INTERNET-DRAFT R. Canetti, B. Pinkas
draft-irtf-smug-taxonomy-01.txt IBM Research, InterTrust Technologies
Expire in six months August 2000
A taxonomy of multicast security issues
(updated version)
Status of this Memo
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1. Abstract
With the growth and commercialization of the Internet, the need for
secure IP multicast is growing. In this draft we present a taxonomy
of multicast security issues. We first sketch some multicast group
parameters that are relevant to security, and outline the basic
security issues concerning multicast in general, with emphasis on IP
multicast. Next we suggest two `benchmark' scenarios for secure
multicast solutions. Lastly we review some previous works.
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Table of Contents:
1. Abstract ................................................. i
2. Introduction ............................................. 1
3. A Taxonomy of multicast security issues................... 2
3.1 Multicast group characteristics....................... 2
3.2 Security requirements and trust issues................ 3
3.3 Performance parameters............................... 5
4. Benchmark Scenarios....................................... 5
4.1 Single source broadcast............................... 6
4.2 Virtual Conferences................................... 7
5. A mini-survey of related work............................. 7
5.1 Works on group key management......................... 8
5.2 Works on individual authentication....................10
5.3 Works on membership revocation........................11
5.4 Working prototypes....................................13
5.5 Architectures.........................................13
5.6 Fighting piracy.......................................14
Acknowledgments..............................................15
References...................................................16
Authors address..............................................18
2. Introduction
In addition to traditional unicast communication, the Internet
Protocol supports a multicast mode where a packet is addressed to
a group of recipients. The main motivation behind this mode is
efficiency, both in sender resources (one transmission serves all
recipients) and in network resources (far less traffic). The main
challenge in efficient multicast transmission is routing: how to get
a packet to its intended recipients with minimal latency and
bandwidth consumption. See work done at the MBONED and IDMR working
groups. Reliable multicast is being studied in the IRTF Reliable
Multicast working group.
The growth and commercialization of the Internet offers a large
variety of scenarios where multicast transmission will greatly save
in bandwidth and sender resources. Immediate examples include news
feeds and stock quotes, video transmissions, teleconferencing,
software updates, and more. (See [Quinn] for a more complete survey
on multicast applications.) Yet, multicast transmission introduces
security concerns that are far more complex than those of simple
unicast. Even dealing with the `standard' issues of message and
source authentication and secrecy becomes much more complex; in
addition other concerns arise, such as access control, trust in
group centers, trust in routers, dynamic group membership, and
others.
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Security solutions should mesh well with current multicast
routing protocols, and should have as small overhead as possible.
In particular, a realistic solution must maintain the current way by
which {\em data packets} are being routed; yet additional control
messages may be introduced, for key exchange and access control.
These messages need not necessarily be sent via multicast.
As a first step towards a workable solution, we present a taxonomy
of multicast security concerns and scenarios, with a strong emphasis
on IP multicast. First we list multicast group characteristics
that are relevant to security. Next we list security concerns and
some trust issues. We also discuss important performance parameters.
It soon becomes clear that the scenarios are so diverse that there
is little hope for a single security solution that accommodates all
scenarios. Thus we suggest two `benchmark' scenarios for
multicast security solutions. One scenario involves a single sender
(say, an on-line stock-quotes distributor) and a large number of
passive recipients (say, hundreds of thousands). The second scenario
depicts relatively small interactive groups of up to few thousands
of participants.
Lastly we present a brief survey of existing work on multicast
security. (The authors apologize in advance for any
misinterpretations and omissions. Please write and complain. They
will be happy to update and correct the draft.) Two main issues
emerge, where the performance of current solutions leaves much to be
desired:
- Source authentication: How to make sure that information
is arriving unmodified from a particular group member
(as opposed to information coming from "one of the group members").
- Membership revocation: How to prevent a leaving member from
future access to the group resources.
3. A Taxonomy of multicast security issues
3.1 Multicast group characteristics
We list salient parameters of multicast groups. These parameters
crucially affect the security architecture that should be used.
Group size: Can vary from several tens of participants in small
discussion groups, through thousands in virtual conferences
and classes, and up to several millions in large broadcasts.
