Internet Draft Mark Baugher (Cisco)
IETF MSEC WG Ran Canetti (IBM)
Expires: December 08, 2004 Lakshminath Dondeti (Nortel)
Category: Informational Fredrik Lindholm (Ericsson)
June 09, 2004
MSEC Group Key Management Architecture
<draft-ietf-msec-gkmarch-08.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
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Abstract
This document defines the common architecture for Multicast Security
(MSEC) key management protocols that support a variety of
application, transport, and network layer security protocols. It
also defines the group security association (GSA), and describes the
key management protocols that help establish a GSA. The framework
and guidelines described in this document allow for a modular and
flexible design of group key management protocols for a variety of
different settings that are specialized to applications needs. MSEC
key management protocols may be used to facilitate secure one-to-
many, many-to-many, or one-to-one communication.
Comments on this document should be sent to msec@securemulticast.org.
Baugher, Canetti, Dondeti, Lindholm June 2004
Table of Contents
Status of this Memo................................................1
Abstract...........................................................1
1.0 Introduction: Purpose of this Document.........................3
2.0 Requirements of a Group Key Management Protocol................4
3.0 Overall Design of the Group Key Management Architecture........6
3.1 Overview.......................................................6
3.2 Detailed Description of the GKM Architecture...................8
3.3 Properties of the Design .....................................11
3.4 Group Key Management Block Diagram............................11
4.0 Registration protocol.........................................13
4.1 Registration protocol via Piggybacking or Protocol Reuse......13
4.2 Properties of Alternative registration Exchange Types.........14
4.3 Infrastructure for Alternative registration Exchange Types....15
4.4 De-registration Exchange......................................15
5.0 Rekey protocol................................................16
5.1 Goals of the rekey protocol...................................16
5.2 Rekey message Transport and Protection........................17
5.3 Reliable Transport of rekey messages..........................18
5.4 State-of-the-art on Reliable Multicast Infrastructure.........20
5.5 Implosion.....................................................20
5.6 Issues in Incorporating Group Key Management Algorithms.......22
5.7 Stateless, Stateful, and Self-healing Rekeying Algorithms.....22
5.8 Interoperability of a GKMA....................................23
6.0 Group Security Association....................................23
6.1 Group policy..................................................24
6.2 Contents of the Rekey SA......................................25
6.2.1 Rekey SA Policy.............................................25
6.2.2 Group Identity..............................................26
6.2.3 KEKs........................................................26
6.2.4 Authentication Key..........................................26
6.2.5 Replay Protection...........................................26
6.2.6 Security Parameter Index (SPI)..............................26
6.3 Contents of the Data SA.......................................27
6.3.1 Group Identity..............................................27
6.3.2 Source Identity.............................................27
6.3.3 Traffic Protection Keys.....................................27
6.3.4 Data Authentication Keys....................................27
6.3.5 Sequence Numbers............................................27
6.3.6 Security Parameter Index (SPI)..............................27
6.3.7 Data SA policy..............................................28
7.0 Scalability Considerations....................................28
8.0 Security Considerations.......................................30
9.0 Acknowledgments...............................................31
10.0 References and Bibliography..................................32
11.0 Authors' Addresses...........................................36
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1.0 Introduction: Purpose of this Document
Group and multicast applications have diverse requirements in IP
networks [TAXONOMY]. Their key management requirements - briefly
reviewed in Section 2.0 - include support for internetwork,
transport, and application-layer protocols. Some applications may
achieve simpler operation by running key-management messaging over a
pre-established secure channel (e.g., TLS, IPsec). Other security
protocols may benefit from a key management protocol that can run
over already deployed session initiation or management protocol
(e.g., SIP or RTSP). Finally, some may benefit from a light-weight
key management protocol that finishes in fewest round trips. For
these reasons, different application, transport, and internetwork-
layer data security protocols (e.g., SRTP [RFC3711] and IPsec
[RFC2401]) may benefit from using different group key management
systems. The purpose of this document is to define a common
architecture and design for group key-management protocols for
internet, transport, and application services.
The common architecture for group key management is called the MSEC
key management architecture and is based on the group control or key
server model developed in GKMP [RFC2094] and assumed by group key
management algorithms such as LKH [RFC2627], OFT [OFT], and MARKS
[MARKS]. There are other approaches that are not considered in this
architecture such as the highly distributed Cliques group key
management protocol [CLIQUES] and broadcast key management schemes
[FN93, Wool]. MSEC (Multicast Security) key management may in fact
be complementary to other group key management designs, but these
are not considered in this document. The integration of MSEC group
key management with Cliques, broadcast key management and other
group key systems is not considered in this document.
Indeed, key-management protocols are difficult to design and
validate. The common architecture described in this document eases
this burden by defining common abstractions and overall design that
can be specialized for different uses.
This document builds on and extends the Group Key Management Building
Block document of the IRTF SMuG research group [GKMBB] and is part of
the MSEC document roadmap. The MSEC architecture [MSEC-Arch] is a
reference for a complete multicast or group security architecture, of
which key management is a component.
The rest of this document is organized as follows. Section 2
discusses the security, performance and architectural requirements
for a group key management protocol. Section 3 presents the overall
architectural design principles. Section 4 describes the registration
protocol in detail and Section 5 does the same for rekey protocol.
Section 6 considers the interface to the Group Security Association
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(GSA). Section 7 reviews the scalability issues for group key
management protocols and Section 8 discusses security considerations.
2.0 Requirements of a Group Key Management Protocol
A group key management protocol supports protected communication
between members of a secure group. A secure group is a collection of
principals, called members, who may be senders, receivers or both
receivers and senders to other members of the group. (Note that group
membership may vary over time.) A group key management protocol
helps to ensure that only members of a secure group gain access to
group data (by gaining access to group keys) and can authenticate
group data. The goal of a group key management protocol is to
provide legitimate group members with the up-to-date cryptographic
state they need for their secrecy and authenticity requirements.
Multicast applications, such as video broadcast and multicast file
transfer, typically have the following key-management requirements
(see also [TAXONOMY]). Note that the list is neither applicable to
all applications, nor exhaustive.
1. The group members receive security associations including
encryption keys, authentication/integrity keys, cryptographic
policy that describes the keys, and attributes such as an index
for referencing the security association (SA) or particular
objects contained in the SA.
2. In addition to the policy associated with group keys, the group
owner or the Group Controller and Key Server (GCKS) may define
and enforce group membership, key management, data security and
other policies that may or may not be communicated to the
membership-at-large.
3. Keys will have a predetermined lifetime and may be periodically
refreshed.
4. Key material should be delivered securely to members of the
group so that they are secret, integrity-protected and can be
verified as coming from an authorized source.
5. The key-management protocol should be secure against replay
attacks and Denial of Service(DoS) attacks (see the Security
Considerations section of this memo).
6. The protocol should facilitate addition and removal of group
members so that members who are added may optionally be denied
access to the key material used before they joined the group,
and that removed members lose access to the key material
following their departure.
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7. The protocol should support a scalable group rekey operation
without unicast exchanges between members and a group
controller/key server, to avoid overwhelming a GCKS managing a
large group.
8. The protocol should be compatible with the infrastructure and
performance needs of the data-security application, such as
IPsec security protocols, AH and ESP, and/or application-layer
security protocols, such as SRTP.
9. The key management protocol should offer a framework for
replacing or renewing transforms, authorization infrastructure
and authentication systems.
10. The key management protocol should be secure against collusion
among excluded members and non-members. Specifically,
collusion must not result in attackers gaining any additional
group secrets than each of them individually are privy to. In
other words, combining the knowledge of the colluding entities
must not result in revealing additional group secrets.
12. The key management protocol should provide a mechanism to
securely recover from a compromise of some or all of the key
material.
13. Key management protocols may need to address real-world
deployment issues such as NAT-traversal and may need to
interface with legacy authentication mechanisms already
deployed.
In contrast to typical unicast key and SA negotiation protocols such
as TLS and IKE, group key management protocols provide SA and key
download capability. This feature may be useful for point-to-point
communication as well. Thus, a group key management protocol may
also be useful to unicast applications. In other words, group key
management protocols may be used for protecting multicast
communications, or unicast communications between members of a secure
group. Secure sub-group communication is also plausible using the
group SA.
There are other requirements for small group operation where there
will be many senders or in which all members may potentially be
senders. In this case, the group setup time may need to be optimized
to support a small, highly interactive group environment [RFC2627].
