Network Working Group T. Hardjono
Request for Comments: 3740 Verisign
Category: Informational B. Weis
Cisco
March 2004
The Multicast Group Security Architecture
Status of this Memo
This memo provides information for the Internet community. It does
not specify an Internet standard of any kind. Distribution of this
memo is unlimited.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document provides an overview and rationale of the multicast
security architecture used to secure data packets of large multicast
groups. The document begins by introducing a Multicast Security
Reference Framework, and proceeds to identify the security services
that may be part of a secure multicast solution.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Scope. . . . . . . . . . . . . . . . . . . . . . . . . . 2
1.2. Summary of Contents of Document. . . . . . . . . . . . . 3
1.3. Audience . . . . . . . . . . . . . . . . . . . . . . . . 4
1.4. Terminology. . . . . . . . . . . . . . . . . . . . . . . 4
2. Architectural Design: The Multicast Security Reference
Framework. . . . . . . . . . . . . . . . . . . . . . . . . . . 4
2.1. The Reference Framework. . . . . . . . . . . . . . . . . 4
2.2. Elements of the Centralized Reference Framework. . . . . 5
2.2.1. Group Controller and Key Server. . . . . . . . . 6
2.2.2. Sender and Receiver. . . . . . . . . . . . . . . 7
2.2.3. Policy Server. . . . . . . . . . . . . . . . . . 7
2.3. Elements of the Distributed Reference Framework. . . . . 8
3. Functional Areas . . . . . . . . . . . . . . . . . . . . . . . 9
3.1. Multicast Data Handling. . . . . . . . . . . . . . . . . 9
3.2. Group Key Management . . . . . . . . . . . . . . . . . . 10
3.3. Multicast Security Policies. . . . . . . . . . . . . . . 11
4. Group Security Associations (GSA). . . . . . . . . . . . . . . 12
4.1. The Security Association . . . . . . . . . . . . . . . . 12
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4.2. Structure of a GSA: Introduction . . . . . . . . . . . . 13
4.3. Structure of a GSA: Reasoning. . . . . . . . . . . . . . 14
4.4. Definition of GSA. . . . . . . . . . . . . . . . . . . . 15
4.5. Typical Compositions of a GSA. . . . . . . . . . . . . . 17
5. Security Services. . . . . . . . . . . . . . . . . . . . . . . 17
5.1. Multicast Data Confidentiality . . . . . . . . . . . . . 18
5.2. Multicast Source Authentication and Data Integrity . . . 18
5.3. Multicast Group Authentication . . . . . . . . . . . . . 19
5.4. Multicast Group Membership Management. . . . . . . . . . 19
5.5. Multicast Key Management . . . . . . . . . . . . . . . . 20
5.6. Multicast Policy Management. . . . . . . . . . . . . . . 21
6. Security Considerations. . . . . . . . . . . . . . . . . . . . 22
6.1. Multicast Data Handling. . . . . . . . . . . . . . . . . 22
6.2. Group Key Management . . . . . . . . . . . . . . . . . . 22
6.3. Multicast Security Policies. . . . . . . . . . . . . . . 22
7. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 23
8. References . . . . . . . . . . . . . . . . . . . . . . . . . . 23
8.1. Normative References . . . . . . . . . . . . . . . . . . 23
8.2. Informative References . . . . . . . . . . . . . . . . . 23
9. Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . 25
10. Full Copyright Statement . . . . . . . . . . . . . . . . . . . 26
1. Introduction
Securing IP multicast group communication is a complex task that
involves many aspects. Consequently, a secure IP multicast protocol
suite must have a number of functional areas that address different
aspects of the solution. This document describes those functional
areas and how they are related.
1.1. Scope
This architecture is concerned with the securing of large multicast
groups. Whereas it can also be used for smaller groups, it is not
necessarily the most efficient means. Other architectures (e.g., the
Cliques architecture [STW]) can be more efficient for small ad-hoc
group communication.
This architecture is "end to end", and does not require multicast
routing protocols (e.g., PIM [RFC2362]) to participate in this
architecture. Inappropriate routing may cause denial of service to
application layer groups conforming to this architecture. However
the routing cannot affect the authenticity or secrecy of group data
or management packets. The multicast routing protocols could
themselves use this architecture to protect their own multicast and
group packets. However, this would be independent of any secure
application layer group.
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This architecture does not require IP multicast admission control
protocols (e.g., IGMP [RFC3376], MLD [RFC3019]) to be a part of
secure multicast groups. As such, a "join" or "leave" operation for
a secure group is independent of a "join" or "leave" of an IP
multicast group. For example, the process of joining a secure group
requires being authenticated and authorized by a security device,
while the process of joining an IP multicast group entails contacting
a multicast-aware router. Admission control protocols could
themselves use this architecture to protect their own multicast
packets. However, this would be independent of any secure
application layer group.
This architecture does not explicitly describe how secure multicast
groups deal with Network Address Translation (NAT) [RFC2663].
Multicast routing protocols generally require the source and
destination addresses and ports of an IP multicast packet to remain
unchanged. This allows consistent multicast distribution trees to be
created throughout the network. If NAT is used in a network, then
the connectivity of senders and receivers may be adversely affected.
This situation is neither improved or degraded as a result of
deploying this architecture.