Member characteristics: These include computing power (do all members
have similar computing power or can some members be loaded more
than others?) and attention (are members on-line at all times?).
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Membership dynamics: Is the group membership static and known in
advance? Otherwise, do members only join, or do members also
leave? how frequently does membership change and how fast should
changes be updated? Are membership changes bursty?
Expected life time: Is the group expected to last several minutes?
days? an unbounded amount of time?
Number and type of senders: Is there a single party that sends data?
several such parties? all parties? Do few senders generate most
of the traffic? Is the identity of the senders known in advance?
Are non-members expected to send data to the group?
Volume and type of traffic: Is there heavy volume of communication?
Must the communication arrive in real-time? what is the allowed
latency? For instance, is it data communication (less stringent
real-time requirements, low volume), audio (must be real-time,
low volume), or video (real-time, high volume)?
3.2 Security requirements and trust issues
We list several security requirements and trust-related concerns. Not
all issues are relevant to all multicast applications; yet they
should be kept in mind when designing a system.
Group management and access control: Making sure that only registered
and legitimate parties have access to the communication addressed
to the group. (Sometimes it may be necessary also to allow only
group members to send data to the group.) Sometimes this is
enforced by having a group key that is known only to group members;
other, more hierarchical solutions exist as well. Here several
security concerns are involved:
* How to authenticate potential group members
* How to securely distribute the group key(s).
* How to revoke membership of leaving members
* How to prevent joining members from access to past group
communication.
* How to periodically refresh the group key(s).
* How to log information and allow for external auditing.
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Ephemeral secrecy: Preventing non group-members from having easy
access to the transmitted data. Here a mechanism that only delays
access, or prevents access only to crucial parts of the data may be
sufficient. (For instance, to maintain ephemeral secrecy when
transmitting a video it is sufficient to encrypt only the
low-order Fourier coefficients in an MPEG encoding.) Ephemeral
secrecy is often sufficient to protect multicasted contents, in
cases where the content itself is not confidential.
Long-term secrecy: Making sure that the data remains secret to
non-group members, for a substantial amount of time after
transmission. This may often not be a requirement for multicast
traffic. In particular, the larger the multicast group the
weaker the secrecy assurance is (even if the cryptography
is perfect).
Forward Secrecy: Making sure that encrypted data remains secret
even if the key is compromised (either by cryptanalysis or
by break-in) at a later date. This requirement is needed only for
applications that require long-term secrecy. Thus in many
multicast applications it is not necessary.
Sender and data authenticity: Making sure that the received data
originates with the claimed sender and was not modified on
the way. Authenticity takes two flavors: Group authenticity
means that a group member can recognize whether a message
was sent by a group member. Source authenticity means that
it is possible to identify the particular sender within the group.
It may also be desirable to verify the origin of messages
even if the originator is not a group member.
Anonymity: Several flavors are possible. One is keeping the identity
of group members secret from outsiders or from other group members.
Another is keeping the identity of the sender of a message secret.
A related concern is protection from traffic analysis.
Non-repudiability: This refers to the ability of receivers of data to
prove to third parties that the data has been transmitted, together
with the source. Non-repudiability is somewhat contradictory to
anonymity, and it is not clear whether it should be implemented in
an IP-layer protocol.
Service availability: Maintaining service availability against
malicious attack is ever more challenging in a multicast setting,
since clogging attacks are easier to mount and are much more
harmful.
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3.3 Performance parameters
We list relevant performance parameters. Relative importance of
these parameters may vary from application to application. These
parameters should always be measured against the degree of security
achieved.
Latency, bandwidth and work overhead per data packets. These are
the most immediate costs and should typically be minimized at the
highest priority. Here distinction should be made between the load
on strong server machines and on weak end-users. An additional
important parameter here is the amount of buffering needed at the
sending side and at the receiving side, both in terms of required
space and in terms of packet delay.
Latency, bandwidth and work overhead per control packets. These are
typically less frequent, thus efficiency here is somewhat less
crucial. The control messages usually deal with key distribution
and refreshment.
Group initialization, and member addition and deletion overheads.
