Internet Engineering Task Force         Mark Baugher (Cisco)
INTERNET-DRAFT                          Ran Canetti (IBM)
                                        Lakshminath Dondeti (Nortel)

                                        October 23, 2001

Group Key Management 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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This document presents a group key-management architecture for MSEC.
The purpose of this document is to define the common architecture for
MSEC group key-management protocols that support a variety of
application, transport, and internetwork security protocols.  To
address these diverse uses, MSEC may need to standardize two or more
group key management protocols that have common requirements,
abstractions, overall design, and messages. The framework and
guidelines in this document allow for a modular and flexible design
of group key management protocols for a variety different settings
that are specialized to application needs.

Comments on this document should be sent to

Table of Contents

1.0 Introduction: Purpose of this Document      3
2.0 Requirements for a group key management protocol    3
3.0 Overall Design      5
3.1 Overview    5
3.2 Detailed description        7
3.3 Properties of the design    9
3.4 Implementation Diagram      9
4.0 Group Security Association  11
4.1 Group policy        11
4.2 Contents of the Re-key SA   12
4.3 Contents of the Data SA     14
5.0 Scalability Considerations  15
6.0 Security Considerations     17
7.0 References and Bibliography 18
8.0 Authors' Addresses  20
Appendix: MSEC Security Documents Roadmap       22

1.0 Introduction: Purpose of this Document

Group and multicast applications have diverse requirements in IP
networks [CP00].  Their key-management requirements, which are
briefly reviewed below (see "Requirements"), include support for
internetwork, transport, and application-layer protocols.  In
particular, while Internet-standard ISAKMP and IKE protocols purport
to manage keys for any and all services in a host, some applications
may achieve simpler operation by running key-management messaging
over TLS or IPsec security services.  For these reasons, application,
transport, and internetwork-layer security protocols such as SRTP,
IPsec, and AMESP may benefit from using different group key
management systems. Some security protocols will benefit from a key
management protocol that can run over IPsec or TLS [GSAKMP].  Other
security protocols may run over SIP or RTSP [KMMS]. Extensions to IKE
may be the best solution for running IPsec protocols over IP
multicast services [GDOI].  The purpose of this document is to define
a common architecture and design for these different group key-
management protocols for internet, transport, and application

Indeed, key-management protocols are difficult to design and
validate.  The common architecture described in this document eases
this burden by defining common abstractions and overall design that
can be specialized for different uses.

This document builds on and extends the Group Key Management Building
Block document of the IRTF SMuG research group [HBH01] and is part of
the MSEC document roadmap. To correctly place the current document in
the context of the MSEC literature we include a copy of the MSEC
draft tree in the appendix.

Section 2 discusses the security, performance and architectural
requirements for a group key management protocol. Section 3 presents
the overall architectural design. Section 4 specifies the interface
to the Group Security Association (GSA) using the standard keywords
of RFC 2119. Section 5 discusses scalability issues and Section 6
discusses Security Considerations.
2.0 Requirements for a group key management protocol

A group key management protocol supports multicast applications that
need a secure group.  A "secure group" is a collection of principals,
called "members," who may be senders, receivers or both receivers and
senders to other members of the group. (Note that group membership
may vary over time.) A "group key management protocol" helps to
ensure that only members of a secure group gain access to group data
(by gaining access to group keys) and can authenticate group data.
The goal of a group key management protocol is to provide legitimate
group members with the up-to-date cryptographic state they need for
their secrecy and authenticity requirements.

Multicast applications, such as video broadcast and multicast file
transfer, have the following key-management requirements (see also

1. The group members must receive "security associations" including
   encryption keys, authentication/integrity keys, cryptographic
   policy that describes the keys, and attributes such as
   an index for referencing the security association.
2. Keys will have a predetermined lifetime and will be periodically
3. Key material must be delivered securely to members of the
   group so that they are secret and authenticated to group members
   during the key lifetime and refreshed securely at the end of
   the key lifetime.
4. The key-management protocol must be secure against man-in-the-
   middle, connection-hijacking, and reflection/replay attacks; it
   must use best-known practices to thwart denial-of-service attacks
   (see Security Considerations section of this memo).
5. It must be possible to add and remove group members so that
   members who are added may optionally be denied access to the key
   material used before they joined the group, and that members who
   are removed lose access to the key material following their
6. It must be possible to provide re-key for the group without
   requiring unicast exchange between a group controller/key server
   (GCKS) and individual members, which would overwhelm a GCKS when
   the group is large.
7. The key management protocol must be suitable for IPsec security
   protocols, AH and ESP, and/or application-layer security protocols
   such as AMESP and SRTP.
8. The key management protocol should allow keys and algorithms to be
   renewed and the authorization infrastructure and authentication
   systems to be replaced.

Although it is not a requirement for a multicast security protocol,
the group key management protocol may also be useful to unicast
applications that share many of the requirements of multicast
applications, such as for Internet entertainment applications that
exhibit a high degree of synchronization among receivers.  For
example, consider a video on demand application scenario where the
top 10 movies are offered.  On a Friday night, a large population of
users is likely to request any one of the small set of movies (files
or streams) and place great demand on the GCKS for keys to the pre-
encrypted data.  If asymmetric cryptography is used to establish
security associations as is done in TLS or IKE, the GCKS will
probably not be able to exceed 20-60 key management sessions per
second (on a 500 MHz Pentium-class server with RSA, DSS, or EC-DSS
keys ranging from about 160 to 1024 bits in length).  If the video-
on-demand community of users is modeled as a group, however, then a
key management implementation that supports requirement 6, above,
will satisfy the requirements of the VOD application as well.

