Network Working Group Debby M. Wallner
Category: Informational Eric J. Harder
Ryan C. Agee
National Security Agency
September 15, 1998
Key Management for Multicast: Issues and Architectures
<draft-wallner-key-arch-01.txt>
Status of this memo
This document is being submitted as an Informational RFC (Request
for Comment) to the Internet Engineering Task Force (IETF) for
consideration as a method for the establishment of multicast
communication sessions. It is an update to a previous document,
submitted July 1, 1998, and contains minor revisions and corrections.
Comments and suggestions for improvements are requested and should
be addressed to the authors. The views expressed in this paper are
those of the authors and do not necessarily reflect the views of
the U.S. Department of Defense or any of its agencies.
Distribution of this document is unlimited.
Abstract
This report contains a discussion of the difficult problem of key
management for multicast communication sessions. It focuses on two
main areas of concern with respect to key management, which are,
initializing the multicast group with a common net key and rekeying
the multicast group. A rekey may be necessary upon the compromise of
a user or for other reasons (e.g., periodic rekey). In particular,
this report identifies a technique which allows for secure compromise
recovery, while also being robust against collusion of excluded users.
This is one important feature of multicast key management which has
not been addressed in detail by most other multicast key management
proposals [1,2,4]. The benefits of this proposed technique are that
it minimizes the number of transmissions required to rekey the
multicast group and it imposes minimal storage requirements on the
multicast group.
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1.0 MOTIVATION
It is recognized that future networks will have requirements that will
strain the capabilities of current key management architectures. One
of these requirements will be the secure multicast requirement. The
need for high bandwidth, very dynamic secure multicast communications
is increasingly evident in a wide variety of commercial, government, and
Internet communities. Specifically, the secure multicast requirement
is the necessity for multiple users who share the same security
attributes and communication requirements to securely communicate with
every other member of the multicast group using a common multicast
group net key. The largest benefit of the multicast communication
being that multiple receivers simultaneously get the same
transmission. Thus the problem is enabling each user to
determine/obtain the same net key without permitting unauthorized
parties to do likewise (initializing the multicast group) and securely
rekeying the users of the multicast group when necessary. At first
glance, this may not appear to be any different than current key
management scenarios. This paper will show, however, that future
multicast scenarios will have very divergent and dynamically changing
requirements which will make it very challenging from a key management
perspective to address.
2.0 INTRODUCTION
The networks of the future will be able to support gigabit
bandwidths for individual users, to large groups of users. These
users will possess various quality of service options and multimedia
applications that include video, voice, and data, all on the same
network backbone. The desire to create small groups of users all
interconnected and capable of communicating with each other, but who
are securely isolated from all other users on the network is being
expressed strongly by users in a variety of communities.
The key management infrastructure must support bandwidths ranging
from kilobits/second to gigabits/second, handle a range of multicast
group sizes, and be flexible enough for example to handle such
communications environments as wireless and mobile technologies. In
addition to these performance and communications requirements, the
security requirements of different scenarios are also wide ranging.
It is required that users can be added and removed securely and
efficiently, both individually and in bulk. The system must be
resistant to compromise, insofar as users who have been dropped should
not be able to read any subsequent traffic, even if they share their
secret information. The costs we seek to minimize are time required
for setup, storage space for each end user, and total number of
transmissions required for setup, rekey and maintenance. It is also
envisioned that any proposed multicast security mechanisms will be
implemented no lower than any layer with the characteristics of the
network layer of the protocol stack. Bandwidth efficiency for any key
management system must also be considered. The trade-off between
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security and performance of the entire multicast session establishment
will be discussed in further detail later in this document.
The following section will explain several potential scenarios where
multicast capabilities may be needed, and quantify their requirements from
both a performance and security perspective. It will be followed in
Section 4.0 by a list of factors one must consider when designing a
potential solution. While there are several security services that will be
covered at some point in this document, much of the focus of this document
has been on the generation and distribution of multicast group net keys. It
is assumed that all potential multicast participants either through some
manual or automated, centralized or decentralized mechanism have received
initialization keying material (e.g. certificates). This document does not
address the initialization key distribution issue. Section 5 will then
detail several potential multicast key management architectures, manual
(symmetric) and public key based (asymmetric), and highlight their relative
advantages and disadvantages (Note:The list of advantages and disadvantages
is by no means all inclusive.). In particular, this section emphasizes our
technique which allows for secure compromise recovery.
3.0 MULTICAST SCENARIOS
There are a variety of potential scenarios that may stress the key
management infrastructure. These scenarios include, but are not limited to,
wargaming, law enforcement, teleconferencing, command and control
conferencing, disaster relief, and distributed computing. Potential
performance and security requirements, particularly in terms of multicast
groups that may be formed by these users for each scenario, consists of the
potential multicast group sizes, initialization requirements (how fast do
users need to be brought on-line), add/drop requirements (how fast a user
needs to be added or deleted from the multicast group subsequent to
initialization), size dynamics (the relative number of people joining/
leaving these groups per given unit of time), top level security
requirements, and miscellaneous special issues for each scenario. While
some scenarios describe future secure multicast requirements, others have
immediate security needs.