Group initialization occurs once. In groups with highly dynamic
membership, efficient addition (and especially deletion) of
members may be an important concern.
Sender initialization, the overhead of a sender when it starts
transmitting to the group.
Congestion control, especially around centralized control services
at peak sign-on and sign-off times. (A quintessential scenario
is a real-time broadcast where many people join right before the
broadcast begins and leave right after it ends.)
Resume overhead: The work incurred when a group member becomes
active after being dormant (say, off-line) for a while.
4. Benchmark Scenarios
As seen above, it takes many parameters to characterize a multicast
security scenario, and a large number of potential scenarios exist.
Different scenarios call for different solutions; it seems unlikely
that a single solution will accommodate all scenarios.
We present two very different scenarios for secure multicast,
and sketch possible solutions and challenges. These scenarios seem
to be the ones that require most urgent solutions; in addition, they
span a large fraction of the concerns described above, and solutions
here may well be useful in other scenarios as well. Thus we suggest
these scenarios as benchmarks for evaluating security solutions.
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4.1 Single source broadcast
Here a single source wishes to continuously broadcast data to a large
number of passive recipients. The source can be a news agency that
broadcasts stock-quotes and news-feeds to paying customers, or
possibly a Pay-TV station. We list a number of characteristics:
The number of recipients can be up to hundreds of thousands and more.
The source is typically a top-end machine with ample resources.
It can also be parallelized or split to several sources in different
locations. The recipients are typically lower-end machines
with limited resources. Consequently, the security solution must
optimize for efficiency at the recipient side.
The life-time of the group is usually long. Yet, the group
membership is dynamic: members join and leave at a relatively high
rate. In addition, at peak times (say, before and after important
broadcasts) a high volume of sign-on/sign-off requests are expected.
It can be assumed that members have a long-term relationship with the
group; this may facilitate processing of sign-on/sign-off requests.
The volume of transmitted data may vary considerably: if only
text is being transmitted then the volume is relatively low (and
the latency requirements are quite relaxed); if audio/video is
transmitted then the volume can be very high and very little latency
is allowed.
Authenticity of the transmitted data is a crucial concern and
should be strictly maintained: a client must never accept a forged
stock-quote as authentic. In particular, it should not accept a stock
quote that originated with any other group member than the specified
sender. Another important concern is preventing
non-members from using the service. This can be achieved by
encrypting the data; yet the encryption may be relatively
weak/ephemeral since there is no real secrecy requirement - only
prevention from easy unauthorized use.
The required latency of the communication varies from application
to application. Member revocation would be performed within
minutes or seconds from the time it is requested (but it is typically
not required to remove members within fractions of a second)
There is typically a natural group owner that manages access-control
as well as key management. However, the sender of data may be a
different entity (say, Yahoo! broadcasting Reuters stock-quotes
via its home-page).
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4.2 Virtual Conferences
Typical virtual conference scenarios may include on-line meetings
of corporate executives or committees, town-hall type meetings,
interactive lectures and classes, or multiparty video games.
A virtual conference involves several tens to hundreds of peers,
often with roughly similar computational resources. Usually most,
or all, group members may a-priori wish to transmit data
(although often there is a small set of members that generate
most of the bandwidth).
The group is often formed per event and is relatively short-lived
(say, few minutes or hours). Membership is often static: members
join at start-up, and remain signed on throughout. Furthermore,
even if a member leaves it is often not crucial to
cryptographically revoke their group membership.
Bandwidth and latency requirements vary from application to
application, similarly to the case of single source
broadcast. However, latency (and especially sender initialization)
should typically be very small in order to facilitate the
simultaneity and interactivity of virtual conferences.
Authenticity of data {\em and sender} may be the most crucial
security concern. In some scenarios maintaining secrecy of data and
anonymity of members may be important as well; in other
scenarios secrecy of data is not a concern at all. There is often a
natural group owner that may serve as a trusted center. Yet,
it is always beneficial to distribute trust as much as possible.
5. A mini-survey of known related work
Following is a short survey of multicast security related work. The
authors apologize in advance for any misinterpretations and
omissions. Please write and complain. They will be happy to update
and correct the draft.
