although it is in violation of the pure P2P
model and exposes the system to attacks typical for server-based
solutions (e.g., denial of service attacks targeted to the
central authority). The latter, the group authority could either
be a fixed number of peers or it could be a dynamic number based
on the total membership of the group. The authors argue that
even if the group charter requires a prospective member to get
votes from peers, the group membership certificate must be
issued by a distinct entity. The reason for this is that voters
need to accompany their votes with a certificate that proves
their own membership. Possible signature schemes that could be
used in voting such as plain digital signature, threshold
signature and accountable subgroup multisignature are also
described. Saxena et. al performed
experiments with the different signature schemes and suggest the
use of plain signatures for groups of moderate size and where
bandwidth is not a concern. For larger groups and where
bandwidth is a concern, they suggest threshold signature and multisignature schemes .
Another way of handling admission would be to use mechanisms
based on trust and recommendation where each new applicant has
to be known and vouched for by at least N existing members. The
difficulties that such models represent include identity
assertion and preventing bot/worm attacks. A compromised node
could have a valid certificate identifying a trustworthy peer
and it would be difficult to detect this. Possible solutions
include sending graphic or logic puzzles easily addressed by
humans but hard to solve by computers, also known as CAPTCHA.
For ring based DHT overlays such as Chord, Kademlia and Pastry, when a node joins the overlay, it
uses a numeric identifier (ID) to determine its position in the
ring. The positioning of a node determines what information it
stores and which nodes it serves. To provide a degree of
robustness, content and services are often replicated across
multiple nodes. However it is possible for an adversary with
sufficient resources to undermine the redundancy deployed in the
overlay by representing multiple identities. Such an attack is
called a sybil attack. This makes
the assignment of IDs very important. One possible scheme to
tackle such attacks on the ID mapping is to have a temporal
mechanism in which nodes need to re-join the network after some
time . Such
temporal solutions, however have the drawback that they increase
the maintenance traffic and possibly deteriorate the efficiency
of caching. Danezis et. al
suggest mechanisms to mitigate the effect of sybil attacks by
reducing the amount of information received from malicious
nodes. Their idea is to vary the nodes used for routing with
time and thus avoid a trust bottleneck. Other solutions suggest
making the joining process harder by introducing cryptographic
puzzles as suggested by Rowaihy
et. al. The assumption is that the adversary has limited
computational resources which may not be true if the adversary
has control over a botnet. Another drawback of such methods is
that non-malicious nodes would also have to perform the extra
computations before they can join the overlay.
A possible heuristic to hamper sybil attacks is to employ
redundancy at nodes with diametrically opposite IDs (in the DHT
ID space) instead of successive IDs as in Chord. The idea behind
choosing diametrically opposite nodes is based on the fact that
a malicious peer can grant admission to others as its successor
without them actually possessing the required IP address (whose
hash is adjacent to the former's), and then they can cooperate
to control access to that part of the ring. If however admission
decisions and redundant content (for robustness), also involve
nodes which are the furthest away (diametrically opposite) from
a given position, then the adversary would require double
resources (IP addresses) to attack. This happens because the
adversary would need presence in the overlay at two independent
positions in the ring.
Another approach proposed by Yu et
al. to limit sybil attacks is based on the usage of the
social relations between users. Authors use the fact that as a
result of sybil attacks, affected P2P overlays end up containing
a large set of sybil nodes connected to the rest of the peers
through an irregularly small number of edges. The SybilGuard protocol defines a method that
allows to discover such kind of discontinuities in the topology
by using a special kind of a verifiable random walk and hence
without the need of one node having a global vision of the
graph.
It is also worth mentioning that in DHT overlays using different
geometric concepts, (e.g., hypercubes instead of rings), peer
positions are usually not related to identifiers. In the content addressable network (CAN), for
example, the position of an entering node may be either selected
by the node itself, or, with little modification to the original
algorithm, assigned by peers already in the overlay. However,
even when malicious nodes do not know their position before
joining, the overlay is still vulnerable to sybil attacks.
Making overlays robust against even a small percentage of
malicious nodes is difficult . It is
therefore important for other peers to identify such nodes and
keep track of their number. There are two aspects to this
problem. One is the identification itself and the second is the
dissemination of this information amongst the peers. Different
metrics need to be defined depending on the peer group for the
former and reputation management systems are needed for the
latter.
For identifying a node as malicious, malicious activity has to
be observed first. This could be done in either a proactive
way, or a reactive way.