There are other requirements for small group operation where there
will be many senders or in which all members may potentially be
senders.  In this case, the group setup time may need to be optimized
to support a small, highly interactive group environment [RFC2627].
A single group controller (or GCKS) may not be the best design for
small, interactive groups.  But for large, single-source multicast
groups, it is generally undesirable to distribute key management
functions among group members: Unlike small, interactive groups,
large single-source multicast groups generally need a specialized
GCKS to support large numbers of group members.  Large distributed
simulations, moreover, may combine the need for large-group operation
with many senders.

We take as a basic requirement the support of large single-sender
groups, such as source-specific (single-source) multicast groups.
Thus, group key management should support high-capacity operation to
large groups that have one or very few senders.  Nonetheless,
scalable operation to a range of group sizes is a desirable feature,
and a better group key management protocol will support large,
single-sender groups as well as groups that have many senders. It may
be that no single key management protocol can satisfy the scalability
requirements of all group-security applications.   In this case, this
architecture allows two or more key management protocols, where each
protocol is suitable to a different scenario such large single-source
groups or small interactive groups.

In addition to these requirements, it is useful to emphasize two non-
requirements, namely, technical protection measures (TPM) and
broadcast key management.  TPM are used for such things as copy
protection by preventing the user of a device to get easy access to
the group keys. Although we should expect that a device under the
control of an attacker would lose its secrets to that attacker, some
TPM advocates see tamper-resistant technologies as a means to keep
honest people honest [MT] and want TPM for that purpose.  There is no
reason why a group key management protocol cannot be used in an
environment where the keys are kept in a "tamper-resistant" store
using various types of hardware or software to implement TPM.  The
group key management architecture described in this document,
however, is for key management protocols and not a design for
technical protection measures, which are outside the scope of this

The second non-requirement is broadcast key management where there is
no back channel [FN93, JKKV94] or the device is not on a network,
such as a digital videodisk player.  We assume IP network operation
where there is two-way communication, however asymmetric, and that
authenticated key-exchange procedures can be used for member
registration.  It is possible that broadcast applications can make
use of a one-way Internet group key management protocol message, and
a one-way Re-key message is described below.
3.0 Overall Design

This section describes the overall structure of a group key
management protocol, and provides a reference implementation diagram
for group key management.  This design is based upon a "group
controller" model [RFC2093, RFC2094, RFC2627, OFT, GSAKMP, GDOI] with
a single group owner as the root-of-trust.  The group owner
designates a group controller for member registration and re-key.
3.1 Overview

The main goal of a group key management protocol is to provide the
group members with an up-to-date security association (SA), which
contains the needed information for securing group communication
(i.e., the group data). We call this SA the "Data Security Protocol
SA", or "Data SA" for short. In order to obtain this goal, the Group
Key Management Architecture consists of the following protocols.

  (1) Registration protocol.
  This is a two-way unicast protocol between the group controller/key
server (GCKS) and a joining group member. In this protocol the GCKS
and joining member mutually authenticate each other. If the
authentication succeeds and the GCKS finds that the joining member is
authorized, then the GCKS supplies the joining member with the
following information:

    (a) Sufficient information to initialize a "Re-key Protocol SA"
within the joining member (see more details about this SA  below).
This information is given only in case that the group security policy
calls for using a Re-key protocol.
    (b) Sufficient information to initialize the Data Security
Protocol SA within the joining member. This information is given only
in the case that the group security policy calls for initializing the
Data Security Protocol SA at Registration, instead of or in addition
to at Re-key.

  The Registration Protocol must ensure that the transfer of
information from GCKS to member is done in an authenticated and
confidential manner over a security association.  We call this SA the
"Registration Protocol SA".

  (2) Re-key protocol.
  This is an optional protocol where the GCKS periodically sends re-
key information to the group members. Re-key messages may result from
group membership changes, the creation of new traffic-encrypting keys
(TEKs, see next section) for the particular Group, or from key
expiration. Re-key messages are protected by the Re-key protocol SA,
which is initialized in the Registration protocol. The Re-key message
includes information for updating both the Re-key protocol SA and/or
the Data Security Protocol SA.  The Re-key messages can be sent
multicast to group members or unicast from the GCKS to a particular
group member.

The Re-key protocol is optional as there are other means for managing
(e.g. expiring or refreshing) the keys locally without interaction
between the GCKS and member [MARKS].  When Re-key is used, it MUST
include authentication data for the re-key.  There are two cases.

 o The first and primary option is to use source authentication.
   That is, each group member MUST verify that Re-key data
   originates with the GCKS and none other.

 o The second option is to use only group-based authentication using
   a symmetric key, such as a message authentication code.  Members
   can only be assured that the Re-key messages originated within
   the group.  Therefore, this is applicable only when all members
   of the group are trusted not to impersonate the GCKS.  Group
   authentication for Re-key messages is typically used when public-
   key cryptography is not suitable for the particular group.

The Re-key protocol SHOULD ensure that all members receive the re-key
information in a timely manner. In addition, the Re-key protocol
SHOULD specify mechanisms for the parties to contact the GCKS and
"re-synch" in case that their keys expired and an updated key has not
yet been received.  The Re-key protocol must be able to avoid
implosion problems and ensure the needed reliability in its delivery
of keying material.