As examples, let us consider two scenarios, distributed gaming and
teleconferencing.
Distributed gaming deals with the government's need to simulate
a conflict scenario for the purposes of training and evaluation. In
addition to actual communications equipment being used, this concept would
include a massive interconnection of computer simulations containing, for
example, video conferencing and image processing. Distributed
gaming could be more demanding from a key management perspective
than an actual scenario for several reasons. First, the nodes of
the simulation net may be dispersed throughout the country.
Second, very large bandwidth communications, which enable the
possibility for real time simulation capabilities, will drive the
need to drop users in and out of the simulation quickly. This is
potentially the most demanding scenario of any considered.
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This scenario may involve group sizes of potentially 1000 or more
participants, some of which may be collected in smaller subgroups. These
groups must be initialized very rapidly, for example, in a ten second total
initialization time. This scenario is also very demanding in that users may
be required to be added or dropped from the group within one second. From
a size dynamics perspective, we estimate that approximately ten percent of
the group members may change over a one minute time period. Data rate
requirements are broad, ranging from kilobits per second (simulating
tactical users) to gigabits per second (multicast video). The distributed
gaming scenario has a fairly thorough set of security requirements
covering access control, user to user authentication, data
confidentiality, and data integrity. It also must be "robust"
which implies the need to handle noisy operating environments that
are typical for some tactical devices. Finally, the notion of
availability is applied to this scenario which implies that
the communications network supplying the multicast capability must be up
and functioning a specified percentage of the time.
The teleconference scenario may involve group sizes of
potentially 1000 or more participants. These groups may take up to
minutes to be initialized. This scenario is less demanding in that
users may be required to be added or dropped from the group within
seconds. From a size dynamics perspective, we estimate that
approximately ten percent of the group members may change over
a period of minutes. Data rate requirements are broad, ranging from
kilobits per second to 100's of Mb per second. The teleconference
scenario also has a fairly thorough set of security requirements
covering access control, user to user authentication, data
confidentiality, data integrity, and non-repudiation. The notion
of availability is also applicable to this scenario. The time
frame for when this scenario must be provided is now.
4.0 ARCHITECTURAL ISSUES
There are many factors that must be taken into account when developing the
desired key management architecture. Important issues for key management
architectures include level (strength) of security, cost, initializing the
system, policy concerns, access control procedures, performance requirements
and support mechanisms. In addition, issues particular to multicast groups
include:
1. What are the security requirements of the group members? Most likely
there will be some group controller, or controllers. Do the other
members possess the same security requirements as the controller(s)?
2. Interdomain issues - When crossing from one "group domain" to another
domain with a potentially different security policy, which policy is
enforced? An example would be two users wishing to communicate, but
having different cryptoperiods and/or key length policies.
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3. How does the formation of the multicast group occur? Will the group
controller initiate the user joining process, or will the users
initiate when they join the formation of the multicast group?
4. How does one handle the case where certain group members have inferior
processing capabilities which could delay the formation of the net key?
Do these users delay the formation of the whole multicast group, or do
they come on-line later enabling the remaining participants to be
brought up more quickly?
5. One must minimize the number of bits required for multicast group net
key distribution. This greatly impacts bandwidth limited equipments.
All of these and other issues need to be taken into account, along with the
communication protocols that will be used which support the desired
multicast capability. The next section addresses some of these issues and
presents some candidate architectures that could be used to tackle the key
management problem for multicasting.
5.0 CANDIDATE ARCHITECTURES
There are several basic functions that must be performed in order for a
secure multicast session to occur. The order in which these functions will
be performed, and the efficiency of the overall solution results from making
trade-offs of the various factors listed above. Before looking at specific
architectures, these basic functions will be outlined, along with some
definition of terms that will be used in the representative architectures.
These definitions and functions are as follows:
1. Someone determines the need for a multicast session, sets the security
attributes for that particular session (e.g., classification levels of
traffic, algorithms to be used, key variable bit lengths, etc.), and
creates the group access control list which we will call the initial
multicast group participant list. The entity which performs these
functions will be called the INITIATOR. At this point, the multicast
group participant list is strictly a list of users who the initiator
wants to be in the multicast group.
2. The initiator determines who will control the multicast group. This
controller will be called the ROOT (or equivalently the SERVER). Often,
the initiator will become the root, but the possibility exists where
this control may be passed off to someone other than the initiator.
(Some key management architectures employ multiple roots, see [4].)