When acting proactively, peers perform periodic operations
with the purpose of detecting malicious activity. A
malicious node could prevent access to content it is
responsible for (e.g., by claiming the object doesn't
exist), or return references to content that does not match
the original queries . With this
approach, publishers of content can later perform lookups
for it at periodic intervals and verify the integrity of
whatever is returned. Any inconsistencies could then be
interpreted as malicious activity. The problem with
proactive identification is the management of the overhead
it implies: if checks are performed too often, they may
actually hinder scalability, while, if they are performed
too rarely, they would probably be useless.
In a reactive strategy, the peers perform normal operations
and if they happen to detect some malicious activity, then
they can label the responsible node as malicious. In a
file-sharing application for example, after downloading
content from a node, if the peer observes that data does not
match its original query it can identify the corresponding
node as malicious. Poon et. al
suggest a strategy based on the forwarding of queries. If
routing is done in an iterative way, then dropping of
packets, forwarding to an incorrect node and delay in
forwarding arouse suspicion and the corresponding peer is
identified as malicious.
Reputation management systems are used to allow peers to share
information about other peers based on their own experience
and thus help in making better judgments. Most reputation
management systems proposed in the literature
are for file-sharing applications. In
reputation systems, it should not be possible for a
misbehaving peer with low reputation to simply rejoin the
network with a different ID and therefore start from a clean
slate. To counter this, Kwon et. al
store not only the reputation of a peer but also the
reputation of files based on file name and content to avoid
spreading of a bad file. Another method is to make the
reputation of a new peer the minimum possible . Kamvar et. al
define five design considerations for reputation management
systems;
Self policing.
Anonymity.
No profit to new comers.
Minimal overhead.
Robustness to malicious peers.
Unstructured reputation management systems have been
proposed by Uzun et. al and Damiani et. al. The basic idea of
these is that each peer maintains information about its own
experience with other peers and resources, and shares it
with others on demand. In the system proposed by Uzun et. al, each node maintains trust
and distrust vectors for every other node that it has
interacted with. When reputation information about a peer is
required, a node first checks its local database, and if
insufficient information is present, it sends a query to its
neighbors just as it would when looking up content.
However, such an approach requires peers to get reputation
information from as many sources as possible; otherwise,
malicious nodes may succesfully place targeted attacks
returning false values for their victims.
One of the problems with unstructured reputation management
systems is that they either take the feedback from few
peers, or if they do from all, then the they incur large
traffic overhead. Systems such as those proposed by try to resolve it in
a structured manner. The idea of the eigen trust algorithm for example, is
transitivity of trust. If a node trusts peer X then it would
also trust the feedback it gives about other peers. A node
builds such information in an iterative way. The algorithm
has fast convergence properties . For maintaining this information in a
structured way, the authors use a content addressable network (CAN)
DHT. The information of each peer is stored and
replicated on different peers to provide robustness against
malicious nodes. They also suggest favoring peers
probabilistically with high trust values instead of doing it
deterministically, to allow new peers to slowly develop a
reputation. Eventually, they suggest the use of incentives
for peers with high reputation values.
Preserving integrity of routing and data, or, in other words,
preventing peers from returning corrupt responses to queries and
routing through malicious peers, is an important security issue
in P2P networks. The data stored on a P2P overlay depends on the
applications that are using it. For file-sharing, this data
would be the files themselves, their location, and owner
information. For realtime communication, this would include user
location bindings and other routing information. We describe
such data integrity issues separately in .
For file-sharing applications, insertion of wrong content
(e.g. files not matching their names or descriptions) or
introduction of corrupt data chunks (often referred to as
poisoning and pollution) are a significant
problem. Bit-Torrent uses voluntary moderators to weed out
bogus files and the SHA-1 algorithm to determine the hash of
each piece of a file to allow verification of integrity. If a
peer detects a bad chunk, it can download that chunk from
another peer. With this strategy, different peers download
different pieces of a file before the original peer disappears
from the network. However, if a malicious peer modifies the
pieces that are only available on it and the original peer
disappears, then the object distribution will fail . An analysis of BitTorrent in terms of
integrity and performance can be found in the work of Pouwelse et. al.
To enhance the integrity of routing, it is important to reduce
the number of queries forwarded to malicious nodes. Marti et. al developed a system that
uses social network information to route queries over trusted
nodes. Their algorithm uses trusted nodes to forward queries
(if one exists and is closer to the required ID in the ID
space). Otherwise they use the regular Chord routing table to forward
queries. While their results indicate good average
performance, it can not guarantee log$N$ hops for all cases.