The Re-key message is protected by a Re-key SA, which may be
established by the Registration Protocol.  The Re-key SA, one or more
Data SAs, and the Registration SA SHOULD be destroyed by a member who
leaves the particular Group to which these SAs belong. Alternatively,
a member MAY allow the SAs to expire.  A particular group key
management protocol MAY provide a De-Registration message to inform
the GCKS that it has destroyed it SAs, or is about to destroy them.
Such a message may prompt the GCKS to cryptographically remove the
member from the group (i.e., to prevent the member from having access
to future group communication). In large-scale multicast
applications, however, De-registration has the potential to cause
implosion at the GCKS.  A De-registration message is not specified in
this document.
3.2 Detailed description

Figure 1 depicts the overall design [HBH01].  Each group member,
sender or receiver, MAY use the Registration Protocol to get
authorized, authenticated access to a particular Group, its policies,
and its keys. The two types of group keys are the KEK (key-encrypting
key) and the TEK (traffic-encrypting key).  The KEK may be a single
key that encrypts the TEK or it may be a vector of keys in a key
revocation algorithm [RFC2627, OFT, CP00, LNN01] that encrypts the
TEK and other KEKs.  The KEK is used by the Re-key protocol.  The TEK
is used by the Data Security Protocol to protect streams, files, or
other data sent and received by the Data Security Protocol.  Thus the
Registration Protocol and/or the Re-key Protocol establish the KEK
and TEK.
 There are a few, distinct outcomes to a successful Registration
Protocol exchange.
     o If the GCKS uses Re-key messages, then the admitted member
       receives the Group KEK; if it uses a key revocation
       algorithm, then the member receives a set of KEKs according to
       the particular algorithm.
     o If Re-key messages are not used for the Group, then the
       admitted member will receive TEKs (in SAs) that are passed to
       the member's Data Security Protocol as IKE does for IPsec.
     o The GCKS may pass one or more TEKS to the member even if Re-
       key messages are used, for efficiency reasons according to
       group policy.

| +-----------------+                          +-----------------+ |
| |     POLICY      |                          |  AUTHORIZATION  | |
| | INFRASTRUCTURE  |                          | INFRASTRUCTURE  | |
| +-----------------+                          +-----------------+ |
|         ^                                            ^           |
|         |                                            |           |
|         v                                            v           |
| +--------------------------------------------------------------+ |
| |                                                              | |
| |                    +--------------------+                    | |
| |            +------>|        GCKS        |<------+            | |
| |            |       +--------------------+       |            | |
| |            |                 |                  |            | |
| |       REGISTRATION           |             REGISTRATION      | |
| |         PROTOCOL             |               PROTOCOL        | |
| |            |                 |                  |            | |
| |            v               RE-KEY               v            | |
| |   +-----------------+     PROTOCOL     +-----------------+   | |
| |   |                 |    (OPTIONAL)    |                 |   | |
| |   |    SENDER(S)    |<-------+-------->|   RECEIVER(S)   |   | |
| |   |                 |                  |                 |   | |
| |   +-----------------+                  +-----------------+   | |
| |            |                                    ^            | |
| |            v                                    |            | |
| |            +-------DATA SECURITY PROTOCOL-------+            | |
| |                                                              | |
| +--------------------------------------------------------------+ |
|                                                                  |
FIGURE 1: Group Security Association Model

The GCKS creates the KEK and TEKs and downloads them to each member -
as the KEK and TEKs are common to the entire Group.  The GCKS is a
separate, logical entity that performs member authentication and
authorization according to the Group policy that is set by the Group
Owner.  The GCKS MAY present a credential to the Group member that is
signed by the Group Owner so the member can check the GCKS's
authorization.  The GCKS, which may be co-located with a member or be
a separate physical entity, MAY run the Re-key Protocol to push Re-
key messages of refreshed KEKs, new TEKs, and refreshed TEKs to
members.  Alternatively, the sender may forward Re-key messages on
behalf of the GCKS when it uses a credential mechanism that supports
delegation. Thus, it is possible for the sender or other member to
source keying material (a TEK encrypted in the Group KEK) as it
sources multicast or unicast data.  As mentioned above, the Re-key
message can be sent using unicast or multicast delivery.  Upon
receipt of a TEK from a Re-key Message or a Registration protocol
exchange, the member's group key management will provide a security
association (SA) to a Data Security Protocol for the data sent from
sender to receiver.

The "Security Protocol SA" protects the data sent on the arc labeled
"DATA SECURITY PROTOCOL" in Figure 1.  A second SA, the "Re-key
SA," is optionally established by the key-management protocol for Re-
key messages, and the arc labeled "RE-KEY PROTOCOL" in Figure 1
depicts this.  The Re-key message is optional because all keys, KEK
and TEKs, can be delivered by the Registration Protocol exchanges
shown in Figure 1, and those keys may not need to be updated.  The
Registration Protocol is protected by a third, symmetric, unicast SA
between the GCKS and each member; this is called the "Registration
Protocol SA."  There may be no need for the Registration Protocol SA
to remain in place after the completion of the Registration Protocol
exchanges.  The three SAs comprise the Group Security Association.
Only one SA is optional and that is the Re-key SA.

