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Versions: 01 RFC 2094

Network Working Group                    Hugh Harney, Carl Muckenhirn
INTERNET DRAFT                                            SPARTA, Inc.
6/18/96                           Secure Systems Engineering Division
                                                Expires in six months


        Group Key Management Protocol (GKMP) Architecture
                 <draft-harney-gkmp-arch-01.txt>

Status of this Memo

     This document is an Internet-Draft.  Internet-Drafts are working
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Table of Contents

      Abstract                                                         1
   1. Introduction                                                     2
   2. Multicast Key Management Architectures                           4
   3. GKMP Protocol Overview                                          11
   4. Issues                                                          21
   Author's Address                                                   23



                Abstract

            This specification proposes a protocol to create grouped
      symmetric keys and distribute them amongst communicating
      peers. This protocol has the following advantages: 1) vir-
      tually invisible to operator, 2) no central key distribution
      site is needed, 3) only group members have the key, 4)
      sender or receiver oriented operation, 5) can make use of
      multicast communications protocols.




Harney/Muckenhirn                                               [Page 1]

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1 Introduction


This document describes an architecture for the management of
cryptographic keys for multicast communications.  We identify the roles
and responsibilities of communications system elements in accomplishing
multicast key management, define security and functional requirements of
each, and provide a detailed introduction to the Group Key Management
Protocol (GKMP) which provides the ability to create and distribute
keys within arbitrary-sized groups without the intervention of a
global/centralized key manager.  The GKMP combines techniques developed
for creation of pairwise keys with techniques used to distribute keys
from a KDC (i.e., symmetric encryption of keys) to distribute symmetric
key to a group of hosts.


1.1 Multicast Communications Environments


The work leading to this report was primarily concerned with military
command and control and weapons control systems, these systems tend
to have top--down, commander--commanded, communications flows.  The
choice of what parties will be members of a particular communication
(a multicast group for example) is at the discretion of the ``higher''
level party(ies).  This ``sender-initiated'' (assuming the higher-level
party is sending) model maps well to broadcast (as in electromagnetic,
free-space, transmission) and circuit switched communications media
(e.g., video teleconferencing, ATM multicast).

In looking to apply this technology to the Internet, a somewhat
different model appears to be at work (at least for some portion of
Internet multicast traffic).  IDRP and Distance Vector Multicast
Routing Protocol (DVMRP) use multicast as a mechanism for parties
to relay common information to their peers.  Each party both sends
and receives information in the multicast channel.  As appropriate,
a party may choose to leave or join the communication without the
express permission of any of the other parties (this begs the question
of meta-authorizations which allow the parties to cooperate).  More
interestingly, the multicast IP model has the receiver telling the
network to add it to the distribution for a particular multicast
address, whether it exists yet or not, and the transmitter not being
consulted as to the addition of the receiver.

Other applications of multicast communications in the Internet, for
example NASA Select broadcasts, can be viewed as implementing the sender
model since the sender selects the broadcast time, channel, and content,
though not the destinations.

It is our intention to provide key management services which support
both communications (and implied access control) models and operate in
either a circuit switched or packet switched environment.




Harney/Muckenhirn                                            [Page 2]

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1.2 Security for Multicast


Multicast communications, as with unicast, may require any of
the security services defined in ISO 7498, access control, data
confidentiality, traffic confidentiality, integrity/data authentication,
source authentication, sender and receiver non-repudiation and service
assurance.  From the perspective of key management processes, only
data confidentiality, data authentication, and source authentication
can be supported.  The other services, traffic confidentiality,
non-repudiation, and service assurance must be provided by the
communications protocol, they may rely on cryptographic services but are
not guaranteed by them.



2 Multicast Key Management Architectures


2.1 Current Operations


There are several electronic mechanisms for generating and distributing
symmetric keys to several computers (i.e., communications groups).
These techniques, generally, rely on a key distribution center (KDC) to
act as a go between in setting up the symmetric key groups.  Military
systems, such as BLACKER, STU-II/BELLFIELD, and EKMS, and commercial
systems, such as X9.17 and Kerberos, all operate using dedicated
KDCs.  A group key request is sent to the KDC via various means (on-
or off-line) The KDC acting as an access controller decides whether
or not the request is proper (i.e., all members of a group are cleared
to receive all the data on a group).  The KDC would then call up each
individual member of the group and down load the symmetric key.  When
each member had the key the KDC would notify the requester.  Then secure
group communication could begin.  While this was certainly faster then
anything that requires human intervention.  It still requires quite a
bit of set-up time.  Also, a third party, whose primary interest isn't
the communication, needs to get involved.