This architecture does not require the use of reliable mechanisms,
for either data or management protocols. The use of reliable
multicast routing techniques (e.g., FEC [RFC3453]) enhance the
availability of secure multicast groups. However the authenticity or
secrecy of group data or management packets is not affected by the
omission of that capability from a deployment.
1.2. Summary of Contents of Document
This document provides an architectural overview that outlines the
security services required to secure large multicast groups. It
provides a Reference Framework for organizing the various elements
within the architecture, and explains the elements of the Reference
Framework.
The Reference Framework organizes the elements of the architecture
along three Functional Areas pertaining to security. These elements
cover the treatment of data when it is to be sent to a group, the
management of keying material used to protect the data, and the
policies governing a group.
Another important item in this document is the definition and
explanation of Group Security Associations (GSA), which is the
multicast counterpart of the unicast Security Association (SA). The
GSA is specific to multicast security, and is the foundation of the
work on group key management.
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1.3. Audience
This document is addressed to the technical community, implementers
of IP multicast security technology, and others interested in gaining
a general background understanding of multicast security. This
document assumes that the reader is familiar with the Internet
Protocol, the IPsec suite of protocols (e.g., [RFC2401]), related
networking technology, and general security terms and concepts.
1.4. Terminology
The following key terms are used throughout this document.
1-to-N
A group which has one sender and many receivers.
Group Security Association (GSA)
A bundling of Security Associations (SAs) that together define how
a group communicates securely. The GSA may include a registration
protocol SA, a rekey protocol SA, and one or more data security
protocol SAs.
M-to-N
A group which has many senders and many receivers, where M and N
are not necessarily the same value.
Security Association (SA)
A set of policy and cryptographic keys that provide security
services to network traffic that matches that policy.
2. Architectural Design: The Multicast Security Reference Framework
This section considers the complex issues of multicast security in
the context of a Reference Framework. This Reference Framework is
used to classify functional areas, functional elements, and
interfaces. Two designs of the Reference Framework are shown: a
centralized design, and a distributed design that extends the
centralized design for very large groups.
2.1. The Reference Framework
The Reference Framework is based on three broad functional areas (as
shown in Figure 1). The Reference Framework incorporates the main
entities and functions relating to multicast security, and depicts
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the inter-relations among them. It also expresses multicast security
from the perspective of multicast group types (1-to-N and M-to-N),
and classes of protocols (the exchanged messages) needed to secure
multicast packets.
The aim of the Reference Framework is to provide some general context
around the functional areas, and the relationships between the
functional areas. Note that some issues span more than one
functional area. In fact, the framework encourages the precise
identification and formulation of issues that involve more than one
functional area or those which are difficult to express in terms of a
single functional area. An example of such a case is the expression
of policies concerning group keys, which involves both the functional
areas of group key management and multicast policies.
When considering the Reference Framework diagrams, it is important to
realize that the singular "boxes" in the framework do not necessarily
imply a corresponding singular entity implementing a given function.
Rather, a box in the framework should be interpreted loosely as
pertaining to a given function related to a functional area. Whether
that function is in reality implemented as one or more physical
entities is dependent on the particular solution. As an example, the
box labeled "Key Server" must be interpreted in broad terms as
referring to the functions of key management.
Similarly, the Reference Framework acknowledges that some
implementations may in fact merge a number of the "boxes" into a
single physical entity. This could be true even across functional
areas. For example, an entity in a group could act as both a Group
Controller and a Sender to a group.
The protocols to be standardized are depicted in the Reference
Framework diagrams by the arrows that connect the various boxes. See
more details in Section 4, below.
2.2. Elements of the Centralized Reference Framework
The Reference Framework diagram of Figure 1 contains boxes and
arrows. The boxes are the functional entities and the arrows are the
interfaces between them. Standard protocols are needed for the
interfaces, which support the multicast services between the
functional entities.
In some cases, a system implementing the multicast security
architecture may not need to implement protocols to account for every
interface. Rather, those interfaces may be satisfied through the use
of manual configuration, or even omitted if they are not necessary
for the application.
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There are three sets of functional entities. Each is discussed
below.
+--------------------------------------+
| |
| |
| FUNCTIONAL |
| AREAS |
| |
| +------+ |
| Multicast |Policy| |
| Security |Server| |
| Policies +------+ |
| ^ |
| | |
| | |
| v |
| +------+ |
| Group |Group | |
| Key |Ctrl/ |<---------+ |
| Management |Key | | |
| |Server| V |
| +------+ +--------+ |
| ^ | | |
| | |Receiver| |
| | | | |
| v +--------+ |
| +------+ ^ |
| | | | |
| Multicast |Sender|----------+ |
| Data | | |
| Handling | | |
| +------+ |
| |
+--------------------------------------+
Figure 1: Centralized Multicast Security Reference Framework
2.2.1. Group Controller and Key Server
The Group Controller and Key Server (GCKS) represent both the entity
and functions relating to the issuance and management of
cryptographic keys used by a multicast group. The GCKS also conducts
user-authentication and authorization checks on the candidate members
of the multicast group.
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The Key Server (KS) and the Group Controller (GC) have somewhat
different functionality and may in principle be regarded as separate
entities. Currently the framework regards the two entities as one
"box" in order to simplify the design, and in order not to mandate
standardization of the protocol between the KS and the GC. It is
stressed that the KS and GC need not be co-located. Furthermore,
future designs may choose to standardize the protocol between the GC
and the KS, without altering other components.