The current key management architecture covers secure communication
in large single-sender groups, such as source-specific multicast
groups. Scalable operation to a range of group sizes is also a
desirable feature, and a better group key management protocol will
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support large, single-sender groups as well as groups that have many
senders. It may be that no single key management protocol can satisfy
the scalability requirements of all group-security applications.
In addition to these requirements, it is useful to emphasize two non-
requirements, namely, technical protection measures (TPM) [TPM] and
broadcast key management. TPM are used for such things as copy
protection by preventing the user of a device to get easy access to
the group keys. There is no reason why a group key management
protocol cannot be used in an environment where the keys are kept in
a tamper-resistant store using various types of hardware or software
to implement TPM. However, for simplicity, the MSEC key management
architecture described in this document considers design for
technical protection measures out of scope.
The second non-requirement is broadcast key management where there is
no back channel [FN93, JKKV94] or where the device is not on a
network, such as a digital videodisk player. We assume IP network
operation where there is two-way communication, however asymmetric,
and that authenticated key-exchange procedures can be used for member
registration. It is possible that broadcast applications can make
use of a one-way Internet group key management protocol message, and
a one-way rekey message as described below.
3.0 Overall Design of the Group Key Management Architecture
This section describes the overall structure of a group key
management protocol. The design is based upon a group controller
model [RFC2093, RFC2094, RFC2627, OFT, GSAKMP, and RFC3547] with a
single group owner as the root-of-trust. The group owner designates
a group controller for member registration and GSA rekeying.
3.1 Overview
The main goal of a group key management protocol is to securely
provide the group members with an up-to-date security association
(SA), which contains the needed information for securing group
communication (i.e., the group data). We call this SA the Data SA.
In order to obtain this goal, the Group Key Management Architecture
defines the following protocols.
(1) Registration protocol.
=====================
This is a unicast protocol between the group controller/key server
(GCKS) and a joining group member. In this protocol, the GCKS and
joining member mutually authenticate each other. If the
authentication succeeds and the GCKS finds that the joining member is
authorized, then the GCKS supplies the joining member with the
following information:
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(a) Sufficient information to initialize the Data SA within the
joining member. This information is given only in the case that the
group security policy calls for initializing the Data SA at
registration, instead of or in addition to as part of the rekey
protocol.
(b) Sufficient information to initialize a Rekey SA within the
joining member (see more details about this SA below). This
information is given only in case that the group security policy
calls for using a rekey protocol.
The registration protocol must ensure that the transfer of
information from GCKS to member is done in an authenticated and
confidential manner over a security association. We call this SA the
Registration SA. A complementary de-registration protocol serves to
explicitly remove Registration SA state. Members may choose to
delete Registration SA state on their own volition.
(2) Rekey protocol.
==============
A GCKS may periodically update or change the Data SA, by sending
rekey information to the group members. Rekey messages may result
from group membership changes, change in group security policy, the
creation of new traffic-protection keys (TPKs, see next section) for
the particular group, or from key expiration. Rekey messages are
protected by the Rekey SA, which is initialized in the registration
protocol. They contain information for updating the Rekey SA and/or
the Data SA. Rekey messages can be sent via multicast to group
members or unicast from the GCKS to a particular group member.
Note that there are other means for managing (e.g. expiring or
refreshing) the Data SA without interaction between the GCKS and the
members. For example in MARKS [MARKS], the GCKS pre-determines TPKs
for different periods in the lifetime of the secure group and
distributes keys to members based on their membership periods.
Alternative schemes such as the GCKS disbanding the secure group and
starting a new group with a new Data SA are also possible, although
this type of operation is typically limited to small groups.
Rekey messages are authenticated using one of the two following
options:
o The first option is to use source authentication [TAXONOMY], that
is to enable each group member to verify that a rekey message
originates with the GCKS and none other.
o The second option is to use only group-based authentication
using a symmetric key. Members can only be assured that the
rekey messages originated within the group. Therefore, this is
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applicable only when all members of the group are trusted not
to impersonate the GCKS. Group authentication for rekey
messages is typically used when public-key cryptography is not
suitable for the particular group.
The rekey protocol ensures that all members receive the rekey
information in a timely manner. In addition, the rekey protocol
specifies mechanisms for the parties to contact the GCKS and re-synch
in case that their keys expired and an updated key has not yet been
received. The rekey protocol for large-scale groups offers
mechanisms to avoid implosion problems and ensure the needed
reliability in its delivery of keying material.
Rekey messages are protected by a Rekey SA, which is established by
the registration protocol, and updated using rekey protocol. When a
member leaves the group, it destroys its local copy of the GSA. Use
of a de-registration message may be an efficient mechanisms for a
member to inform the GCKS that it has destroyed the SAs, or is about
to destroy them. Such a message may prompt the GCKS to
cryptographically remove the member from the group (i.e., to prevent
the member from having access to future group communication). In
large-scale multicast applications, however, de-registration has the
potential to cause implosion at the GCKS.
3.2 Detailed Description of the GKM Architecture
Figure 1 depicts the overall design of a GKM protocol. Each group
member, sender or receiver, uses the registration protocol to get
authorized, authenticated access to a particular Group, its policies,
and its keys. The two types of group keys are the key encryption keys
(KEKs) and the traffic encryption keys (TEKs). For group
authentication of rekey messages or data, key integrity keys or
traffic integrity keys may be used as well. We use the term
protection keys to refer to both integrity keys and the encryption
keys. For example, the term traffic protection key (TPK) is used to
denote the combination of a TEK and a traffic integrity key, or key
material used to generate them.
The KEK may be a single key that protects the rekey message,
typically containing a new Rekey SA (containing a KEK) and/or Data SA
(containing a TEK). A Rekey SA may also contain a vector of keys
that are part of a group key membership algorithm [RFC2627, OFT,
TAXONOMY, SD1, SD2]. The TPKs are used by the data security protocol
to protect streams, files, or other data sent and received by the
data security protocol. Thus the registration protocol and/or the
rekey protocol establish the KEK(s) and/or the TPKs.
Internet Draft Group Key Management Architecture [PAGE 8]
Baugher, Canetti, Dondeti, Lindholm June 2004
+------------------------------------------------------------------+
| +-----------------+ +-----------------+ |
| | POLICY | | AUTHORIZATION | |
| | INFRASTRUCTURE | | INFRASTRUCTURE | |
| +-----------------+ +-----------------+ |
| ^ ^ |
| | | |
| v v |
| +--------------------------------------------------------------+ |
| | | |
| | +--------------------+ | |
| | +------>| GCKS |<------+ | |
| | | +--------------------+ | | |
| | REGISTRATION or | REGISTRATION or | |
| | DE-REGISTRATION | DE-REGISTRATION | |
| | PROTOCOL | PROTOCOL | |
| | | | | | |
| | v REKEY v | |
| | +-----------------+ PROTOCOL +-----------------+ | |
| | | | (OPTIONAL) | | | |
| | | SENDER(S) |<-------+-------->| RECEIVER(S) | | |
| | | | | | | |
| | +-----------------+ +-----------------+ | |
| | | ^ | |
| | v | | |
| | +-------DATA SECURITY PROTOCOL-------+ | |
| | | |
| +--------------------------------------------------------------+ |
| |
+------------------------------------------------------------------+
FIGURE 1: Group Security Association Model
There are a few, distinct outcomes to a successful registration
Protocol exchange.
o If the GCKS uses rekey messages, then the admitted member
receives the Rekey SA. The Rekey SA contains the groups
rekey policy (note that not all of the policy need to be
revealed to members), and at least a group KEK. In
addition, the GCKS may send a group key integrity key, and
if the group uses a group key management algorithm, a set
of KEKs (or key material used to derive the KEKs) according
to the particular algorithm.
o If rekey messages are not used for the Group, then the
admitted member will receive TPKs (as part of the Data
Security SAs) that are passed to the members Data Security
Protocol (as IKE does for IPsec).
Internet Draft Group Key Management Architecture [PAGE 9]
Baugher, Canetti, Dondeti, Lindholm June 2004
o The GCKS may pass one or more TPKs to the member even if
rekey messages are used, for efficiency reasons according
to group policy.
The GCKS creates the KEK and TPKs and downloads them to each member -
as the KEK and TPKs are common to the entire group. The GCKS is a
separate, logical entity that performs member authentication and
authorization according to the group policy that is set by the group
owner. The GCKS may present a credential to the group member that is
signed by the group owner so the member can check the GCKSs
authorization. The GCKS, which may be co-located with a member or be
a separate physical entity, runs the rekey protocol to push rekey
messages containing refreshed KEKs, new TPKs, and/or refreshed TPKs
to members. Note that some group key management algorithms refresh
any of the KEKs (potentially), whereas others only refresh the group
KEK.