The root's job is to perform the addition and deletion of group
participants, perform user access control against the security
attributes of that session, and distribute the traffic encryption key
for the session which we will call the multicast group NET KEY. After
initialization, the entity with the authority to accept or reject the
addition of future group participants, or delete current group
participants is called the LIST CONTROLLER.
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This may or may not be the initiator. The list controller has
been distinguished from the root for reasons which will become clear
later. In short, it may be desirable for someone to have the
authority to accept or reject new members, while another party
(the root) would actually perform the function.
3. Every participant in the multicast session will be referred to as a
GROUP PARTICIPANT. Specific group participants other than the root or
list controller will be referred to as LEAVES.
4. After the root checks the security attributes of the participants
listed on the multicast group participant list to make sure that they
all support the required security attributes, the root will then pass
the multicast group list to all other participants and create and
distribute the Net Key. If a participant on the multicast group list
did not meet the required security attributes, the leaf must be
deleted from the list.
Multiple issues can be raised with the distribution of the multicast
group list and Net Key.
a. An issue exists with the time ordering of these functions. The
multicast group list could be distributed before or after the link
is secured (i.e. the Net Key is distributed).
b. An issue exists when a leaf refuses to join the session. If a
leaf refuses to join a session, we can send out a modified list
before sending out the Net Key, however sending out modified
lists, potentially multiple times, would be inefficient. Instead,
the root could continue on, and would not send the Net Key to
those participants on the list who rejected the session.
For the scenario architectures which follow, we assume the multicast
group list will be distributed to the group participants once before
the Net Key is distributed. Unlike the scheme described in [4], we
recommend that the multicast group participant list be provided to all
leaves. By distributing this list to the leaves, it allows them to
determine upfront whether they desire to participate in the multicast
group or not, thus saving potentially unnecessary key exchanges.
Four potential key management architectures to distribute keying material
for multicast sessions are presented. Recall that the features that are
highly desirable for the architecture to possess include the time required
to setup the multicast group should be minimized, the number of transmissions
should be minimized, and memory/storage requirements should be minimized.
As will be seen, the first three proposals each fall short in a different
aspect of these desired qualities, whereas the fourth proposal appears to
strike a balance in the features desired. Thus, the fourth proposal is the
one recommended for general implementation and use.
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Please note that these approaches also address securely eliminating users
from the multicast group, but don't specifically address adding new users to
the multicast group following initial setup because this is viewed as
evident as to how it would be performed.
5.1 MANUAL KEY DISTRIBUTION
Through manual key distribution, symmetric key is delivered without the use
of public key exchanges. To set up a multicast group Net Key utilizing
manual key distribution would require a sequence of events where Net Key and
spare Net Keys would be ordered by the root of the multicast session group.
Alternate (supersession) Net Keys are ordered (by the root) to be used in
case of a compromise of a group participant(s). The Net Keys would be
distributed to each individual group participant, often through some
centralized physical intermediate location. At some predetermined time, all
group participants would switch to the new Net Key. Group participants use
this Net Key until a predetermined time when they need another new Net Key.
If the Net Key is compromised during this time, the alternate Net Key is
used. Group participants switch to the alternate Net Key as soon as they
receive it, or upon notification from the root that everyone has the new Net
Key and thus the switch over should take place. This procedure is repeated
for each cryptoperiod.
A scheme like this may be attractive because the methods exist today and are
understood by users. Unfortunately, this type of scheme can be time
consuming to set up the multicast group based on time necessary to order
keying material and having it delivered. For most real time scenarios, this
method is much too slow.
5.2 N Root/Leaf Pairwise Keys Approach
This approach is a brute force method to provide a common multicast group
Net Key to the group participants. In this scheme, the initiator sets the
security attributes for a particular session, generates a list of desired
group participants and transmits the list to all group participants. The
leaves then respond with an initial acceptance or rejection of participation.
By sending the list up front, time can be saved by not performing key
exchanges with people who rejected participation in the session. The root
(who for this and future examples is assumed to be the initiator) generates a
pairwise key with one of the participants (leaves) in the multicast group
using some standard public key exchange technique (e.g., a Diffie-Hellman
public key exchange.) The root will then provide the security association
parameters of the multicast (which may be different from the parameters of
the initial pairwise key) to this first leaf. Parameters may include items
such as classification and policy. Some negotiation (through the use of a
Security Association Management Protocol, or SAMP) of the parameters may be
necessary. The possibility exists for the leaf to reject the connection to
the multicast group based on the above parameters and multicast group list.
If the leaf rejects this session, the root will repeat this process with
another leaf.
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Once a leaf accepts participation in the multicast session, these two then
choose a Net Key to be used by the multicast group. The Net Key could be
generated through another public key exchange between the two entities, or
simply chosen by the root, depending upon the policy which is in place for
the multicast group ( i.e. this policy decision will not be a real time
choice). The issue here is the level of trust that the leaf has in the
root. If the initial pairwise key exchange provides some level of user
authentication, then it seems adequate to just have the root select the Net
Key at this stage. Another issue is the level of trust in the strength of
the security of the generated key. Through a cooperative process, both
entities (leaf and root) will be providing information to be used in the
formation of the Net Key.
