Network Working Group H. Schulzrinne
Internet-Draft Columbia University
Intended status: Informational E. Marocco
Expires: August 26, 2009 Telecom Italia
E. Ivov
SIP Communicator
February 22, 2009
Security Issues and Solutions in Peer-to-peer Systems for Realtime
Communications
draft-schulzrinne-p2prg-rtc-security-00
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Abstract
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.
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 4
1.1. Purpose of this document . . . . . . . . . . . . . . . . . 6
2. The attackers . . . . . . . . . . . . . . . . . . . . . . . . 6
2.1. Incentive of the attacker . . . . . . . . . . . . . . . . 6
2.2. Resources available to the attacker . . . . . . . . . . . 7
2.3. Victim of the attack . . . . . . . . . . . . . . . . . . . 7
2.4. Time of attack . . . . . . . . . . . . . . . . . . . . . . 7
3. Admission control . . . . . . . . . . . . . . . . . . . . . . 8
4. Determining the position in the overlay . . . . . . . . . . . 9
5. Resilience against malicious peers . . . . . . . . . . . . . . 10
5.1. Identification of malicious peers . . . . . . . . . . . . 10
5.1.1. Proactive identification . . . . . . . . . . . . . . . 10
5.1.2. Reactive identification . . . . . . . . . . . . . . . 11
5.2. Reputation management systems . . . . . . . . . . . . . . 11
5.2.1. Unstructured reputation management . . . . . . . . . . 11
5.2.2. Structured reputation management . . . . . . . . . . . 12
6. Routing and data integrity . . . . . . . . . . . . . . . . . . 12
6.1. Data integrity . . . . . . . . . . . . . . . . . . . . . . 12
6.2. Routing integrity . . . . . . . . . . . . . . . . . . . . 13
7. Peer-to-peer in realtime communication . . . . . . . . . . . . 13
7.1. Admission . . . . . . . . . . . . . . . . . . . . . . . . 14
7.1.1. Active vs. passive upgrades . . . . . . . . . . . . . 14
7.1.2. When to upgrade . . . . . . . . . . . . . . . . . . . 15
7.1.3. Which clients to upgrade . . . . . . . . . . . . . . . 15
7.1.4. Incentives for clients . . . . . . . . . . . . . . . . 15
7.2. Security . . . . . . . . . . . . . . . . . . . . . . . . . 16
7.2.1. Targeted denial of service . . . . . . . . . . . . . . 16
7.2.2. Man in the middle attack . . . . . . . . . . . . . . . 16
7.2.3. Trust between peers . . . . . . . . . . . . . . . . . 16
7.2.4. Routing call signalization . . . . . . . . . . . . . . 17
7.2.5. Integrity of location bindings . . . . . . . . . . . . 17
7.2.6. Encrypting content . . . . . . . . . . . . . . . . . . 18
7.2.7. Other issues . . . . . . . . . . . . . . . . . . . . . 18
8. Security Considerations . . . . . . . . . . . . . . . . . . . 19
9. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 19
10. Informative references . . . . . . . . . . . . . . . . . . . . 19
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 22
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1. Introduction
Peer to Peer (P2P) overlays have become quite popular with the advent
of file-sharing applications such as Napster [NAPSTER], KaZaa [KAZAA]
and BitTorrent [BITTORRENT]. After their success in file-sharing and
content distribution [Androutsellis-Theotokis], P2P networks are now
also being used for applications such as Voice over IP (VoIP) [SKYPE]
[Singh] and television [JOOST] [COOLSTREAM]. However most of these
systems are not purely P2P and have centralized components like the
login server in Skype [Baset] or moderators and trackers in
BitTorrent [Pouwelse]. Securing pure P2P networks is therefore still
a field of very active research [Wallach]. P2P overlays can be
broadly classified as structured and unstructured. Unstructured
overlays are often relatively simple but search operations in them
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 [Kim]. 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, Table 1 shows that some important
differences, mostly related to privacy and availability,
significantly increase security requirements.
