One document matched: draft-ietf-msec-gkmarch-05.txt
Differences from draft-ietf-msec-gkmarch-04.txt
Internet Draft Mark Baugher (Cisco)
IETF MSEC WG Ran Canetti (IBM)
Expires: December 2003 Lakshminath Dondeti (Nortel)
Fredrik Lindholm (Ericsson)
June 27, 2003
Group Key Management Architecture
<draft-ietf-msec-gkmarch-05.txt>
Status of this Memo
This document is an Internet-Draft and is in full conformance
with all provisions of Section 10 of RFC2026.
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Abstract
This document presents the 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 of
different settings that are specialized to application needs.
Comments on this document should be sent to msec@securemulticast.org.
Baugher, Canetti, Dondeti, Lindholm June 2003
Table of Contents
Status of this Memo................................................1
Abstract...........................................................1
1.0 Introduction: Purpose of this Document.........................3
2.0 Requirements for a group key management protocol...............4
3.0 Overall Design.................................................6
3.1 Overview.....................................................6
3.2 Detailed description.........................................8
3.3 Properties of the design....................................10
3.4 Implementation Diagram......................................11
4.0 Registration Protocol.........................................13
4.1 Registration Protocol Message Exchange......................13
4.2 Properties of Alternative Registration Exchange Types.......14
4.3 Infrastructure for Alternative Registration Exchange Types..15
4.2 De-Registration Exchange....................................15
5.0 Rekey protocol................................................16
5.1 Goals of the Rekey protocol.................................16
5.2 Rekey messages..............................................17
5.3 Reliable transport of rekey messages........................17
5.4 Implosion...................................................18
5.5 Issues in incorporating group key management algorithms....19
5.5.1 Stateless vs. stateful rekeying..........................19
5.6 Interoperability of a GKMA..................................20
6.0 Group Security Association....................................21
6.1 Group policy................................................21
6.2 Contents of the Re-key SA...................................22
6.2.1 Re-key SA policy.........................................22
6.2.2 Group identity...........................................23
6.2.3 Key encrypting key(s)....................................23
6.2.4 Authentication key.......................................23
6.2.5 Replay protection information............................24
6.2.6 Security Parameter Index (SPI)...........................24
6.3 Contents of the Data SA.....................................24
6.3.1 Group identity...........................................24
6.3.2 Source identity..........................................24
6.3.3 Traffic encrypting key...................................24
6.3.4 Authentication key.......................................24
6.3.5 Sequence numbers.........................................25
6.3.6 Security Parameter Index (SPI)...........................25
6.3.7 Data SA policy...........................................25
7.0 Scalability Considerations....................................25
8.0 Security Considerations.......................................29
8.0 References and Bibliography...................................31
9.0 Authors' Addresses............................................34
Appendix: MSEC Security Documents Roadmap.........................35
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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 Section 2.0), include support for
internetwork, transport, and application-layer protocols. In
particular, while 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 a pre-established
secure channel (e.g., TLS, IPsec). Other security protocols may
benefit from a key management protocol that can run over already
deployed session initiation or management protocol (e.g., SIP or
RTSP). Finally, some may benefit from a light-weight keye management
protocol that finishes in fewest round trips. 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. The most popular key management protocols
deployed today provide support for a number of real-world scenarios
such as NAT traversal (e.g., IKE). Thus any general purpose group
key management protocol might also need to provide such
functionalities. 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 services.
The common architecture for group key management is called the "MSEC
Key Management Architecture" and is based on the group control or
key server model developed in GKMP [RFC2094] and assumed by group
key management algorithms such as LKH [RFC2627], OFT [McGrew and
Sherman], SDR [NNL], and MARKS. There are other approaches that are
not considered in this architecture such as the highly distributed
Cliques group key management protocol [Tsudik, et. al.] and
broadcast key management schemes [Fiat & Naor, Wool]. MSEC
(Multicast SECurity) key management may in fact be complementary to
other group key management designs, but these are not considered in
this document. The integration of MSEC group key management with
Cliques, broadcast key management and other group key systems is not
considered in this document, which seeks to provide a group key
management framework for the MSEC suite of multicast security
protocols.
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.
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Section 2 discusses the security, performance and architectural
requirements for a group key management protocol. Section 3 presents
the overall architectural design principles. Section 4 describes the
Registration protocol in detail and Section 5 does the same for Rekey
protocol. Section 6 considers the interface to the Group Security
Association (GSA) using the standard keywords of RFC 2119. Section 7
reviews the scalability issues for group key management protocols and
Section 8 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
[CP00]).
1. The group members receive "security associations" including
encryption keys, authentication/integrity keys, cryptographic
policy that describes the keys, and attributes such as an index
for referencing the security association (SA) or particular
objects contained in the SA.
2. Keys will have a predetermined lifetime and will be periodically
refreshed.
3. Key material are delivered securely to members of the group so
that they are secret, integrity-protected and can be verified as
coming from an authorized source.
4. The key-management protocol is also secure against replay attacks
and Denial of Service(DoS)attacks(see the Security Considerations
section of this memo).