Alternatively, the sender may forward rekey messages on behalf of the
GCKS when it uses a credential mechanism that supports delegation.
Thus, it is possible for the sender (or other members) to source
keying material - TPKs encrypted in the Group KEK - as it sources
multicast or unicast data. As mentioned above, the rekey message can
be sent using unicast or multicast delivery. Upon receipt of a TPK
(as part of a Data SA) from a rekey message or a registration
protocol exchange, the members group key management functional block
will provide the new or updated security association (SA) to the data
security protocol to protect the data sent from sender to receiver.
The Data SA protects the data sent on the arc labeled DATA SECURITY
PROTOCOL shown in Figure 1. A second SA, the Rekey SA, is optionally
established by the key-management protocol for rekey messages as
shown in Figure 1 by the arc labeled REKEY PROTOCOL. The rekey
message is optional because all keys, KEKs and TPKs, can be delivered
by the registration protocol exchanges shown in Figure 1, and those
keys may not need to be updated. The registration protocol is
protected by a third, unicast, SA between the GCKS and each member;
this is called the Registration SA. There may be no need for the
Registration SA to remain in place after the completion of the
registration protocol exchanges. The de-registration protocol may be
used when explicit teardown of the SA is desirable (such as when a
phone call or conference terminates). The three SAs compose the GSA.
Only one SA is optional and that is the Rekey SA.
Figure 1 shows two blocks that are external to the group key
management protocol: The policy and authorization infrastructures
are discussed in Section 6.1. The Multicast Security Architecture
document further clarifies the SAs and their use as part of the
complete architecture of a multicast security solution [MSEC-Arch].
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3.3 Properties of the Design
The design of Section 3.2 achieves scalable operation by (1) allowing
the de-coupling of authenticated key exchange in a registration
protocol from a rekey protocol, (2) allowing the rekey protocol to
use unicast push or multicast distribution of group and data keys as
an option, (3) allowing all keys to be obtained by the unicast
registration protocol, and (4) delegating the functionality of the
GCKS among multiple entities, i.e., to permit distributed operation
of the GCKS.
High-capacity operation is obtained by (1) amortizing
computationally-expensive asymmetric cryptography over multiple data
keys used by data security protocols, (2) supporting multicast
distribution of symmetric group and data keys, and (3) supporting key
revocation algorithms such as LKH [RFC2627, OFT, SDR] that allow
members to be added or removed at logarithmic rather than linear
space/time complexity. The registration protocol may use asymmetric
cryptography to authenticate joining members and optionally establish
the group KEK. Asymmetric cryptography such as Diffie-Hellman key
agreement and/or digital signatures are amortized over the life of
the group KEK: A Data SA can be established without the use of
asymmetric cryptography - the TPKs are simply encrypted in the
symmetric KEK and sent unicast or multicast in the rekey protocol.
The design of the registration and rekey protocols is flexible. The
registration protocol establishes either a Rekey SA or one or more
Data SAs or both types of SAs. At least one of the SAs is present
(otherwise, there is no purpose to the Registration SA). The Rekey
SA may update the Rekey SA, or establish or update one or more Data
SAs. Individual protocols or configurations may take advantage of
this flexibility for efficient operation.
3.4 Group Key Management Block Diagram
In the block diagram of Figure 2, group key management protocols run
between a GCKS and member principal to establish a Group Security
Association (GSA). The GSA consists of a Data SA, an optional Rekey
SA, and a Registration SA. The GCKS may use a delegated principal,
such as the sender, which has a delegation credential signed by the
GCKS. The Member of Figure 2 may be a sender or receiver of
multicast or unicast data. There are two functional blocks in Figure
2 labeled GKM, and there are two arcs between them depicting the
group key-management registration (reg) and rekey (rek) protocols.
The message exchanges are the GSA establishment protocols, which are
the registration protocol and the rekey protocol described above.
Figure 2 shows that a complete group-key management functional
specification includes much more than the message exchange. Some of
these functional blocks and the arcs between them are peculiar to an
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Baugher, Canetti, Dondeti, Lindholm June 2004
operating system (OS) or vendor product, such as vendor
specifications for products that support updates to the IPsec
Security Association Database (SAD) and Security Policy Database
(SPD) [RFC2367]. Various vendors also define the functions and
interface of credential stores, CRED in Figure 2.
+----------------------------------------------------------+
| |
| +-------------+ +------------+ |
| | CONTROL | | CONTROL | |
| +------^------+ +------|-----+ +--------+ |
| | | +-----| CRED | |
| | | | +--------+ |
| +----v----+ +----v--v-+ +--------+ |
| | <-----Reg-----> |<->| SAD | |
| | GKM -----Rek-----> GKM | +--------+ |
| | | | | +--------+ |
| | ------+ | |<->| SPD | |
| +---------+ | +-^-------+ +--------+ |
| +--------+ | | | | |
| | CRED |----->+ | | +-------------------+ |
| +--------+ | | +--------------------+ | |
| +--------+ | +-V-------+ +--------+ | | |
| | SAD <----->+ | |<->| SAD <-+ | |
| +--------+ | |SECURITY | +--------+ | |
| +--------+ | |PROTOCOL | +--------+ | |
| | SPD <----->+ | |<->| SPD <----+ |
| +--------+ +---------+ +--------+ |
| |
| (A) GCKS (B) MEMBER |
+----------------------------------------------------------+
Figure 2: Group key management block diagram for a host computer
The CONTROL function directs the GCKS to establish a group, admit a
member, or remove a member, or it directs a member to join or leave a
group. CONTROL includes authorization, which is subject to group
policy [GSPT], but how this is done is specific to the GCKS
implementation. For large-scale multicast sessions, CONTROL could
perform session announcement functions to inform a potential group
member that it may join a group or receive group data (e.g. a stream
of file transfer protected by a data security protocol).
Announcements notify group members to establish multicast SAs in
advance of secure multicast data transmission. Session Description
Protocol (SDP) is one form that the announcements might take
[RFC2327]. The announcement function may be implemented in a
session-directory tool, an electronic program guide (EPG), or by
other means. The Data Security or the announcement function directs
group key management using an application-programming interface
(API), which is peculiar to the host OS in its specifics. A generic
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API for group key management is for further study, but this function
is necessary to allow Group (KEK) and Data (TPKs) key establishment
to be done in a way that is scalable to the particular application.
A GCKS application program will use the API to initiate the
procedures to establish SAs on behalf of a Security Protocol in which
members join secure groups and receive keys for streams, files or
other data.
The goal of the exchanges is to establish a GSA through updates to
the SAD of a key-management implementation and particular Security
Protocol. The data security protocol of Figure 2 may span
internetwork and application layers or operate at the internetwork
layer, such as AH and ESP.
4.0 Registration protocol
The design of the registration protocol is flexible, and can support
different application scenarios. The chosen registration protocol
solution reflects the specific requirements of specific scenarios.
In principle, it is possible to base a registration protocol on any
secure-channel protocol, such as IPsec and TLS, which is the case in
tunneled GSAKMP [tGSAKMP]. GDOI [RFC3547] reuses IKE Phase 1 as the
secure channel to download Rekey and/or Data SAs. Other protocols,
such as MIKEY and GSAKMP, use authenticated Diffie-Hellman exchanges
similar to IKE Phase 1, but specifically tailored for key download
to achieve efficient operation. We discuss the design of a
registration protocol in detail in the rest of this section.
4.1 Registration protocol via Piggybacking or Protocol Reuse
Some registration protocols need to tunnel through a data-signaling
protocol to take advantage of already existing security
functionality, and/or to optimize the total session setup time. For
example, a telephone call has strict bounds for delay in setup time.
It is not feasible to run security exchanges in parallel with call
setup since the latter often resolves the address: Call setup must
complete before the caller knows the address of the callee. In this
case, it may be advantageous to tunnel the key exchange procedures
inside call establishment [H.235, MIKEY] so both can complete (or
fail, see below) at the same time.
The registration protocol has different requirements depending on
the particular integration/tunneling approach. These requirements
are not necessarily security requirements, but will have an impact
on the chosen security solution. For example, the security
association will certainly fail if the call setup fails in the case
of IP telephony.