The first three sections of the survey describe work on three main
issues described above: group key management, individual
authentication, and membership revocation. The last section describes
work on prototypes which implement various elements of multicast
security.
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5.1 Works on group key management
The works below concentrate on establishing and managing a common
key among all group members. This key can be used for encryption
and group authentication, but is insufficient for individual
authentication. Group management is closely related to user
revocation methods (see Section 5.3) since it should prevent a
leaving group member from further decrypting the group
communication.
The GKMP protocol [GKMPA,GKMPS] generates and maintains symmetric
keys for the members of a multicast group. In this protocol a
multicast group has a dedicated Group Controller (GC) which is
responsible for managing the group keys. The GC generates the
group keys in a joint operation with a selected group
member. Afterwards it contacts each group member validates its
permissions, and sends it the group keys (encrypted using a key which
is mutually shared between the GC and that member). This approach may
have scalability problems since a single entity, the GC, is
responsible for sending the keys to all group members.
The Scalable Multicast Key Distribution scheme (SMKD) [Ballardie] is
based on the Core-Based Tree (CBT) routing protocol and provides
secure join to a CBT group tree in a scalable approach. It utilizes
the hard-state approach of CBT in which routers on the delivery tree
know the identities of their tree-neighbors. When a CBT group is
initiated in this scheme the core of the tree operates as the group
controller and generates the group session keys and key distribution
keys. As routers join the delivery tree they are delegated the
ability to authenticate joining members and provide them with the
group key. This approach is highly scalable. However, it is tied to a
specific routing protocol, and does not provide a separation between
the routing and the security mechanism. (In particular, it puts high
trust in the routers, since each router in the delivery tree obtains
the same keys as the group controller.)
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The MKMP [MKMP] key management protocol enables the initial Group Key
Manager to delegate the key distribution authority to other
parties in a dynamic way. It first generates the group key.
Then it delegates the key distribution ability to
selected parties by sending a message to the multicast group
soliciting these parties. This message contains keys and access
lists which can only be decrypted by the solicited parties. After
they obtain this material they can operate as Group Key Managers.
This dynamic approach has the advantage that the group topology can
be adapted on-line. MKMP uses a single key for the entire group and
thus does not require hop-by-hop decryption/re-encryption of the
payload.
The Iolus scheme [Mittra] handles the scalability problem
by introducing a "secure distribution tree". The multicast group is
divided into subgroups which are arranged hierarchically. There is a
Group Security Controller (GSC) managing the top-level group, and
Group Security Intermediaries (GSIs) for managing the different
subgroups. Each subgroup has its own sub-key which is chosen by its
manager. A GSI knows the keys of its subgroup and of a higher level
subgroup, so it can "translate" messages to/from higher levels.
A disadvantage of this approach is the latency incurred by GSIs
decrypting and re-encrypting each data packet (although the use of
encryption indirection enables this latency to be constant and
independent of the packets length). The removal of an untrusted GSI
is also complex.
The work of Poovendran et al [PACB] identifies two major drawbacks of
the GKMP protocol which result from the use of a single group
controller: the group controller is a single point of failure, and
its heavy load might affect scalability. It is suggested to use a
panel of three controllers, where every two panel members can operate
as a group controller. It is furthermore proposed to improve
scalability by segmenting the group into clusters, which are each
managed by a sub-controller panel.
The Internet draft of Hardjono et al [HCD] suggests a hierarchical
framework for group key management, similar to that of Iolus. The
network is divided into regions: many "leaf regions" and a single
"trunk region" which which is used to connect leaf regions and does
not contain any member hosts. Each region can have a different key
management protocol. In particular, different leaf regions can have
different intra-region group key management protocols, and the trunk
region operates an inter-region group key management protocol.