Figure 1 shows two blocks that are external to the group key
management protocol:  The Policy and Authorization Infrastructures
are discussed in Section 4.1.
3.3 Properties of the design

The design of Section 3.2 achieves scalable operation by (1) allowing
the de-coupling of authenticated key exchange in a "Registration
Protocol" from a "Re-key Protocol," (2) allowing the Re-key Protocol
to use unicast push or multicast distribution of group and data keys
as an option, and (3) allowing all keys to be obtained by the unicast
Registration Protocol.  The GCKS functionality, moreover, can be
delegated when the authorization infrastructure supports delegation
to permit distributed operation of the GCKS.

High-capacity operation is obtained by (1) amortizing
computationally-expensive asymmetric cryptography over multiple data
keys used by data security protocols, (2) supporting unicast push or
multicast distribution of symmetric group and data keys, and (3)
supporting key revocation algorithms such as LKH [RFC2627, OFT,
LNN01] that allow members to be added or removed at logarithmic
rather than linear space/time complexity.  The Registration protocol
often uses asymmetric cryptography to authenticate joining members
and optionally establish the group KEK.  Asymmetric cryptography such
as Diffie-Hellman key agreement and/or digital signatures are
amortized over the life of the group KEK:  A TEK SA can be
established without the use of asymmetric cryptography - the TEK is
simply encrypted in the symmetric KEK and sent unicast or multicast
in the Re-key protocol.

The design of the Registration and Re-key Protocols is flexible. The
Registration protocol establishes one KEK or multiple TEKs or both
KEK and TEKs.  The TEK (or "data key") is associated with a data
security protocol SA; there may in fact be multiple keys pushed with
or derived from the TEK.  The Re-key Protocol establishes KEKs or
TEKs or both.
3.4 Implementation Diagram

In the block diagram of Figure 2, group key management protocols run
between a GCKS and member principals to establish a Group Security
Association (GSA) consisting of a Security Protocol SA, an optional
Re-key SA, and a Registration Protocol SA.  The GCKS MAY use a
delegated principal, such as an SRTP [SRTP] sender, which has a
delegation credential signed by the GCKS.  The "Member" of Figure 2
may be a sender or receiver of multicast or unicast data [HCBD].
There are two functional blocks in Figure 2 labeled "GKM," and there
are two arcs between them depicting the group key-management
Registration ("reg") and Re-key ("rek") protocols.  The message
exchanges are the GSA establishment protocols, which are the
Registration Protocol and the Re-key Protocol described above.

   |                                                          |
   | +-------------+         +------------+                   |
   | |AUTHORIZATION|         |ANNOUNCEMENT|                   |
   | +------^------+         +------|-----+  +--------+       |
   |        |                       |  +-----| CRED   |       |
   |        |                       |  |     +--------+       |
   |   +----v----+             +----v--v-+   +--------+       |
   |   |         <-----Reg----->         |<->|  SAD   |       |
   |   |   GKM    -----Rek----->   GKM   |   +--------+       |
   |   |         |             |         |   +--------+       |
   |   |         ------+       |         |<->|  SPD   |       |
   |   +---------+     |       +-^-------+   +--------+       |
   |   +--------+      |         | |   |                      |
   |   | CRED   |----->+         | |   +-------------------+  |
   |   +--------+      |         | +--------------------+  |  |
   |   +--------+      |       +-V-------+   +--------+ |  |  |
   |   |  SAD   <----->+       |         |<->|  SAD   <-+  |  |
   |   +--------+      |       |SECURITY |   +--------+    |  |
   |   +--------+      |       |PROTOCOL |   +--------+    |  |
   |   |  SPD   <----->+       |         |<->|  SPD   <----+  |
   |   +--------+              +---------+   +--------+       |
   |                                                          |
   |     (A) GCKS                     (B) MEMBER               |
   Figure 2: Group key management block diagram for a host computer

Figure 2 shows that a complete group-key management functional
specification includes much more than the message exchange.  Some of
these functional blocks and the arcs between them are peculiar to an
operating system (OS) or vendor product, such as vendor
specifications for products that support updates to the IPsec
Security Association Database (SAD) and Security Policy Database
(SPD) [RFC2367].  Various vendors also define the functions and
interface of credential stores, "CRED" in Figure 2.  AUTHORIZATION is
subject to Group Policy [HH], but how this is done is specific to the
GCKS implementation.

Beside the AUTHORIZATION block in Figure 2, there is an ANNOUNCEMENT
block. The announcement function is needed to inform a potential
group member that it may join a group or receive group data (e.g. a
stream of file transfer protected by a Data Security protocol).
Announcements notify group members to establish multicast SAs in
advance of secure multicast data transmission.  Session Description
Protocol (SDP) is one form that the announcements might take
[RFC2327].  The announcement function may be implemented in a
session-directory tool, an electronic program guide (EPG), or by
other means.  The Data Security or the announcement function directs
group key management using an application-programming interface
(API), which is peculiar to the host OS in its specifics.  A generic
API for group key management is for further study, but this function
is necessary to allow Group (KEK) and Data (TEK) key establishment to
be done in a way that is scalable to the particular application.  A
GCKS application program will use the API to initiate the procedures
to establish SAs on behalf of a Security Protocol in which members
join secure groups and receive keys for streams, files or other data.