The root then performs a pairwise key exchange with another leaf and
optionally performs the negotiation discussed earlier. Upon acceptance by
the leaf to join the multicast group, the root sends the leaf the Net Key.
This pairwise key exchange and Net Key distribution continues for all N
users of the multicast group.
Root/leaves cache pairwise keys for future use. These keys serve as Key
Encryption Keys (KEKs) used for rekeying leaves in the net at a later time.
Only the root will cache all of the leaves' pairwise keys. Each individual
leaf will cache only its own unique pairwise Key Encryption Key.
There are two cases to consider when caching the KEKs. The first case is
when the Net key and KEK are per session keys. In this case, if one wants to
exclude a group participant from the multicast session (and rekey the
remaining participants with a new Net Key), the root would distribute a new
Net key encrypted with each individual KEK to every legitimate remaining
participant. These KEKs are deleted once the multicast session is
completed.
The second case to consider is when the KEKs are valid for more than one
session. In this case, the Net Key may also be valid for multiple sessions,
or the Net Key may still only be valid for one session as in the above case.
Whether the Net Key is valid for one session or more than one session, the
KEK will be cached. If the Net Key is only valid per session, the KEKs will
be used to encrypt new Net Keys for subsequent multicast sessions. The
deleting of group participants occurs as in the previous case described
above, regardless of whether the Net Key is per session or to be used for
multiple sessions.
A scheme like this may be attractive to a user because it is a
straightforward extension of certifiable public key exchange techniques.
It may also be attractive because it does not involve third parties. Only
the participants who are part of the multicast session participate in the
keying mechanism. What makes this scheme so undesirable is that it will be
transmission intensive as we scale up in numbers, even for the most
computationally efficient participants, not to mention those with less
capable hardware (tactical, wireless, etc.). Every time the need arises to
drop an "unauthorized" participant, a new Net Key must be distributed.
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This distribution requires a transmission from the Root to each remaining
participant, whereby the new Net Key will be encrypted under the cover of
each participant's unique pairwise Key Encryption Key (KEK).
Note: This approach is essentially the same as one proposal to the Internet
Engineering Task Force (IETF) Security Subworking Group [Ref 1,2].
Also note that there exist multiple twists to an approach like this. For
example, instead of having the root do all N key exchanges, the root could
pass some of this functionality (and control) to a number of leaves beneath
him. For example, the multicast group list could be split in half and the
root tells one leaf to take half of the users and perform a key exchange
with them (and then distribute the Net key) while the root will take care of
the other half of the list. (The chosen leaves are thus functioning as a
root and we can call them "subroots." These subroots will have leaves
beneath them, and the subroots will maintain the KEK of each leaf beneath
it.) This scales better than original approach as N becomes large.
Specifically, it will require less time to set up (or rekey) the multicast
net because the singular responsibility of performing pairwise key exchanges
and distributing Net Key will be shared among multiple group participants
and can be performed in parallel, as opposed to the root only distributing
the Net Key to all of the participants.
This scheme is not without its own security concerns. This scheme pushes
trust down to each subgroup controller - the root assumes that these
"subroot" controllers are acting in a trustworthy way. Every control
element (root and subroots) must remain in the system throughout the
multicast. This effectively makes removing someone from the net (especially
the subroots) harder and slower due to the distributed control. When
removing a participant from the multicast group which has functioned on
behalf of the root, as a subroot, to distribute Net Key, additional steps
will be necessary. A new subroot must be delegated by the root to replace
the removed subroot. A key exchange (to generate a new pairwise KEK) must
occur between the new subroot and each leaf the removed subroot was
responsible for. A new Net Key will now be distributed from the root, to
the subroots, and to the leaves. Note that this last step would have been
the only step required if the removed party was a leaf with no controlling
responsibilities.
5.3 COMPLEMENTARY VARIABLE APPROACH
Let us suppose we have N leaves. The Root performs a public key exchange
with each leaf i (i= 1,2, ..., N). The Root will cache each pairwise KEK.
Each leaf stores their own KEK. The root would provide the multicast group
list of participants and attributes to all users. Participants would accept
or reject participation in the multicast session as described in previous
sections. The root encrypts the Net Key for the Multicast group to each
leaf, using their own unique KEK(i). (The Root either generated this Net
Key himself, or cooperatively generated with one of the leaves as was
discussed earlier). In addition to the encrypted Net Key, the root will
also encrypt something called complementary variables and send them to the
leaves.