Pairwise keys can be created autonomously by the host on a network by
using any number of key generation protocols (FireFly, Diffe-Hellman,
RSA). These protocols all rely on cooperative key generation algorithms
to create a cryptographic key.  These algorithms rely on random
information generated by each host.  These algorithms also rely on peer
review of permissions to ensure that the communication partners are who
they claim to be and have authorization to receive the information being
transmitted.  This peer review process relies on a trusted authority
assigning permissions to each host in the network that wants the ability
to create these keys.  The real beauty of these pairwise key management
protocols is that they can be integrated into the communication
protocol or the application.  This means that the key management becomes
relatively invisible to the people in the system.



Harney/Muckenhirn                                            [Page 3]

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2.2 GKMP-Based Operations


The GKMP described below, delegates the access control, key generation,
and distribution functions to the communicating entities themselves
rather than relying on a third party (KDC) for these functions.  As
prelude to actually distributing key, a few things must be assumed
(for purposes of this document):  there exists a ``security manager''
responsible for creating and distributing to parties authentic
identification and security permission information(1); communicating
parties are online for the keys formed and distributed by the GKMP


2.2.1 Sender Initiated Operations


This section describes the basic operational concept for multicast
key management for sender initiated multicast support.  This model
of multicast communications was the basis for our original work on
multicast key management.  From a security viewpoint the sending
application is able to control access to the transmission through
both key distribution and communications distribution (not sending the
transmission to some addresses).


Identification of Group Key Controller --   The originator of the
multicast group creates or obtains a group management certificate from
its certification hierarchy.  The certificate identifies the holder as
responsible for generation and distribution of the group key(2).  The
originator relays the membership list to the Group Key Management (GKM)
application.


Group Key Creation --   The GKM application, operating on behalf of the
originator, selects one member of the group, contacts it, and creates
a Group Key Packet (GKP). A GKP contains the current group traffic
----------------------------
 1. The security manager function may be accomplished through a strictly
hierarchical system (a la STU-III) or a more ad hoc system of
cooperating peer ``domain managers,'' the implementation of the
certification hierarchy is not addressed in this document.

 2. Naming standards are not addressed here, the name should reflect the
naming structures appropriate for the supported cryptographic service.
For example, IP-level encryptors should use
naming reflecting ``host'' identities (IP addresses, or DNS host names),
RTP encryptor would use session names.





Harney/Muckenhirn                                            [Page 4]

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encrypting key (GTEK) and future group key encrypting key (GKEK). The
GKM application then identifies itself as the group key controller,
which the member validates, under cover of the GTEK.

     Group Key Packet (GKP) = [GTEKn,GKEKn+1]

As part of group key packet formation, usage parameters, appropriate
for the underlying crypto-system, are selected.  Unlike normal parameter
negotiation, where common security-level/range, and services are arrived
at, the originator's GKM application selects these parameters and the
member must comply.


Group Key Distribution --   After creation of the GKP, the group
controller contacts each member of the group, creates a Session Key
Package (SKP), validates their permissions (check member's certificate
against group parameters), and create a Group Rekey Package for that
member.  A SKP contains a session TEK and a session KEK for a particular
member.  A GRP contains the GKP encrypted in a KEK and signed using the
originator's certificate.

     Session Key Package (SKP) = [STEK, SKEK]


     Group Rekey Package (GRP) = {[GKP]KEK} SignatureController


Group Rekey --   When the group needs to be rekeyed, the originating
GKM application selects a member, creates a new GKP, creates a new
GRP (which is encrypted in the previously distributed next GKEK) and
broadcasts it to the group.

This procedure is fairly complex, but other than for the distribution of
site-specific certificates, no centralized key management resources are
needed.  The only parties to the key management communications are the
same parties which will be participating in the group.


2.2.2 Receiver Initiated Operations


This section describes key management operational concept for receiver
initiated multicast communication support.  The receiver initiated
model presents some interesting problems from a security view point
since the end-participants are not known a priori.  Also, in a purely
receiver initiated application (such as DVMRP), there is no concept of
an ``originator'' and the participants in the group may be quite dynamic
with participants changing on a minute by minute basis.