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+-----------------+-----------------------+-------------------------+
| | File-sharing | Realtime communication |
+-----------------+-----------------------+-------------------------+
| Distributed | Shared file locations | User locations are |
| database | are indexed in a | indexed in a table |
| | table distributed | distributed among |
| | among peers; often | peers; rarely more than |
| | hundreds or thousands | one per user. |
| | per user. | |
| Availability | Same files are | Users are unique; |
| | usually available at | attacks targeting |
| | multiple locations | single users may be |
| | and failures | addressed both to the |
| | involving single | distributed index and |
| | istances are overcame | to the user's device |
| | by abundancy of | directly. |
| | resources; attacks | |
| | targeting single | |
| | files need to be | |
| | addressed to the | |
| | distributed index. | |
| Integrity | Attackers may want to | Attackers may want to |
| | share corrupted files | impersonate different |
| | in place of popular | users in order to |
| | content, e.g. to | handle calls directed |
| | discourage users from | to them; constitute a |
| | acquiring copyrighted | particular threat for |
| | material; constitute | the user as, in case of |
| | a threat for the | success, the attacker |
| | service, but not for | acquires full control |
| | the users. | on the victim's |
| | | personal |
| | | communications. |
| Confidentiality | Shared files are, by | Communications are |
| | definition, readable | usually meant to be |
| | by all users; in some | private and need to be |
| | cases encryption is | encrypted; evesdropping |
| | used to avoid | may reveal sensitive |
| | elements not involved | data and is a serious |
| | in the service to | threat for users. |
| | detect traffic. | |
+-----------------+-----------------------+-------------------------+
Main differences between P2P applications used for file-sharing and
for realtime communication.
Table 1
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The rest of the document is organized as follows. In Section 2, 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 Section 3, we
discuss admission control problems. In Section 4, we identify the
problem of where a node joins in the overlay. In Section 5, we
describe problems related to identification of malicious nodes and
the dissemination of this information. In Section 6, we describe the
issues of routing and data integrity in P2P networks. Finally, in
Section 7 we discuss how issues and solutions previously presented
apply in P2P overlays for realtime communication.
1.1. Purpose of this document
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.
2. The attackers
2.1. Incentive of the attacker
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 [McCue]. 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
[Sit]. However with replication of key-value pairs at multiple
locations, such threats can be mitigated.
Attackers may also have other incentives apart from 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 [Liang] and in hope that
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users would get frustrated and stop using the service. 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.
2.2. Resources available to the attacker
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
[Douceur] described in Section 4, IP addresses are also an important
resource for the attacker since in DHTs such as Chord [Stoica], the
position in the overlay is determined by using a base hash function
such as SHA-1 [SHA1]on the node's IP address. The cryptographic
puzzles [Rowaihy] 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 [Douceur] 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.
2.3. Victim of the attack
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 [Sit] 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.
2.4. Time of attack
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 [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
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been recorded in routing tables). Similarly for reputation
management systems, it is important that they adapt to the current
behavior of a peer.
3. Admission control
Admission control depends on who decides whether or not to admit a
node and how this permission is granted. Kim et. al [Kim] 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 [Saxena] 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 [Kong] and multisignature schemes [Ohta].
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
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easily addressed by humans but hard to solve by computers, also known
as CAPTCHA [Ahn].
4. Determining the position in the overlay
For ring based DHT overlays such as Chord [Stoica], Kademlia
[KADEMLIA] and Pastry [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 [Douceur]. 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 [Condie]
[Scheideler]. Such temporal solutions, however have the drawback
that they increase the maintenance traffic and possibly deteriorate
the efficiency of caching. Danezis et. al [Danezis] 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 [Rowaihy]. 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 [Yu]. to limit sybil attacks is
based on the usage of the social relations between users. Authors
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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 [Yu] 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) [Ratnasamy], 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.
5. Resilience against malicious peers
Making overlays robust against even a small percentage of malicious
nodes is difficult [Castro]. 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.
5.1. Identification of malicious peers
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.
5.1.1. Proactive identification
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 [Sit]. 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
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scalability, while, if they are performed too rarely, they would
probably be useless.
5.1.2. Reactive identification
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 [Poon] 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.
5.2. Reputation management systems
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 [Uzun] [Damiani] [Lee] [Kamvar] 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 [Lee] 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].
Kamvar et. al [Kamvar] define five design considerations for
reputation management systems;
o Self policing.
o Anonymity.
o No profit to new comers.
o Minimal overhead.
o Robustness to malicious peers.
5.2.1. Unstructured reputation management
Unstructured reputation management systems have been proposed by Uzun
et. al [Uzun] and Damiani et. al [Damiani]. 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 [Uzun], 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
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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.
5.2.2. Structured reputation management
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 [Lee] [Kamvar] try to resolve it in a structured
manner. The idea of the eigen trust algorithm [Kamvar] 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 [Haveliwala]. For maintaining this information in a
structured way, the authors use a content addressable network (CAN)
DHT [Ratnasamy]. 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.
6. Routing and data integrity
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 Section 7.
6.1. Data integrity
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,
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if a malicious peer modifies the pieces that are only available on it
and the original peer disappears, then the object distribution will
fail [Zhang]. An analysis of BitTorrent in terms of integrity and
performance can be found in the work of Pouwelse et. al [Pouwelse].