2.2.2. Sender and Receiver
The Sender is an entity that sends data to the multicast group. In a
1-to-N multicast group only a single sender is authorized to transmit
data to the group. In an M-to-N multicast group, two or more group
members are authorized to be senders. In some groups all members are
authorized as senders.
Both Sender and Receiver must interact with the GCKS entity for the
purpose of key management. This includes user and/or device
authentication, user and/or device authorization, the obtaining of
keying material in accordance with some key management policies for
the group, obtaining new keys during key-updates, and obtaining other
messages relating to the management of keying material and security
parameters.
Senders and Receivers may receive much of their policy from the GCKS
entities. The event of joining a multicast group is typically
coupled with the Sender/Receiver obtaining keying material from a
GCKS entity. This does not preclude the direct interaction between
the Sender/Receiver and the Policy Server.
2.2.3. Policy Server
The Policy Server represents both the entity and functions used to
create and manage security policies specific to a multicast group.
The Policy Server interacts with the GCKS entity in order to install
and manage the security policies related to the membership of a given
multicast group and those related to keying material for a multicast
group.
The interactions between the Policy Server and other entities in the
Reference Framework is dependent to a large extent on the security
circumstances being addressed by a given policy.
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2.3. Elements of the Distributed Reference Framework
The need for solutions to be scalable to large groups across wide
geographic regions of the Internet requires the elements of the
framework to also function as a distributed system. Figure 2 shows
how distributed designs supporting large group scalability fit into
the Reference Framework.
+-----------------------------------------------------------------+
| |
| |
| FUNCTIONAL |
| AREAS |
| +------+ +------+ |
| Multicast |Policy|<-------------------------------->|Policy| |
| Security |Server| |Server| |
| Policies +------+ +------+ |
| ^ ^ |
| | | |
| | | |
| v v |
| +------+ +------+ |
| Group |Group |<-------------------------------> |Group | |
| Key |Ctrl/ |<---------+ |Ctlr/ | |
| Management |Key | | |Key | |
| |Server| V |Server| |
| +------+ +--------+ +------+ |
| ^ | | ^ |
| | |Receiver| | |
| | | | | |
| v +--------+ | |
| +------+ ^ V |
| | | | +--------+ |
| Multicast |Sender|----------+ | | |
| Data | |-------------------------------->|Receiver| |
| Handling | | | | |
| +------+ +--------+ |
+-----------------------------------------------------------------+
Figure 2: Distributed Multicast Security Reference Framework
In a distributed design the GCKS entity interacts with other GCKS
entities to achieve scalability in the key management related
services. GCKS entities will require a means of authenticating their
peer GCKS entities, a means of authorization, and a means of
interacting securely to pass keys and policy.
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Similarly, Policy Servers must interact with each other securely to
allow the communication and enforcement of policies across the
Internet.
Two Receiver boxes are displayed corresponding to the situation where
both the Sender and Receiver employ the same GCKS entity (centralized
architecture) and where the Sender and Receiver employ different GCKS
entities (distributed architecture). In the distributed design, all
Receivers must obtain identical keys and policy. Each member of a
multicast group may interact with a primary GCKS entity (e.g., the
"nearest" GCKS entity, measured in terms of a well-defined and
consistent metric). Similarly, a GCKS entity may interact with one
or more Policy Servers, also arranged in a distributed architecture.
3. Functional Areas
The Reference Framework identifies three functional areas. They are:
- Multicast data handling. This area covers the security-related
treatments of multicast data by the sender and the receiver.
This functional area is further discussed in Section 3.1.
- Group Key Management. This area is concerned with the secure
distribution and refreshment of keying material. This
functional area is further discussed in Section 3.2.
- Multicast Security Policies. This area covers aspects of
policy in the context of multicast security, taking into
consideration the fact that policies may be expressed in
different ways: that they may exist at different levels in a
given multicast security architecture, and that they may be
interpreted differently according to the context in which they
are specified and implemented. This functional area is further
discussed in Section 3.3.
3.1. Multicast Data Handling
In a secure multicast group, the data typically needs to be:
1. Encrypted using the group key, mainly for access control and
possibly also for confidentiality.
2. Authenticated, for verifying the source and integrity of the
data. Authentication takes two flavors:
a. Source authentication and data integrity. This
functionality guarantees that the data originated with the
claimed source and was not modified en route (either by a
group member or an external attacker).
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b. Group authentication. This type of authentication only
guarantees that the data was generated (or last modified) by
some group member. It does not guarantee data integrity
unless all group members are trusted.
While multicast encryption and group authentication are fairly
standard and similar to encrypting and authenticating a point-to-
point communication, source authentication for multicast is
considerably more involved. Consequently, off-the-shelf solutions
(e.g., taken from IPsec [RFC2406]) may be sufficient for encryption
and group authentication. For source authentication, however,
special-purpose transformations are necessary. See [CCPRRS] for
further elaboration on the concerns regarding the data transforms.
Multicast data encrypted and/or authenticated by a sender should be
handled the same way by both centralized and distributed receivers,
(as shown in Figure 2).
The "Multicast Encapsulating Security Payload" [BCCR] provides the
definition for Multicast ESP for data traffic. The "Multicast Source
Authentication Transform Specification" [PCW] defines the use of the
TESLA algorithm for source authentication in multicast.