For secure group communications to take place, all members must obtain
the same key.  This may be achieved by either using deterministic key
generation techniques (using a secret, shared seed) or by making one



Harney/Muckenhirn                                            [Page 5]

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member of the group responsible for creation of the key.  The use of
a deterministic key generator presents security problems, particularly
regarding loss of the seed (it compromises both past and future
traffic).  The assignment of a member to the role of key ``controller''
also presents drawbacks, but these relate to determining which one
should be the controller and the need for each member to contact him.
The remainder of this discussion will look at how the ``controller''
concept from above could work in the receiver initiated case.


Selection of Group Key Controller --   A group member will be made
responsible for initial group establishment and periodic generation
and dissemination of new GRPs.  There is no need for the selected
controller to be the controller for all time, but at any one time only
one controller may be active for each group.  Selection of controller
may be made through a voting system, by a simple default (the first to
transmit to the group is the controller), or configuration.

The current controller's identity must be made available to all members,
and potential members, for initial group key load and error recovery.
The information may be relayed by broacast on a key management
``channel,'' or through a directory service.


Group Key Creation --   The GKP is created and distributed in much the
same way as in sender initiated operations.  The controller creates
a GKP with the first group member to initiate contact.  The GKM
application then identifies itself as the group key controller, which
the member validates, under cover of the GTEK. Parameter negotiation is
performed and the first group member is keyed.


Group Key Distribution --   After creation of the GKP, as other members
contact the controller, a SKP is created, member permissions are
validated and a GRP is loaded to the member.

For widely distributed groups, a form of distributed dissemination may
be used.  Some number of regional GKM applications are enabled with the
ability to validate the permissions of new members and upon validation
send to them the current GKP.(3) These regional key distributors perform
the same functions as the controller, except that they do not create the
GKP. This concept can be expanded to the point where all current members
are capable of downloading the GKP, and passing on that capability.

----------------------------
 3. Access control is not defined in this document, but it is assumed
that both hierarchical and discretionaly (rule-based and identity-based)
access control will be supported.







Harney/Muckenhirn                                            [Page 6]

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Group Rekey --   When the group need rekeying the procedure would be
identical to the sender initiated case.  The controlling GKM application
selects a member, creates a new GKP, creates a new GRP (which is
encrypted in the previously distributed next GKEK) and broadcasts it to
the group.



2.3 GKMP Features


This section highlights areas which we believe the GKMP approach has
advantages over the ``traditional'' KDC based approaches.


2.3.1 Multicast


Multicast protocols are a growing area of interest for the Internet.
The largest benefit of a multicast protocol is the ability of several
receivers to simultaneously get the same transmission.  If the
transmission is of a sensitive nature, it should be encrypted.  This
means that the all members of the group must share the same encryption
key to take benefit of the multicast transmission.

To date the only way of setting up a group of symmetric keys is with
the assistance of a centralized key management facility.  This facility
would act as a key broker creating a distributing key to qualified group
members.  There are several problems with this centralized concept.
These problems give rise to many of the following motivations for
creating a distributed key management protocol.


2.3.2 Increase the autonomy of key groups


The GKMP proposes to extend the pairwise key paradigm to grouped keys.
This protocol can be integrated into the communication protocols or
applications and can become invisible to the host's operator.  We will
use peer review to enforce our security policy.

The GKMP allows any host on a network to create and manage a secure
group.  Maintenance of these group keys can be performed by the hosts
interested in the group.  The groups themselves will be relatively
autonomous.  This simplifies the installation of this technology
allowing more host to use secure multicast communications.









Harney/Muckenhirn                                            [Page 7]

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2.3.3 Latency


Latency refers to the time to set-up or tear down or to re-key a group.
In short this corresponds to the length of time it would take to set-up
a multicast address.

The GKMP can allow delegation of group creation authority to any host
in the network.  In essence, when a host needs a group it will have the
tools needed to create that group and manage it.  Additionally, since
the host only needs to create a single group it can concentrate on that
particular group.

In the current centralized key distribution approach.  The group must
be requested from the central site.  The central site would process
that request in accordance with it's priority and current workload.
Latencies would develop if the workload of the central site gets
unwieldy or if the communications to the site become overloaded.


2.3.4 Extendibility


One of the problems with a centralized key distribution system is the
concentration of key management workload at a single site.  The process
of creating key groups -- key creation, access review, communication to
group members takes time and effort.  As the number of groups on the
network grows and the number of group members group.  The workload at
that central sight quickly reaches capacity.