5. The protocol adds and removes group members so that members who
are added may optionally be denied access to the key material used
before they joined the group, and that removed members lose access
to the key material following their departure.
6. The protocol supports a scalable group re-key operation without
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unicast exchange between members and a group controller/key
server, which might overwhelm a GCKS when the group is large.
7. The protocol is compatible with the infrastructure and performance
needs of the data-security application, such as IPsec security
protocols, AH and ESP, and/or application-layer security
protocols, AMESP and SRTP. (note: needs further clarification)
8. The key management protocol offers a framework for replacing or
renewing transforms, authorization infrastructure and
authentication systems.
9. The key management protocol must be secure against collusions
among excluded members and non-members. Specifically, collusions
must not result in gaining any additional group secrets than the
colluding entities themselves are privy to.
10. The key management protocol must provide a mechanism to securely
recover from a compromise of some or all of the key material.
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. In other words group key management protocols may be
used for protecting multicast communications, or communications in
groups where members communicate among themselves mainly via unicast.
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. However, large single-source multicast
groups generally may benefit from the use of a specialized GCKS.
Large distributed simulations, moreover, may combine the need for
large-group operation with many senders.
We also take as a 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. The group key
management 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.
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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
document.
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 securely
provide the group members with an up-to-date security association
(SA), which contains the needed information for securing group
communication (i.e., the group data). We call this SA the "Data
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
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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". A complementary "De-registration
protocol" serves to explicitly remove Registration Protocol SA state.
(2) Re-key protocol.
================
This is an optional protocol where a 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
(TPKs, 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 via
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]. The Re-key SA that is
established includes 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 verifies 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-
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key cryptography is not suitable for the particular group.
The Re-key protocol ensures that all members receive the re-key
information in a timely manner. In addition, the Re-key protocol
specifies mechanisms for the parties to contact the GCKS and "re-
synch" in case that their keys expired and an updated key has not yet
been received. The Re-key protocol for large-scale groups offers
mechanisms 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 is established
by the Registration Protocol. It is a recommended practice that a
member who leaves the group destroys the Re-key SA, one or more Data
SAs, and the Registration SA to which these SAs belong. Use of a De-
Registration message is often an efficient mechanisms for a member 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.
3.2 Detailed description
Figure 1 depicts the overall design [HBH01]. Each group member,
sender or receiver, uses the Registration Protocol to get authorized,
authenticated access to a particular Group, its policies, and its
keys. The two types of group keys are the KEK (key-encrypting key)
and the Traffic Protection Keys or TPKs (TPKs refer to both Traffic
Encryption Keys or TPKs, and Traffic integrity protection keys). The
KEK may be a single key that encrypts the TPKs or it may be a vector
of keys in a group key membership algorithm [RFC2627, OFT, CP00,
LNN01, SD] that encrypts the TPKs and other KEKs. The KEK is used by
the Re-key protocol. The TPKs are used by the Data Security Protocol
to protect streams, files, or other data sent and received by the
Data Security Protocol. Thus the Registration Protocol and/or the
Re-key Protocol establish the KEK and the TPKs.
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 group key management
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 TPKs (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 TPKs to the member even if Re-
key messages are used, for efficiency reasons according to
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Baugher, Canetti, Dondeti, Lindholm June 2003
group policy.
+------------------------------------------------------------------+
| +-----------------+ +-----------------+ |
| | POLICY | | AUTHORIZATION | |
| | INFRASTRUCTURE | | INFRASTRUCTURE | |
| +-----------------+ +-----------------+ |
| ^ ^ |
| | | |
| v v |
| +--------------------------------------------------------------+ |
| | | |
| | +--------------------+ | |
| | +------>| GCKS |<------+ | |
| | | +--------------------+ | | |
| | REGISTRATION or | REGISTRATION or | |
| | DE-REGISTRATION | DE-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 TPKs and downloads them to each member -
as the KEK and TPKs are common to the entire Group. The GCKS is a
separate, logical entity that performs member authentication and
authorization according to the Group policy that is set by the Group
Owner. The GCKS MAY present a credential to the Group member that is
signed by the Group Owner so the member can check the GCKS's
authorization. The GCKS, which may be co-located with a member or be
a separate physical entity, runs the Re-key Protocol to push Re-key
messages of refreshed KEKs, new TPKs, and refreshed TPKs 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 TPKs encrypted in the Group KEK) as it
sources multicast or unicast data. As mentioned above, the Re-key
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message can be sent using unicast or multicast delivery. Upon
receipt of a TPKs 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 TPKs, can be delivered by the Registration Protocol exchanges
shown in Figure 1, and those keys may not need to be updated. The
Registration Protocol is protected by a third, 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 De-registration protocol is also optional and is used
when explicit teardown or the SA is desirable (such as when a phone
call or conference terminates). 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 6.1.
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, (3) allowing all keys to be obtained by the unicast
Registration Protocol, and (4) delegating the functionality of the
GCKS among multiple entities, i.e., 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
may use asymmetric cryptography to authenticate joining members and
optionally establish the group KEK. Asymmetric cryptography such as
Diffie-Hellman key agreement and/or digital signatures are amortized
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over the life of the group KEK: A Data Security SA can be established
without the use of asymmetric cryptography - the TPKs are simply
encrypted in the symmetric KEK and sent unicast or multicast in the
Re-key protocol.