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A leaf will NOT receive his own complementary variable, but he
will receive the other N-1 leaf complementary variables. The root sends the
Net Key and complementary variables j, where j=1,2,...,N and j not equal to
i, encrypted by KEK(i) to each leaf. Thus, every leaf receives and
stores N variables which are the Net key, and N-1 complementary variables.
Thus to cut a user from the multicast group and get the remaining
participants back up again on a new Net Key would involve the following.
Basically, to cut leaf number 20 out of the net, one message is sent out
that says "cut leaf 20 from the net." All of the other leaves (and Root)
generate a new Net Key based on the current Net Key and Complementary
variable 20. [Thus some type of deterministic key variable generation
process will be necessary for all participants of the multicast group]. This
newly generated variable will be used as the new Net Key by all remaining
participants of the multicast group. Everyone except leaf 20 is able to
generate the new Net Key, because they have complementary variable 20, but
leaf 20 does not.
A scheme like this seems very desirable from the viewpoint of transmission
savings since a rekey message encrypted with each individual KEK to every
leaf does not have to be sent to delete someone from the net. In other
words, there will be one plaintext message to the multicast group versus N
encrypted rekey messages. There exists two major drawbacks with this
scheme. First are the storage requirements necessary for the (N-1)
complementary variables. Secondly, when deleting multiple users from the
multicast group, collusion will be a concern. What this means is that these
deleted users could work together and share their individual complementary
variables to regain access to the multicast session.
5.4 HIERARCHICAL TREE APPROACH
The Hierarchical Tree Approach is our recommended approach to address the
multicast key management problem. This approach provides for the following
requisite features:
1. Provides for the secure removal of a compromised user from the
multicast group
2. Provides for transmission efficiency
3. Provides for storage efficiency
This approach balances the costs of time, storage and number of required
message transmissions, using a hierarchical system of auxiliary keys to
facilitate distribution of new Net Key. The result is that the storage
requirement for each user and the transmissions required for key replacement
are both logarithmic in the number of users, with no background
transmissions required. This approach is robust against collusion of
excluded users. Moreover, while the scheme is hierarchical in nature, no
infrastructure is needed beyond a server (e.g., a root), though the presence
of such elements could be used to advantage (See Figure 1).
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--------------------------
| |
| S E R V E R |
| |
--------------------------
| | |
| | . . . . |
- - -
|1| |2| |n|
- - -
Figure 1: Assumed Communication Architecture
The scheme, advantages and disadvantages are enumerated in more detail
below. Consider Figure 2 below. This figure illustrates the logical key
distribution architecture, where keys exist only at the server and at the
users. Thus, the server in this architecture would hold Keys A through O,
and the KEKs of each user. User 11 in this architecture would hold its own
unique KEK, and Keys F, K, N, and O.
net key Key O
-------------------------------------
intermediate | |
keys | |
Key M Key N
----------------- --------------------
| | | |
| | | |
Key I Key J Key K Key L
-------- -------- --------- ----------
| | | | | | | |
| | | | | | | |
Key A Key B Key C Key D Key E Key F Key G Key H
--- --- --- --- --- ---- ---- ----
| | | | | | | | | | | | | | | |
- - - - - - - - - -- -- -- -- -- -- --
|1| |2| |3| |4| |5| |6| |7| |8| |9| |10| |11| |12| |13| |14| |15| |16|
- - - - - - - - - -- -- -- -- -- -- --
users
Figure 2: Logical Key Distribution Architecture
We now describe the organization of the key hierarchy and the setup process.
It will be clear from the description how to add users after the hierarchy
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is in place; we will also describe the removal of a user. Note: The passing
of the multicast group list and any negotiation protocols is not included in
this discussion for simplicity purposes.
We construct a rooted tree (from the bottom up) with one leaf corresponding
to each user, as in Figure 2. (Though we have drawn a balanced binary tree
for convenience, there is no need for the tree to be either balanced or
binary - some preliminary analysis on tree shaping has been performed.) Each
user establishes a unique pairwise key with the server. For users with
transmission capability, this can be done using the public key exchange
protocol. The situation is more complicated for receive-only users; it is
easiest to assume these users have pre-placed key.
Once each user has a pairwise key known to the server, the server generates
(according to the security policy in place for that session) a key for each
remaining node in the tree. The keys themselves should be generated by a
robust process. We will also assume users have no information about keys
they don't need. (Note: There are no users at these remaining nodes, (i.e.,
they are logical nodes) and the key for each node need only be generated by
the server via secure means.) Starting with those nodes all of whose
children are leaves and proceeding towards the root, the server transmits
the key for each node, encrypted using the keys for each of that node's
children. At the end of the process, each user can determine the keys
corresponding to those nodes above her leaf. In particular, all users hold
the root key, which serves as the common Net Key for the group. The storage
requirement for a user at depth d is d+1 keys (Thus for the example in
Figure 2, a user at depth d=4 would hold five keys. That is, the unique Key
Encryption Key generated as a result of the pairwise key exchange, three
intermediate node keys - each separately encrypted and transmitted, and the
common Net Key for the multicast group which is also separately encrypted.)
