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INTERNET-DRAFT S. Sakane
Intended Status: Informational Ken'ichi Kamada
Expires: January 31, 2010 Yokogawa Electric Corp.
S. Zrelli
JAIST
M. Ishiyama
Toshiba Corp.
July 30, 2009
Problem statement on the cross-realm operation of Kerberos
draft-ietf-krb-wg-cross-problem-statement-04.txt
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
other groups may also distribute working documents as Internet-
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and may be updated, replaced, or obsoleted by other documents at any
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The list of current Internet-Drafts can be accessed at
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This Internet-Draft expires in January 31, 2010.
Copyright Notice
Copyright (c) 2009 IETF Trust and the persons identified as the
document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents in effect on the date of
publication of this document (http://trustee.ietf.org/license-info).
S.Sakane, et al. [Page 1]
Internet-Draft July 2009
Please review these documents carefully, as they describe your rights
and restrictions with respect to this document.
Abstract
As industrial automation is moving towards wider adoption of Internet
standards, the Kerberos authentication protocol represents one of the
best alternatives for ensuring the confidentiality and the integrity
of communications in control networks while meeting performance and
security requirements.
However, the use of Kerberos cross-realm operations in large scale
industrial systems may introduce issues that could cause performance
and reliability problems. This document describes some examples of
actual large scale industrial systems, and lists requirements and
restriction regarding authentication operations in such environments.
The document then describes standing issues in the Kerberos cross-
realm authentication model that should be fixed before Kerberos can
be adopted in large scale industrial systems.
Conventions used in this document
It is assumed that the readers are familiar with the terms and
concepts described in the Kerberos Version 5 [RFC4120].
S.Sakane, et al. [Page 2]
Internet-Draft July 2009
Table of Contents
1. Introduction ................................................. 4
2. Kerberos system .............................................. 4
2.1. Kerberos basic operation ................................ 4
2.2. Cross-realm operation ................................... 5
3. Applying Cross-Realm Kerberos in Complex Environments ........ 6
4. Requirements ................................................. 7
5. Issues ....................................................... 8
5.1. Unreliability of authentication chain ................... 8
5.2. Possibility of MITM in case of the indirect trust model . 9
5.3. Scalability of the direct trust model ................... 9
5.4. Exposure to DoS Attacks ................................. 9
5.5. Client's performance .................................... 10
5.6. Pre-authentication problem in roaming scenarios ......... 10
6. Implementation consideration ................................. 11
7. IANA Considerations .......................................... 11
8. Security Considerations ...................................... 11
9. Acknowledgments .............................................. 11
10. References ................................................... 11
10.1. Normative References ................................... 11
10.2. Informative References ................................. 12
Authors' Addresses ............................................... 12
S.Sakane, et al. [Page 3]
Internet-Draft July 2009
1. Introduction
Kerberos Version 5 is a widely deployed mechanism that enables a
server to authenticate a client's access. Each client belongs to a
managed domain called realm. Kerberos supports authentication when a
client and a server belong to different realms. This is called
cross-realm operation.
Meanwhile, there are lots of manners of operation in actual systems,
where Kerberos could be applied. Large systems or distributed
systems are typically split into several managed domains. For
example, systems could be split into multiple domains for
geographical reasons, or to implement different management policies.
Even in such systems, a common authentication mechanism for the
different managed domains is required. When the cross-realm
operation of Kerberos is applied to such systems, some issues come
out.
This document briefly describes the Kerberos Version 5 system and its
cross-realm mode of operation. Then, it describes two actual systems
that Kerberos could be applied to. and describes seven requirements
of those systems in term both of management and operation. Finally,
it lists six issues of the cross-realm operation when it is applied
to those system.
Note that this document might not describe all of the issues of
cross-realm operation. New issues might be found in the future. It
also does not propose any solution to solve the issues. Furthermore,
publication of this document does not mean that each of the issues
have to be solved by the IETF members. Hence, in further step, we
will analyze the issues, define problems and explore the solutions.
These works will be described in another document.
This document is assumed that the readers are familiar with the terms
and concepts described in the Kerberos Version 5 [RFC4120].