The design of the Registration and Re-key Protocols is flexible. The
Registration protocol establishes one KEK or multiple TPKs or both
KEK and TPKs. The TPKs(or ôdata keyö) is associated with a data
security protocol SA; there may in fact be multiple keys pushed with
or derived from the TPKs. The Re-key Protocol establishes KEKs or
TPKs or both.
3.4 Implementation Diagram
In the block diagram of Figure 2, group key management protocols run
between a GCKS and member principal to establish a Group Security
Association (GSA). The GSA consists of a 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.
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.
The CONTROL function directs the GCKS to establish a group, admit a
member or remove a member, or it directs a member to join or leave a
group. CONTROL includes authorization, which is subject to Group
Policy [HH], but how this is done is specific to the GCKS
implementation. CONTROL may be a telephony signaling protocol such
as SIP with the GCKS function operating on a caller's phone. For
large-scale multicast sessions, CONTROL could perform session
announcement functions to inform a potential group member that it may
join a group or receive group data (e.g. a stream of file transfer
protected by a Data Security protocol). Announcements notify group
members to establish multicast SAs in advance of secure multicast
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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 (TPKs) key establishment to be done in a way that is
scalable to the particular application. A GCKS application program
will use the API to initiate the procedures to establish SAs on
behalf of a Security Protocol in which members join secure groups and
receive keys for streams, files or other data.
+----------------------------------------------------------+
| |
| +-------------+ +------------+ |
| | CONTROL | | CONTROL | |
| +------^------+ +------|-----+ +--------+ |
| | | +-----| CRED | |
| | | | +--------+ |
| +----v----+ +----v--v-+ +--------+ |
| | <-----Reg-----> |<->| SAD | |
| | GKM -----Rek-----> GKM | +--------+ |
| | | | | +--------+ |
| | ------+ | |<->| SPD | |
| +---------+ | +-^-------+ +--------+ |
| +--------+ | | | | |
| | CRED |----->+ | | +-------------------+ |
| +--------+ | | +--------------------+ | |
| +--------+ | +-V-------+ +--------+ | | |
| | SAD <----->+ | |<->| SAD <-+ | |
| +--------+ | |SECURITY | +--------+ | |
| +--------+ | |PROTOCOL | +--------+ | |
| | SPD <----->+ | |<->| SPD <----+ |
| +--------+ +---------+ +--------+ |
| |
| (A) GCKS (B) MEMBER |
+----------------------------------------------------------+
Figure 2: Group key management block diagram for a host computer
The 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.
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4.0 Registration Protocol
The design of the Registration is flexible. The Registration
protocol establishes one Rekey SA or multiple Data Security SAs or
both Rekey and Data Security SAs. Traffic protection keys (or "data
keys") associated with a data security protocol SA; there may in
fact be multiple keys pushed with or derived from the TPKs. A
particular group key management protocol MAY restrict these many
options according to its particular requirements.
Each registration protocol supports different scenarios. The chosen
registration protocol solution reflects the specific requirements of
specific scenarios. In principle, it is possible to base a
registration protocol on any secure-channel protocol, such as IPsec
and TLS, which is the case in GSAKMP [GSAKMP]. However, registration
protocols that address other scenarios, such as GDOI [GDOI] and
MIKEY [MIKEY], use other methods to secure SA establishment. Some of
the different solutions that arise from specific scenarios are
discussed in the sections below. This document also refers in more
detail to the specific registration protocols GDOI and MIKEY, and
shows how these fit within the general architecture.
4.1 Registration Protocol Message Exchange
Some registration protocols need "tunnel" through a data-signaling
protocol. The reason may e.g. be to take advantage of already
existing (security) functionality, and/or to optimize the total
session setup time. For example, a telephone call has strict bounds
for delay in setup time; we donÆt like to wait a second longer than
we have to. It is not feasible to run security exchanges in parallel
with call setup since the latter often resolves the address: Call
setup must complete before the caller knows the address of the
callee. A better solution is to tunnel the key exchange procedures
inside call establishment [H.235, MIKEY] so both can complete (or
fail, see below) at the same time.
The registration protocol has different requirements depending on
the particular integration/tunneling approach. These requirements
are not necessarily security requirements, but will have an impact
on the chosen security solution. For example, the security
association will certainly fail if the call setup fails in the case
of IP telephony.
Conversely, the registration protocol imposes requirements on the
protocol that tunnels it. In the case of IP telephony, the call
setup usually will fail when the security association is not
successfully established. In the case of video-on-demand, protocols
such as RTSP that convey key management data will fail when a needed
security association cannot be established.
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Both GDOI and MIKEY use this approach, but in different ways. MIKEY
can be tunneled in SIP and RTSP. It takes advantage of the session
information contained in these protocols and the possibility to
optimize the setup time for the registration procedure. SIP requires
that a tunneled protocol must use at most one roundtrip (i.e. two
messages). This is also desirable requirement from RTSP as well.