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Conversely, the registration protocol imposes requirements on the
protocol that tunnels it. In the case of IP telephony, the call
setup usually will fail when the security association is not
successfully established. In the case of video-on-demand, protocols
such as RTSP that convey key management data will fail when a needed
security association cannot be established.
Both GDOI and MIKEY use this approach, but in different ways. MIKEY
can be tunneled in SIP and RTSP. It takes advantage of the session
information contained in these protocols and the possibility to
optimize the setup time for the registration procedure. SIP
requires that a tunneled protocol must use at most one roundtrip
(i.e. two messages). This is also desirable requirement from RTSP
as well.
The GDOI approach takes advantage of the already defined ISAKMP
phase 1 exchange [RFC2409], and extends the phase 2 exchange for the
registration. The advantage here is the reuse of a successfully
deployed protocol and the code base, where the defined phase 2
exchange is protected by the SA created by phase 1. GDOI also
inherits other functionality of the ISAKMP, and thus it is readily
suitable for running IPsec protocols over IP multicast services.
4.2 Properties of Alternative registration Exchange Types
The required design properties of a registration protocol have
different tradeoffs. A protocol that provides perfect forward
secrecy and identity protection trades performance or efficiency for
better security, while a protocol that completes in one or two
messages may trade security functionality (e.g. identity protection)
for efficiency.
Replay protection generally uses either a timestamp or a sequence
number. The first requires synchronized clocks, while the latter
requires that it is possible to keep state. In a timestamp-based
protocol, a replay cache is needed to store the authenticated
messages (or the hashes of the messages) received within the
allowable clock skew. The size of the replay cache depends on the
number of authenticated messages received during the allowable clock
skew. During a DoS attack, the replay cache might become
overloaded. One solution is to over-provision the replay cache.
However, this may lead to a large replay cache. Another solution is
to let the allowable clock skew be changed dynamically during
runtime. During a suspected DoS attack, the allowable clock skew is
decreased so that the replay cache becomes manageable.
A challenge-response mechanism (using Nonces) obviates the need for
synchronized clocks for replay protection when the exchange uses
three or more messages [MVV].
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Additional security functions become possible as the number of
allowable messages in the registration protocol increase. ISAKMP
offers identity protection, for example, as part of a six-message
exchange. With additional security features, however, comes added
complexity: Identity protection, for example, not only requires
additional messages, but may result in DoS vulnerabilities since
authentication is performed in a late stage of the exchange after
resources already have been devoted.
In all cases, there are tradeoffs with the number of message
exchanged, the desired security services, and the amount of
infrastructure that is needed to support the group key management
service. Whereas protocols that use two or even one-message setup
have low latency and computation requirements, they may require more
infrastructure such as secure time or offer less security such as
the absence of identity protection. What tradeoffs are acceptable
and what are not is very much dictated by the application and
application environment.
4.3 Infrastructure for Alternative registration Exchange Types
The registration protocol may need external infrastructures to be
able to handle authentication and authorization, replay protection,
protocol-run integrity, and potentially other security services such
as secure, synchronized clocks. For example, authentication and
authorization may need a PKI deployment (with either authorization-
based certificates or a separate management for this) or may be
handled by using AAA infrastructure. Replay protection using
timestamps requires an external infrastructure or protocol for clock
synchronization.
However, external infrastructures may not always be needed, if for
example pre-shared keys are used for authentication and
authorization; this may be the case if the subscription base is
relatively small. In a conversational multimedia scenario (e.g., a
VoIP call between two or more people), it may very well be the end
user who handles the authorization by manually accepting/rejecting
the incoming calls. Thus, infrastructure support may not be
required in that case.
4.4 De-registration Exchange
The session-establishment protocol (e.g., SIP, RTSP) that conveys a
registration exchange often has a session-disestablishment protocol
such as RTSP TEARDOWN [RFC2326] or SIP BYE [RFC2543]. The session-
disestablishment exchange between endpoints offers an opportunity to
signal the end of the GSA state at the endpoints. This exchange
need only be a uni-directional notification by one side that the GSA
is to be destroyed. For authentication of this notification, we may
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use a proof-of-possession of the group key(s) by one side to the
other. Some applications benefit from acknowledgement in a mutual,
two-message exchange signaling disestablishment of the GSA
concomitant with disestablishment of the session, e.g., RTSP or SIP
session. In this case, a two-way proof-of-possession might serve
for mutual acknowledgement of the GSA disestablishment.
5.0 Rekey protocol
The group rekey protocol is for transport of keys and SAs between a
GCKS and the members of a secure communications group. The GCKS
sends rekey messages to update a Rekey SA, or initialize/update a
Data SA or both. Rekey messages are protected by a Rekey SA. The
GCKS may update the Rekey SA when group membership changes or when
KEKs or TPKs expire. Recall that KEKs correspond to a Rekey SA and
TPKs correspond to a Data SA.
The following are some desirable properties of the rekey protocol:
o Rekey protocol ensures that all members receive the rekey
information in a timely manner.
o Rekey protocol specifies mechanisms for the parties
involved, to contact the GCKS and re-sync when their keys expire
and no updates have been received.
o Rekey protocol avoids implosion problems and ensures the
needed reliability in delivering Rekey information.
We further note that the rekey protocol is primarily responsible for
scalability of the group key management architecture. Hence it is
imperative that we provide the above listed properties in a scalable
manner. Note that solutions exist in the literature (both IETF
standards and research articles) for parts of the problem. For
instance, the rekey protocol may use a scalable group key management
algorithm (GKMA) to reduce the number of keys sent in a rekey
message. Examples of a GKMA include LKH, OFT, Subset difference
based schemes etc.
5.1 Goals of the rekey protocol
The goals of the rekey protocol are:
o to synchronize a GSA
o to provide privacy and (symmetric or asymmetric)
authentication, replay protection and DoS protection
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o efficient rekeying after changes in group membership, or when
keys (KEKs) expire,
o reliable delivery of rekey messages,
o provide methods for members to recover from an out-of-sync
GSA,
o high throughput and low latency, and
o to use IP Multicast or multi-unicast.
We identify several major issues in the design of a rekey protocol:
1. rekey message format
2. reliable transport of rekey messages
3. implosion
4. recovery from out-of-sync GSA
5. incorporating GKMAs in rekey messages
6. interoperability of GKMAs
Note that for a GCKS to successfully rekey a group, it is not
sufficient that rekey protocol implementations interoperate. We also
need to ensure that the GKMA also interoperates, i.e., standards
versions of group key management algorithms, such as LKH, OFT, subset
difference and others need to be used.
In the rest of this section we discuss these topics in detail.
5.2 Rekey message Transport and Protection
Rekey messages contain Rekey and/or Data SAs along with KEKs and
TPKs. These messages need to be confidential, authenticated, and
protected against replay and DoS attacks. They are sent via
multicast or multi-unicast from the GCKS to the members.
Rekey messages are encrypted with the Group KEK for confidentiality.
When used in conjunction with a GKMA, portions of the rekey message
are first encrypted with the appropriate KEKs as specified by the
GKMA. The GCKS authenticates rekey messages using either a MAC -
computed using the group Authentication key - or a digital signature.
In both cases, a sequence number is included in computation of the
MAC or the signature to protect against replay attacks.
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When group authentication is provided - with a symmetric key - rekey
messages are vulnerable to attacks by other members of the group.
Rekey messages are digitally signed when group members do not trust
each other. When asymmetric authentication is used, members
receiving rekey messages are vulnerable to DoS attacks. An external
adversary may send a bogus rekey message, which a member cannot
identify until after it performs an expensive digital signature
operation. To protect against such an attack, a MAC may be sent as
part of the rekey message. Members verify the signature only upon
successful verification of the MAC.
Rekey messages contain group key updates corresponding to a single
[RFC2627, OFT] or multiple membership changes [SD, BatchRekey] and
may contain group key initialization messages [OFT].
5.3 Reliable Transport of rekey messages
The GCKS needs to ensure that all members have the current Data
Security and Rekey SAs. Otherwise, authorized members may be
inadvertently excluded from receiving group communications. Thus,
the GCKS needs to use a rekey algorithm that is inherently reliable
or employ some reliable transport mechanism to send rekey messages.
There are two dimensions to the problem: Messages that update group
keys may be lost in transit or may be missed by a host when it is
offline. LKH and OFT group key management algorithms rely on past
history of updates being received by the host. If the host goes
offline, it will need to resynchronize its group-key state when it
comes online; this may require a unicast exchange with the GCKS.
