One document matched: draft-ietf-msec-arch-00.txt
Internet Engineering Task Force Thomas Hardjono (VeriSign)
INTERNET-DRAFT Brian Weis (Cisco)
draft-ietf-msec-arch-00.txt Expires May 2003
November 2002
The Multicast Security (MSEC) Architecture
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 provides a foundation for the protocols developed by
the Multicast Security (MSEC) group. The document begins by
introducing a Reference Framework, and proceeds to identify
functional areas which must be addressed as part of a secure
multicast solution.
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Table of Contents
1. Introduction.......................................................2
1.1 Summary of Contents of Document.................................2
1.2 Audience........................................................3
1.3 Related Documents...............................................3
2. Architectural Design: The MSEC Reference Framework.................4
2.1 A Reference Framework...........................................4
2.2 Elements of the Reference Framework.............................5
2.2.1 Group Controller and Key Server.............................5
2.2.2 Sender and Receiver.........................................6
2.2.3 Policy Server...............................................6
2.2.4 Centralized and Distributed Designs.........................7
3. Functional Areas...................................................7
3.1 Multicast Data..................................................7
3.2 Management of Keying Material...................................8
3.3 Multicast Security Policies.....................................8
4. Group Security Associations (GSA).................................10
4.1 SAs and Multicast..............................................10
4.1 Structure of a GSA: Reasoning..................................11
5. Security Services.................................................14
5.2.1 Multicast Data Confidentiality.............................14
5.2.2 Multicast Source Authentication and Data Integrity.........15
5.2.3. Multicast Group Authentication............................15
5.2.4 Multicast Group Membership Management......................16
5.2.5 Multicast Key Management...................................16
5.2.6 Multicast Policy Management................................17
6. MSEC Documents Roadmap............................................18
7. Conclusion........................................................19
8. Acknowledgments...................................................19
9. References........................................................19
9.1 Normative References...........................................19
9.2 Informative References.........................................20
Authors Addresses....................................................21
1. Introduction
Securing IP multicast 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 problem. This document describes those functional areas, and
protocols which have been developed which fit into those component
areas.
1.1 Summary of Contents of Document
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This document provides an architectural overview of the work being
conducted in the MSEC Working Group. It provides a Reference
Framework for covering the scope of the problems in multicast
security, and explains the elements of the Reference Framework.
The Reference Framework, in turn, provides the division of labor
along three Functional Areas pertaining to security. These cover the
treatment of data from a security perspective 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.
1.2 Audience
This document is addressed to the technical community and
implementers of IP multicast security technology 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. IPsec, IKE, ISAKMP),
related networking technology, and general security terms and
concepts.
1.3 Related Documents
Other documents provide detailed explanations of the Functional Areas
within the MSEC Reference Framework. These include the following set
of documents:
a. "Group Key Management Architecture" document [BCDL] -- a document
that provides the key management architecture for multicast
security, building on the Group Security Association (GSA)
concept defined in the current document.
b. "Group Domain of Interpretation" [BHHW] and "GSAKMP Light" [HSC],
which are two instances of protocols implementing the group key
management function.
c. "Multicast Encapsulating Security Payload" [BCCR], which provides
the definition for Multicast ESP, for data traffic.
d. "Multicast Source Authentication Transform Specification" [PCW],
which defines the use of the TESLA algorithm for source
authentication in multicast.
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2. Architectural Design: The MSEC Reference Framework
This section considers the complex problems of multicast security in
the context of a heuristic device, the Reference Framework diagram,
shown in Figure 1. The Reference Framework is used to classify
functional areas, functional elements, and interfaces.
2.1 A Reference Framework
Based on the three broad functional areas, a reference framework is
proposed (Figure 1). The reference framework attempts to incorporate
the main entities and functions relating to multicast security, and
to depict the inter-relations among them. At the same time, it also
tries to express the complex multicast security question from the
perspective of problem classification (i.e., the three functional
areas), from the perspective of architectures (centralized
distributed), of multicast types (1-to-M or M-to-N), and protocols
(the exchanged messages).
The aim of the reference framework is to provide some general context
within which functional areas can be identified and classified and
the relationships among the functional areas can be recognized. Note
that some issues span more than one so-called 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 (Figure 1) 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.
The reference framework can be viewed horizontally and vertically.
Horizontally, it displays both the entities and functions as singular
boxes, expressing each of the three broad functional areas.
Vertically, it expresses the basic architecture designs for
solutions, namely a centralized architecture and a distributed
architecture.
The protocols to be standardized are depicted in Figure 1 by the
arrows that connect the various boxes. See more details in Section 4,
below.