The goal of the exchanges is to establish a GSA through updates to
the SAD of a key-management implementation and particular Security
Protocol.  The "Security Protocol" of Figure 2 may span internetwork
and application layers [AMESP] or operate at the internetwork layer,
such as AH and ESP.
4.0 Group Security Association

The GKM Architecture defines the interfaces between the Registration,
Re-key, and Data Security protocols in terms of the Security
Associations (SAs) of those protocols.  By isolating these protocols
behind a uniform interface, our architecture allows implementations
to use protocols best suited to their needs.  For example, a Re-key
protocol for a small group could use multiple unicast transmissions
with symmetric authentication, while that for a large group could use
IP Multicast with packet-level Forward Error Correction and source

The Group Key Management Architecture provides an interface between
the security protocols and the group SA (GSA), which consists of
three SAs, viz., Registration SA, Re-key SA and Data SA.  The Re-key
SA is optional.  There are two cases in defining the relationships
between the three SAs.  In both cases, the Registration SA protects
the Registration protocol.

In Case 1, Group key management is done WITHOUT using a Re-key SA.
The Registration protocol initializes and updates one or more Data
SAs (having TEKs to protect files or streams).  Each Data SA
corresponds to a single group û and a group may have more than one
data SA.

In Case 2, group key management USES a Re-key SA to protect the Re-
key protocol. The Registration protocol initializes the Re-key SAs
(one or more) as well as zero or more Data SAs upon successful
completion.  When a Data SA is not initialized in the Registration
protocol, this MUST be done in the Re-key protocol.  The Re-key
protocol updates Re-key SA(s) AND establishes Data SA(s).
4.1 Group policy

Group-policy is currently being defined [GSPT].  It will likely be
distributed through both Announcement and Key Management protocols.
The group key management MUST carry cryptographic policies of the SA
keys it establishes and MAY carry additional policies for the group
as well.

The acceptable cryptographic policies for the Registration Protocol,
which may run over TLS, IPsec, or IKE, are not conveyed in the group
key-management protocol since they precede any of the key management
exchanges.  Thus, a security policy repository having some access
protocol may need to be queried prior to key-management session
establishment to determine what the initial cryptographic policies
are for that establishment.  This document assumes the existence of
such a repository and protocol for GCKS and member policy queries.
Thus group security policy will be represented in a policy repository
and accessible using a policy protocol.

This memo assumes that at least the following group-policy
information is
externally managed.
  o Group owner, authentication method, and delegation method for
    identifying a GCKS for the group
  o Group GCKS, authentication method, and any method used for
    delegating other GCKSs for the group
  o Group membership rules or list and authentication method

There are also two additional policy-related requirements external to
group key management.

  o There is an authorization and authentication infrastructure such
    as X.509, SPKI, or pre-shared key scheme in accordance with the
    group policy for a particular group.
  o There is an announcement mechanism for secure groups and events
    that operates according to group policy for a particular group.

Group policy determines how the Registration and Re-key protocols
initialize or update Re-key and Data SAs.  The following sections
describe the information that are sent by the GCKS for the Re-key and
Data SAs.  A member needs to have the information specified in the
next sections to establish Re-key and Data SAs.
4.2 Contents of the Re-key SA

The Re-key SA protects the Re-key protocol.  It contains
cryptographic policy, Security Parameter Index (SPI) [RFC2401] to
uniquely identify an SA, replay protection information, and secret

4.2.1 Re-key SA policy

The MEMBERSHIP MANAGEMENT ALGORITHM represents the group key
revocation algorithm that enforces forward and backward access
control.  Examples of key revocation algorithms include LKH, LKH+,
OFT, OFC and Subset Difference [RFC2627, OFT, CP00, LNN01].  The key
revocation algorithm could also be NULL.  In that case, the Re-key SA
contains only one KEK, which serves as the group KEK.  The Re-key
messages initialize or update Data SAs as usual.  But, the Re-key SA
itself can be updated (group KEK can be re-keyed) when members join
or the KEK is about to expire.  Leave re-keying is done by re-
initializing the Re-key SA through the Re-key Protocol.

The KEK ENCRYPTION ALGORITHM MUST use a standard encryption algorithm
such as 3DES or AES.  The KEK KEY LENGTH MUST also be specified.

authentication (since all shared secrets are known to some or all
members of the group), or it MAY use a symmetric secret in computing
MACs for group authentication, which provides weaker authentication
meaning that any group member can impersonate a particular source.

The CONTROL GROUP ADDRESS MUST be used for multicast transmission of
Re-key messages.  This MAY be sent via the ANNOUNCEMENT protocol.
However, the Group Owner MAY decide to send this information only to
authorized members.

The REKEY SERVER ADDRESS allows the registration server to be a
different entity from the server used for re-key, such as for future
invocations of the Registration and Re-key protocols.  If the
registration server and the re-key server are two different entities,
the registration server MUST send the re-key server's address as part
of the Re-key SA.

4.2.2 Group identity

The Group identity MUST accompany the SA (payload) information as an
identifier if the specific group key management protocol allows
multiple groups to be initialized in a single invocation of the
Registration protocol or multiple groups to be updated in a single
Re-key message.  While it MAY be much simpler to restrict each
Registration invocation to a single group, this Group Key Management
architecture mandates no such restriction.  There is always a need to
identify the group when establishing a Re-key SA either implicitly
through an SPI or explicitly as an SA parameter.

4.2.3 Key encrypting key(s)

Corresponding to the key management algorithm, the Re-key SA contains
one or more KEKs.  The GCKS holds the key encrypting keys of the
group, while the members receive keys following the specification of
the key-management algorithm.  When there are multiple KEKs for a
group (as in an LKH tree), each KEK MAY be associated with a Key ID,
which MAY be used to identify the key needed to decrypt it.  Each KEK
MUST have a LIFETIME associated with it, after which the KEK expires.