It is also possible to transmit all of the intermediate node keys and root
node key in one message, where the node keys would all be encrypted with the
unique pairwise key of the individual leaf. In this manner, only one
transmission (of a larger message) is required per user to receive all of
the node keys (as compared to d transmissions). It is noted for this
method, that the leaf would require some means to determine which key
corresponds to which node level.
It is important to note that this approach requires additional processing
capabilities at the server where other alternative approaches may not. In
the worst case, a server will be responsible for generating the intermediate
keys required in the architecture.
5.4.1 The Exclusion Principle
Suppose that User 11 (marked on Figure 2 in black) needs to be deleted from
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the multicast group. Then all of the keys held by User 11 (bolded Keys F, K,
N, O) must be changed and distributed to the users who need them, without
permitting User 11 or anyone else from obtaining them. To do this, we must
replace the bolded keys held by User 11, proceeding from the bottom up. The
server chooses a new key for the lowest node, then transmits it encrypted
with the appropriate daughter keys (These transmissions are represented by
the dotted lines). Thus for this example, the first key replaced is Key F,
and this new key will be sent encrypted with User 12's unique pairwise key.
Since we are proceeding from the bottom up, each of the replacement keys
will have been replaced before it is used to encrypt another key. (Thus, for
the replacement of Key K, this new key will be sent encrypted in the newly
replaced Key F (for User 12) and will also be sent as one multicast
transmission encrypted in the node key shared by Users 9 and 10 (Key E). For
the replacement of Key N, this new key will be sent encrypted in the newly
replaced Key K (for Users 9, 10, and 12) and will also be encrypted in the
node key shared by Users 13, 14, 15, and 16 (Key L). For the replacement of
Key O, this new key will be sent encrypted in the newly replaced Key N (for
Users 9, 10, 12, 13, 14, 15, and 16) and will also be encrypted in the node
key shared by Users 1, 2 , 3, 4, 5, 6, 7, and 8 (Key M).) The number of
transmissions required is the sum of the degrees of the replaced nodes. In a
k-ary tree in which a sits at depth d, this comes to at most kd-1
transmissions. Thus in this example, seven transmissions will be required
to exclude User 11 from the multicast group and to get the other 15 users
back onto a new multicast group Net Key that User 11 does not have access
to. It is easy to see that the system is robust against collusion, in that
no set of users together can read any message unless one of them could have
read it individually.
If the same strategy is taken as in the previous section to send multiple
keys in one message, the number of transmissions required can be reduced
even further to four transmissions. Note once again that the messages will
be larger in the number of bits being transmitted. Additionally, there must
exist a means for each leaf to determine which key in the message
corresponds to which node of the hierarchy. Thus, in this example, for the
replacement of keys F, K, N, and O to User 12, the four keys will be
encrypted in one message under User 12's unique pairwise key. To replace
keys K, N, and O for Users 9 and 10, the three keys will be encrypted in one
message under the node key shared by Users 9 and 10 (Key E). To replace
keys N and O for Users 13, 14, 15, 16, the two keys will be encrypted in
one message under the node key shared by Users 13, 14, 15, and 16 (Key L).
Finally, to replace key O for Users 1, 2 , 3, 4, 5, 6, 7, and 8, key O will
be encrypted under the node key shared by Users 1, 2 , 3, 4, 5, 6, 7, and 8
(Key M). Thus the number of transmission required is at most (k-1)d.
The following table demonstrates the removal of a user, and how the storage
and transmission requirements grow with the number of users.
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Table 1: Storage and Transmission Costs
Number Degree Storage per user Transmissions to Transmissions to
of users (k) (d+1) rekey remaining rekey remaining
participants of participants of
multicast group - multicast group -
one key per message multiple keys per
(kd-1) message
(k-1)d
8 2 4 5 3
9 3 3 5 4
16 2 5 7 4
2048 2 12 21 11
2187 3 8 20 14
131072 2 18 33 17
177147 3 12 32 22
The benefits of a scheme such as this are:
1. The costs of user storage and rekey transmissions are balanced and
scalable as the number of users increases. This is not the case for
[1], [2], or [4].
2. The auxiliary keys can be used to transmit not only other keys, but
also messages. Thus the hierarchy can be designed to place subgroups
that wish to communicate securely (i.e. without transmitting to the
rest of the large multicast group) under particular nodes, eliminating
the need for maintenance of separate Net Keys for these subgroups.
This works best if the users operate in a hierarchy to begin with
(e.g., military operations), which can be reflected by the key
hierarchy.
3. The hierarchy can be designed to reflect network architecture,
increasing efficiency (each user receives fewer irrelevant messages).