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In [HCM] an intra-region group key management protocol is presented
in detail. To enable scalability a domain is further divided into
areas, where each host belongs to a single area. There is a single
Domain-Key-Distributor and many Area-Key-Distributors which are
responsible for each area. A host only communicates with the AKD of
its area. A key for the members of a multicast group in a domain is
generated by the DKD and is propagated to the hosts through the
AKD's. This scheme presents an interesting new concept: the group key
is common to members in the entire domain, while the control
messages for key updates are transferred via the
Area-Key-Distributors, using two levels of keys. This method enjoys
"the best of two worlds": First, the data packets need not be
re-encrypted en-route and can be routed using any multicast routing
protocol. Second, the group (or domain) controller need not keep
track of all group members; instead, it can keep track only of the
AKDs. This facilitates scalability while maintaining independence
from the data routing mechanism. Note that this protocol is for
managing a multicast group inside a domain (a "leaf" in the terms of
[HCD]) whereas a different protocol can be used for inter-domain
("trunk") key management of the group.
Kruus [Kruus] focuses on identifying and surveying security related
issues for multicast group key management. The paper describes
several approaches for group key management and for user revocation.
Banerjee and Bhattacharjee [BB] suggest using spatial clustering of
group members in order to scale rekeying and other multicast based
applications. The group members are divided to clusters, which each
have a cluster leader. In a higher level, the cluster leaders are
divided to clusters, which have their own leaders, and so on. An
analysis of the rekeying algorithm shows that the amortized cost of
rekeying is constant.
The previous schemes have a single group controller (GC), which is a
single point of failure, or otherwise use several GCs in a way which
compromises the security of the whole group, or part of the group, if
any one of the GCs is broken into. Alternatively, one could use a
distributed pseudo-random system [NPR] which uses several servers to
produce keys. The system has n servers, and a host must contact k
servers in order to obtain a key. The system ensures that two or more
hosts, that need to obtain the same key, learn the same key value
even if they contact different, non-intersecting, sets of k
servers. The system provides better security in the sense that an
adversary must break into at least k servers in order to learn keys.
Rodeh et al. [Rodeh] introduced algorithms for management of
group-keys in group-communication systems. Unlike prior work, based
on centralized key-servers, this solution is completely distributed
and fault-tolerant and its performance is comparable to the
centralized solution.
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5.2 Works on individual authentication
In order to authenticate that a message was sent by one of the group
members it is sufficient to use Message Authentication Codes (MACs,
see e.g. [HMAC]) with a single shared key known to all group
members. However, this method does not suffice to enable individual
authentication, i.e. it cannot be used to authenticate a message as
originating from a specific party.
Source authentication can be achieved if the sender of the
message signs it using a digital signature scheme. However, the
computational complexity of computing and verifying digital
signatures, as well as the length of the signature, is
significant. RSA signatures might be an appealing choice of a
signature scheme since it is possible to use them in a mode which
considerably reduces the running time of the verification algorithm.
(Furthermore, the use of Batch RSA [Fiat] enables the source to sign
many messages in parallel, with a computational overhead which is not
considerably larger than signing a single message. The sender should,
however, know all the messages that should be signed before it can
sign the first message.)
It is possible to use signature schemes based on elliptic curves,
which are very efficient both in processing time and in bandwidth.
Another interesting approach is to use on-line/off-line signature
schemes. These enable the signer to perform most of its
computation off-line, even before it learns the message that it
should sign. When this message becomes known the signer only has
to perform a very efficient computation in order to complete the
signature.
The schemes of Gennaro and Rohatgi [GR] enable to efficiently sign
streams of data. Basically, the idea is to partition data packets
into chains. Each data packet includes a hash of the next packets in
the chain, and then only the first packet in the chain needs to be
signed. There are two types of schemes, for data streams which are
available off-line (and therefore the whole stream can be examined by
the source before it sends the first packet), and for real-time data.
A major drawback of the suggested schemes is that they do not deal
well with unreliable communication channels and might therefore not
be suitable for large scale multicast groups. Furthermore, the scheme
for real-time data introduces a considerable communication overhead
per packet.
Wong and Lam [WL] address the problem of source authentication in the
presence of unreliable communication. Their scheme allows a receiver
to individually authenticate each packet. The idea is to let the
sender buffer a number of packets; once enough packets are buffered
the sender computes a hash-tree of the packets (a la Merkle
[Merkle]), and signs the root. This signature is attached to each
packet, together with the appropriate hashes that allow verification
of the packet independently of other packets. It is also suggested
that signatures will be performed using a variant of Fiat-Shamir
signatures, which (using some heuristics) are more efficient than
other common signature schemes.