4.2.4 Authentication key

The GCKS MAY provide a symmetric or public key for authentication of
its Re-key messages.  Symmetric-key authentication is appropriate
only when all group members can be trusted not to impersonate the
GCKS.  The architecture does not rule out methods for deriving
symmetric authentication keys at the member [RFC2409] rather than
being pushed from the GCKS.

4.2.5 Replay protection information

Re-key messages MUST be protected from replay/reflection attacks.
Sequence numbers are used for this purpose and the Re-key SA (or
protocol) contains this information.

4.2.6 Security Parameter Index (SPI)

The triple (Group identity, SPI, an identifier for "Re-key SA")
uniquely identifies an SA.  The SPI changes each time the KEKs
4.3 Contents of the Data SA

The GCKS specifies the Data Security protocol used for secure
transmission of data from sender(s) to receiving members.  Examples
of Data Security protocols include IPsec ESP, SRTP, MESP, and AMESP.
While the content of each of these protocols is out of the scope of
this document, we list the information sent by the Registration
protocol (or the Re-key Protocol) to initialize or update the Data

4.3.1 Group identity

The Group identity MUST accompany SA information when Data SAs are
initialized or re-keyed for multiple groups in a single invocation of
the Registration protocol or in a single Re-key message (see 4.2.2).

4.3.2 Source identity

The Group Owner MAY choose to reveal Source identity to authorized
members only.  Alternatively, it MAY choose the ANNOUNCEMENT protocol
to list the Source(s) corresponding to the Source identities.  Thus
an SA MAY include source-identity information.

4.3.3 Traffic encrypting key

Irrespective of the Data Security Protocol used, the GCKS supplies
the TEKs (or information to derive TEKs) used in secure data
transmission, source and group authentication.

4.3.4 Authentication key

Depending on the data-authentication method used by the Data Security
protocol, group key management may pass one or more keys, functions
(e.g., TESLA), or other parameters used for authenticating streams or

4.3.5 Sequence numbers

The GCKS MAY initialize and pass sequence numbers used by the Data
Security protocol, for replay protection.

4.3.6 Security Parameter Index (SPI)

In the event that a Data Security protocol uses an SPI, the GCKS MUST
send that information as part of the Data SA contents.

4.3.7 Data SA policy

The Data SA parameters are specific to the Data Security Protocol but
SHOULD include encryption algorithm and parameters, the source
authentication algorithm and parameters, the group authentication
algorithm and parameters, and/or replay protection information.
Generally, specification of source or group authentication is
mutually exclusive.
5.0 Scalability Considerations

Group communications is quite diverse.  In commercial
teleconferencing, a multipoint control unit (MCU) may be used to
aggregate a number of teleconferencing members into a single session;
MCUs may be hierarchically organized as well.  A "loosely coupled"
teleconferencing session [RFC 1889] has no central controller but is
fully distributed and end-to-end.  Teleconferencing sessions tend to
have at most dozens of participants whereas video broadcast, which
uses multicast communications, and media on demand, which uses
unicast, are large-scale groups numbering hundreds to millions of

As described in the Requirements section above, group key management
must support source-specific multicast.  One-to-many (single-sender)
applications are well suited to source-specific multicast, which tend
to have large numbers of participants and problems with
synchronization among the participants.  "Flash crowds" are one
manifestation of the problem with synchronized participants who make
concurrent request for group data with concomitant requests for
secure group keys.  Thus, a group key management protocol designed
for single-source multicast applications must support large-scale
operation.  The architecture described in this paper supports large-
scale operation through the following features.

1. There is no need for a unicast exchange to provide data keys to a
security protocol for members who have previously-registered in the
particular group; data keys can be pushed in the Re-key protocol.

2. The Registration and Re-key protocols are separable to allow
flexibility in how members get group secrets.  A group can use a
smart-card based system in place of the Registration protocol, for
example, to allow the Re-key protocol to be used with no back channel
for broadcast applications such as television conditional access

3. The Registration and Re-key protocols should support new keys,
algorithms, authorization infrastructures and authentication
mechanisms in the architecture.  When the authorization
infrastructure supports delegation, as does X.509 and SPKI, the GCKS
function can be distributed as shown in Figure 3.

|       +-------+                        |
|       |  GCKS |                        |
|       +-------+                        |
|         |   ^                          |
|         |   |                          |
|         |   +---------------+          |
|         |       ^           ^          |
|         |       |    ...    |          |
|         |   +--------+  +--------+     |
|         |   | MEMBER |  | MEMBER |     |
|         |   +--------+  +--------+     |
|         v                              |
|         +-------------+                |
|         |             |                |
|         v      ...    v                |
|     +-------+   +-------+              |
|     |  GCKS |   |  GCKS |              |
|     +-------+   +-------+              |
|         |   ^                          |
|         |   |                          |
|         |   +---------------+          |
|         |       ^           ^          |
|         |       |    ...    |          |
|         |   +--------+  +--------+     |
|         |   | MEMBER |  | MEMBER |     |
|         |   +--------+  +--------+     |
|         v                              |
|        ...                             |
Figure 3: Hierarchically-organized Key Distribution

The first feature in the list allows fast keying of Data Security
protocols when the member already belongs to the group.  While this
is realistic for subscriber groups and customers of service providers
who offer content events, it may be too restrictive for applications
that allow member enrollment at the time of the event.  The recourse
for handling member registration in the context of a "flash crowd" is
Figure 3, which will require the use of many GCKSs to accommodate the
load.  The Figure 3 configuration may be needed when conventional
clustering and load-balancing solutions of a central GCKS site cannot
meet customer requirements.  Unlike conventional caching and content-
distribution networks, however, the configuration shown in Figure 3
has additional security ramifications for physical security of a

More analysis and work needs to be done on the protocol
instantiations of the Group Key Management architecture to determine
how effectively and securely the architecture can operate in large-
scale environments such as source-specific multicast and video on
demand.  Specifically, the requirements for a Figure 3 configuration
must be determined such as the need for additional protocols between
the GCKS designated by the Group Owner and GCKSs that have been
delegated to serve keys on behalf of the designated GCKS.