GKMP should allow a great number of groups to exist on the Internet
without overloading any particular host.  Delegation of the net wide
group creation and management workload places the burden of maintaining
groups on the hosts interested in using those groups.  Not only is this
more efficient, but it places the burden in an appropriate location.

The GKMP distributes the communication requirements to manage groups
across the network.  Each group manages the group using the same
communication resources needed to pass traffic.  It is likely that if a
communication group can support the traffic of a group, it will be able
to support the minimal traffic needed to management the keys for that
group.

GKMP provides it's own access control, based on signed netwide
permission certificates.  This partially disseminates the burden of
access control and permission management.  A system wide authority
must assign the permission certificates, but day to day access control
decisions are a GKMP responsibility.







Harney/Muckenhirn                                            [Page 8]

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2.3.5 Operating expense


A centralized key distribution site contains, at one time or another,
the keys for the net.  This is a valuable target for someone to
compromise.  To protect this site physical and procedural security
mechanisms are employed (e.g., guards, fences, intrusion alarms, two
person safes, no-alone zones).  These mechanisms do not come cheap.

Allowing the hosts to create and manage their keys eliminates the need
for an on-line centralized key distribution site.  The protocol approach
restricts access to the keys to the hosts using them (the minimal set).
Since, the encryption mechanisms will have already incurred the cost to
be physically secured there is no additional cost levied on the system
by the key management system.


2.3.6 Communication Resources


Because a centralized site is involved in creating, distributing,
rekeying, and providing access control for every group, it is frequently
accessed.  The communication resources available to this site often
become a bottle neck for the groups.  Therefore a big pipe is usually
installed to this facility.

The GKMP proposes delegating most of the key creation, distribution,
rekey and access control mission to the hosts that need the secure
communication.  There no longer is a single third party that must be
consulted prior to every group key management action.  Hence, the
communications requirements to manage the keys have shifted to the
groups themselves.  The need for special high capacity communications
has been eliminated.


2.3.7 Reliability


Delegating key management responsibility to the groups eliminates
the centralized key management site as a single point of failure.
The groups that will use the key are responsible for it.  If the
communications system fails for the key management it is also down for
the communications.

The GKMP will attempt to delegate as many functions to the group as
possible.  There will be some functions which still need to be performed
outside of the group (granting of privileges).  These functions can
still fail.  The GKMP will operate on the old set of permissions.  These
functions need not be in-line.  They are performed separate from the key
management actions and are not crucial to day-to-day operation.





Harney/Muckenhirn                                           [Page 9]

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2.3.8 Security


People are the most risky element for security.  A distributed protocol
eliminates many people from the key distribution chain.  This limits
``exposure'' of the key.



3 GKMP Protocol Overview


3.1 Supporting functions


A secure key management protocol needs a number of supporting functions,
especially in a military environment.  The two major support functions
are security management and network group management.  In the commercial
world a company could provide these support functions.

The issue of Security Management is permission management, in a military
environment separation of data occurs along classical classification
lines (i.e., TOP SECRET to UNCLASSIFIED). In the commercial world these
levels are proprietary or need to know access.

Network group management provides an interface to the communications
system and control of network resources.  Some entity either a
commercial or military system, the host or network operations center,
must provide the key management protocol with a list of the group
members.  Also, if the network resources, bandwidth and processing, are
considered scarce a management structure must allocate them.


3.1.1 Security management


Security management is a role performed for the entire network.  It
involves netwide issues of permission management, initialization
of software, and compromise recovery.  The GKMP relies on security
management to operate.  Refer to figure 1:  Security management view.

The GKMP must assume trusted handling of the protocol software prior
and during installation.  If the GKMP is to use peer to peer access
control the system must control the assignment of permissions.  These
permissions must be monitored and updated as needed.  Finally, overview
of these permissions must include the maintenance of a Certificate
Revocation List.


Secure start-up  We need to control the process of loading GKMP software
onto a host and initializing it.  The protocol needs keys, public and




Harney/Muckenhirn                                           [Page 10]

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                  Figure 1:  Security Management View

private, to operate.  It also must have identify information of the host
on whose behalf it will act.

There are some life cycle and security concerns with the software while
in transit, stored, distributed, and installed.  A one time start-up
procedure must verify the identity of the host.  Procedural and physical
identification techniques will verify the identity of the host (i.e.,
the Armed Forces Courier Service (ARFCS) accounting, or registered
mail).  Upon key delivery the security manager logs it's receipt and
assumes responsibility for the key.