The Subset Difference algorithm, however, conveys all the needed
state in its rekey messages and does not need members to be always
online, nor keeping state. Subset difference algorithm does not
require a backchannel and can operate on a broadcast network. If a
rekey message is lost in transmission, subset difference algorithm
cannot decrypt messages encrypted with the TPK sent via the lost
rekey message. There are self-healing GKMAs proposed in the
literature that allow a member to recover lost rekey messages, as
long as rekey messages before and after the lost rekey message are
received.
Rekey messages are typically short (for single membership change as
well as for small groups) which makes it easy to design a reliable
delivery protocol. On the other hand, the security requirements may
add an additional dimension to address. Also there are some special
cases where membership changes are processed as a batch, which
reduces the frequency of rekey messages, but increases their size.
Furthermore, among all the KEKs sent in a rekey message, as many as
half the members need only a single KEK. We may take
advantage of these properties in designing a rekey message(s) and a
protocol for their reliable delivery.
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Three categories of solutions have been proposed:
1. Repeatedly transmit the rekey message: Recall that in many
cases rekey messages translate to only one or two IP packets.
2. Use an existing reliable multicast protocol/infrastructure
3. Use FEC for encoding rekey packets (with NACKs as feedback)
[BatchRekey]
Note that for small messages, category 3 is essentially the same as
category 1.
The group member might be out of synchrony with the GCKS if it
receives a rekey message having a sequence number that is more than
one greater than the last sequence number processed. This is one
means by which the GCKS member detects that it has missed a rekey
message. Alternatively, the data-security application might detect
that it is using an out-of-date key and notifies the group key
management module of this condition. What action the GCKS member
takes is a matter of group policy: The GCKS member should log the
condition and may contact the GCKS to re-run the re-registration
protocol to obtain a fresh group key. The group policy needs to
take into account boundary conditions, such as re-ordered rekey
messages when rekeying is so frequent that two messages might get
reordered in an IP network. The group key policy also needs to
take into account the potential for denial of service attacks where
an attacker delays or deletes a rekey message in order to force a
subnetwork or subset of the members to synchronously contact the
GCKS.
If a group member becomes out-of-synch with the GSA then it should
re-register with the GCKS. However, in many cases there are other,
simpler methods for re-synching with the group:
o The member can open a simple, unprotected connection (say, TCP)
with the GCKS and obtain the current (or several recent) rekey
messages. Note that there is no need for authentication or
encryption here, since the rekey message is already signed and
is anyway multicasted in the clear. One may think that this
opens the GCKS to DoS attacks by many bogus such requests. But
this does not seem to worsen the situation: in fact, bombarding
the GCKS with bogus resynch requests would be much more
problematic.
o The GCKS can post the rekey messages on some public site (say,
web site) and the out-of-synch memeber can obtain the rekey
messages from that site.
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It is suggested that the GCKS always provide all three ways of
resynching (i.e., re-registration, simple TCP, and public posting).
This way, it is up to the member to choose how to resynch; it also
avoids adding yet another field to the policy token [GSPT].
Alternatively, a policy token may contain a field specifying one or
more methods supported for resynchronization of a GSA.
5.4 State-of-the-art on Reliable Multicast Infrastructure
The rekey message may be sent using reliable multicast. There are
multiple types of reliable multicast protocols and products, which
have different properties. However, there are no standard reliable
multicast protocols at the present time. Thus, this document makes
no recommendation for use of a particular reliable multicast
protocol or set of protocols for the purposes group key management.
The suitability of NAK-based, ACK-based or other reliable multicast
methods are determined by the particular needs of the group key
management application and environment. In the future, group key
management protocols may choose to use particular standards-based
approaches that meet the needs of the particular application. A
secure announcement facility is needed to signal the use of a
reliable multicast protocol, which must be specified as part of
group policy. The reliable multicast announcement and policy
specification, however, can only follow the establishment of
reliable multicast standards and are not considered further in this
document.
Today, the several MSEC group key management protocols support
sequencing of the rekey messages through a sequence number, which is
authenticated along with the rekey message. A sender of rekey
messages may re-transmit multiple copies of the message provided
that they have the same sequence number. Thus, re-sending the
message is a rudimentary means of overcoming loss along the network
path. A member who receives the rekey message will check the
sequence number to detect duplicate and missing rekey messages. The
member receiver will discard duplicate messages that it receives.
Large rekey messages, such as those that contain LKH or OFT tree
structures, might benefit from transport-layer FEC when standard
methods are available in the future. It is unlikely that forward
error correction (FEC) methods will benefit rekey messages that are
short and fit within a single message. In this case, FEC
degenerates to simple retransmission of the message.
5.5 Implosion
Implosion may occur due to one of two reasons. First, recall that
one of the goals of the rekey protocol is to synchronize a GSA. When
a rekey or Data SA expires, members may contact the GCKS for an
update. If all or even many members contact the GCKS at about the
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same time, the GCKS cannot handle all those messages. We refer to
this as an out-of-sync implosion.
The second case is in the reliable delivery of rekey messages.
Reliable multicast protocols use feedback (NACK or ACK) to determine
which packets must be retransmitted. Packet losses may result in
many members sending NACKs to the GCKS. We refer to this as feedback
implosion.
The implosion problem has been studied extensively in the context of
reliable multicasting. Some of the proposed solutions viz., feedback
suppression and aggregation, might be useful in the GKM context as
well.
Members may wait for a random time before sending an out-of-sync or
feedback message. Meanwhile, members might receive the key updates
they need and therefore will not send a feedback message.
An alternative solution is to have the members contact one of several
registration servers when they are out-of-sync. This requires GSA
synchronization between the multiple registration servers.
Feedback aggregation and local recovery employed by some reliable
multicast protocols are not easily adaptable to transport of rekey
messages. There are authentication issues to address in aggregation.
Local recovery is more complex in that members need to establish SAs
with the local repair server (Any member of the group or a
subordinate GCKS might serve as a repair server. Repair servers may
be responsible for resending rekey messages).
Members may use the group SA, more specifically the Rekey SA, to
authenticate requests sent to the repair server; however, replay
protection requires maintaining state at members as well as repair
servers. Authentication of repair requests is to protect against DoS
attacks. Note also that an out-of-sync member may use an expired
Rekey SA to authenticate repair requests, which requires repair
servers to accept messages protected by old SAs.
Alternatively, a simple mechanism may be employed to achieve local
repair efficiently. Each member receives a set of local repair
server addresses as part of group operation policy information. When
a member does not receive a rekey message, it can send a "retransmit
replay message(s) with sequence number n and higher" to one of the
local repair servers. The repair server can do one of two things:
ignore the request if it is busy, or retransmit the requested rekey
messages as received from the GCKS. The repair server, which is also
another member may choose to serve only m requests in a given time
period (i.e., rate limits responses) or per a given rekey message.
Rate limiting the requests and responses protects the repair servers
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as well other members of the group from being vulnerable to DoS
attacks.
5.6 Issues in Incorporating Group Key Management Algorithms
Group key management algorithms make Rekeying scalable. Large group
Rekeying without employing GKMAs is prohibitively expensive.
First we list some requirements to consider in selecting a GKMA:
o Protection against Collusion: Members (or non members) should
not be able to collaborate to deduce keys that they are not
privileged to (following the GKMA key distribution rules).
o Forward access control: Ensure that departing members cannot get
access to future group data.
o Backward access control: Ensure that joining members cannot
decrypt past data.
5.7 Stateless, Stateful, and Self-healing Rekeying Algorithms
We classify group key management algorithms into three categories,
viz., stateful, stateless, and self-healing algorithms.
Stateful algorithms [RFC2627, OFT] use KEKs from past rekeying
instances to encrypt (protect) KEKS corresponding to the current and
future rekeying instances. The main disadvantage in these schemes is
that if a member were offline or otherwise fails to receive KEKs from
a past rekeying instance, it may no longer be able to synchronize its
GSA even though it can receive KEKs from all future rekeying
instances. The only solution is to contact the GCKS explicitly for
resynchronization. Note that the KEKs for the first rekeying
instance are protected by the Registration SA. Recall that
communication in that phase is one to one, and therefore it is easy
to ensure reliable delivery.
Stateless GKMAs [SD1, SD2] encrypt rekey messages with KEKs sent
during the registration protocol. Since rekey messages are
independent of any past rekey messages (i.e. not protected by KEKs
therein), a member may go offline, but continue to be able to
decipher future communications. However, they offer no mechanisms to
recover past rekeying messages. Stateless rekeying may be relatively
inefficient, particularly for immediate (in contrast to batch)
rekeying in highly dynamic groups.