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Rohatgi [Rohatgi] describes a scheme that gets rid of buffering
altogether, even at the sender side. The idea is to prepare and sign
the hash tree in the [WL] scheme ahead of time, where the leaves
correspond to public keys of one-time signatures a la Lamport. This
way, each packet can be signed (using the one-time signature scheme)
and sent without delay. Similarly, each packet can be verified upon
arrival independently of other packets. The main drawback of that
scheme is the size of the signature, which is up to 270 byes per
packet.
Another approach to making the [GR] scheme resilient to packet loss
was taken by Perrig et. al. [PCTS]. (A similar idea appears in [G].)
First, they let the hash of each packet appear in the next packet
(rather than in the previous one). Then, they include the hash
of each packet in several additional packets "down the stream"
from the hashed packet. This provides resilience to packet loss,
provided that the choice of packets that contain the hash of each
packet is a good one. They also suggest other optimizations that
reduce the bandwidth overhead of their scheme.
Building on previous work [FN1,DFFT], Canetti et al [CGIMNP]
suggest individual authentication schemes which are based on
efficient MACs rather than on public key signatures. These schemes
are designed to be secure against coalitions of up to k of group
members, where k is a parameter which affects the overhead. To
explain the approach, let us present a simplified example: The idea
is to use some number, n, of MAC keys. The pre-designated sender has
all keys, where each one of the receivers has n/2 keys, chosen at
random from the n keys. Now, each message is MACed with each one of
the n keys, and a recipient verifies the MACs whose keys it knows.
A coalition of bad parties can make some `victim' accept forged
messages only it the coalition knows all the MAC keys that the victim
knows. The parameters are set so that the probability that such a
bad event occurs is small.
Yet another approach to providing source authentication uses only
symmetric cryptography, more specifically on message authentication
codes (MACs), and is based on delayed disclosure of keys by the
sender. The idea, common to all above schemes, is to have the sender
attach to each packet a MAC computed using a key $k$ known only to
itself. The receiver buffers the received packet without being able
to authenticate it. If the packet is received after some
``deadline'', it is discarded. A short while later, the sender
discloses $k$ and the receiver is able to authenticate the packet.
(See more details within.) Consequently, a single MAC per packet
suffices to provide source authentication, provided that the sender
has synchronized its clock with the sender ahead of time.
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This technique was first used by Cheung [Cheung] in the context
of authenticating communication among routers. It was then used in the
Guy Fawks protocol [Fawks] for interactive unicast communication.
In the context of multicast streamed data it was proposed by several
authors [BCC,Briscoe,PCTS]. An Internet Draft based on the TESLA scheme
of [PCTS] was recently written [PCBST].
5.3 Works on membership revocation
In order to prevent new group members (respectively, leaving members)
from accessing data sent before they joined (respectively, after they
leave), the group controller needs to change a multicast group key
whenever membership in the group changes. While it is rather
straightforward to efficiently update the group key when a new member
joins the group, this is not the case when a member is removed from
the group since this member already knows the group key.
The approach taken in many group key management protocols
[GKMPA,SMKD,MKMP] to remove untrusted members is to generate a new
group key and send it independently to each of the remaining group
members (using secret keys which are shared between each of the
members and the group controller), thus essentially creating a new
multicast group without the untrusted member. This approach is
non-scalable.
As discussed in Section 5.1, an alternative approach is to divide the
multicast group to subgroups with independent subgroup keys. When a
member is removed it is only required to send individually encrypted
messages to members of the subgroup of the removed member. This
approach, taken in [Mittra,PACB,HCD,HCM], is more scalable. It
requires that each subgroup contains a trusted controller (e.g. the
Group Security Intermediary in the Iolus system of [Mittra]). If this
party becomes untrusted then a more complex revocation procedure
(which is not described in these drafts) should be run. (Note
that the scheme of [HCM] suggests that subgroups (domains) are
further partitioned to smaller subgroups (areas) with their own
controllers, to provide better scalability).