After proper installation of the software a paper trail verifies the
recipient.  The computer would initiate an association with the security
management function to initialize the protocol software (create a unique
public and private key pair for network operation and receive network
permissions).  This activation process uses keys distributed with the
software (good only for initialization) to secure an exchange with the
security manager.  The host then creates a unique public and private
pair and sends the public key to the security manager.  The security
manager creates a credential that uniquely identifies the host and it
permissions.  This credential is signed by the security management with
its private key and can be verified by all net members with the public
key.


Permission management  Each host on the network is given a permissions
certificate signed by the security management which uniquely identify
that host and identifies the access permissions it is allowed.  These
permission certificates are used by the network hosts to assign
permissions to other hosts.





Harney/Muckenhirn                                           [Page 11]

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This process assigns permissions to equipment or human beings in
accordance with their duties.  This process involves security clearances
and human judgment therefore it is outside the scope of this protocol.

The security management function, especially in military operations,
would be responsible for managing permissions and classifications at
each host.  In the commercial world, permission management corresponds
to projects or duties.


Compromise recovery management  If a group member is found compromised,
the protocol must facilitate the exclusion of the compromised member
and return to secure operations.  The security management function will
provide control of compromise recovery.

Usually, physical inspections or accounting techniques find compromises.
These separate systems report the compromise to the key management
system.  We must assume the loss of all key resident at that host.  The
security management function will rescind the permission allocated
to this compromised host.  We create a list of all know compromised
hosts and distribution that list across the network.  Each host is
then responsible for reviewing the propriety of each association and
enforcing access control to data.


3.1.2 Group management


The group manager interacts with other management functions in the
network to provide the GKMP with group membership lists and group
relevant commands.  The GKMP deals strictly with cryptographic key.
It relies on external communication and network management services to
supply network related information.  Primarily, it relies on the network
management service to provide it with the addresses of group members (if
the group is sender initiated).

The GKMP allows an external entity to determine the controller of
a group.  The controller of the group should be able to handle the
additional processing and communication requirements associated with
the role.  If this is not a necessary function given the implementation,
this assignment of controller duties can be set to some automated
default.  However, even if defaulted some external management entity
determines how the role of controller is allocated.

The group manager can receive group progress reports from the group
controller.  The GKMP provides a service for the network.  It makes
sense that someone in the network is interested in the progress of
this service.  The GKMP can provide progress reports.  It is up to the
network management to determine the manner and recipient of the reports.
Reference figure 2:  Network manager interaction.



Harney/Muckenhirn                                           [Page 12]

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                 Figure 2:  Network Manager Interaction

Group to member mapping  When the GKMP is implemented in sender
initiated group establishment mode, a list of group member addresses
must be provided as part of the group establishment command.  The GKMP
will use these addresses to contact the group members and create the
group.

The creation of groups involves the assignment of a group address,
update of router databases, and distribution of this group address to
the group members.  This is a classic function of network management.
The GKMP group controller would be another recipient of this
information.


Protocol role allocation  The Group Management Protocol assigns roles
to members of a particular group.  These roles are binary one is either
the control over the group or a member of a group.  Some external entity
will allocate the identity of the group controller and group receiver.
This is a desirable aspect because some computers are more capable
(i.e., central site, great deal of process power available to control
a group).  We allow some external entity to allocate these roles to
individual group members, this is important in the military application
do to the fact that in a commercial application the allocating authority
and group controller may very well always be the same.


Group key progress reporting  The Group Key Management Protocol has
to be able to report to somebody.  If we create a group, we should
report it to group requester.  Contrarily if we are not able to create


Harney/Muckenhirn                                           [Page 13]

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                Figure 3:  Distributed Group Management


a group we should report that especially since failure to create a
group at least as a first study will highly correlate with a failure
of the underlying communications.  The Group Key Management Protocol
does not have an ability to fix the underlying communications so the
communication management function must deal with these failures.


3.2 Protocol Roles


Creation and distribution of grouped key require assignment of roles.
These identify what functions the individual hosts perform in the
protocol.  The two primary roles are those of controller and receiver.
The controller initiates the creation of the key, forms the key
distribution messages, and collects acknowledgment of key receipt from
the receivers.  The receivers wait for a distribution message, decrypt,
validate, and acknowledge the receipt of new key.