In self-healing schemes [Self-healing], a member can reconstruct a
lost rekey message, as long as it receives some past rekey messages
and some future rekey messages.
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5.8 Interoperability of a GKMA
Most GKMA specifications do not specify packet formats although any
group key management algorithms need to, for the purposes of
interoperability. In particular there are several alternative ways
to managing key trees and numbering nodes within key trees. The
following information is generally needed during initialization of a
Rekey SA or included with each GKMA packet.
o GKMA name (e.g. LKH, OFT, Subset difference)
o GKMA version number (implementation specific). Version may imply
several things such as the degree of a key tree, proprietary
enhancements, and qualify another field such as a key id.
o Number of keys or Largest ID
o Version specific data
o Per key information
- Key ID
- Key lifetime (creation/expiration data)
- Encrypted key
- Encryption key's ID (optional)
Key IDs may change in some implementations in which case we need to
send:
o List of <old id, new id>
6.0 Group Security Association
The GKM Architecture defines the interfaces between the registration,
Rekey, and data security protocols in terms of the Security
Associations (SAs) of those protocols. By isolating these protocols
behind a uniform interface, the architecture allows implementations
to use protocols best suited to their needs. For example, a rekey
protocol for a small group could use multiple unicast transmissions
with symmetric authentication, while that for a large group could use
IP Multicast with packet-level Forward Error Correction and source
authentication.
The Group Key Management Architecture provides an interface between
the security protocols and the group SA (GSA), which consists of
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three SAs, viz., Registration SA, Rekey SA and Data SA. The Rekey SA
is optional. There are two cases in defining the relationships
between the three SAs. In both cases, the Registration SA protects
the registration protocol.
In Case 1, group key management is done WITHOUT using a Rekey SA. The
registration protocol initializes and updates one or more Data SAs
(having TPKs to protect files or streams). Each Data SA corresponds
to a single group - and a group may have more than one Data SA.
In Case 2, group key management is done WITH a Rekey SA to protect
the rekey protocol. The registration protocol initializes the Rekey
SAs (one or more) as well as zero or more Data SAs upon successful
completion. When a Data SA is not initialized in the registration
protocol, this is done in the rekey protocol. The rekey protocol
updates Rekey SA(s) AND establishes Data SA(s).
6.1 Group policy
Group policy is described in detail in the Group Security Policy
Token document [GSPT]. Group policy can be distributed through group
announcements, key management protocols, and other out-of-band means
(e.g., via a web page). The group key management protocol carries
cryptographic policies of the SAs and keys it establishes as well as
additional policies for the secure operation of the group.
The acceptable cryptographic policies for the registration protocol,
which may run over TLS [TLS], IPsec, or IKE, are not conveyed in the
group key-management protocol since they precede any of the key
management exchanges. Thus, a security policy repository having some
access protocol may need to be queried prior to key-management
session establishment to determine the initial cryptographic policies
for that establishment. This document assumes the existence of such
a repository and protocol for GCKS and member policy queries.
Thus group security policy will be represented in a policy repository
and accessible using a policy protocol. Policy distribution may be a
push or a pull operation.
The group key management architecture assumes that the following
group-policy information may be externally managed, e.g., by the
content owner, group conference administrator or group owner.
o Identity of the Group owner, and authentication method, and
delegation method for identifying a GCKS for the group
o Group GCKS, authentication method, and delegation method for any
subordinate GCKSs for the group
o Group membership rules or list and authentication method
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There are also two additional policy-related requirements external to
group key management.
o There is an authentication and authorization infrastructure such
as X.509 [RFC 2459], SPKI [RFC 2693], or a pre-shared key scheme
in accordance with the group policy for a particular group.
o There is an announcement mechanism for secure groups and events
that operates according to group policy for a particular group.
Group policy determines how the registration and rekey protocols
initialize or update Rekey and Data SAs. The following sections
describe potential information sent by the GCKS for the Rekey and
Data SAs. A member needs to have the information specified in the
next sections to establish Rekey and Data SAs.
6.2 Contents of the Rekey SA
The Rekey SA protects the rekey protocol. It contains cryptographic
policy, Group Identity and Security Parameter Index (SPI) [RFC2401]
to uniquely identify an SA, replay protection information, and key
protection keys.
6.2.1 Rekey SA Policy
The GROUP KEY MANAGEMENT ALGORITHM represents the group key
revocation algorithm that enforces forward and backward access
control. Examples of key revocation algorithms include LKH, LKH+,
OFT, OFC and Subset Difference [RFC2627, OFT, TAXONOMY, SDR]. The
key revocation algorithm could also be NULL. In that case, the Rekey
SA contains only one KEK, which serves as the group KEK. The rekey
messages initialize or update Data SAs as usual. But, the Rekey SA
itself can be updated (group KEK can be Rekeyed) when members join or
the KEK is about to expire. Leave Rekeying is done by re-
initializing the Rekey SA through the rekey protocol.
The KEK ENCRYPTION ALGORITHM uses a standard encryption algorithm
such as 3DES or AES. The KEK KEY LENGTH is also specified.
The AUTHENTICATION ALGORITHM uses digital signatures for GCKS
authentication (since all shared secrets are known to some or all
members of the group), or some symmetric secret in computing MACs for
group authentication. Symmetric authentication provides weaker
authentication in that any group member can impersonate a particular
source. The AUTHENTICATION KEY LENGTH is also be specified.
The CONTROL GROUP ADDRESS is used for multicast transmission of rekey
messages. This information is sent over the control channel such as
in an ANNOUNCEMENT protocol or call setup message. The degree to
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which the control group address is protected is a matter of group
policy.
The REKEY SERVER ADDRESS allows the registration server to be a
different entity from the server used for Rekey, such as for future
invocations of the registration and rekey protocols. If the
registration server and the Rekey server are two different entities,
the registration server sends the Rekey servers address as part of
the Rekey SA.
6.2.2 Group Identity
The Group identity accompanies the SA (payload) information as an
identifier if the specific group key management protocol allows
multiple groups to be initialized in a single invocation of the
registration protocol or multiple groups to be updated in a single
rekey message. It is often much simpler to restrict each
registration invocation to a single group; no such restriction is
necessary. There is always a need to identify the group when
establishing a Rekey SA either implicitly through an SPI or
explicitly as an SA parameter.
6.2.3 KEKs
Corresponding to the key management algorithm, the Rekey SA contains
one or more KEKs. The GCKS holds the key encrypting keys of the
group, while the members receive keys following the specification of
the key-management algorithm. When there are multiple KEKs for a
group (as in an LKH tree), each KEK needs to be associated with a Key
ID, which is used to identify the key needed to decrypt it. Each KEK
has a LIFETIME associated with it, after which the KEK expires.
6.2.4 Authentication Key
The GCKS provides a symmetric or public key for authentication of its
rekey messages. Symmetric-key authentication is appropriate only
when all group members can be trusted not to impersonate the GCKS.
The architecture does not rule out methods for deriving symmetric
authentication keys at the member [RFC2409] rather than being pushed
from the GCKS.
6.2.5 Replay Protection
Rekey messages need to be protected from replay/reflection attacks.
Sequence numbers are used for this purpose and the Rekey SA (or
protocol) contains this information.
6.2.6 Security Parameter Index (SPI)
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The tuple <Group identity, SPI > uniquely identifies a Rekey SA. The
SPI changes each time the KEKs change.
6.3 Contents of the Data SA
The GCKS specifies the data security protocol used for secure
transmission of data from sender(s) to receiving members. Examples
of data security protocols include IPsec ESP [RFC 2401] and SRTP [RFC
3711]. While the content of each of these protocols is out of the
scope of this document, we list the information sent by the
registration protocol (or the rekey protocol) to initialize or update
the Data SA.
6.3.1 Group Identity
The Group identity accompanies SA information when Data SAs are
initialized or Rekeyed for multiple groups in a single invocation of
the registration protocol or in a single rekey message.
6.3.2 Source Identity
The SA includes source identity information when the group owner
chooses to reveal Source identity to authorized members only. A
public channel such as announcement protocol is only appropriate when
there is no need to protect source or group identities.
6.3.3 Traffic Protection Keys
Irrespective of the data security protocol used, the GCKS supplies
the TEKs or information to derive TEKs, used for data encryption.
6.3.4 Data Authentication Keys
Depending on the data-authentication method used by the data security
protocol, group key management may pass one or more keys, functions
(e.g., TESLA [TESLA]), or other parameters used for authenticating
streams or files.