The GDOI approach takes advantage of the already defined ISAKMP
phase 1 exchange [RFC2409], and extends the phase 2 exchange for the
registration. This is a good example of reusing security
functionality, where the defined phase 2 exchange is protected by
the SA created by phase 1. The GDOI will also inherent other
functionality of the ISAKMP. This may e.g. make the solution very
suitable for running IPsec protocols over IP multicast services.
4.2 Properties of Alternative Registration Exchange Types
The required design properties of a registration protocol has
different tradeoffs. A protocol that provides perfect forward
secrecy and identity protection trades security for performance or
efficiency, while a protocol that completes in one or two messages
may trade security functionality (e.g. identity protection) for
efficiency.
In a one- or two-message protocol, replay protection generally uses
either a timestamp or a sequence number. The first requires
synchronized clocks, while the latter requires that it is possible
to keep state. In a timestamp-based protocol, a replay cache is
needed to store the messages (or the hashes of the messages)
received within the allowable "clock skew". The size of the replay
cache depends on the number of messages received during the
allowable clock skew. During a DoS attack, the replay cache might
become overloaded. One solution is to over provision the replay
cache. However, this may lead to a large replay cache. Another
solution is to let the allowable clock skew be changed dynamically
during runtime. During a suspected DoS attack, the allowable clock
skew is then decreased so that the replay cache becomes manageable.
A challenge-response mechanism (using Nonce) obviates the need for
synchronized clocks for replay protection when the exchange uses
three or more messages [MVV]. This does not guarantee the replay
protection of individual messages (unless the protocols record all
Nonce), but on the exchange itself. "Cookies", such as stateless
cookies are means to protect against the replay of individual
messages [Photuris].
Additional security functions become possible as the number of
allowable messages in the registration protocol increase. ISAKMP
offers identity protection, for example, as part of a six-message
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exchange. With additional security features, however, comes added
complexity: Identity protection, for example, not only requires
additional messages, but may result in DoS vulnerabilities since
authentication is performed in a late stage of the exchange after
resources already have been devoted.
In all cases, there are tradeoffs with the number of message
exchanged, the desired security services, and the amount of
infrastructure that is needed to support the group key management
service. Whereas protocols that use two or even one-message setup
have low latency and computation requirements, they may require more
infrastructure such as secure time or offer less security such as
the absence of identity protection. What tradeoffs are acceptable
and what are not is very much dictated by the application and
application environment.
4.3 Infrastructure for Alternative Registration Exchange Types
The registration protocols need external infrastructures to be able
to handle authentication, replay protection, protocol-run integrity,
authorization and potentially other security services such as
secure, synchronized clocks. These may be solved by e.g. deploying a
PKI (with either authorization-based certificates or a separate
management for this). Other existing solutions may be employed such
as AAA infrastructure. Depending on the registration protocol and
its application, other external infrastructures may also be needed
e.g. timestamp-based protocols may need an infrastructure to
synchronize the clocks.
However, external infrastructures may not always be needed. This
could be the case when e.g. pre-shared keys are used and the
subscription base is very small. In a conversational multimedia
scenario (e.g. a VoIP call between two or more people), it may very
well be the end user who handles the authorization by manually
accepting/rejecting the incoming calls.
In general, protocols that use fewer messages require more
infrastructure (such as synchronized clocks) or fewer security
features such as PFS or identity protection.
4.2 De-Registration Exchange
The session-establishment protocol (e.g. SIP, RTSP) that conveys a
Registration exchange often has a session-disestablishment protocol
such as RTSP TEARDOWN [RFC2326] or SIP BYE [RFC2543]. The session-
disestablishment exchange between endpoints offers an opportunity to
signal the end of the GSA state at the endpoints. This "exchange"
need only be a uni-directional notification by one side that the GSA
is to be destroyed. Authentication of this notification can use a
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proof-of-possession of the group key(s) by one side to the other.
Some applications benefit from acknowledgement in a mutual, two-
message exchange signaling disestablishment of the GSA concomitant
with disestablishment of the session, e.g. RTSP or SIP session. In
this case, a two-way proof-of-possession might serve for mutual
acknowledgement of the GSA disestablishment.
5.0 Rekey protocol
Group Rekey protocol is for transport of keys and SAs between a GCKS
and the members of a secure communications group. The GCKS sends
Rekey messages to update a Rekey SA, or initialize/update a Data
Security SA or both. Rekey messages are protected by a Rekey SA. The
GCKS may update the Rekey SA when group membership changes or when
KEKs or TPKs expire. Recall that KEKs correspond to a Rekey SA and
TPKs correspond to a Data Security SA.
The following are some desirable properties of the Rekey protocol:
o Rekey protocol ensures that all members receive the rekey
information in a timely manner.
o Rekey protocol specifies mechanisms for the parties
involved, to contact the GCKS and re-sync when their keys expire
and no updates have been received.
o Rekey protocol avoids implosion problems and ensures the
needed reliability in delivering Rekey information.