One of the essential concepts behind the GKMP is delegation of group
control.  Since each host in the network has the capability to act
as a group controller, the processing and communication requirements
of controlling the groups in the network can be distributed equitably
throughout the network.  This avoids potential single points of failure,
communication congestion, and processor overloading.  Refer to figure 3:
Distributed group management.



Harney/Muckenhirn                                           [Page 14]

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3.2.1 Group controller


The group controller is the a group member with authority to perform
critical protocol actions (i.e., create key, distribute key, create
group rekey messages, and report on the progress of these actions).  All
group members have the capability to be a group controller and could
assume this duty upon assignment.

The group controller helps the cryptographic group reach and maintain
key synchronization.  A group must operate on the same symmetric
cryptographic key.  If part of the group loses or inappropriately
changes it's key, it will not be able to send or receive data to another
host operating on the correct key.  Therefor, it is important that those
operations that create or change key are unambiguous and controlled
(i.e., it would not be appropriate for multiple hosts to try to rekey
a net simultaneously).


3.2.2 Group receiver


Simply stated a group receiver is any group member who is not acting
as the controller.  The group receivers will:  assist the controller in
creating key, validate the controller authorization to perform actions,
accept key from the controller, request key from the controller,
maintain local CRL lists, perform peer review of key management actions,
and manage local key.



3.3 Scenarios


3.3.1 Group establishment


The protocol to establish a group of host that share a cryptographic
key must create a high quality key, verify that all intended recipients
have permission to join the group, distribute the key to all qualified
members, and report on the progress.  This process consists of two
phases:  creation of the key and distribution of the key.  Refer to
figure 4:  Group Establishment.

The group establishment process is proceeds in the following manner.
First, a ``create group'' command is issued to the group commander.
The group controller validates the command to ensure it came from an
authorized commander and the group is within the controller's permission
range.  Next, the controller creates a key.  Then that key is passed to
the group members, after they pass the peer to peer review process.



Harney/Muckenhirn                                           [Page 15]

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                     Figure 4:  Group Establishment


Validate command  The create group command is signed by the group
commander ( they may be the same device).  This signature should be
asymmetric in nature.  The public key to validate this command can be
sent with the command itself, if the public bound to the identity of the
commander.

The group controller receives the command.  It verifies that the
signature, thereby ensuring the message was sent by the claimed source
and the message has not been modified in transit.


Creation of group keys  The controller initiates the creation of two
keys for use in the group.  The creation of a cryptographic key requires
that the key be sufficiently random.  Randomizers, capable of creating
high grade cryptographic key, tend to be hardware based and are not
likely to be practical for this protocol.  There are several established
key creation protocols based in software (e.g., Diffe-Hellman, FireFly,
RSA). All these software based algorithms involve two hosts cooperating
to create a cryptographic key.  These software algorithms are more
appropriate for this protocol.

Also important, in the creation of these keys, is verification of the
authorization of the key creation partner.  Authorization to posses
the keys include permissions that equal or exceed the group traffic and
identity verification.


Distribution of group keys  The controller distributes the group
keys to the net members.  The controller must verify the identity
and permissions of each member prior to the key being distributed.


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                         Figure 5:  Group Rekey


Likewise, the net member must verify the controller's identity,
authorization to perform this action, and permissions.

The key being distributed is the same level as the data that it will
encrypt.  Hence, we must encrypt the key during distribution.  If no
suitable key exists between the controller and member, a new key must be
created.  This new key is cooperatively created between the controller
and net member in a similar manner as the net keys.

The controller creates a message for encryption in the key held between
the controller and member.  This message will include key management
information and the keys.


3.3.2 Group rekey


Cryptographic key has a life span.  New key must replace ``old'' key
prior to the end of its cryptographic life.  This process is rekey.

Rekey has the advantage of using an existing cryptographic association
to distribute key.  Also, there is no requirement to verify the identity
and authorization for the other members.  Identify and authorization are
assumed.

A group rekey consists of two stages.  First the Group Controller
creates new group keys.  Second these ``new'' keys are sent to the Group
Members in a multicast message.  Refer to figure 5:  Group Rekey.


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Creation of group keys  The controller of the rekey will create the new
keys in exactly the same manner as used during group establishment.


Distribution of group keys  The GKMP creates a message for the group
address.  This message uses one of the keys distributed during
group establishment to encrypt the new keys.  It also contains an
authorization token identifying the controller as the rekey agent
and new management data.  All members of the group using a multicast
protocol (if one exists) accept this message.

The message which rekeys the group encrypts the new keys in the existing
KEK. Since all group members possess the KEK the entire group can
decrypt this message.