3.2. Group Key Management
The term "keying material" refers to the cryptographic keys belonging
to a group, the state associated with the keys, and the other
security parameters related to the keys. Hence, the management of
the cryptographic keys belonging to a group necessarily requires the
management of their associated state and parameters. A number of
solutions for specific issues must be addressed. These may include
the following:
- Methods for member identification and authentication.
- Methods to verify the membership to groups.
- Methods to establish a secure channel between a GCKS entity and
the member, for the purpose of delivery of shorter-term keying
material pertaining to a group.
- Methods to establish a long-term secure channel between one GCKS
entity and another, for the purpose of distributing shorter-term
keying material pertaining to a group.
- Methods to effect the changing of keys and keying material.
- Methods to detect and signal failures and perceived compromises to
keys and keying material.
The requirements related to the management of keying material must be
seen in the context of the policies that prevail within the given
circumstance.
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Core to the area of key management is Security Association (SA)
Management, which will be discussed further below.
A "Group Key Management Architecture" document [BCDL] further defines
the key management architecture for multicast security. It builds on
the Group Security Association (GSA) concept, and further defines the
roles of the Key Server and Group Controller.
"The Group Domain of Interpretation" [RFC3547], "GSAKMP" [GSAKMP],
and "MIKEY" [ACLNM] are three instances of protocols implementing the
group key management function.
3.3. Multicast Security Policies
Multicast Security Policies must provide the rules for operation for
the other elements of the Reference Framework. Security Policies may
be distributed in an ad-hoc fashion in some instances. However,
better coordination and higher levels of assurance are achieved if a
Policy Controller distributes Security Policies policy to the group.
Multicast security policies must represent, or contain, more
information than a traditional peer-to-peer policy. In addition to
representing the security mechanisms for the group communication, the
policy must also represent the rules for the governance of the secure
group. For example, policy would specify the authorization level
necessary in order for an entity to join a group. More advanced
operations would include the conditions when a group member must be
forcibly removed from the group, and what to do if the group members
need to resynchronize because of lost key management messages.
The application of policy at the Group Controller element and the
member (sender and receiver) elements must be described. While there
is already a basis for security policy management in the IETF,
multicast security policy management extends the concepts developed
for unicast communication in the areas of:
- Policy creation,
- High-level policy translation, and
- Policy representation.
Examples of work in multicast security policies include the Dynamic
Cryptographic Context Management project [Din], Group Key Management
Protocol [Har1, Har2], and Antigone [McD].
Policy creation for secure multicast has several more dimensions than
the single administrator specified policy assumed in the existing
unicast policy frameworks. Secure multicast groups are usually large
and by their very nature extend over several administrative domains,
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if not spanning a different domain for each user. There are several
methods that need to be considered in the creation of a single,
coherent group security policy. They include a top-down
specification of the group policy from the group initiator and
negotiation of the policy between the group members (or prospective
members). Negotiation can be as simple as a strict intersection of
the policies of the members or extremely complicated using weighted
voting systems.
The translation of policy rules from one data model to another is
much more difficult in a multicast group environment. This is
especially true when group membership spans multiple administrative
domains. Policies specified at a high level with a Policy Management
tool must be translated into more precise rules that the available
security policy mechanisms can both understand and implement. When
dealing with multicast communication and its multiple participants,
it is essential that the individual translation performed for each
participant result in the use of a mechanism that is interoperable
with the results of all of the other translations. Typically, the
translation from high-level policy to specific policy objects must
result in the same objects in order to achieve communication between
all of the group members. The requirement that policy translation
results in the same objects places constraints on the use and
representations in the high-level policies.
It is also important that policy negotiation and translation be
performed as an integral part of joining a group. Adding a member to
a group is meaningless if they will not be able to participate in the
group communications.
4. Group Security Associations (GSA)
4.1. The Security Association
A security association is a commonly used term in cryptographic
systems (e.g., [RFC2401, RFC2406bis, RFC2409]). This document uses
the term to mean any set of policy and cryptographic keys that
provide security services for the network traffic matching that
policy. A Security Association usually contains the following
attributes:
- selectors, such as source and destination transport addresses.
- properties, such as an security parameter index (SPI) or cookie
pair, and identities.
- cryptographic policy, such as the algorithms, modes, key
lifetimes, and key lengths used for authentication or
confidentiality.
- keys, such as authentication, encryption and signing keys.
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Group key management uses a different set of abstractions than
point-to-point key management systems (such as IKE [RFC2409]).
Notwithstanding, the abstractions used in the Group Key Management
functional area may be built from the point-to-point key management
abstractions.
4.2. Structure of a GSA: Introduction
Security associations (SAs) for group key management are more
complex, and are usually more numerous, than for point-to-point key
management algorithms. The latter establishes a key management SA to
protect application SAs (usually one or two, depending on the
protocol). However, group key management may require up to three or
more SAs. These SAs are described in later sections.
A GSA contains all of the SA attributes identified in the previous
section, as well some additional attributes pertaining to the group.
As shown in Figure 3, the GSA builds on the SA in two distinct ways.
- First, the GSA is a superset of an SA (Figure 3(a)). A GSA has
group policy attributes. For example, the kind of signed
credentials needed for group membership, whether group members
will be given new keys when a member is added (called "backward
re-key" below), or whether group members will be given new keys
when a member is removed from the group ("forward re-key"). A GSA
also includes an SA as an attribute of itself.
- Second, the GSA is an aggregation of SAs (Figure 3(b)). A GSA is
comprised of multiple SAs, and these SAs may be used for several
independent purposes.