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+-----------------------------------------------------------------+
| CENTRALIZED \ DISTRIBUTED |
| DESIGNS \ DESIGNS |
| 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 1: MSEC Reference Framework
2.2 Elements of the 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. There are three sets of functional entities in
both centralized and distributed designs as discussed below.
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, which is subject to the
user-authentication and authorization checks conducted on the
candidate member of the multicast group.
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In a distributed architecture the GCKS entity also interacts with
other GCKS entities to achieve scalability in the key management
related services. In such a case, each member of a multicast group
may interact with one or more GCKS entity (say, the "nearest" GCKS
entity, measured in terms of a well-defined and consistent metric).
Similarly, in a distributed architecture a GCKS entity may interact
with one or more Policy Servers, also arranged in a distributed
architecture.
We remark that 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 allowed to transmit
data to the group. In an M-to-N multicast group, many (or even all)
group members can transmit data to the group.
Both Sender and Receiver must interact with the GCKS entity for the
purpose of key management. This includes user-authentication, 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.
The influence of policies on both Senders and Receivers is seen as
coming indirectly through the GCKS entities, since 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.
The reference framework displays two Receiver boxes 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).
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.
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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.
2.2.4 Centralized and Distributed Designs
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. This implies
that a GCKS entity must be able to interact securely with other
GCKS entities in a different location. Similarly, Policy Servers
must interact with each other securely to allow the communication and
enforcement of policies across the Internet.
3. Functional Areas
In order to begin to address the problems in securing IP multicast,
we identify three functional area emanating from the reference
framework. The three functional area are:
¡ Multicast data handling. This area covers problems concerning
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
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:
2.1 Source authentication and data integrity. This
functionality guarantees that the data originate with the
claimed source and was not modified en route (either by a
group member or an external attacker).
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2.2 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 point-to-point
communication, source authentication for multicast is considerably
more involved. Consequently, off-the-shelf solutions (e.g., taken
from IPSec [RFC2406], TLS [RFC2246]) may be sufficient for
encryption. For source authentication, however, special-purpose
transformations are necessary. See [CP99] for further elaboration
on the concerns regarding the data transforms, on present solutions
and remaining challenges.
3.2 Management of Keying Material
The term "keying material" refers to the cryptographic key 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 problems 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 needs related to the management of keying material must be seen
in the context of the policies that prevail within the given
circumstance.
Core to the problem of key management is Security Association (SA)
Management, which will be discussed further below.
3.3 Multicast Security Policies
Multicast Security Policies must provide the rules for operation for
the other elements of the Reference Framework. While much of the
work for the Multicast Security Policy area is focused in the Policy
Controller, there are potential areas for work in the application of
policy at the Group Controller element and the member (sender and
receiver) elements. While there is already a basis for security
policy management in the IETF between the Policy Working Group and
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the IP Security Policy Working Group, multicast security policy
management will extend 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,
if not spanning a different domain for each user. There are several
methods that need to be explored for 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.
High-level policy translation is much more difficult in a multicast
group environment, especially when group membership spans multiple
administrative domains. When policies are specified at a high level
with a Policy Management tool, they must then be translated into more
precise rules that the available security 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
implementation mechanisms must result in the same mechanism in order
to achieve communication between all of the group members. The
requirement that policy translation results in the same mechanism
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.
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. Policy must be established for the basic group operations of
add and remove, as well as more advanced operations such as leave,
rejoin, or resynchronize.
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4. Group Security Associations (GSA)
4.1 SAs and Multicast
It is clear that the security associations (SA) for group key
management are more complex, or at least more numerous, than for
Internet key management [RFC2409]. The latter establishes a key
management SA to protect application SAs (where a minimum of two are
needed to key an Internet application process). However, group key
management requires at least three: There is a registration SA
between the group member and the GCKS, a rekey SA between the GCKS
and all the group members, and an SA to protect application data from
sender-members to receiver-members. In fact, each sender to the
group may use a unique key for their data and use a separate SA:
there may be more SAs than there are group senders.
Group key management, therefore, uses a different set of abstractions
than ISAKMP and IKE. Notwithstanding, the abstractions used in our
Group Key Management functional area may be built from the ISAKMP
abstractions. In our approach the Group Security Association (GSA)
includes the attributes of the Internet Security Architecture SA,
which is succinctly defined as the encapsulation of keys and policies
[RFC2409] as follows.
- An SA has selectors, such as source and destination transport
addresses.
- An SA has properties, such as an security parameter index (SPI)
or cookie pair, and identities.
- An SA has cryptographic policy, such as the algorithms, modes,
key lifetimes, and key lengths used for authentication or
confidentiality.