The token authorizing the group controller to perform this rekey is also
included.  This token is asymmetrically signed by the group commander.
It uniquely identifies the group controller's authority to rekey this
group.  It also identifies the group the level of traffic and rekey
interval.


3.3.3 Deletion


It is desirable to be able to delete group members for either
administrative purposes or security reasons.  Administrative deletion is
the deletion of a trusted group member.  It is possible to confirm the
deletion of trusted group members.  Security relevant deletion is the
deletion of an untrusted member.  It assumes that the member is ignore
all deletion commands.


Administrative delete  Administrative deletion removes the group keys
from trusted group members.  This deletion consists of two messages
the first sends a command to the group encrypted in the groups TEK.
The command essentially says:  acknowledge receipt and then delete
group keys.  This command is signed by the group controller to prevent
unauthorized deletions.

The acknowledgment message is also encrypted under the group TEK and
is sent to acknowledge receipt of the command.  We could acknowledge
accomplishment of the command if the net is willing to accept the burden
of creating pairwise keys between the exiting group members and the
group controller.


Compromise recovery  Compromise recovery is the deletion of untrusted
group members.  This actually involves the creation of an entirely new
group, without the untrusted member.  Once the new group is created,
net operations can be shifted to the new group, effectively denying the


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untrusted member access to the data.

There is always a trade-off between security and continued net
operations when a member is found to be compromised.  The security
first position states that if a member is compromised, the group must
be destroyed and then a new secure group created.  However, operational
concerns sometimes out weigh the security concerns.  The operational
position is that the group will continue to operate with the compromised
member and will shift to a new secure group when it becomes available.

The GKMP does not mandate either position.  However, the speed
and flexibility of the GKMP does allow a new secure group to be
created quickly.  Thereby, restricting the potential damage done by a
compromised member.

Once a member is found to be compromised, that members certificate
is added to a Certificate Revocation List (CRL). The CRL is an
asymmetrically signed piece of data, signed by a security manager.  The
list is made up of compromised resource ID's, a version of the CRL, and
perhaps an identifier of the security manager.  The CRL is accessed
every time a new key is negotiated.  If one of the key creators is on
the CRL the key is destroyed and interaction terminated.

The idea behind a CRL is each host would keep records of all open
associations and compromised resources.  The host would then make
sure that it does not have and will not create a secure association
open with anyone who is on the CRL. The CRL concept of becomes more
complicated in the case of groups.  This is because it is not necessary
for every member in the group to know who the other group members are.
Hence, a group member does not have sufficient information to identify
compromised group associations.  The GKMP proposes that the group
controllers be responsible for reviewing the CRL and taking appropriate
actions should a group member be compromised.

Another issue with CRLs is the speed that they can be distributed across
a network.  Every time a key is created the cooperating hosts exchange
the version number of their current CRL. If the versions do not match.
The most current version is passed to the host with the old version.
Hence, CRLs propagate when keys are created.  If this is infrequently
and there is a single CRL insertion point, the list may take a few days
to move across the net.  The GKMP allows a speedier distribution of the
CRL.

The GKMP delegates control of groups to specific group controllers
(a subset of the network).  These controllers are responsible for
maintaining the security of the group.  If quicker distribution of
the CRL were desired, the CRL generator ( security management function
could seed the CRL at these controllers.  Controllers are points of key
management activity and are logical CRL staging areas.




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4 Issues


What are the unresolved issues with this protocol?



4.1 Access Control


One interesting issue with a grouped key protocol is access control.
This is because we are moving away from having humans in the loop or
having a central authority to check the propriety of the group.

The group protocol must police itself.  It must ensure that each member
of a group meets the classic military access control policy ( i.e.,
a group member `s classification level must be higher or equal to the
classification of the group that it's in).

Is allocation of permissions by a higher authority sufficient to provide
access control?  Or is a more discretionary mechanism necessary?


4.2 MLS


A GKMP must be capable of operating in a multi-level secure environment.
The integration of a key management protocol capable of creating keys
of several different classifications with an operating system capable of
operating with multiple classifications in non -trivial.

Classified label standards needed to be incorporated.  The
classification labels used by the key management protocol should
coincide with the labels used by the MLS operating system.  These
interoperability issues need to be addressed.


4.3 Error Conditions


A group protocol is more complex than a pairwise protocol hence there
are more possible error conditions.  In a pairwise protocol you have
two parties; they must communicate between themselves.  It is relatively
simple to define an take care of all the potential error conditions.