6.2. Routing integrity
To enhance the integrity of routing, it is important to reduce the
number of queries forwarded to malicious nodes. Marti et. al [Marti]
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 [Stoica] 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 [Danezis] 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 [PASTRY] and Kademlia
[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.
7. Peer-to-peer in realtime communication
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 [P2PSIP] 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:
o 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
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available at any given time as users only register a limited
number of locations (rarely more than one).
o 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.
7.1. Admission
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.
7.1.1. Active vs. passive upgrades
Most existing P2P networks [KAZAA] [BITTORRENT] [JOOST] 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 [SKYPE], 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
[Baset]. 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):
o An attacker cannot estimate in advance when and if it would be
invited to join the overlay as a peer.
o 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.
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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.
o 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).
7.1.2. When to upgrade
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. The criteria for
determining load can be:
o Number of clients attached.
o Bandwidth usage for DHT maintenance, forwarding requests and
responses to and from peers and from the attached clients.
o Memory usage for DHT routing table, DHT neighborhood table,
application specific data and information about the attached
clients.
7.1.3. Which clients to upgrade
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:
o 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.
o 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.
o 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.
7.1.4. Incentives for clients
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:
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o 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.
o 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.
7.2. Security
7.2.1. Targeted denial of service
In addition to bombardment with queries as described in Section 2,
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 [Danezis] can also
provide protection against such attacks as in their solution peers
vary the nodes used in queries.
7.2.2. Man in the middle attack
The man in the middle attack is well described by Seedorf [Seedorf06]
in the particular case of P2PSIP [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'' [Danezis]. 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 [Seedorf06].
7.2.3. Trust between peers
The effect of malicious peers could be mitigated by introducing the
concept of trust within an overlay. This can be done in different
ways:
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o Using certificates assigned by an external authority. The
drawback with this approach is that it requires a centralized
element.
o Using certificates reciprocally signed by peers. This mechanism
is quite similar to PGP [Zimmermann]; 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.
7.2.4. Routing call signalization
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 [RFC3261] 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 called SIPSEC [I-D.gurbani-sip-sipsec]. It 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 [RFC2818].
7.2.5. Integrity of location bindings
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 [Seedorf08]. 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
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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.
7.2.6. Encrypting content
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
[RFC3711]. 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.
7.2.7. Other issues
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.
o To whom do the users pay?
o Do the users only pay when accessing the public telephone network?
o 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 [Baumann] 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.
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8. Security Considerations
This document, informative in nature, discusses some of the security
issues of peer-to-peer systems used for realtime communications.
9. Acknowledgments
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.
10. Informative references
[Ahn] Ahn, Luis., Blum, Manuel., and John. Langford, "Telling
humans and computers apart automatically".
[Androutsellis-Theotokis]
Androutsellis-Theotokis, S. and D. Spinellis, "A survey of
peer-to-peer content distribution technologies".
[BITTORRENT]
"BitTorrent", <http://www.bittorrent.com/>.
[Baset] Baset, S. and H. Schulzrinne, "An analysis of the skype
peer-to-peer internet telephony protocol".
[Baumann] Baumann, R., Cavin, S., and S. Schmid, "Voice Over IP -
Security and SPIT".
[COOLSTREAM]
"COOLSTREAMING", <http://www.coolstreaming.us>.
[Castro] Castro, M., Druschel, P., Ganesh, A., Rowstron, A., and D.
Wallach, "Secure routing for structured peer-to-peer
overlay networks".
[Condie] Condie, T., Kacholia, V., Sankararaman, S., Hellerstein,
J., and P. Maniatis, "Maelstorm: Churn as Shelter".
[Damiani] Damiani, E., Vimercati, D., Paraboschi, S., Samarati, P.,
and F. Violante, "A Reputation-Based Approach for Choosing
Reliable Resources in Peer-to-Peer Networks".
[Danezis] Danezis, G., Lesniewski-Laas, C., Kaashoek, M., and R.
Anderson, "Sybil-resistant DHT routing".
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[Douceur] Douceur, J., "The Sybil Attack".
[Haveliwala]
Haveliwala, T. and S. Kamvar, "The second value eigenvalue
of the google matrix".
[I-D.gurbani-sip-sipsec]
Gurbani, V., Audet, F., and D. Willis, "The SIPSEC Uniform
Resource Identifier (URI)", draft-gurbani-sip-sipsec-01
(work in progress), June 2007.
[JOOST] "Joost", <http://www.joost.com>.
[KADEMLIA]
Maymounkov, P. and D. Mazi, "Kademlia: A Peer-to-peer
Information System Based on the XOR Metric".
[KAZAA] "KaZaa", <http://www.kazaa.com/>.