Danezis et. al suggest a method
for routing in the presence of a large number of sybil
nodes. Their method is to ensure that a peer queries a diverse
set of nodes and does not place too much trust in a node. Both
the above works have been described based on Chord. However,
unlike Chord, in DHTs like Pastry
and Kademlia there is
flexibility in selecting nodes for any row in a peer's routing
table. Potentially many nodes have a common ID prefix of a
given length and are candidates for routing a given query. To
exploit the social network information and still guarantee
log(N) hops, a peer should select its friends to route a
query, but only when they are present in the appropriate row
selected by the DHT algorithm.
The idea of using P2P in realtime communication boils down to
distributing centralized entities from conventional
architectures over peer-to-peer overlays and thus reducing the
costs of deployment and increasing reliability of the different
services. Initiatives such as the P2PSIP
working group in IETF are currently concentrating on
achieving this by using a DHT for services such as registration,
location lookup, and support for NAT traversal, which are
normally handled by dedicated servers.
Even if based on the same technology, overlays used for realtime
communication differ from those used for file sharing in at
least two aspects:
Resource consumption. Contrary to file sharing systems where
the DHT is used to store huge amounts of data (even if the
distributed database is used only for storing file
locations, each user usually indexes hundreds or thousands
of files), realtime communication overlays only require a
subset of the resources available at any given time as users
only register a limited number of locations (rarely more
than one).
Confidentiality. While in file sharing applications, where
shared files are supposed to be made publicly available,
eavesdropping and identity theft do not constitute real
threats, in realtime communication, since exchanges of data
are usually meant to happen privately, it is essential to
have mechanisms to assert identities and to guarantee
confidentiality.
In this section we go over the admission issues, and security
problems discussed in previous sections, and discuss solutions
that would be applicable to realtime communication in P2P.
In order to keep as much compatibility with existing user
agents as possible, nodes in P2P communication architectures
would probably have to participate as either peers or
clients. If a node participates as a client, then it would use
the overlay network by simply attaching to a peer or a proxy
instead of registering with a server. In most cases users
would be able to benefit from the overlay by only acting as
clients. However, in order to keep the solution scalable, at
some point clients would have to be promoted to peers
(admission to the DHT). This requires addressing the following
issues.
Most existing P2P networks would generally
make it the responsibility of clients to determine if and
when they would apply for becoming peers. A well known
exception to this trend is the Skype
network, arguably one of the most popular overlay
networks used for realtime communications today. Instances
of the Skype application are supposed to operate as either
super-nodes, directly contributing to the distributed
provision of the service, or ordinary-nodes, simply using
the service, and the ``promotions'' are decided by the
higher levels of the hierarchy . Even
if there is not much difference for a client whether it has
to actively ask for authorization to join an overlay, or
passively wait for an invitation, the latter approach has
some advantages which fit well in overlays where only a
subset of the peers is required to provide the service (as
in realtime communication):
An attacker cannot estimate in advance when and if it
would be invited to join the overlay as a peer.
Allows peers to perform long-lasting measurements on
sets of candidates, in order to accurately select the
most appropriate for upgrading and only invite it when
they are ``ready'' to do so. The opposite approach, that
is when clients initiate the join themselves, adds an
extra constraint for the peer that has to act upon the
request since it doesn't know if and when the peer would
attempt to join again.
Discourages malicious peers from attempting sybil and,
more generally, brute force attacks, as only a small
ratio of clients has chances to join the overlay
(possibly after an accurate examination).
In order to answer this question one would have to define
some criteria that would allow to determine the load on a
peer and a reasonable threshold. When the load exceeds this
threshold, a client is invited to become a peer and share
the load. Several mechanisms to diagnose the status of P2P
systems have recently been proposed ; in general,
reasonable criteria for determining load can be:
Number of clients attached.
Bandwidth usage for DHT maintenance, forwarding requests
and responses to and from peers and from the attached
clients.
Memory usage for DHT routing table, DHT neighborhood
table, application specific data and information about
the attached clients.
Selecting which clients to upgrade would require defining
and keeping track of new metrics. The exact set of metrics
and how they influence decisions should be the subject of
serious analysis and experimentation. These could be based
on the following observations:
Uptime. A peer could easily record the amount of time
that it has been maintaining a connection with a client
and take it into account when trying to determine
whether or not to upgrade it.