2. Kerberos system
2.1. Kerberos basic operation
Kerberos [RFC4120] is a widely deployed authentication system. The
authentication process in Kerberos involves principals and a Key
Distribution Center (KDC). The principals can be users or services.
Each KDC maintains a database of principals and shares a secret key
with each registered principal.
S.Sakane, et al. [Page 4]
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The authentication process allows a user to acquire the needed
credentials from the KDC. These credentials allow services to
authenticate the users before granting them access to the resources.
An important part of the credentials are called Tickets. There are
two kind of tickets: Ticket Granting Ticket (TGT) and Service Ticket.
The TGT is obtained periodically from the KDC and has a limited limit
after which it expires and the user must renew it. The TGT is used
to obtain the other kind of tickets, Service Tickets. The user
obtains a TGT from the Authentication Service (AS), a logical
component of the KDC. The process of obtaining a TGT is referred to
as 'AS exchange'. When a TGT request is issued by an user, the AS
responds by sending a reply packet containing the credentials which
consists of the TGT along with a random key called 'TGS Session Key'.
The TGT contains a set of information encrypted using a secret key
associated with a special service referred to as TGS (Ticket Granting
Service). The TGS session key is encrypted using the user's key so
that the user can obtain the TGS session key only if she knows the
secret key shared with the KDC. The TGT then is used to obtain
Service Tickets from the Ticket Granting Service (TGS)- the second
component of the KDC. The process of obtaining service tickets is
referred to as 'TGS exchange'. The request for a service ticket
consists on a packet containing a TGT and an 'Authenticator'. The
Authenticator is encrypted using the TGS session key and contains the
identity of the user as well as time stamps (for protection against
replay attacks). After decrypting the TGT (which was encrypted by
the AS using the TGS's secret key), the TGS extracts the TGS session
key. Using that session key, it decrypts the Authenticator and
authenticates the user. Then, the TGS issues credentials requested
by the user. These credentials consist on a service ticket and a
session key that will be used to authenticate the user with the
desired application service.
2.2. Cross-realm operation
The Kerberos protocol provides cross-realm authentication
capabilities. This allows users to obtain service tickets to access
services in foreign realms. In order to access such services, the
users first contact their home KDC asking for a TGT that will be used
with the TGS of the foreign realm. If there is a direct trust
relationship between the home realm and the foreign realm, namely
both realms share keys (this is called inter-realm keys), the home
KDC delivers the requested TGT.
However, if the home realm does not share inter-realm keys with the
foreign realm the home KDC will provide a TGT that can be used with
an intermediary foreign realm that is likely to be sharing inter-
realm keys with the target realm. The client can use this
S.Sakane, et al. [Page 5]
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'intermediary TGT' to communicate with the intermediary KDC which
will iterate the actions taken by the home KDC. If the intermediary
KDC does not share inter-realm keys with the target foreign realm it
will point the user to another intermediary KDC (just as in the first
exchange between the user and its home KDC). However, in the other
case (when it shares inter-realm keys with the target realm), the
intermediary KDC will issue a TGT that can be used with the KDC of
the target realm. This is so-called indirect trust model. After
obtaining a TGT for the desired foreign realm, the client uses it to
obtain service tickets from the TGS of the foreign realm. Finally,
the user access the service using the service ticket.
When the realms belong to the same institution, a chain of trust can
be determined by the client or the KDC by following the DNS domain
hierarchy and supposing that the parent domains share keys with all
its child sub-domains. However, because the inter-realm trust model
is not necessarily constructing the hierarchic approach anytime, the
trust path must be specified manually. When intermediary realms are
involved, the success of the cross-realm operation completely depends
on the realms that are part of the authentication path.
3. Applying Cross-Realm Kerberos in Complex Environments
In order to help understanding both requirements and restriction,
this section describes scale and operation of two actual systems that
could be supported by cross-realm Kerberos. The two systems would be
most naturally implemented using different models, which will imply
different requirements for cross-realm Kerberos.
We refer to actual petrochemical enterprise [SHELLCHEM], and show two
examples among its plants. The enterprise produces bulk
petrochemicals and their delivery to large industrial customers.