[Kamvar] Kamvar, S., Garcia-Molina, H., and M. Schlosser, "The
EigenTrust Algorithm for Reputation Management in P2P
Networks".
[Kim] Kim, Y., Mazzocchi, D., and G. Tsudik, "Admission Control
in Peer Groups".
[Kong] Kong, J., Zerfos, P., Luo, H., Lu, S., and L. Zhang,
"Providing robust and ubiquitous security support for
MANET".
[Lee] Lee, S., Kwon, O., Kim, J., and S. Hong, "A Reputation
Management System in Structured Peer-to-Peer Networks".
[Liang] Liang, J., Kumar, R., Xi, Y., and K. Ross, "Pollution in
p2p file sharing systems".
[Marti] Marti, S., Ganesan, P., and H. Garcia-Molina, "SPROUT: P2P
Routing with Social Networks".
[McCue] McCue, Andy., "Bookie reveals 100,000 cost of denial-of-
service extortion attacks", <http://software.silicon.com/
security/0,39024655,39121278,00.htm>.
[NAPSTER] "Napster", <http://www.napster.com/>.
[Ohta] Ohta, K., Micali, S., and L. Reyzin, "Accountable Subgroup
Multisignatures".
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[P2PSIP] "Peer-to-Peer Session Initiation Protocol (P2PSIP) IETF
Working Group",
<http://ietf.org/html.charters/p2psip-charter.html>.
[PASTRY] Rowstron, A. and P. Druschel, "Pastry: Scalable,
distributed object location and routing for large-scale
peer-to-peer systems".
[Poon] Poon, W. and R. Chang, "Robust Forwarding in Structured
Peer-to-Peer Overlay Networks".
[Pouwelse]
Pouwelse, J., Garbacki, P., Epema, D., and H. Sips, "The
Bittorent P2P File-Sharing System: Measurements and
Analysis".
[RFC2818] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[RFC3261] Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston,
A., Peterson, J., Sparks, R., Handley, M., and E.
Schooler, "SIP: Session Initiation Protocol", RFC 3261,
June 2002.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
[Ratnasamy]
Ratnasamy, S., Francis, P., Handley, M., Karp, R., and S.
Shenker, "A Scalable Content-Addressable Network".
[Rowaihy] Rowaihy, H., Enck, W., McDaniel, P., and T. Porta,
"Limiting Sybil attacks in structured peer-to-peer
networks".
[SHA1] 180-1, FIPS., "Secure Hash Standard".
[SKYPE] "Skype", <http://www.skype.com/>.
[Saxena] Saxena, N., Tsudik, G., and J. Yi, "Admission Control in
Peer-to-Peer: Design and Performance Evaluation".
[Scheideler]
Scheideler, C., "How to Spread Adversarial Nodes?:
Rotate!".
[Seedorf06]
Seedorf, J., "Security Challenges for Peer-to-Peer SIP".
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[Seedorf08]
Seedorf, J., "Using Cryptographically Generated SIP-URIs
to Protect the Integrity of Content in P2P-SIP".
[Singh] Singh, K. and H. Schulzrinne, "Peer-to-Peer Internet
Telephony using SIP".
[Sit] Sit, E. and R. Morris, "Security considerations for peer-
to-peer distributed hash tables".
[Stoica] Stoica, I., Morris, R., Karger, D., Kaashoek, M., and H.
Balakrishnan, "Chord: A Scalable Peer-to-peer Lookup
Service for Internet Applications".
[Uzun] Uzun, E., Pariente, M., and A. Selpk, "A Reputation-Based
Trust Management System for P2P Networks".
[Wallach] Wallach, D., "A Survey of Peer-to-Peer Security Issues",
<http://www.cs.rice.edu/~dwallach/pub/tokyo-p2p2002.pdf>.
[Yu] Yu, H., Kaminsky, M., Gibbons, P., and A. Flaxman,
"SybilGuard: Defending Against Sybil Attacks via Social
Networks".
[Zhang] Zhang, X., Chen, S., and R. Sandhu, "Enhancing Data
Authenticity and Integrity in P2P Systems".
[Zimmermann]
Zimmermann, Philip., "Pretty good privacy: public key
encryption for the masses".
Authors' Addresses
Henning Schulzrinne
Columbia University
1214 Amsterdam Avenue
New York, NY 10027
USA
Email: hgs@cs.columbia.edu
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Enrico Marocco
Telecom Italia
Via G. Reiss Romoli, 274
Turin 10148
Italy
Email: enrico.marocco@telecomitalia.it
Emil Ivov
SIP Communicator
4 rue Blaise Pascal
Strasbourg Cedex F-67070
France
Email: emcho@sip-communicator.org
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