Level of activity. It is reasonable to assume that the
more a client uses the service (e.g. making phone
calls), the less they would be willing to degrade it.
Keeping track of history. Peers could record history of
the clients they invite and the way they contribute to
the overlay.
Other metrics such as public vs. private IP addresses,
computation power, and bandwidth should also be taken into
account even though they do not necessarily have a direct
impact on security.
Clients need to have incentives for accepting upgrades in
order to prevent excessive burden on existing peers. One way
to handle this would be to maintain separate incentive
management through the use of currency or credits. Another
option would involve embedding these incentives inside the
protocol itself:
Peers share with clients only a fraction of their
bandwidth (uplink and downlink). This would result in
higher latency when using the services of the overlay as
a client and better service quality for peers.
Peers could restrict the number or types of calls that
they allow clients to make.
Introducing such incentives, however, may turn out to be
somewhat risky. Differences in quality would probably be
perceptible for end users who would not always be able to
understand the difference between the roles that their user
agent is playing in the overlay. Such behavior may therefore
be interpreted as arbitrary and make the service look
unreliable.
In addition to bombardment with queries as described in
, the denial of service attack
against an individual node can be conducted in DHTs used for
realtime communications if the peers which surround a
particular ID are compromised. These peers which act as
proxy servers for the victim, can fake the responses from
the victim by sending fictitious error messages back to
peers trying to establish a session. Danezis et al.'s solution can also
provide protection against such attacks as in their solution
peers vary the nodes used in queries.
The man in the middle attack is well described by Seedorf in the particular case of
P2PSIP and consist of an attack
that exploits the lack of integrity when routing
information. A malicious node could return IP addresses of
other malicious nodes when queried for a particular ID. The
requesting peer would then establish a session with a second
malicious node which would again return a ``poisoned''
reply. This could go on until the TTL expires and the
requester gives up the ``wild goose chase'' . A simple way for entities to verify the
correctness of the routing lookup is to employ iterative
routing and to check the node-ID of every routing hop that
it is returned and it should get closer to the desired ID
with every hop. However, this is not a strong check and can
be defeated .
The effect of malicious peers could be mitigated by
introducing the concept of trust within an overlay. This can
be done in different ways:
Using certificates assigned by an external authority.
The drawback with this approach is that it requires a
centralized element.
Using certificates reciprocally signed by peers. This
mechanism is quite similar to PGP; every peer signs
certificates of ``friend'' peers and trusts any other
peer with a certificate signed by one of its friends.
However even though it might be theoretically possible,
in reality it is extremely difficult to obtain long
enough trust chains.
One way for implementing realtime communication overlays (as
we have mentioned in earlier sections) would be to simply
replace centralized entities in signalling protocols like
SIP with distributed
services. In some cases this might imply reusing existing
protocol mechanisms for routing signalling messages. In the
case of SIP this would imply regarding peers as SIP
proxies. However the design of SIP supposes that such
proxies are trusted, and makes it possible for them to fork
requests or change their destination, add or remove header
fields, act as the remote party, and generally manipulate
message content and semantics
However, in a P2P environment where messages may be routed
through numerous successive peers, some of which might be
compromised, it is important not to treat them as trusted
proxies. One way to limit what peers can do is by protecting
signalling with some kind of end-to-end encryption.
Another option would be to extend existing signalling
protocols and modify the way they route messages in order to
guarantee secure end-to-end transmission. Gurbani et
al. define a similar mechanism for SIP that allows nodes to establish a secure
channel by sending a CONNECT SIP request, and then tunnel
all SIP messages through it, adopting a similar mechanism to
the one used for upgrading from HTTP to HTTPS.
It is important to ensure that the location that a user
registers, usually a (URI, IP) pair, is what is returned to
the requesting party. Or the entities that issue the lookup
request must be able to verify the integrity of this pair. A
pure P2P approach to allow verification of the integrity of
location binding information is presented in . The idea is for an entity to choose an
asymmetric key pair and hash its public key to generate its
URI. The entity then signs its present location with its
private key and registers with the quadruple (URI, IP,
signature, public key). Any entity which looks up for the
URI and receives such a quadruple can then verify its
integrity by using the public key and the certificate.
Another possible merit of such an approach could be that it
is possible to identify the malicious nodes and maintain a
black list. However, the resulting URIs are not easy to
remember and associate with entities. Discovering these URIs
and associating them with entities would therefore require
some sort of a directory service. The authors suggest using
existing authentication infrastructure for this such as a
certified web service using SSL which can publish an
``online phone book'' mapping users to URIs.