+---------------+ +-------------------+
| GSA | | GSA |
| | | +-----+ +-----+ |
| | | | SA1 | | SA2 | |
| +----+ | | +-----+ +-----+ |
| | SA | | | +-----+ |
| +----+ | | | SA3 | |
| | | +-----+ |
+---------------+ +-------------------+
(a) superset (b) aggregation
Figure 3: Relationship of GSA to SA
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4.3. Structure of a GSA: Reasoning
Figure 4 shows three categories of SAs that can be aggregated into a
GSA.
+------------------------------------------------------------+
| |
| +------------------+ |
| | GCKS | |
| | | |
| | REG REG | |
| | / REKEY \ | |
| +---/-----|----\---+ |
| / | \ |
| / | \ |
| / | \ |
| / | \ |
| / | \ |
| +----------/------+ | +------\----------+ |
| | REG | | | REG | |
| | REKEY-----+----REKEY | |
| | Sender | | Receiver | |
| | DATA----------DATA | |
| +-----------------+ +-----------------+ |
| |
| |
+------------------------------------------------------------+
Figure 4: GSA Structure and 3 categories of SAs
The three categories of SAs are:
- Registration SA (REG): A separate unicast SA between the GCKS and
each group member, regardless of whether the group member is a
sender or a receiver or acting in both roles.
- Re-key SA (REKEY): A single multicast SA between the GCKS and all
of the group members.
- Data Security SA (DATA): A multicast SA between each multicast
source speaker and the group's receivers. There may be as many
data SAs as there are multicast sources allowed by the group's
policy.
Each of these SAs are defined in more detail in the next section.
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4.4. Definition of GSA
The three categories of SAs correspond to three different kinds of
communications commonly required for group communications. This
section describes the SAs depicted in Figure 4 in detail.
- Registration SA (REG):
An SA is required for (bi-directional) unicast communications
between the GCKS and a group member (be it a Sender or Receiver).
This SA is established only between the GCKS and a Member. The
GCKS entity is charged with access control to the group keys, with
policy distribution to members (or prospective members), and with
group key dissemination to Sender and Receiver members. This use
of a (unicast) SA as a starting point for key management is common
in a number of group key management environments [RFC3547, GSAKMP,
CCPRRS, RFC2627, BMS].
The Registration SA is initiated by the member to pull GSA
information from the GCKS. This is how the member requests to
join the secure group, or has its GSA keys re-initialized after
being disconnected from the group (e.g., when its host computer
has been turned off during re-key operations). The GSA
information pulled down from the GCKS is related to the other two
SAs defined as part of the GSA.
Note that this (unicast) SA is used to protect the other elements
of the GSA. As such, the Registration SA is crucial and is
inseparable from the other two SAs in the definition of a GSA.
However, the requirement of a registration SA does not imply the
need of a registration protocol to create that Registration SA.
The registration SA could instead be setup through some manual
means, such as distributed on a smart card. Thus, what is
important is that a Registration SA exists, and is used to protect
the other SAs.
From the perspective of one given GCKS, there are as many unique
registration SAs as there are members (Senders and/or Receivers)
in the group. This may constitute a scalability concern for some
applications. A registration SA may be established on-demand with
a short lifetime, whereas re-key and data security SAs are
established at least for the life of the sessions that they
support.
Conversely the registration SA could be left in place for the
duration of the group lifetime, if scalability is not an issue.
Such a long term registration SA would be useful for re-
synchronization or deregistration purposes.
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- Re-key SA (REKEY):
In some cases, a GCKS needs the ability to "push" new SAs as part
of the GSA. These new SAs must be sent to all group members. In
other cases, the GCKS needs the ability to quickly revoke access
to one or more group members. Both of these needs are satisfied
with the Re-key SA.
This Re-key SA is a unidirectional multicast transmission of key
management messages from the GCKS to all group members. As such,
this SA is known by the GCKS and by all members of the group.
This SA is not negotiated, since all the group members must share
it. Thus, the GCKS must be the authentic source and act as the
sole point of contact for the group members to obtain this SA.
A rekey SA is not absolutely required to be part of a GSA. For
example, the lifetime of some groups may be short enough such that
a rekey is not necessary. Conversely, the policy for the group
could specify multiple rekey SAs of different types. For example,
if the GC and KS are separate entities, the GC may deliver rekey
messages that adjust the group membership, and the KS may deliver
rekey messages with new DATA SAs.
- Data Security SA (DATA):
The Data Security SA protects data between member senders and
member receivers.
One or more SAs are required for the multicast transmission of
data-messages from the Sender to other group members. This SA is
known by the GCKS and by all members of the group.
Regardless of the number of instances of this third category of
SA, this SA is not negotiated. Rather, all group members obtain
it from the GCKS. The GCKS itself does not use this category of
SA.
From the perspective of the Receivers, there is at least one data
security SA for the member sender (one or more) in the group. If
the group has more than one data security SA, the data security
protocol must have a means of differentiating the SAs (e.g., with
a SPI).
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There are a number of possibilities with respect to the number of
data security SAs:
1. Each sender in the group could be assigned a unique data
security SA, thereby resulting in each receiver having to
maintain as many data security SAs as there are senders in the
group. In this case, each sender may be verified using source
origin authentication techniques.
2. The entire group deploys a single data security SA for all
senders. Receivers would then be able to maintain only one
data security SA.