- An SA has keys, such as authentication, encryption and signing
keys.
As is discussed in the next section of this memo, a GSA contains the
SA attributes plus some additional ones. As shown in Figure 2 (a),
the GSA is a superset of the SA.
¡ A GSA has group policy attributes, such as the kind of signed
credential needed for group membership and whether the group
will be given new keys when a member is added (called "backward
re-key" below) or whether group members will be given new key
when a member is removed from the group ("forward re-key").
- A GSA has SAs as attributes.
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+------------------------------------------------------------+
| |
| +---------------+ +-------------------+ |
| | GSA | | GSA | |
| | | | +-----+ +-----+ | |
| | | | | SA1 | | SA2 | | |
| | +----+ | | +-----+ +-----+ | |
| | | SA | | | +-----+ | |
| | +----+ | | | SA3 | | |
| | | | +-----+ | |
| +---------------+ +-------------------+ |
| |
| (a) superset (b) aggregation |
| |
+------------------------------------------------------------+
Figure 2: Relationship of GSA to SA
4.1 Structure of a GSA: Reasoning
There are three categories of SAs aggregated into a GSA in Figure
2(b). We choose this structure to better realize a GSA in our key
management environment. There is a need to maintain SAs between a Key
Server and a group member (either a sender, a receiver or both) and
among members. The Key Server is called the "GCKS," which is charged
with access control to the group keys, with policy distribution to
client members or prospective members, and with group key
dissemination to sender and receiver client members. This structure
is common in many group key management environments [HH, CP99,
RFC2627, BMS]. There are two SAs established between the GCKS and the
members, and there is an SA established among the sending and
receiving members as shown in Figure 3.
The first category of SA (namely REG in Figure 3, for "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 as described below). The GSA information pulled down from
the GCKS include the SA, keys and policy used to secure the data
transmission between sending and receiving members; this is DATA in
Figure 3, "for data security SA". Note that DATA is a category of
SA, and this implies that there may be multiple SAs established
between member senders and member receivers - at least as an option.
There may exist, for example, a single DATA SA in which all senders
share common keys and associated information. On the other hand,
there may be one or more DATA SAs that are unique to the particular
sender. A DATA SA may be reestablished or have its keys modified
through re-key operations, which occur over a REKEY SA (for "rekey
SA). Keys are pushed through a REKEY SA to support subscription
groups.
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Thus, despite the fact that the data to be protected are multicast,
registration exchanges through a REG SA should be unicast or point-
to-point key determination exchanges. Some group key management
solutions rely solely point-to-point. Most others combine unicast
exchanges for initialization with multicast distribution for re-key.
In some cases, such as in a pure "pay-per-session" application, all
of the SA information needed for the session may be distributed at
the time of registration or selection of a session, i.e. over a REG
SA; re-key and re-initialization may not be necessary, so there is no
REKEY SA. For subscription groups where keying material is changed
as membership changes, a REKEY SA is needed to re-initialize a DATA
SA.
+------------------------------------------------------------+
| |
| +------------------+ |
| | GCKS | |
| | | |
| | REG REG | |
| | / REKEY \ | |
| +---/-----|----\---+ |
| / | \ |
| / | \ |
| / | \ |
| / | \ |
| / | \ |
| +-------------/------+ | +------\-------------+ |
| | REG | | | REG | |
| | REKEY-----+----REKEY | |
| | MEMBER SENDER | | MEMBER RECEIVER| |
| | DATA----------DATA | |
| +--------------------+ +--------------------+ |
| |
| |
+------------------------------------------------------------+
Figure 3: GSA Structure and 3 categories of SAs
4.2 Definition of GSA
The GSA includes an aggregate of the three aforementioned categories
of SAs. The three categories of SAs correspond to the three kinds of
communications as seen from the point of view of the Receiver
(Member). Figure 3 depicts this concept:
- Registration (REG) SA:
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
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common in a number of group key management environments [HH,
CP99, RFC2627, BMS, Bris99].
Note that this (unicast) SA is used to protect the other elements
of the GSA (such as the other following two categories of SAs),
either in a "push" or "pull" model. As such, this SA is crucial
and is inseparable from the other two SAs as the definition of a
GSA.
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, so a registration SA may be used on-demand whereas
re-key and data security SAs are established at least for the
life of the sessions that they support.
- Re-key (REKEY) SA:
An SA is required for the multicast transmission of key
management messages (unidirectional) 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.
From the perspective of each participant in a group (GCKS and all
members), there is at least one registration SA for the group.
Note that this allows for the possibility of the GCKS deploying
multiple re-key SAs for group key management purposes.