6.3.5 Sequence Numbers
The GCKS passes sequence numbers when needed by the data security
protocol, for SA synchronization and replay protection.
6.3.6 Security Parameter Index (SPI)
The GCKS may provide an identifier as part of the Data SA contents
for data security protocols that use an SPI or similar mechanism to
identify an SA or keys within an SA.
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Baugher, Canetti, Dondeti, Lindholm June 2004
6.3.7 Data SA policy
The Data SA parameters are specific to the data security protocol but
generally include encryption algorithm and parameters, the source
authentication algorithm and parameters, the group authentication
algorithm and parameters, and/or replay protection information.
7.0 Scalability Considerations
The area of group communications is quite diverse. In
teleconferencing, a multipoint control unit (MCU) may be used to
aggregate a number of teleconferencing members into a single session;
MCUs may be hierarchically organized as well. A loosely coupled
teleconferencing session [RFC3550] has no central controller but is
fully distributed and end-to-end. Teleconferencing sessions tend to
have at most dozens of participants whereas video broadcast, which
uses multicast communications, and media on demand, which uses
unicast, are large-scale groups numbering hundreds to millions of
participants.
As described in the Requirements section above, the group key
management architecture supports multicast applications with a single
sender. The architecture described in this paper supports large-
scale operation through the following features.
1. There is no need for a unicast exchange to provide data keys to a
security protocol for members who have previously-registered in the
particular group; data keys can be pushed in the rekey protocol.
2. The registration and rekey protocols are separable to allow
flexibility in how members receive group secrets. A group can use a
smart-card based system in place of the registration protocol, for
example, to allow the rekey protocol to be used with no back channel
for broadcast applications such as television conditional access
systems.
3. The registration and rekey protocols support new keys, algorithms,
authentication mechanisms and authorization infrastructures in the
architecture. When the authorization infrastructure supports
delegation, as in X.509 and SPKI, the GCKS function can be
distributed as shown in Figure 3.
Internet Draft Group Key Management Architecture [PAGE 28]
Baugher, Canetti, Dondeti, Lindholm June 2004
+----------------------------------------+
| +-------+ |
| | GCKS | |
| +-------+ |
| | ^ |
| | | |
| | +---------------+ |
| | ^ ^ |
| | | ... | |
| | +--------+ +--------+ |
| | | MEMBER | | MEMBER | |
| | +--------+ +--------+ |
| v |
| +-------------+ |
| | | |
| v ... v |
| +-------+ +-------+ |
| | GCKS | | GCKS | |
| +-------+ +-------+ |
| | ^ |
| | | |
| | +---------------+ |
| | ^ ^ |
| | | ... | |
| | +--------+ +--------+ |
| | | MEMBER | | MEMBER | |
| | +--------+ +--------+ |
| v |
| ... |
+----------------------------------------+
Figure 3: Hierarchically-organized Key Distribution
The first feature in the list allows fast keying of data security
protocols when the member already belongs to the group. While this
is realistic for subscriber groups and customers of service providers
who offer content events, it may be too restrictive for applications
that allow member enrollment at the time of the event. The MSEC
group key management architecture suggests hierarchically organized
key distribution to handle potential mass simultaneous registration
requests. The Figure 3 configuration may be needed when conventional
clustering and load-balancing solutions of a central GCKS site cannot
meet customer requirements. Unlike conventional caching and content-
distribution networks, however, the configuration shown in Figure 3
has additional security ramifications for physical security of a
GCKS.
More analysis and work needs to be done on the protocol
instantiations of the group key management architecture to determine
how effectively and securely the architecture can support large-
scale multicast applications. In addition to being as secure as
Internet Draft Group Key Management Architecture [PAGE 29]
Baugher, Canetti, Dondeti, Lindholm June 2004
pairwise key management against man-in-the-middle, replay, and
reflection attacks, group key management protocols have additional
security needs. Unlike pairwise key management, group key
management needs to be secure against attacks not only by non-
members but by members who may attempt to impersonate a GCKS or
disrupt the operation of a GCKS. Thus, secure groups need to
converge to a common group key under the conditions of members
attacking the group, joining and leaving the group, and being
evicted from the group. Group key management protocols also need to
be robust when denial of service attacks or network partitions lead
to large numbers of synchronized requests. An instantiation of
group key management, therefore, needs to consider how GCKS
operation might be distributed across multiple GCKS as designated by
the group owner to serve keys on behalf of a designated GCKS.
GSAKMP [GSAKMP] protocol uses the policy token and allows
designating some of the members as subordinate GCKSs to address this
scalability issue.
8.0 Security Considerations
This memo describes MSEC key management architecture. This
architecture will be instantiated in one or more group key management
protocols, which must be protected against man-in-the-middle,
connection hijacking, replay or reflection of past messages, and
denial of service attacks.
Authenticated key exchange [STS, SKEME, RFC2408, RFC2412, RFC2409]
techniques limit the effects of man-in-the-middle and connection-
hijacking attacks. Sequence numbers and low-computation message
authentication techniques can be effective against replay and
reflection attacks. Cookies [RFC2522], when properly implemented,
provide an efficient means to reduce the effects of denial of service
attacks.
This memo does not address attacks against key management or security
protocol implementations such as so-called type attacks that aim to
disrupt an implementation by such means as buffer overflow. The
focus of this memo is on securing the protocol, not an implementation
of the protocol.
While classical techniques of authenticated key exchange can be
applied to group key management, new problems arise with the sharing
of secrets among a group of members: Group secrets may be disclosed
by a member of the group and group senders may be impersonated by
other members of the group. Key management messages from the GCKS
should not be authenticated using shared symmetric secrets unless all
members of the group can be trusted not to impersonate the GCKS or
each other. Similarly, members who disclose group secrets undermine
the security of the entire group. group owners and GCKS
Internet Draft Group Key Management Architecture [PAGE 30]
Baugher, Canetti, Dondeti, Lindholm June 2004
administrators must be aware of these inherent limitations of group
key management.
Another limitation of group key management is policy complexity:
Whereas peer-to-peer security policy is an intersection of the policy
of the individual peers, a group owner sets group security policy
externally in secure groups. This document assumes there is no
negotiation of cryptographic or other security parameters in group
key management. Group security policy, therefore, poses new risks to
members who send and receive data from secure groups. Security
administrators, GCKS operators, and users need to determine minimal
acceptable levels of security (e.g., authentication and admission
policy of the group, key lengths, cryptographic algorithms and
protocols used etc.) when joining secure groups.
Given the limitations and risks of group security, the security of
the group key management registration protocol should be as good as
the base protocols on which it is developed such as IKE, IPsec, TLS,
or SSL. The particular instantiations of this Group Key Management
architecture must ensure that the high standards for authenticated
key exchange are preserved in their protocol specifications, which
will be Internet standards-track documents that are subject to
review, analysis and testing.
The second protocol, the group key management rekey protocol, is new
and has unknown risks associated with it. The source-authentication
risks described above are obviated by the use of public-key
cryptography. The use of multicast delivery may raise additional
security issues such as reliability, implosion, and denial of service
attacks based upon the use of multicast. The rekey protocol
specification needs to offer secure solutions to these problems.
Each instantiation of the rekey protocol, such as the GSAKMP Rekey or
the GDOI Groupkey-push operations, need to validate the security of
their Rekey specifications.
Novelty and complexity are the biggest risks to group key management
protocols. Much more analysis and experience are needed to ensure
that the architecture described in this document can provide a well-
articulate standard for security and risks of group key management.
9.0 Acknowledgments
The GKM Building Block [GKMBB) I-D by SMuG was a precursor to this
document; thanks to Thomas Hardjono and Hugh Harney for their
efforts. During the course of preparing this document, Andrea
Colegrove, Brian Weis, George Gross and several others in MSEC WG
and GSEC and SMuG research groups provided valuable comments that
helped improve this document. The authors appreciate their
contributions to this document.
Internet Draft Group Key Management Architecture [PAGE 31]
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10.0 References and Bibliography
[BatchRekey] Yang, Y. R., et al., Reliable Group Rekeying: Design and
Performance Analysis, in Proc. of ACM SIGCOMM, San Diego, CA, August
2001.
[CLIQUES] M. Steiner, G. Tsudik and M. Waidner, CLIQUES: A New
Approach to Group Key Agreement, IEEE ICDCS 97, May 1997
[FN93] A. Fiat, M. Naor, Broadcast Encryption, Advances in
Cryptology - CRYPTO 93 Proceedings, Lecture Notes in Computer
Science, Vol. 773, 1994, pp. 480 -- 491.