3. A combination of 1. and 2.
4.5. Typical Compositions of a GSA
Depending on the multicast group policy, many compositions of a GSA
are possible. For illustrative purposes, this section describes a
few possible compositions.
- A group of memory-constrained members may require only a REG SA,
and a single DATA SA.
- A "pay-per-session" application, where all of the SA information
needed for the session may be distributed over a REG SA. Re-key
and re-initialization of DATA SAs may not be necessary, so there
is no REKEY SA.
- A subscription group, where keying material is changed as
membership changes. A REG SA is needed to distribute other SAs; a
REKEY SA is needed to re-initialize a DATA SA at the time
membership changes.
5. Security Services
This section identifies security services for designated interfaces
of Figure 2. Distinct security services are assigned to specific
interfaces. For example, multicast source authentication, data
authentication, and confidentiality occur on the multicast data
interface between Senders and Receivers in Figure 2. Authentication
and confidentiality services may also be needed between the Key
Server and group members (i.e., the Senders and Receivers of Figure
2), but the services that are needed for multicast key management may
be unicast as well as multicast. A security service in the Multicast
Security Reference Framework therefore identifies a specific function
along one or more Figure 2 interfaces.
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This paper does not attempt to analyze the trust relationships,
detailed functional requirements, performance requirements, suitable
algorithms, and protocol specifications for IP multicast and
application-layer multicast security. Instead, that work will occur
as the security services are further defined and realized in
algorithms and protocols.
5.1. Multicast Data Confidentiality
This security service handles the encryption of multicast data at the
Sender's end and the decryption at the Receiver's end. This security
service may also apply the keying material that is provided by
Multicast Key Management in accordance with Multicast Policy
Management, but it is independent of both.
An important part of the Multicast Data Confidentiality security
service is in the identification of and motivation for specific
ciphers that should be used for multicast data. Obviously, not all
ciphers will be suitable for IP multicast and application-layer
multicast traffic. Since this traffic will usually be connectionless
UDP flows, stream ciphers may be unsuitable, though hybrid
stream/block ciphers may have advantages over some block ciphers.
Regarding application-layer multicast, some consideration is needed
to consider the effects of sending encrypted data in a multicast
environment lacking admission-control, where practically any
application program can join a multicast event independently of its
participation in a multicast security protocol. Thus, this security
service is also concerned with the effects of multicast
confidentiality services (intended and otherwise) on application
programs. Effects to both Senders and Receivers are considered.
In Figure 2, the Multicast Data Confidentiality security service is
placed in Multicast Data Handling Area along the interface between
Senders and Receivers. The algorithms and protocols that are
realized from work on this security service may be applied to other
interfaces and areas of Figure 2 when multicast data confidentiality
is needed.
5.2. Multicast Source Authentication and Data Integrity
This security service handles source authentication and integrity
verification of multicast data. It includes the transforms to be
made both at the Sender's end and at the Receiver's end. It assumes
that the appropriate signature and verification keys are provided via
Multicast Key Management in accordance with Multicast Policy
Management as described below. This is one of the harder areas of
multicast security due to the connectionless and real-time
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requirements of many IP multicast applications. There are classes of
application-layer multicast security, however, where offline source
and data authentication will suffice. As discussed previously, not
all multicast applications require real-time authentication and
data-packet integrity. A robust solution to multicast source and
data authentication, however, is necessary for a complete solution to
multicast security.
In Figure 2, the Multicast Source and Data Authentication security
service is placed in Multicast Data Handling Area along the interface
between Senders and Receivers. The algorithms and protocols that are
produced for this functional area may have applicability to security
services in other functional area that use multicast services such as
Group Key Management.
5.3. Multicast Group Authentication
This security service provides a limited amount of authenticity of
the transmitted data: It only guarantees that the data originated
with (or was last modified by) one of the group members. It does not
guarantee authenticity of the data in case that other group members
are not trusted.
The advantage of group authentication is that it is guaranteed via
relatively simple and efficient cryptographic transforms. Therefore,
when source authentication is not paramount, group authentication
becomes useful. In addition, performing group authentication is
useful even when source authentication is later performed: it
provides a simple-to-verify weak integrity check that is useful as a
measure against denial-of-service attacks.
The Multicast Group Authentication security service is placed in the
Multicast Data Handling Area along the interface between Senders and
Receivers.
5.4. Multicast Group Membership Management
This security service describes the functionality of registration of
members with the Group Controller, and de-registration of members
from the Group Controller. These are security functions, which are
independent from IP multicast group "join" and "leave" operations
that the member may need to perform as a part of group admission
control protocols (i.e., IGMP [RFC3376], MLD [RFC3019]).
Registration includes member authentication, notification and
negotiation of security parameters, and logging of information
according to the policies of the group controller and the would-be
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member. (Typically, an out-of-band advertisement of group information
would occur before the registration takes place. The registration
process will typically be invoked by the would-be member.)
De-registration may occur either at the initiative of the member or
at the initiative of the group controller. It would result in
logging of the de-registration event by the group controller and an
invocation of the appropriate mechanism for terminating the
membership of the de-registering member (see Section 5.5).
This security service also describes the functionality of the
communication related to group membership among different GCKS
servers in a distributed group design.
In Figure 2, the Multicast Group Membership security service is
placed in the Group Key Management Area and has interfaces to Senders
and Receivers.