- Data Security (DATA) SA:
One or more SAs are required for the multicast transmission of
data-messages (unidirectional) from the Sender to other group
members. This SA is known by the GCKS and by all members of the
group.
Similarly, 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.
This allows for the possibility of including group IDs (GID) in
transmission of data packets from the senders in the group.
There are a number of possibilities with respect to the number of
data security SAs and the use of Group IDs (GIDs):
(i) Each sender in the group could be assigned a unique dta
security SA, thereby resulting in each receiver having to
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maintain as many data security SAs as there are senders in
the group.
(ii) The entire group deploys a single data security SA for all
senders, together with the use of GIDs. Receivers would
then be able to filter based on the GIDs, whilst maintaining
only one data security SA.
(iii) A combination of (i) and (ii) above.
5. Security Services
Referring to our Reference Diagram, this section identifies security
services for designated interfaces of Figure 1. In this section,
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 1. Authentication and confidentiality
services may also be needed between the Key Server and key clients
(i.e., the Senders and Receivers of Figure 1), but the services that
are needed for multicast key management may be unicast as well as
multicast. A security service for multicast security, therefore,
identifies a specific function along one or more Figure 1 interfaces.
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, we propose these
tasks as future work that will occur as the functional building
blocks are further defined and realized in algorithms and protocols.
We identify a set of security services in the following sections.
This preliminary list of services is intended to serve as a basis for
discussion in the MSEC working group.
5.2.1 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 future work on the Multicast Data
Confidentiality building block 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. Those working on this security service will need to
evaluate the real-time and other requirements of multicast senders
and receivers, and recommend a small set of promising ciphers and
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data protocols for IP multicast and application-layer multicast data
confidentiality.
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 in all senders and receivers.
In Figure 1, 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 1 when multicast data confidentiality
is needed.
5.2.2 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. Work done by MSEC Working Group
members suggests that this is one of the harder areas of multicast
security. This is due to the connectionless and real-time
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 Whole Protocol solution
to multicast security.
In Figure 1, 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 building
blocks in other functional area that use multicast services such as
Group Key Management.
5.2.3. 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.
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MSEC Architecture October, 2002
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.
5.2.4 Multicast Group Membership Management
This security service describes the functionality of registration and
de-registration of members. 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 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 5.2.5).
This security service also describes the functionality of the
communication related to group membership among different GC+KS
servers in a distributed group design.
In Figure 1, the Multicast Group Membership security service is
placed in the Group Key Management Area and has interfaces to Senders
and Receivers.
5.2.5 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 building may include:
- GC+KS to Client (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.
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MSEC Architecture October, 2002
- Termination of groups in a secure manner, including the
multicast group itself and the associated keying material.
Among the problems to be solved by this security service is the
secure management of keys between Key Servers and Clients, the
addressing issues for the multicast distribution of keying material,
and the scalability or other performance requirements for multicast
key management [RFC2627, BMS].
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, work on this
security service must take into account the key management
requirements for IP 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 an ISAKMP Domain of
Interpretation for IP multicast or application-layer multicast
security).
This security service also describes the functionality of the
communication related to key management among different GC+KS servers
in a distributed group design.
Multicast Key Management appears in both the centralized and
distributed designs as shown in Figure 1 and is placed in the Group
Key Management Area.
5.2.6 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.
Thus, it may not be necessary to re-invent a separate architecture
for multicast security policy; we expect that this work will evaluate
use of the products of IETF efforts in the areas of network and
security policy. At minimum, however, this security service will be
Hardjono, Weis Expires May, 2003 17
MSEC Architecture October, 2002
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 GC+KS to
an instance 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. Thus, the Multicast Policy Management security
service is placed in Problem Area 3, along the interface between Key
Servers and Policy Servers. Group members are not expected to
directly participate in this security service. However, this option
is not ruled out.
6. MSEC Documents Roadmap
The roadmap of MSEC WG documents is shown the in the following.
+--------------+
| MSEC |
| Requirements |
+--------------+
:
:
+--------------+
| MSEC |
| Architecture |
+--------------+
:
.....................:.......................
: : :
+--------------+ +--------------+ +--------------+
| Policy | | GKM | | Data Security|
| Architecture | | Architecture | | Architecture |
+--------------+ +--------------+ +--------------+
: : :
: : :
. +------------+ : +------------+ :
. | GDOI | : |TESLA/MESP | :
| Resolution |-: | |-:
| | : | | :
+------------+ : +------------+ :
: :
: :
+------------+ : +------------+ :
| GSAKMP- | : | | :
| Resolution |-: | TBD |-:
| | : | | :
+------------+ : +------------+ :
: :
Hardjono, Weis Expires May, 2003 18
MSEC Architecture October, 2002
: :
+------------+ : +------------+ :
| | : | | :
| RE-KEY |-: | TBD |-:
| | : | | :
+------------+ : +------------+ :
: :
. .