6.0 Security Considerations

This memo describes an architecture for group key management.  This
architecture will be instantiated in one or more group key management
protocols, which must be protected against man-in-the-middle,
connection hijacking, replay or reflection of past messages, and
denial of service attacks.

Authenticated key exchange [STS, SKEME, RFC2408, RFC2412, RFC2409]
techniques limit the effects of man-in-the-middle and connection-
hijacking attacks.  Sequence numbers and low-computation message
authentication techniques can be effective against replay and
reflection attacks. Cookies [RFC2522], when properly implemented,
provide an efficient means to reduce the effects of denial of service

This memo does not address attacks against key management or security
protocol implementations such as so-called "type attacks" that aim to
disrupt an implementation by such means as buffer overflow.  The
focus of this memo is on securing the protocol, not an implementation
of the protocol.

While classical techniques of authenticated key exchange can be
applied to group key management, new problems arise with the sharing
of secrets among a group of members:  Group secrets may be disclosed
by a member of the group and group senders may be impersonated by
other members of the group.  Key management messages from the GCKS
should not be authenticated using shared symmetric secrets unless all
members of the group can be trusted not to impersonate the GCKS.
Similarly, members who disclose group secrets undermine the security
of the entire group. Group Owners and GCKS administrators must be
aware of these inherent limitations of group key management.

Another limitation of group key management is policy complexity:
Whereas peer-to-peer security policy is an intersection of the policy
of the individual peers, a Group Owner sets group security policy
externally in secure groups.  This document assumes there is no
negotiation of cryptographic or other security parameters in group
key management.  Group security policy, therefore, poses new risks to
members who send and receive data from secure groups.  Security
administrators, GCKS operators, and users need to determine minimal
acceptable levels of trust, authenticity and confidentiality when
joining secure groups.  Unfortunately, group policy is at a very
early stage of development so little guidance is available to the
technical community at the present time.

Given the limitations and risks of group security, the security of
the group key management Registration protocol should be as good as
the base protocols on which it is developed such as IKE, IPsec, TLS,
or SSL.  The particular instantiations of this Group Key Management
architecture must ensure that the high standards for authenticated
key exchange are preserved in their protocol specifications, which
will be Internet standards-track documents that are subject to
review, analysis and testing.

The second protocol, the group key management Re-key protocol, is new
and has unknown risks associated with it.  The source-authentication
risks describe above are obviated by the use of public-key
cryptography.  The use of multicast delivery may raise additional
security issues such as reliability, implosion, and denial of service
attacks based upon the use of multicast.  The Re-key protocol
specification (see Appendix A for the drafts roadmap) needs to offer
secure solutions to these problems.  Each instantiation of the Re-key
protocol, such as the GSAKMP Re-key or the GDOI Groupkey-push
operations, need to validate the security of their Re-key

Novelty and complexity are the biggest risks to group key management
protocols.  Much more analysis and experience are needed to ensure
that the architecture described in this document can provide a well-
articulate standard for security and risks of group key management.

7.0 References and Bibliography

[AMESP] R. Canetti, P. Rohatgi, Pau-Chen Cheng, Multicast Data
Security Transformations: Requirements, Considerations, and Prominent
transforms.txt, Work In Progress, 2000.

[CP00] R. Canetti, B. Pinkas, A taxonomy of multicast security
01.txt, Work in Progress, August 2000.

[FN93]A. Fiat, M. Naor, Broadcast Encryption, Advances in Cryptology
- CRYPTO '93 Proceedings, Lecture Notes in Computer Science, Vol.
773, 1994, pp. 480û491.

[FS00] N. Ferguson and B. Schneier, A Cryptographic Evaluation of
IPsec, CounterPane,

[GDOI] M. Baugher, T. Hardjono, H. Harney, B. Weis, The Group Domain
of Interpretation,
msec-gdoi-00.txt, February 2001, Work in Progress.

[GSAKMP] H.Harney, A.Colegrove, E.Harder, U.Meth, R.Fleischer, Group
Secure Association Key Management Protocol,
00.txt, March 2001, Work in Progress.

[GSPT] T.Hardjono, H.Harney, P.McDaniel, A.Colegrove, P.Dinsmore,
Group Security Policy Token,
drafts/draft-ietf-msec-gspt-00.txt, Work in Progress, September 2001.

[HBH] H. Harney, M. Baugher, T. Hardjono, GKM Building Block: Group
Security Association (GSA) Definition,
00.txt, Work in Progress 2000.

[HBH01] Group Security Association Management in IP Multicast,  T.
Hardjono, M. Baugher, H. Harney, Proceedings of 16th IFIP/SEC
International Conference on Information Security, Paris, France May

[HCBD] T. Hardjono, R. Canetti, M. Baugher, P. Dinsmore, Secure IP
Multicast: Problem areas, Framework, and Building Blocks,,
Work in Progress 1999.