Also, server responsibilities can be divided up among subroots (all of
which must be secure).
4. The security risk associated with receive-only users can be minimized
by collecting such users in a particular area of the tree.
5. This approach is resistant to collusion among arbitrarily many users.
As noted earlier, in the rekeying process after one user is compromised, in
the case of one key per message, each replaced key must be decrypted
successfully before the next key can be replaced (unless users can cache the
rekey messages). This bottleneck could be a problem on a noisy or slow
network. (If multiple users are being removed, this can be parallelized, so
the expected time to rekey is roughly independent of the number of users
removed.)
By increasing the valences and decreasing the depth of the tree, one can
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reduce the storage requirements for users at the price of increased
transmissions. For example, in the one key per message case, if n users are
arranged in a k-ary tree, each user will need storage. Rekeying after one
user is removed now requires transmissions. As k approaches n, this
approaches the pairwise key scheme described earlier in the paper.
5.4.2 Hierarchical Tree Approach Options
5.4.2.1 Distributed Hierarchical Tree Approach
The Hierarchical Tree Approach outlined in this section could be distributed
as indicated in Section 5.2 to more closely resemble the proposal put forth
in [4]. Subroots could exist at each of the nodes to handle any joining or
rekeying that is necessary for any of the subordinate users. This could be
particularly attractive to users which do not have a direct connection back
to the Root. Recall as indicated in Section 5.2, that the trust placed in
these subroots to act with the authority and security of a Root, is a
potentially dangerous proposition. This thought is also echoed in [4].
Some practical recommendations that might be made for these subroots include
the following. The subroots should not be allowed to change the multicast
group participant list that has been provided to them from the Root. One
method to accomplish this, would be for the Root to sign the list before
providing it to the subroots. Authorized subroots could though be allowed
to set up new multicast groups for users below them in the hierarchy.
It is important to note that although this distribution may appear to
provide some benefits with respect to the time required to initialize the
multicast group (as compared to the time required to initialize the group as
described in Section 5.4) and for periodic rekeying, it does not appear to
provide any benefit in rekeying the multicast group when a user has been
compromised.
It is also noted that whatever the key management scheme is (hierarchical
tree, distributed hierarchical tree, core based tree, GKMP, etc.), there
will be a "hit" incurred to initialize the multicast group with the first
multicast group net key. Thus, the hierarchical tree approach does not
suffer from additional complexity with comparison to the other schemes with
respect to initialization.
5.4.2.2 Multicast Group Formation
Although this paper has presented the formation of the multicast group as
being Root initiated, the hierarchical approach is consistent with user
initiated joining. User initiated joining is the method of multicast group
formation presented in [4]. User initiated joining may be desirable when
some core subset of users in the multicast group need to be brought up on-
line and communicating more quickly. Other participants in the multicast
group can then be brought in when they wish. In this type of approach
though, there does not exist a finite period of time by when it can be
ensured all participants will be a part of the multicast group.
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For example, in the case of a single root, the hierarchy is set up once, in
the beginnning, by the initiator (also usually the root) who also generates
the group participant list. The group of keys for each participant can then
be individually requested (pulled) as soon as, but not until, each
participant wishes to join the session.
5.4.2.3 Sender Specific Authentication
In the multicast environment, the possibility exists that participants of
the group at times may want to uniquely identify which participant is the
sender of a multicast group message. In the multicast key distribution
system described by Ballardie [4], the notion of "sender specific keys" is
presented.
Another option to allow participants of a multicast group to uniquely
determine the sender of a message is through the use of a signature process.
When a member of the multicast group signs a message with their own private
signature key, the recipients of that signed message in the multicast group
can use the sender's public verification key to determine if indeed the
message is from who it is claimed to be from.
Another related idea to this is the case when two users of a multicast group
want to communicate strictly with each other, and want no one else to listen
in on the communication. If this communication relationship is known when
the multicast group is originally set up, then these two participants could
simply be placed adjacent to one another at the lowest level of the
hierarchy (below a binary node). Thus, they would naturally share a secret
pairwise key. Otherwise, a simple way to accomplish this is to perform a
public key based pairwise key exchange between the two users to generate a
traffic encryption key for their private unicast communications. Through
this process, not only will the encrypted transmissions between them be
readable only by them, but unique sender authentication can be accomplished
via the public key based pairwise exchange.
5.4.2.4 Rekeying the Multicast Group and the Use of Group Key Encryption Keys
Reference [4] makes use of a Group Key Encryption Key that can be shared by
the multicast group for use in periodic rekeys of the multicast group.
Aside from the potential security drawbacks of implementing a shared key for
encrypting future keys, the use of a Group Key Encryption Key is of no
benefit to a multicast group if a rekey is necessary due to the known
compromise of one of the members. The strategy for rekeying the multicast
group presented in Section 5.4.1 specifically addresses this critical
problem and offers a means to accomplish this task with minimal message
transmissions and storage requirements.