. .
Figure 4: MSEC Document Roadmap
7. Conclusion
This document has provided an architectural overview of the work
being conducted in the MSEC Working Group and introduced several
important aspects of the standardization efforts in the MSEC WG.
A Reference Framework for covering the scope of the problems in
multicast security was introduced, and a division of labor along
three Functional Areas pertaining to security was discussed. These
cover the treatment of data from a security perspective 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.
This document also defined the notion of Group Security Associations
(GSA), which is the foundation of the work on group key management in
the MSEC Working Group.
8. Acknowledgments
This document was derived from an IRTF SMuG Working Group draft that
was originally co-authored by Thomas Hardjono, Ran Canetti, Mark
Baugher, and Pete Dinsmore.
9. References
9.1 Normative References
[BCDL] M. Baugher, R. Canetti, L. Dondeti, F. Lindholm, Group Key
Management Architecture, draft-ietf-msec-gkmarch-03.txt. IETF,
October 2002. Work in Progress.
[HSC] H. Harney, A. Schuett, A. Colegrove, GSAKMP Light. draft-ietf-
msec-gsakmp-light-sec-01.txt. IETF, July 2002. Work in Progress.
[BHHW] M. Baugher, T. Hardjono, H. Harney, B. Weis, The Group Domain
of Interpretation, draft-ietf-msec-gdoi-06.txt. IETF, February 2002.
Work in Progress.
Hardjono, Weis Expires May, 2003 19
MSEC Architecture October, 2002
[BCCR] M. Baugher, R. Canetti, P. Cheng, P. Rohatgi, MESP: Multicast
Encapsulating Security Payload, draft-ietf-msec-mesp-00.txt. IETF,
October 2002. Work in Progress.
[PCW] A. Perrig, R. Canetti, B. Whillock, TESLA: Multicast Source
Authentication Transform Specification. draft-ietf-msec-tesla-spec-
00.txt. IETF, October 2002. Work in Progress.
9.2 Informative References
[BMS] D. Balenson, D. McGrew, A. Sherman, Key Management for Large
Dynamic Groups: One-Way Function Trees and Amortized Initialization,
http://www.ietf.org/internet-drafts/draft-balenson-groupkeymgmt-oft-
00.txt, February 1999, Work in Progress.
[CP99] R. Canetti and B. Pinkas, A taxonomy of multicast security
issues, http://search.ietf.org/internet-drafts/draft-irtf-smug-
taxonomy-01.txt, April 1999, Work in Progress.
[Din] Dinsmore, P., Balenson, D., Heyman, M., Kruus, P., Scace, C.,
and Sherman, A., "Policy-Based Security Management for Large Dynamic
Groups: An Oerview of the DCCM Project," DARPA Information
Survivability Conference and Exposition, To Be Published.
[Har1] Harney, H., and Muckenhirn, C., "Group Key Management
Protocol (GKMP) Specification," RFC 2093, July 1997.
[Har2] Harney, H., and Muckenhirn, C., "Group Key Management
Protocol (GKMP) Architecture," RFC 2094, July 1997.
[HH] H. Harney, E. Harder, Group Secure Association Key Management
Protocol, http://search.ietf.org/internet-drafts/draft-harney-
sparta-gsakmp-sec-00.txt, April 1999, Work in Progress.
[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.
[RFC2246] Dierks, T. and C. Allen, The TLS Protocol Version 1.0,
RFC 2246, January 1999.
[RFC2406] S. Kent, R. Atkinson, IP Encapsulating Security Payload
(ESP),November 1998.
[RFC2408] D. Maughan, M. Shertler, M. Schneider, J. Turner, Internet
Security Association and Key Management Protocol, November 1998.
[RFC2409] D. Harkins, D. Carrel, The Internet Key Exchange (IKE),
November, 1998.
Hardjono, Weis Expires May, 2003 20
MSEC Architecture October, 2002
[RFC2627] D. M. Wallner, E. Harder, R. C. Agee, Key Management for
Multicast: Issues and Architectures, September 1998.
Authors Addresses
Thomas Hardjono
VeriSign
401 Edgewater Place, Suite 280
Wakefield, MA 01880
(781) 245-6996
thardjono@verisign.com
Brian Weis
Cisco Systems
170 W. Tasman Drive,
San Jose, CA 95134-1706, USA
(408) 526-4796
bew@cisco.com
Hardjono, Weis Expires May, 2003 21
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