We further note that the Rekey protocol is primarily responsible for
scalability of the group key management architecture. Hence it is
imperative that we provide the above listed properties in a scalable
manner. Note that solutions exist in the literature (both IETF
standards and research articles) for parts of the problem. For
instance, the Rekey protocol may use a scalable group key management
algorithm (GKMA) to reduce the number of keys sent in a rekey
message. Examples of a GKMA include LKH, OFT, Subset difference based
schemes etc.
5.1 Goals of the Rekey protocol
The goals of the Rekey protocol are:
o to synchronize a GSA
o to provide privacy and (symmetric or asymmetric)
authentication,
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o efficient rekeying after changes in group membership, or when
keys (KEKs) expire,
o (optional) reliable delivery of rekey messages
o high throughput and low latency, and
o to use IP Multicast or multi-unicast.
We identify five major issues in the design of a rekey protocol:
1. rekey message format
2. reliable transport of rekey messages
3. implosion
4. incorporating GKMAs in rekey messages
5. interoperability of GKMAs
Note that for a GCKS to successfully rekey a group, it is not
sufficient that Rekey protocol implementations interoperate. We also
need to ensure that the GKMA also interoperates.
In the rest of this section we discuss these issues in detail.
5.2 Rekey messages
Rekey messages are at the core of the rekey protocol. They contain
Rekey and/or Data Security SAs along with KEKs and TPKs. These
messages need to be confidential, authenticated, and protected
against replay attacks.
Rekey messages contain group key updates corresponding to a
single[LKH,OFT] or multiple membership changes[Subset, BatchRekey]
and often contain group key initialization messages [OFT].
5.3 Reliable transport of rekey messages
The GCKS needs to ensure that all members have the current Data
Security and Rekey SAs. Otherwise, authorized members may be
inadvertently excluded from receiving group communications. Thus,
the GCKS needs to use a rekey algorithm that is inherently reliable
or employ some reliable transport mechanism to send rekey messages.
There are two dimensions to the problem: Messages that update group
keys may be lost in transit or may be missed by a host when it is
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offline. LKH and OFT group key management algorithms rely on past
history of updates being received by the host. If the host is
offline, then it will need to resynchronize its group-key state,
which probably requires a unicast exchange with the GCKS. The
Subset Difference algorithm, however, conveys all needed state in
its re-key message and does not need members to be always on nor
always connected. Subset difference does not require a backchannel
and can operate on a broadcast network. Subset difference, however,
does need to have its key management message received by the member.
Thus Subset difference, LKH and OFT are not inherently reliable.
Reliable multicasting is a hard problem, but there are several
solutions in the literature. We discuss reliable transport of rekey
messages in this section.
Rekey messages are typically short (for single membership change as
well as for small groups) which makes it easy to design a reliable
delivery protocol. On the other hand, the security requirements
may add an additional dimension to address. Also there are some
special cases where membership changes are processed as a batch,
which reduces the frequency of rekey messages, but increases their
size. Furthermore, among all the KEKs sent in a rekey message,
as many as half the members need only a single KEK. We need to take
advantage of these properties in designing a rekey message(s) and
a protocol for their reliable delivery.
Three categories of solutions have been proposed:
1. Repeatedly transmit the rekey message: Recall that in many
cases rekey messages translate to only one or two IP packets.
2. Use an existing reliable multicast protocol/infrastructure
3. Use FEC for encoding rekey packets (with NACKs as
feedback) [BatchRekey]
Note that for small messages, category 3 is essentially the same as
category 1.
5.4 Implosion
Implosion may occur due to one of two reasons. First, recall that
one of the goals of the rekey protocol is to "synchronize a GSA."
When a rekey or data security SA expires, members may contact the
GCKS for an update. If all or even many members contact the GCKS at
about the same time, the GCKS cannot handle all those messages. We
refer to this as an "out-of-sync implosion."
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The second case is in the reliable delivery of rekey messages.
Reliable multicast protocols use feedback (NACK or ACK) to determine
which packets must be retransmitted. Packet losses may result in
many members sending NACKs to the GCKS. We refer to this as feedback
implosion.
The implosion problem has been studied extensively in the context of
reliable multicasting. Some of the proposed solutions viz., feedback
suppression and aggregation, might be useful in this context as well.
The GCKS might send each receiver a random number to be used as time
to wait before sending a NACK or out-of-sync message. Meanwhile,
members might receive the key updates they need and therefore will
not send a feedback message.
An alternative solution is to have the members contact one of several
registration servers when they are out-of-sync. This results in
repetition of the registration process for those members.
Furthermore, there is the need to setup multiple registration servers
and synchronize them.
Feedback aggregation and local recovery employed by some reliable
multicast protocols are not easily adaptable to transport of rekey
messages. There are authentication issues to address in aggregation.
Local recovery is more complex in that members need to establish SAs
with the local repair server.
5.5 Issues in incorporating group key management algorithms
Group key management algorithms make re-keying scalable. Large group
re-keying without employing GKMAs is prohibitively expensive.
First we list some requirements to consider in selecting a GKMA:
o Collusion: Members (or non members) should not be able to
collaborate to deduce keys that they are not privileged
(following the GKMA key distribution rules) to.
o Forward access control: Ensure that departing members cannot get
access to future group data.
o Backward access control: Ensure that joining members cannot
decrypt past data.