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Broadcast encryption [FN] is a scheme to encrypt messages from a
single source to a dynamically changing group of recipients. When a
member is leaving the scheme can be used to send the new group
key to the remaining members. The scheme uses a parameter k which is
the maximum tolerable size of a corrupt coalition of former group
members that might try to learn a key they should not get. The
overhead of the scheme depends on the maximum number of potential
members in the group, and the maximum size of the corrupt coalition,
but not on the number of members which are removed. Therefore
overhead is better than in other user revocation methods if the
number of leaving/joining members is large. The scheme is based on
using a set of keys and applying a clever method of assigning subsets
of these keys to group members. This assignment makes sure that for
every corrupt coalition of k users it is possible to encrypt a
message such that the keys known to its coalition members do not
suffice for decryption, whereas the keys of any other member do.
Wallner et al [WHA] (and, independently, [WGL]) introduce a
scalable, tree based user revocation scheme. For a group of n members
there is a total of 2n keys but each member is only required to store
log(n) keys. When a group member is removed the group controller
sends a single message of size 2log(n) to all members, and each
member performs log(n) (rather efficient) computations in order to
generate the new group key. The removed member cannot compute the new
group key even if it receives this message. The basic idea of the
scheme is to imagine the users as the leaves of a binary tree, assign
a key to each node, give each user the keys in the path from its leaf
to the root and use the root key as the group key. When a user is
removed all the keys it holds are replaced. Two drawbacks of the
scheme are that it requires the center to keep track of 2n keys, and
requires each member to receive and process each member-revocation
message in order to learn the current group key.
The scheme of [WHA] was generalized by [WGL] for trees of arbitrary
degree. It is possible to reduce the length of the member
revocation message that the controller broadcasts to only log(n)
encryptions (instead of 2log(n) in [WHA]). Different schemes that
achieve this property are presented in [CGIMNP] and in [MS].
The scheme of [MS] affects the broadcast overhead of the member
join operation, it increases it from O(1) in the scheme of [WHA]
to log(n). The scheme of [CGIMNP] does not increase the overhead of
member join. The security of the scheme of [CGIMNP] can be
rigorously proven based on the security (i.e. pseudorandomness) of
the cryptographic function that is used.
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A member revocation scheme of a different flavor is suggested in
[NP1]. It enables to revoke the keys of up to k members, and is secure
against a coalition of all the revoked members. Th personal key of
each member is of constant size, and the revocation message is of
size O(k). The main idea is to give each member a personal key which
is a share of a (k+1)-out-of-n secret sharing scheme. When k members
have to be removed their keys are broadcast and the new group key is
set to be the secret. Each other member has k+1 shares and can reveal
the secret, whereas the revoked members have nothing but their k
shares. The scheme was generalized to multiple revocations and to
interleaving with traitor tracing.
5.4 Working prototypes
A prototype of the Iolus system has been implemented [Mittra]. It
uses a client application which interfaces between applications and
the Iolus GSC/GSIs. It is claimed there that the basic prototype is
rather a simple to implement and to use. There is only small
penalty for the decryption/encryption process of a GSI, and
this penalty does not depend on the size of the payload. Note however
that the Iolus system does not provide any individual authentication
mechanism.
A toolkit for secure internet multicast is described in [CEKPS]. It
emphasizes a separation between control and data functions. This
enables applications to have fine grain control over the data path,
while keeping the control plain transparent to the applications. The
toolkit can operate without end-to-end support for multicast, using
data reflectors connected via unicast tunnels. It is written in
Java. Similar to Iolus, a multicast group is divided to subgroups
(domains), however the toolkit offers better flexibility, supports
individual authentication (by using digital signatures), and operates
over non-multicast enabled backbones.
5.5 Architectures
Several works address the architectural issues involved in the key
management aspect of secure multicast [HCD,GMKPA.GMKPS,WHA,WGL].
These works have been described above.
In [CCPRRS] a host architecture for secure IP multicast protocols is
suggested. The architecture is based on the IPSEC architecture [Ken98]
and tries to use IPSEC components (IKE, AH, ESP) as much as possible.
As in IPSEC, the architecture calls for separation of the key
management (to take place in the application layer) from the data
transformations (to take place mostly in the IP layer). Yet, an
additional data-processing module is added in the application/UDP
layer. This module is responsible for source authentication, which is
not taken care of by the IPSEC transformations. The suggested
architecture is flexible and can adopt many of the key management
protocols described above.