[GSAKMP] H.Harney, A.Colegrove, E.Harder, U.Meth, R.Fleischer, Group
Secure Association Key Management Protocol,
http://www.ietf.org/proceedings/03mar/I-D/draft-ietf-msec-gsakmp-sec-
01.txt, February 2003, Work in Progress.
[GSPT] Hardjono, T., H. Harney, P. McDaniel, A. Colegrove, and P.
Dinsmore, The MSEC Group Security Policy Token, draft-ietf-msec-gspt-
02.txt, August 2003, Work in Progress.
[H.235] ITU, Security and encryption for H-Series (H.323 and other
H.245-based) multimedia terminals, ITU-T Recommendation H.235 Version
3, 2001, Work in progress.
[JKKV94] M. Just, E. Kranakis, D. Krizanc, P. van Oorschot, On Key
Distribution via True Broadcasting. In Proceedings of 2nd ACM
Conference on Computer and Communications Security, November 1994,
pp. 81--88.
[MARKS] Briscoe, B., MARKS: Zero Side Effect Multicast Key
Management Using Arbitrarily Revealed Key Sequences, in Proc. of
First International Workshop on Networked Group Communication (NGC),
Pisa, Italy, November 1999.
[MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
Norrman, MIKEY: Multimedia Internet KEYing, Internet Draft,
http://www.ietf.org/proceedings/03mar/I-D/draft-ietf-msec-mikey-
06.txt, February 2003, Work in progress.
[MSEC-Arch] Hardjono, T., and B. Weis, The Multicast Group Security
Architecture, RFC 3740 (Informational), March 2004.
[MVV] A.J.Menzes, P.C.van Oorschot, S.A. Vanstone, Handbook of
Applied Cryptography, CRC Press, 1996.
Internet Draft Group Key Management Architecture [PAGE 32]
Baugher, Canetti, Dondeti, Lindholm June 2004
[OFT] Balenson, D., D. McGrew, and A. Sherman, Key Management for
Large Dynamic Groups: One-Way Function Trees and Amortized
Initialization, draft-irtf-smug-groupkeymgmt-oft-00.txt, IRTF, August
2000, Work in progress.
[RFC2093] Harney, H., and C. Muckenhirn, Group Key Management
Protocol (GKMP) Specification, RFC 2093 (experimental), July 1997.
[RFC2094] Harney, H., and C. Muckenhirn, Group Key Management
Protocol (GKMP) Architecture, RFC 2094 (experimental), July 1997.
[RFC2326] Schulzrinee, H., A. Rao, and R. Lanphier, Real Time
Streaming Protocol (RTSP), RFC 2326 (Proposed Standard), April 1998.
[RFC2327] Handley, M., and V. Jacobson, SDP: Session Description
Protocol, RFC 2327 (Proposed Standard), April 1998.
[RFC2367] McDonald, D., C. Metz, and B. Phan, PF_KEY Key Management
API, Version 2, RFC 2367 (Informational), July 1998.
[RFC2401] Kent, S., and R. Atkinson, Security Architecture for the
Internet Protocol, RFC 2401 (proposed standard), November 1998.
[RFC2406] Kent, S., and R. Atkinson, IP Encapsulating Security
Payload (ESP), RFC 2406 (proposed standard), November 1998.
[RFC2408] Maughan, D., et al., Internet Security Association and Key
Management Protocol (ISAKMP), RFC 2408 (proposed standard), November
1998.
[RFC2409] Harkins, D., and D. Carrel, The Internet Key Exchange
(IKE), RFC 2409 (proposed standard), November 1998.
[RFC2412] H. Orman, The OAKLEY Key Determination Protocol, RFC 2412
(Informational), November 1998.
[RFC2522] Karn, P., and W. Simpson, Photuris: Session-Key Management
Protocol, RFC 2522 (Informational), March 1999.
[RFC2543] Handley, M., et. al., SIP: Session Initiation Protocol,
RFC 2543 (Proposed Standard), March 1999.
[RFC2627] Wallner, D., E. Harder, and R. Agee, Key Management for
Multicast: Issues and Architectures, RFC 2627(informational), IETF,
June 1999.
[RFC3547] M. Baugher, T. Hardjono, H. Harney, B. Weis, The Group
Domain of Interpretation, RFC 3547 (Proposed Standard), July 2003.
Internet Draft Group Key Management Architecture [PAGE 33]
Baugher, Canetti, Dondeti, Lindholm June 2004
[RFC3550] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, RTP:
A Transport Protocol for Real-Time Applications, RFC 3550 (Proposed
Standard), July 2003.
[RFC3711] Baugher, M., et. al., The Secure Real Time Transport
Protocol, RFC 3711 (Proposed Standard), March 2004.
[SD1] Naor, D., M. Naor, and J. Lotspiech, Revocation and Tracing
Schemes for Stateless Receivers, in Advances in Cryptology - CRYPTO,
Santa Barbara, CA: Springer-Verlag Inc., LNCS 2139, August 2001.
[SD2] Moni Naor and Benny Pinkas, Efficient Trace and Revoke
Schemes, In Proceedings of Financial Cryptography 2000, Anguilla,
British West Indies, February 2000.
[Self-Healing] Staddon, J., et. al., Self-healing Key Distribution
with Revocation, In proceedings of the 2002 IEEE Symposium on
Security and Privacy, Oakland, CA, May 2002.
[SKEME] H. Krawczyk, SKEME: A Versatile Secure Key Exchange
Mechanism for Internet, ISOC Secure Networks and Distributed Systems
Symposium, San Diego, 1996.
[STS] Diffie, P. van Oorschot, M. J. Wiener, Authentication and
Authenticated Key Exchanges, Designs, Codes and Cryptography, 2,
107-125 (1992), Kluwer Academic Publishers.
[TAXONOMY] R. Canetti et al, Multicast Security: A taxonomy and some
Efficient Constructions, IEEE INFOCOM, 1999.
[TESLA-INFO] Perrig, A., R. Canetti, D. Song, D. Tygar, and B.
Briscoe, TESLA: Multicast Source Authentication Transform
Introduction, http://www.ietf.org/proceedings/03mar/I-D/draft-ietf-
msec-tesla-intro-01.txt, October 2002, Work in Progress.
[TESLA-SPEC] Perrig, A., R. Canetti, and Whillock, TESLA: Multicast
Source Authentication Transform Specification,
http://www.ietf.org/proceedings/03mar/I-D/draft-ietf-msec-tesla-spec-
00.txt, April 2002, Work in Progress.
[tGSAKMP] Harney, H., et. al., Tunneled Group Secure Association Key
Management Protocol, http://www.ietf.org/internet-drafts/draft-ietf-
msec-tgsakmp-00.txt, May 2003, Work in Progress.
[TPM] D.S. Marks, B.H. Turnbull, Technical protection measures: The
intersection of technology, law, and commercial licenses, Workshop
on Implementation Issues of the WIPO Copyright Treaty (WCT) and the
WIPO Performances and Phonograms Treaty (WPPT), World Intellectual
Property Organization, Geneva, December 6 and 7, 1999
(http://www.wipo.org/eng/meetings/1999/wct_wppt/pdf/imp99_3.pdf).
Internet Draft Group Key Management Architecture [PAGE 34]
Baugher, Canetti, Dondeti, Lindholm June 2004
[Wool] Wool. A., Key Management for Encrypted broadcast, 5th ACM
Conference on Computer and Communications Security, San Francisco,
CA, Nov. 1998.
Internet Draft Group Key Management Architecture [PAGE 35]
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11.0 Authors' Addresses
Mark Baugher
Cisco Systems
5510 SW Orchid St.
Portland, OR 97219, USA
+1 408-853-4418
mbaugher@cisco.com
Ran Canetti
IBM Research
30 Saw Mill River Road
Hawthorne, NY 10532, USA
+1 914-784-7076
canetti@watson.ibm.com
Lakshminath R. Dondeti
Nortel Networks
600 Technology Park Drive
Billerica, MA 01821, USA
+1 978-288-6406
ldondeti@nortelnetworks.com
Fredrik Lindholm
Ericsson Research
SE-16480 Stockholm, Sweden
+46 8 58531705
fredrik.lindholm@ericsson.com
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Baugher, Canetti, Dondeti, Lindholm June 2004
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Internet Draft Group Key Management Architecture [PAGE 38]