5.5.1 Stateless vs. stateful rekeying
We classify group key management algorithms into two categories,
viz., stateful and stateless algorithms.
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Stateful algorithms use KEKs from the ith rekeying instance to
encrypt (protect) KEKS corresponding to the i+1st rekeying instance.
The main disadvantage in these schemes is that if a member was
offline or otherwise fails to receive KEKs from a rekeying instance
i, it can no longer synchronize its GSA even though it can receive
KEKs from all future rekeying instances starting at i+1. The only
solution is to contact the GCKS explicitly for resynchronization.
Note that the KEKs for the first rekeying instance are protected by
the registration SA. Recall that communication in that phase is one
to one, and therefore it is easy to ensure reliable delivery.
Stateless GKMAs encrypt rekey messages with KEKs sent during the
registration protocol. Since rekey messages are independent of any
past rekey messages (i.e. not protected by KEKs therein), a member
may go offline, but continue to be able to decipher future
communications. The catch however is that members can never decrypt
any messages sent while they were offline, even though there are
eligible to (i.e. paid for that content as well). Stateless rekeying
may be relatively inefficient, particularly for immediate (in
contrast to batch) rekeying in highly dynamic groups.
5.6 Interoperability of a GKMA
Most GKMA specifications do not specify packet formats although any
group key management algorithms needs to for the purposes of
interoperability. In particular there are several alternative ways
to managing key trees and numbering nodes within key trees. The
following information is generally needed during initialization of a
rekey SA or included with each GKMA packet.
o GKMA name (e.g. LKH, OFT, Subset difference)
o GKMA version number (implementation specific). Version may imply
several things such as the degree of a key tree, proprietary
enhancements, and qualify another field such as a key id.
o Number of keys or Largest ID
o Version specific data
o Per key information
- Key ID
- Key lifetime (creation/expiration data)
- Encrypted key
- encryption key's ID (optional)
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Key IDs may change in some implementations in which case we need to
send:
o List of <old id, new id>
6.0 Group Security Association
The GKM Architecture defines the interfaces between the Registration,
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
authentication.
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 TPKs 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 is done in the Re-key protocol. The Re-key protocol
updates Re-key SA(s) AND establishes Data SA(s).
6.1 Group policy
Group-policy is currently being defined [GSPT]. It can be
distributed through announcement, key management protocols, and other
means. The group key management protocol carries cryptographic
policies of the SA keys it establishes as well as 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
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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 is 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.
6.2 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
keys.
6.2.1 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
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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 uses a standard encryption algorithm
such as 3DES or AES. The KEK KEY LENGTH is also specified.
The AUTHENTICATION ALGORITHM uses digital signatures for GCKS
authentication (since all shared secrets are known to some or all
members of the group), or some symmetric secret in computing MACs for
group authentication. Symmetric authentication provides weaker
authentication in that any group member can impersonate a particular
source. The AUTHENTICATION KEY LENGTH is also be specified.
The CONTROL GROUP ADDRESS is used for multicast transmission of Re-
key messages. This information is sent over the control channel such
as in an ANNOUNCEMENT protocol or call setup message. The degree to
which the control group address is protected is a matter of group
policy.
The REKEY SERVER ADDRESS allows the registration server to be a
different entity from the server used for 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 sends the re-key server's address as part of
the Re-key SA.
6.2.2 Group identity
The Group identity accompanies the SA (payload) information as an
identifier if the specific group key management protocol allows
multiple groups to be initialized in a single invocation of the
Registration protocol or multiple groups to be updated in a single
Re-key message. It is often 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.
6.2.3 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 needs to be associated with a Key
ID, which is used to identify the key needed to decrypt it. Each KEK
has a LIFETIME associated with it, after which the KEK expires.
6.2.4 Authentication key
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The GCKS provides 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.
6.2.5 Replay protection information
Re-key messages need to be protected from replay/reflection attacks.
Sequence numbers are used for this purpose and the Re-key SA (or
protocol) contains this information.
6.2.6 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
change.
6.3 Contents of the Data SA
The GCKS specifies the Data Security protocol used for secure
transmission of data from sender(s) to receiving members. Examples
of Data Security protocols include IPsec ESP, 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
SA.
6.3.1 Group identity
The Group identity accompanies 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).
6.3.2 Source identity
The SA includes source identity information when the Group Owner
chooses to reveal Source identity to authorized members only. A
public channel such as announcement protocol is only appropriate when
there is no need to protect source or group identities.
6.3.3 Traffic encrypting key
Irrespective of the Data Security Protocol used, the GCKS supplies
the TPKs (or information to derive TPKs) used in secure data
transmission, source and group authentication.
6.3.4 Authentication key
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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
files.
6.3.5 Sequence numbers
The GCKS passes sequence numbers when needed by the Data Security
protocol, for replay protection.
6.3.6 Security Parameter Index (SPI)
The GCKS sends provides an identifier as part of the Data SA contents
for data security protocols that use an SPI or similar mechanism to
identify an SA or keys within an SA.