One assumption with any group protocol is the underlying internet is,
to some degree, always broken.  The protocol designer has to assume that
messages will be delayed or destroyed in transit, all member will not
receive all multicast messages, and acknowledgment of actions may not
be delivered.  This assumption is important if a protocol uses multicast
functions to speed-up actions.


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The protocol must provide recovery mechanisms to allow group members
to recover from loss of messages.  It must recover in a way that is
transparent to the host and underlying communications network.

For example, there is an issue whether or not we can create an
application layer acknowledgment of multi-cast actions.  The issue deals
with the required bandwidth that acknowledgment would take up.  It may
be much more friendly to the underlying communications systems to have
each member identify potential errors and correct them in a pairwise
manner.  The task of handling error conditions in a key management
protocol is double difficult because many error conditions can be
induced error condition (invoked by a third party trying to break the
security of that system) to retrieve there key that is in transit or to
block the successful dissemination of a key thereby attacking the system
security mechanism.



4.4 Commercial vs.  Military


Commercial and military key management differ in many ways.  Commercial
Key management protocols tend to emphasize inter-operability, freedom
of action, and performance.  Military systems tend to emphasize security
and control of operations.

There will be a difference in cryptographic algorithms.  The military
protocol would certainly use high grade encryption because of protecting
classified information.  The commercial system would probably using
algorithms.  and techniques certified for unclassified communication
systems.  The main difference is in the algorithms length and type.

A military protocol would require more management and structure than
a commercial one.  The military has always adopted a hierarchical
communication structure and the commercial system, especially if you
look at the internet, work mainly by anarchist style.


4.4.1 Algorithm Type


Another difference between military and commercial key management
is the type of cryptographic algorithms.  The commercial world uses
encryption algorithms like DES and in the future Skipjack.  The military
uses other cryptographic algorithms that differ in key length and have
more restrictions.  An example of this would be the identification of
ACCORDION, as a military key encryption algorithm as used in the EKMS
program run by NSA.

Any experiments with a grouped key management protocol must consider the
differences between military and commercial algorithms.  The commercial
algorithms tend to be quicker to implement, run faster, involve less

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processing time, and allows an unclassified experiment.  However, we
must be careful not paint an unrealistic picture of the performance of
the protocol based on these commercial algorithms.  A military algorithm
tends to be more cumbersome to process, slow to process, require more
bandwidth, a lot of unpleasant characteristics from the commercial
stand point, but allow for a higher grade of cryptographic security.
One way of dealing with the disparity between algorithms is to use
the commercial cryptographic algorithms and leave the fields the size
used by a comparative DOD cryptographic algorithms and insert delays to
simulate DOD algorithm processing times.


4.4.2 Management Philosophy


Management for a military network is far more structured than a
commercial network.  A military network would restrict the creation of
network groups, the rekeying of those groups, and access to the data
contained in those groups.  In contrast the commercial world would
enable any member in the network to create a group and allow any other
member of the net to join that group.

The group Key Management Protocol must allow for both these
architectures i.e., all for very structure command control hierarchy and
another free form group creation.



4.5 Receiver Initiated Operations


How do they actually work, what are the performance trades,
experimentation needed.

Who is the group leader?

How do we elect a new leader?

Will multiple leaders be created?

Will rule based access control allow fine enough access disgression?

Methods for distributed GKP/GRP dissemination need to be examined.  This
includes:


 o  resolving group identification issues, such as how to notify
    potential members of membership requirements without compromising
    any security-relevant information about the group;

 o  approaches for rapidly identifying GKP/GRP sources must be
    developed, such as a ``Key ARP'' whereby a new member broadcasts

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    into the group a request for key service and existing members
    resolve which will provide service; and,

 o  Security effects of distributing access control decisions must also
    be reviewed.



5 Addresses of Authors

Hugh Harney
SPARTA, Inc.
Secure Systems Engineering Division
9861 Broken Land Parkway, Suite 300
Columbia, MD 21046-1170
United States
telephone:        +1 410 381 9400 (ext.  203)
electronic mail:  hh@columbia.sparta.com



Carl Muckenhirn
SPARTA, Inc.
Secure Systems Engineering Division
9861 Broken Land Parkway, Suite 300
Columbia, MD 21046-1170
United States
telephone:        +1 410 381 9400 (ext.  208)
electronic mail:  cfm@columbia.sparta.com























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