Using P2P overlays for realtime communication implies that
content is likely to traverse numerous intermediate peers
before reaching its destination. A typical example could be
the use of peers as media relays as a way of traversing NATs
in VoIP calls.
Contrary to publicly shared files, communication sessions
are in most cases expected to be private. It is therefore
very important to make sure that no media leaves the client
application without being encrypted and securely transported
through a protocol like SRTP.
However, the extra processing resources required by the
encryption algorithms, the management of keying material
(e.g., retrieving public keys when interacting with unknown
peers) may constitute an expensive task, especially for
mobile devices.
Identifying more specific threats related to the P2P
realtime communications, would require a clearly defined
economic model. Answers to the following questions would be
helpful.
To whom do the users pay?
Do the users only pay when accessing the public
telephone network?
Is the billing done per call or is it fixed?
For instance, the implications of an attack such as taking
control over another's user agent or its identity and using
it for outbound calls would depend on whether or not this
would be economically advantageous for the attacker. Baumann et. al suggests that to
prevent unwanted communication costs, gateways for the
public telephone network should only be accessible via
authenticated servers and dialing authorizations should be
enforced. Also it seems that it would be difficult to do
billing in a pure P2P manner as it would mean keeping the
billing details with untrusted peers.
This document, informative in nature, discusses some of the
security issues of peer-to-peer systems used for realtime
communications.
The authors are particularly grateful to Dhruv Chopra who
contributed to the writing of the article "Peer-to-peer Overlays
for Real-Time Communications: Issues and Solutions" (IEEE
Surveys & Tutorials, Vol. 11, No. 1) this work is partially
derived from.
The authors would also like to thank Vijay Gurbani, Song Haibin
and the many other people who provided useful comments.
SIP: Session Initiation Protocol
The Secure Real-time Transport Protocol (SRTP)
HTTP Over TLS
Survey of Research towards Robust Peer-to-Peer Networks:
Search Methods
Diagnose P2PSIP Overlay Network
Cryptographically Transparent Session Initiation Protocol
(SIP) Proxies
Peer-to-Peer Session Initiation Protocol (P2PSIP) IETF Working Group
Napster
KaZaa
Skype
COOLSTREAMING
PPLive
BitTorrent
Telling humans and computers apart automatically
A survey of peer-to-peer content distribution technologies
A Survey of Peer-to-Peer Security Issues
Enhancing Data Authenticity and Integrity in P2P Systems
The Bittorent P2P File-Sharing System: Measurements and Analysis
Peer-to-Peer Internet Telephony using SIP
Security Challenges for Peer-to-Peer SIP
Chord: A Scalable Peer-to-peer Lookup Service for Internet Applications
Security considerations for peer-to-peer distributed hash tables
An analysis of the skype peer-to-peer internet telephony protocol
Robust Forwarding in Structured Peer-to-Peer Overlay Networks
A Reputation-Based Trust Management System for P2P Networks
Admission Control in Peer Groups
Kademlia: A Peer-to-peer Information System Based on the XOR Metric
Pastry: Scalable, distributed object location and routing for large-scale peer-to-peer systems
A Scalable Content-Addressable Network
Admission Control in Peer-to-Peer: Design and Performance Evaluation
Maelstorm: Churn as Shelter
Sybil-resistant DHT routing
SPROUT: P2P Routing with Social Networks
How to Spread Adversarial Nodes?: Rotate!
Limiting Sybil attacks in structured peer-to-peer networks
The Sybil Attack
SybilGuard: Defending Against Sybil Attacks via Social Networks
Pollution in p2p file sharing systems
Providing robust and ubiquitous security support for MANET
Accountable Subgroup Multisignatures
A Reputation Management System in Structured Peer-to-Peer Networks
Using Cryptographically Generated SIP-URIs to Protect the Integrity of Content in P2P-SIP
Secure routing for structured peer-to-peer overlay networks
Secure Hash Standard
A Reputation-Based Approach for Choosing Reliable Resources in Peer-to-Peer Networks
The EigenTrust Algorithm for Reputation Management in P2P Networks
The second value eigenvalue of the google matrix
Voice Over IP - Security and SPIT
Bookie reveals 100,000 cost of denial-of-service extortion attacks
Pretty good privacy: public key encryption for the masses