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5.6 Fighting piracy
Secure multicast is used to ensure the secrecy of the
communication inside the multicast group. However nothing
prevents one of the legitimate members of the group from helping
other, illegitimate parties to receive the messages that are
sent to the group. This problem is well known in the context of
television broadcasting, and is commonly referred to as
"piracy". There has been a considerable amount of work on this
subject, motivated by the pay-TV industry. It is mostly relevant
to the one-to-many multicast scenario.
The fight against piracy consists of two stages: (1) tracing the
pirates, and (2) taking action to prevent them from further
operation. We describe below some methods which can be used for
tracing. Once the source of piracy is detected it is possible to
prevent it from further decryption using the user revocation
schemes we discussed above. It might be also desirable to take
legal actions against the pirates.
There are two methods by which pirates can operate. They can either
(1) distribute decryption keys that enable to decrypt the group
communication, i.e. perform illegal "key distribution", or
(2) decrypt the communication themselves and distribute the
decrypted content, i.e. perform illegal "content distribution".
The content distribution attack is harder for large scale
implementation and easier to detect, since the pirates should
essentially operate their own broadcast station. The key
distribution attack is therefore preferable to the pirates.
In order to prevent illegal key distribution pay-TV providers
provide decryption keys in smartcards which are supposed to
be "tamper proof". That is, it should be hard to extract from a
key from a smartcard. However, most smartcards have weaknesses
which enable to extract the keys they contain (see e.g. [C]).
It is easier to trace the source of a key distribution attack
than that of a content distribution attack. The reason being that
it is possible to give each user a somewhat different key (and
then, given a pirate decoder, recognize which keys were used to
construct it). It is a much harder task to distribute to each
user a different copy of the data such that each copy would seem
to have the same content, but a pirate copy would identify its
source (even if the content was passed through different
transformations, e.g. lossy compression, and was generated from
several legal copies). This task is known as watermarking, and
is discussed below.
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Fighting illegal key distribution:
A obvious method to trace the source of keys is to give each
user a personal key. The content should be encrypted with a
random key, which is itself encrypted with each of the personal
keys. The drawback of this scheme is that it sends an additional
encryption per user, and this overhead is only reasonable for
small groups. There are more advanced methods whose overhead is
much smaller [CFN,NP,CFNP,BF]. These methods are secure as long as
the pirates use less than k keys, where k is a parameter.
Fighting illegal content distribution:
In order to trace the source of the content it is essential to
insert in it "water marks" which the pirates cannot remove. See
[CKLS,PAK] for a discussion of marking methods. An additional
issue is the distribution of marks into the copies that are
delivered to users, i.e the problem of efficiently generating
unique fingerprints. An efficient fingerprinting method is
suggested in [BS] (however, fingerprinting schemes are less
efficient than schemes for tracing illegal key distribution).
Self enforcement is an interesting concept: it intends to
prevent piracy (rather than trace its source) by discouraging
legitimate users from giving their keys to others. In order to
achieve this property each user's key contains some information
which is private to the user (for example, his credit card
number). It is hoped for that users would be discouraged from
providing keys which contain this information to others.
Efficient tracing schemes are described in [DLN].
Acknowledgments
================
Much of this text is reproduced from [CGIMNP], written
with Juan Garay, Gene Itkis, Daniele Micciancio and Moni Naor.
The authors are grateful to the people with whom they interacted
on this topic, including all the above and in addition Naganand
Doraswamy, Rosario Gennaro, Dan Harkins, Shai Halevi, Dimitris
Pendarakis, Tal Rabin, Pankaj Rohargi and Debanjan Saha.
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Authors' Addresses:
====================
Ran Canetti Benny Pinkas
IBM TJ Watson Research Center STAR Lab
POB. 704, Yorktown Heights, InterTrust Technologies
Tel. 1-914-784-7076 4750 Patrick Henry Drive
canetti@watson.ibm.com Santa Clara, CA 95054-1851
bpinkas@intertrust.com
Canetti, Pinkas [Page 21]