6.3.7 Data SA policy
The Data SA parameters are specific to the Data Security Protocol but
generally include encryption algorithm and parameters, the source
authentication algorithm and parameters, the group authentication
algorithm and parameters, and/or replay protection information.
Generally, specification of source or group authentication is
mutually exclusive.
7.0 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
participants.
As described in the Requirements section above, the group key
management architecture supports 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.
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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
systems.
3. The Registration and Re-key protocols 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
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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
GCKS.
More analysis and work needs to be done on the protocol
instantiations of the Group Key Management architecture to determine
how effectively and securely the architecture can 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.
8.0 GKM protocolsÆ applicability
Several protocols for group key management have been proposed at the
IETF. A few of them are already RFCs (e.g., GKMP) and a few others
are on their way to become RFCs. Three protocols, viz., GDOI,
GSAKMP, MIKEY, are expected to become standards track RFCs. In this
section, we discuss the applicability of these protocols to real-
world applications.
A common theme to group key management protocols proposed at the IETF
is that they all facilitate "download" or push of a common key from a
server to a member. The data security SA (or the rekey SA, where
applicable) is not negotiated, but unilaterally selected by the GCKS.
Key download must be protected from eavesdropping, and from active
attacks such as message modification, MiTM, and replay attacks.
Different protocols may use different mechanisms to provide these
security features, and not all may provide all features. Next,
operational environments have a number of expectations on key
distribution protocols. For example, the unicast key exchange
protocol, IKE, supports remote legacy authentication, and client
configuration (e.g., IP address assignment). The end goal is to
establish a data security SA and an optional rekey SA. Another
requirement is to finish in fewest round trips for efficient
operation. Optionally, protocols may provide protection against
identity attacks and denial of service attacks.
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8.1 GDOI
GDOI is based on ISAKMP model of key establishment. In Phase 1, the
DOI field indicates the nature of the protocol, viz., group key
download. Other than that there is no change to ISAKMP Phase 1;
using main/aggressive mode, a secure channel is established using an
authenticated DH exchange. At the end of the exchange an IKE SA is
established. The member is expected to request to join a group,
optionally prove its authorization to join the group, and prove the
liveness of the exchange. Under the protection of the IKE SA, the
GCKS downloads the rekey SA and the data security SAs to the member.
GDOI has a rekey exchange to update the rekey and/or the data
security SAs. The data security SA can be an IPsec SA or an SRTP SA.
8.1.1 GDOI's Applicability:
GDOI is similar to IKE, owing to the fact that both are derived from
ISAKMP. Thus any proposed/deployed/standardized extensions to IKE
may generally work for GDOI as well. For example, GDOI supports the
use of pre-shared keys for end-point authentication, NAT traversal,
and client configuration. GDOI policy download capabilities are
similar to that of ISAKMP/IKE. In addition to IPsec policy, the GCKS
may download rekey SA and rekey algorithm (e.g., LKH, SDR) policy.
GDOI assumes that there is a single GCKS that downloads keys to any
number of members. Multiple servers may be used to provide the GCKS
service, but they would all serve the same membership. Subordinate
GCKSs serving subgroups is not supported in GDOI.
GDOI's Phase 1 provides all of the listed security features earlier;
however it requires too many round trips (Aggressive mode requires
1.5 + 2 round trips; and main mode requires 3 + 2 round trips in
providing identity protection) compared to the other two protocols.
Some of the additional complexity is due to the rich feature set;
however ISAKMPÆs model of establishing a secure tunnel before
downloading the rekey and data security SAs is also responsible for
the complexity.
8.2 GSAKMP
GSAKMP uses an authenticated (using digital signatures) DH exchange,
downloads keys and a policy token, and supports the concept of a
member ACK for auditing or billing purposes. The policy token can be
transformed into a data security SA, such as an IPsec SA (and perhaps
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Baugher, Canetti, Dondeti, Lindholm June 2003
an SRTP SA). GSAKMP assumes controlled operational environments and
supports DoS avoidance as an optional feature.
GSAKMP supports the concept of subordinate GCKSs and allows members
to be authorized as GCKSs. GSAKMP does not have provisions to
support PSKs and identity protection (active or passive). The policy
token needs to be translated into an IPsec SA, which is out of scope
of the document. GSAKMP requires fewer RTs compared to GDOI, and
allows members to be designated as SGCKSs. GSAKMP requires only 1.5
round trips, however the efficiency comes at the expense of lack of
DoS protection and lack of support for legacy client authentication
methods.
8.2.1 tGSAKMP <to be added>
8.3 MIKEY
MIKEY provides a single message key download using preshared keys or
public keys. It is designed for peer-to-peer key download, or key
download for small one-many or many-to-many interactive groups. It
defines a registration SA and a data security SA only; MIKEY does not
specify a rekey SA.
MIKEY also supports key download using DH exchange via a single RTT
exchange. Replay protection is provided using timestamps, to keep the
latency low. MIKEY is specifically designed for multimedia call
setup, where low latency is a requirement. MIKEY cannot seem to be
sufficient for general purpose group key management. GDOI or GSAKMP
should be used instead.