The question though can now be asked as to whether the rekey of a multicast
group will be necessary in a non-compromise scenario. For example, if a
user decides they do not want to participate in the group any longer, and
requests the list controller to remove them from the multicast group
participant list, will a rekey of the multicast group be necessary? If the
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security policy of the multicast group mandates that deleted users can no
longer receive transmissions, than a rekey of a new net key will be
required. If the multicast group security policy does not care that the
deleted person can still decrypt any transmissions (encrypted in the group
net key that they might still hold), but does care that they can not encrypt
and transmit messages, a rekey will once again be necessary. The only
alternative to rekeying the multicast group under this scenario would
require a recipient to check every received message sender, against the
group participant list. Thus rejecting any message sent by a user not on
the list. This is not a practical option. Thus it is recommended to always
rekey the multicast group when someone is deleted, whether it is because of
compromise reasons or not.
5.4.2.5 Bulk Removal of Participants
As indicated in Section 2, the need may arise to remove users in bulk. If
the users are setup as discussed in Section 5.4.1 into subgroups that wish
to communicate securely all being under the same node, bulk user removal can
be done quite simply if the whole node is to be removed. The same technique
as described in Section 5.4.1 is performed to rekey any shared node key that
the remaining participants hold in common with the removed node.
The problem of bulk removal becomes more difficult when the participants to
be removed are dispersed throughout the tree. Depending on how many
participants are to be removed, and where they are located within the
hierarchy, the number of transmissions required to rekey the multicast group
could be equivalent to brute force rekeying of the remaining participants.
Also the question can be raised as to at what point the remaining users are
restructured into a new hierarchical tree, or should a new multicast group
be formed. Restructuring of the hierarchical tree would most likely be the
preferred option, because it would not necessitate the need to perform
pairwise key exchanges again to form the new user unique KEKs.
5.4.2.6 ISAKMP Compatibility
Thus far this document has had a major focus on the architectural trade-offs
involved in the generation, distribution, and maintenance of traffic
encryption keys (Net Keys) for multicast groups. There are other elements
involved in the establishment of a secure connection among the multicast
participants that have not been discussed in any detail. For example, the
concept of being able to "pick and choose" and negotiating the capabilities
of the key exchange mechanism and various other elements is a very important
and necessary aspect.
The NSA proposal to the Internet Engineering Task Force (IETF) Security
Subworking Group [Ref. 3] entitled "Internet Security Association and Key
Management Protocol (ISAKMP)" has attempted to identify the various
functional elements required for the establishment of a secure connection
for the largest current network, the Internet. While the proposal has
currently focused on the problem of point to point connections, the
functional elements should be the same for multicast connections, with
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appropriate changes to the techniques chosen to implement the individual
functional elements. Thus the implementation of ISAKMP is compatible with
the use of the hierarchical tree approach.
6.0 SUMMARY
As discussed in this report, there are two main areas of concern when
addressing solutions for the multicast key management problem. They are the
secure initialization and rekeying of the multicast group with a common net
key. At the present time, there are multiple papers which address the
initialization of a multicast group, but they do not adequately address how
to efficiently and securely remove a compromised user from the multicast
group.
This paper proposed a hierarchical tree approach to meet this difficult
problem. It is robust against collusion, while at the same time, balancing
the number of transmissions required and storage required to rekey the
multicast group in a time of compromise.
It is also important to note that the proposal recommended in this paper is
consistent with other multicast key management solutions [4], and allows for
multiple options for its implementation.
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7.0 REFERENCES
1. Harney, Hugh; Muckenhirn, Carl; and Rivers, Thomas,
"Group Key Management Protocol Architecture,"
RFC 2094, September 1994.
2. Harney, Hugh; Muckenhirn, Carl; and Rivers, Thomas,
"Group Key Management Protocol Specification,"
RFC 2093, September 1994.
3. Maughan, Douglas; Schertler, Mark; Schneider, Mark; and Turner, Jeff,
"Internet Security Association and Key Management Protocol, Version 7,"
21 February 1997.
4. Tony Ballardie,
"Scalable Multicast Key Distribution,"
RFC 1949, May 1996.
5. Wong, Chung Kei; Gouda, Mohamed; and Lam, Simon S.,
"Secure Group Communications Using Key Graphs,"
Technical Report TR 97-23, Department of Computer Sciences,
The University of Texas at Austin, July 1997.
Address of Authors
The authors are with:
National Security Agency
Attn: R2
9800 Savage Road STE 6451
Ft. Meade, MD. 20755-6451
1. Debby M. Wallner
Phone: 301-688-0331
E-mail: dmwalln@orion.ncsc.mil
2. Eric J. Harder
Phone: 301-688-0850
E-mail: ejh@tycho.ncsc.mil
3. Ryan C. Agee