[HH] H. Harney, E. Harder, Group Secure Association Key Management
gsakmp-sec-02.txt, June 2000, Work in Progress.

[KMMS] E.Carrara, F.Lindholm, M.Naslund, K.Norman, J.Arko, Key
Management for Multimedia Sessions, draft-carrara-mm-key-mgt-sol-
00.txt, July 2001, Work in Progress

[LNN01] J.Lottspiech, M.Naor, D.Naor, Subset-Difference based Key
Management for Secure Multicast,
drafts/draft-irtf-smug-subsetdifference-00.txt, Work in Progress,

[JKKV94] M. Just, E. Kranakis, D. Krizanc, P. van Oorschot, On Key
Distribution via True Broadcasting, On Key Distribution via True
Broadcasting. In Proceedings of 2nd ACM Conference on Computer and
Communications Security, November 1994, pp. 81--88.

[MARKS] B. Briscoe, MARKS: Zero Side Effect Multicast Key Management
using Arbitrarily Revealed Key Sequences, Proceedings of NGC'99,

[MT] D.S. Marks, B.H. Turnbull, Technical protection measures:  The
intersection of technology, law, and commercial licenses, Workshop on
Implementation Issues of the WIPO Copyright Treaty (WCT) and the WIPO
Performances and Phonograms Treaty (WPPT), World Intellectual
Property Organization, Geneva, December 6 and 7, 1999


[OFT] D. Balenson, D. McGrew, A. Sherman, Key Management for Large
Dynamic Groups: One-Way Function Trees and Amortized Initialization,
00.txt, February 1999, Work in Progress.

[RFC1889] H. Schulzrinne, S. Casner, R. Frederick, V. Jacobson, RTP:
A Transport Protocol for Real-Time Applications, January 1996.

[RFC2093] Harney, H., and Muckenhirn, C., "Group Key Management
Protocol (GKMP) Specification," RFC 2093, July 1997.

[RFC2094] Harney, H., and Muckenhirn, C., "Group Key Management
Protocol (GKMP) Architecture," RFC 2094, July 1997.

[RFC2327] M. Handley, V. Jacobson, SDP: Session Description Protocol,
April 1998.

[RFC2367] D. McDonald, C. Metz, B. Phan, PF_KEY Key Management API,
Version 2, July 1998.

[RFC2401] S. Kent, R. Atkinson, Security Architecture for the
Internet Protocol, November 1998

[RFC2406] S. Kent, R. Atkinson, IP Encapsulating Security Payload
(ESP), November 1998.

[RFC2407] D. Piper, The Internet IP Domain of Interpretation for
ISAKMP, 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.

[RFC2412] H. Orman, The OAKLEY Key Determination Protocol, November

[RFC2522] P. Karn, W. Simpson, Photuris: Session-Key Management
Protocol, March 1999.

[RFC2627] D. M. Wallner, E. Harder, R. C. Agee, Key Management for
Multicast: Issues and Architectures, September 1998.

[Schneier] B. Schneier, Applied Cryptography, Second Edition, John
Wiley & Sons, 1996.

[SKEME] H. Krawczyk, SKEME: A Versatile Secure Key Exchange Mechanism
for Internet, ISOC Secure Networks and Distributed Systems Symposium,
San Diego, 1996.

[STS] Diffie, P. van Oorschot, M. J. Wiener, Authentication and
Authenticated Key Exchanges, Designs, Codes and Cryptography, 2, 107-
125 (1992), Kluwer Academic Publishers.

[SRTP] R.Blom, E.Carrara, D.McGrew, M.Nasland, K.Norrman, D. Oran,
The Secure Real Time Transport Protocol,,
February 2001, Work in Progress.

8.0 Authors' Addresses

Mark Baugher
Cisco Systems
5510 SW Orchid St.
Portland, OR  97219
(503) 245-4543

Ran Canetti
IBM Research
30 Saw Mill River Road
Hawthorne, NY 10532
(914) 784 7076

Lakshminath R. Dondeti
Nortel Networks
600 Technology Park Drive
Billerica, MA 01821, USA
(978) 288-6406

Appendix: MSEC Security Documents Roadmap

                         |     MSEC     |
                         | Requirements |
                         |     MSEC     |
                         | Architecture |
            :                    :                      :
    +--------------+     +--------------+      +--------------+
    |    Policy    |     |     GKM      |      | Data Security|
    | Architecture |     | Architecture |      | Architecture |
    +--------------+     +--------------+      +--------------+
                   :                    :                     :
                   :                    :                     :
                   .     +------------+ :      +------------+ :
                   .     |  GDOI      | :      |TESLA/MESP  | :
                         | Resolution |-:      |            |-:
                         |            | :      |            | :
                         +------------+ :      +------------+ :
                                        :                     :
                                        :                     :
                         +------------+ :      +------------+ :
                         | GSAKMP-    | :      |            | :
                         | Resolution |-:      |    TBD     |-:
                         |            | :      |            | :
                         +------------+ :      +------------+ :
                                        :                     :
                                        :                     :
                         +------------+ :      +------------+ :
                         |            | :      |            | :
                         |   RE-KEY   |-:      |    TBD     |-:
                         |            | :      |            | :
                         +------------+ :      +------------+ :
                                        :                     :
                                        .                     .
                                        .                     .

FIGURE A: Graphic rendition of the inter-relations between the I-D's
of MSEC. Note that some of these drafts are still in the process of
being written.