9.0 Security Considerations
This memo describes MSEC key management architecture. This
architecture will be instantiated in one or more group key management
protocols, which must be protected against man-in-the-middle,
connection hijacking, replay or reflection of past messages, and
denial of service attacks.
Authenticated key exchange [STS, SKEME, RFC2408, RFC2412, RFC2409]
techniques limit the effects of man-in-the-middle and connection-
hijacking attacks. Sequence numbers and low-computation message
authentication techniques can be effective against replay and
reflection attacks. Cookies [RFC2522], when properly implemented,
provide an efficient means to reduce the effects of denial of service
attacks.
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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.
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
specifications.
Novelty and complexity are the biggest risks to group key management
protocols. Much more analysis and experience are needed to ensure
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Baugher, Canetti, Dondeti, Lindholm June 2003
that the architecture described in this document can provide a well-
articulate standard for security and risks of group key management.
10.0 References and Bibliography
[AMESP] R. Canetti, P. Rohatgi, Pau-Chen Cheng, Multicast Data
Security Transformations: Requirements, Considerations, and Prominent
Choices, http://search.ietf.org/internet-drafts/draft-irtf-smug-data-
transforms.txt, Work In Progress, 2000.
[CP00] R. Canetti, B. Pinkas, A taxonomy of multicast security
issues, http://www.ietf.org/internet-drafts/draft-irtf-smug-
taxonomy-01.txt, Work in Progress, August 2000.
[FN93]A. Fiat, M. Naor, Broadcast Encryption, Advances in Cryptology
- CRYPTO Æ93 Proceedings, Lecture Notes in Computer Science, Vol.
773, 1994, pp. 480û491.
[FS00] N. Ferguson and B. Schneier, A Cryptographic Evaluation of
IPsec, CounterPane, http://www.counterpane.com/ipsec.html.
[GDOI] M. Baugher, T. Hardjono, H. Harney, B. Weis, The Group Domain
of Interpretation, http://www.ietf.org/internet-drafts/draft-ietf-
msec-gdoi-08.txt, May 2003, Work in Progress.
[GSAKMP] H.Harney, A.Colegrove, E.Harder, U.Meth, R.Fleischer, Group
Secure Association Key Management Protocol,
http://www.ietf.org/internet-drafts/draft-ietf-msec-gsakmp-sec-
01.txt, February 2003, Work in Progress.
[H.235] ITU, Security and encryption for H-Series (H.323 and other
H.245-based) multimedia terminals, ITU-T Recommendation H.235 Version
3, 2001, Work in progress.
[JKKV94] M. Just, E. Kranakis, D. Krizanc, P. van Oorschot, On Key
Distribution via True Broadcasting, On Key Distribution via True
Broadcasting. In Proceedings of 2nd ACM Conference on Computer and
Communications Security, November 1994, pp. 81--88.
[MIKEY] J. Arkko, E. Carrara, F. Lindholm, M. Naslund, and K.
Norrman, "MIKEY: Multimedia Internet KEYing", Internet Draft,
http://www.ietf.org/internet-drafts/draft-ietf-msec-mikey-06.txt,
February 2003, Work in progress.
[MARKS] B. Briscoe, MARKS: Zero Side Effect Multicast Key Management
using Arbitrarily Revealed Key Sequences, Proceedings of NGC'99,
rbriscoe@bt.co.uk.
Internet Draft Group Key Management Architecture [PAGE 31]
Baugher, Canetti, Dondeti, Lindholm June 2003
[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
(http://www.wipo.org/eng/meetings/1999/wct_wppt/pdf/imp99_3.pdf).
[MVV] A.J.Menzes, P.C.van Oorschot, S.A. Vanstone, Handbook of
Applied Cryptography, CRC Press, 1996.
[OFT] 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.
[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.
[RFC2326] ftp://ftp.isi.edu/in-notes/rfc2326.txt
[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
1998.
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Baugher, Canetti, Dondeti, Lindholm June 2003
[RFC2522] P. Karn, W. Simpson, Photuris: Session-Key Management
Protocol, March 1999.
[RFC2543] ftp://ftp.isi.edu/in-notes/rfc2543.txt
[RFC2627] D. M. Wallner, E. Harder, R. C. Agee, Key Management for
Multicast: Issues and Architectures, September 1998.
[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,
http://www.ietf.org/internet-drafts/draft-ietf-avt-srtp-00.txt,
February 2001, Work in Progress.
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11.0 Authors' Addresses
Mark Baugher
Cisco Systems
5510 SW Orchid St.
Portland, OR 97219, USA
+1 408-853-4418
mbaugher@cisco.com
Ran Canetti
IBM Research
30 Saw Mill River Road
Hawthorne, NY 10532, USA
+1 914-784-7076
canetti@watson.ibm.com
Lakshminath R. Dondeti
Nortel Networks
600 Technology Park Drive
Billerica, MA 01821, USA
+1 978-288-6406
ldondeti@nortelnetworks.com
Fredrik Lindholm
Ericsson Research
SE-16480 Stockholm, Sweden
+46 8 58531705
fredrik.lindholm@era.ericsson.se
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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.
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| PAFTECH AB 2003-2026 | 2026-04-23 22:24:06 |