5.5. Multicast Key Management
This security service describes the functionality of distributing and
updating the cryptographic keying material throughout the life of the
group. Components of this security service may include:
- GCKS to group member (Sender or Receiver) notification
regarding current keying material (e.g., group encryption and
authentication keys, auxiliary keys used for group management,
keys for source authentication, etc.).
- Updating of current keying material, depending on circumstances
and policies.
- Termination of groups in a secure manner, including the secure
group itself and the associated keying material.
Among the responsibilities of this security service is the secure
management of keys between Key Servers and group members, the
addressing issues for the multicast distribution of keying material,
and the scalability or other performance requirements for multicast
key management [RFC2627, BMS]. Key Servers and group members may
take advantage of a common Public Key Infrastructure (PKI) for
increased scalability of authentication and authorization.
To allow for an interoperable and secure IP multicast security
protocol, this security service may need to specify host abstractions
such as a group security association database (GSAD) and a group
security policy database (GSPD) for IP multicast security. The
degree of overlap between IP multicast and application-layer
multicast key management needs to be considered. Thus, this security
service takes into account the key management requirements for IP
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multicast, the key management requirements for application-layer
multicast, and to what degree specific realizations of a Multicast
Key Management security service can satisfy both. ISAKMP, moreover,
has been designed to be extensible to multicast key management for
both IP multicast and application-layer multicast security [RFC2408].
Thus, multicast key management protocols may use the existing ISAKMP
standard's Phase 1 and Phase 2 protocols, possibly with needed
extensions (such as GDOI [RFC3547] or application-layer multicast
security).
This security service also describes the functionality of the
communication related to key management among different GCKS servers
in a distributed group design.
Multicast Key Management appears in both the centralized and
distributed designs as shown in Figure 2 and is placed in the Group
Key Management Area.
5.6. Multicast Policy Management
This security service handles all matters related to multicast group
policy including membership policy and multicast key management
policy. Indeed, one of the first tasks in further defining this
security service is identifying the different areas of multicast
policy. Multicast Policy Management includes the design of the
policy server for multicast security, the particular policy
definitions that will be used for IP multicast and application-layer
multicast security, and the communication protocols between the
Policy Server and the Key Server. This security service may be
realized using a standard policy infrastructure such as a Policy
Decision Point (PDP) and Policy Enforcement Point (PEP) architecture
[RFC2748]. Thus, it may not be necessary to re-invent a separate
architecture for multicast security policy. At minimum, however,
this security service will be realized in a set of policy
definitions, such as multicast security conditions and actions.
The Multicast Policy Management security service describes the
functionality of the communication between an instance of a GCKS to
an instance of the Policy Server. The information transmitted may
include policies concerning groups, memberships, keying material
definition and their permissible uses, and other information. This
security service also describes communication between and among
Policy Servers. Group members are not expected to directly
participate in this security service. However, this option is not
ruled out.
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6. Security Considerations
This document describes an architectural framework for protecting
multicast and group traffic with cryptographic protocols. Three
functional areas are identified within the framework. Each
functional area has unique security considerations, and these are
discussed below.
This architectural framework is end-to-end, and does not rely upon
the network that connects group controllers and group members. It
also does not attempt to resolve security issues in the unicast or
multicast routing infrastructures, or in multicast admission control
protocols. As such, denial of service, message deletion, and other
active attacks against the unicast or multicast routing
infrastructures are not addressed by this framework. Section 1.1
describes the relationship of the network infrastructure to the
multicast group security architecture.
6.1. Multicast Data Handling
Cryptographic protocols protecting multicast data are responsible for
providing confidentiality and group authentication. They should also
be able to provide source authentication to uniquely identify senders
to the group. Replay protection of multicast data is also desirable,
but may not always be possible. This is due to the complexity of
maintaining replay protection state for multiple senders. Section
3.1 elaborates on the security requirements for this area.
6.2. Group Key Management
Group key management protocols provide cryptographic keys and policy
to group members. They are responsible for authenticating and
authorizing group members before revealing those keys, and for
providing confidentiality and authentication of those keys during
transit. They are also responsible for providing a means for
rekeying the group, in the case that the policy specifies a lifetime
for the keys. They also are responsible for revocation of group
membership, once one or more group members have had their
authorization to be a group member revoked. Section 3.2 describes
the security requirements of this area in more detail.
6.3. Multicast Security Policies
Cryptographic protocols providing multicast security policies are
responsible for distributing that policy such that the integrity of
the policy is maintained. If the policy itself is confidential, they
also are responsible for authenticating group controllers and group
members, and providing confidentiality of the policy during transit.
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7. Acknowledgements
Much of the text in this document was derived from two research
papers. The framework for this document came from a paper co-
authored by Thomas Hardjono, Ran Canetti, Mark Baugher, and Pete
Dinsmore. Description of the GSA came from a document co-authored by
Hugh Harney, Mark Baugher, and Thomas Hardjono. George Gross
suggested a number of improvements that were included in later
versions of this document.
8. References
8.1. Normative References
[RFC2401] Kent, S. and R. Atkinson, "Security Architecture for the
Internet Protocol", RFC 2401, November 1998.
[RFC2408] Maughan, D., Shertler, M., Schneider, M. and J. Turner,
"Internet Security Association and Key Management
Protocol", RFC 2408, November 1998.
8.2. Informative References
[ACLNM] J. Arkko, et. al., "MIKEY: Multimedia Internet KEYing",
Work in Progress, December 2003.
[BCCR] M. Baugher, R. Canetti, P. Cheng, P. Rohatgi, "MESP: A
Multicast Framework for the IPsec ESP", Work in
Progress, October 2002.
[BCDL] M. Baugher, R. Canetti, L. Dondeti, F. Lindholm, "Group
Key Management Architecture", Work in Progress,
September 2003.
[BMS] D. Balenson, D. McGrew, A. Sherman, Key Management for
Large Dynamic Groups: One-Way Function Trees and
Amortized Initialization,
http://www.securemulticast.org/draft-balenson-
groupkeymgmt-oft-00.txt, Work in Progress, February
1999.
[CCPRRS] Canetti, R., Cheng P. C., Pendarakis D., Rao, J.,
Rohatgi P., Saha D., "An IPSec-based Host Architecture
for Secure Internet Multicast",
http://www.isoc.org/isoc/conferences/ndss/2000/
proceedings/028.pdf, NDSS 2000.
Hardjono & Weis Informational [Page 23]
RFC 3740 Multicast Group Security Architecture March 2004
[Din] Dinsmore, P., Balenson, D., Heyman, M., Kruus, P.,
Scace, C., and Sherman, A., "Policy-Based Security
Management for Large Dynamic Groups: An Overview of the
DCCM Project," DARPA Information Survivability
Conference and Exposition,
http://download.nai.com/products/media/nai/doc/discex-
110199.doc.
[GSAKMP] H. Harney, et. al., "GSAKMP", Work in Progress, October
2003.
[Har1] Harney, H. and C. Muckenhirn, "Group Key Management
Protocol (GKMP) Specification", RFC 2093, July 1997.
[Har2] Harney, H. and C. Muckenhirn, "Group Key Management
Protocol (GKMP) Architecture", RFC 2094, July 1997.
[McD] McDaniel, P., Honeyman, P., and Prakash, A., "Antigone:
A Flexible Framework for Secure Group Communication,"
Proceedings of the Eight USENIX Security Symposium, pp
99-113, August, 1999.
[PCW] Perrig, A., Canetti, R. and B. Whillock, TESLA:
Multicast Source Authentication Transform
Specification", Work in Progress, October 2002.
[RFC2362] Estrin, D., Farinacci, D., Helmy, A., Thaler, D.,
Deering, S., Handley, M., Jacobson, V., Liu, C., Sharma,
P. and L. Wei, "Protocol Independent Multicast-Sparse
Mode (PIM-SM): Protocol Specification", RFC 2362, June
1998.
[RFC2406] Kent, S. and R. Atkinson, "IP Encapsulating Security
Payload (ESP)", RFC 2406, November 1998.
[RFC2406bis] Kent, S., "IP Encapsulating Security Payload (ESP)",
Work in Progress, March 2003.
[RFC2409] Harkins, D. and D. Carrel, "The Internet Key Exchange
(IKE)", RFC 2409, November 1998.
[RFC2627] Wallner, D., Harder, E. and R. Agee, "Key Management for
Multicast: Issues and Architectures", RFC 2627,
September 1998.
[RFC2663] Srisuresh, P. and M. Holdrege, "IP Network Address
Translator (NAT) Terminology and Considerations", RFC
2663, August 1999.
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RFC 3740 Multicast Group Security Architecture March 2004
[RFC2748] Durham, D., Ed., Boyle, J., Cohen, R., Herzong, S.,
Rajan, R. and A. Sastry, "COPS (Common Open Policy
Service) Protocol", RFC 2748, January 2000.
[RFC3019] Haberman, B. and R. Worzella, "IP Version 6 Management
Information Base for The Multicast Listener Discovery
Protocol", RFC 3019, January 2001.
[RFC3376] Cain, B., Deering, S., Kouvelas, I., Fenner, B. and A.
Thyagarajan, "Internet Group Management Protocol,
Version 3", RFC 3376, October 2002.
[RFC3453] Luby, M., Vicisano, L., Gemmell, J., Rizzo, M., Handley,
M. and J. Crowcroft, "The Use of Forward Error
Correction (FEC) in Reliable Multicast", RFC 3453,
December 2002.
[RFC3547] Baugher, M., Weis, B., Hardjono, T. and H. Harney, "The
Group Domain of Interpretation", RFC 3547, December
2002.
[STW] M., Steiner, Tsudik, G., Waidner, M., CLIQUES: A New
Approach to Group key Agreement, IEEE ICDCS'98 , May
1998.
9. Authors' Addresses
Thomas Hardjono
VeriSign
487 E. Middlefield Rd.
Mountain View, CA 94043, USA
Phone:(650) 426-3204
EMail: thardjono@verisign.com
Brian Weis
Cisco Systems
170 W. Tasman Drive,
San Jose, CA 95134-1706, USA
Phone: (408) 526-4796
EMail: bew@cisco.com
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10. Full Copyright Statement
Copyright (C) The Internet Society (2004). This document is subject
to the rights, licenses and restrictions contained in BCP 78 and
except as set forth therein, the authors retain all their rights.
This document and the information contained herein are provided on an
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OR IS SPONSORED BY (IF ANY), THE INTERNET SOCIETY AND THE INTERNET
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INCLUDING BUT NOT LIMITED TO ANY WARRANTY THAT THE USE OF THE
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Internet Society.
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