There are 43 typical plants of the enterprise all over the world.
They are managed by the operation sites placed in 35 countries. This
section shows two examples of them.
One is an example of a centralized system [CSPC]. CSPC is operated
by a joint enterprise of two companies. This system is one of the
largest systems of this enterprise in the world. This is placed in
the area of 3.4 square kilo meters in the north coast of Daya Bay,
Guangdong, which is at the southeast of China. 3,000 network
segments are established in the system. 16,000 control devices are
connected to the local area network. These devices belong to
different 9 sub systems, A control device has some control points,
which are controlled and monitored by other devices remotely. There
are 200,000 control points in all. They are controlled by 3
different control center.
S.Sakane, et al. [Page 6]
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Another example is a distributed system [NAM]. The NAM (Nederlandse
Aardolie Maatschappij) is operated by a partnership company of two
enterprises that represent the oil company. This system is
constituted by some plants that are geographically distributed within
the range of 863 square kilometers in the northern part of
Netherlands. 26 plants, each is named "cluster", are scattered in
the area. They are connected each other by a private ATM WAN. Each
cluster has approximately 500-1,000 control devices. These devices
are managed by each local control center in each cluster. In the
entire system of the NAM, there are one million control points.
In the both of the systems, the end devices are basically connected
to a local network by a twisted pair cable, which is a low band-width
of 32 kbps. Low clock CPU, for example H8 [RNSS-H8] and M16C [RNSS-
M16C], are employed by many control devices. Furthermore, to
suppress power consumption, these CPU may be lowered the number of
clocks. Because there is a requirement of the explosion-proof. The
requirement restricts the amount of total energy in the device.
A device on the network collects data from other devices which are
monitoring condition of the system. The device uses the data to make
a decision how to control another devices. And then the device gives
more than one instruction that controls other devices. If it took
time for data to reach, they could not be associated. The travel
time of data from the device to the other device is demanded within 1
second at least.
A part of the operation, like control of these system, maintenance,
and the environmental monitoring, is consigned to an external
organization. Agents who are consigned walk around the plant to get
their information, or watch the plant from a remote site.
4. Requirements
This section lists the requirements derived from the previous
section. R-1, R-2, R-3 and R-4 are related to the management of the
divided system. R-5, R-6 and R-7 are related to the restriction to
such industrial network.
R-1 It is necessary to partition a management domain into some
domains. Or it is necessary to delegate a management authority
to another independent management domain.
R-2 It is necessary to allow different independent management
domains to coexist on the same network because two or more
organizations need to enter into the system and to management
S.Sakane, et al. [Page 7]
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it.
R-3 It is necessary that a device controls other devices that belong
to a different domain.
R-4 It is necessary to consider that a device is not always
geographically or network topologically close to the other
devices even when the devices belong to a same management
domain.
R-5 It is demanded to reduce the management cost as much as
possible.
R-6 It is necessary to consider the processing performance of the
device. And, it is necessary to suppress the power consumption
of the device.
R-7 It is necessary to consider bandwidth of the communication.
5. Issues
This section lists the issues in the cross-realm operation when we
apply the Kerberos version 5 into the system described in the section
3, and consider the system applied the Kerberos with the requirements
described in the section 4.
5.1. Unreliability of authentication chain
When the relationship of trust is constructed like a chain or
hierarchical, the authentication path is not dependable since it
strongly depends on intermediary realms that might not be under the
same authority. If any of the realms in the authentication path is
not available, then the principals of the end-realms can not perform
the cross-realm operation.
The end-point realms do not have full control and responsibility of
the success of the operations even if their respective KDCs are fully
functional. Dependability of a system decreases if the system relies
on uncontrolled components. We can not be sure at 100% about the
result of the authentication since we do not know how is it going in
intermediary realms.
This issue will happen as a by-product of a result meeting the
requirements R-1 and R-2.
S.Sakane, et al. [Page 8]
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5.2. Possibility of MITM in case of the indirect trust model
Every KDC in the authentication path knows the shared secret between
the client and the remaining KDCs in the authentication path. This
allows a malicious KDC to perform MITM attacks on communications
between the client and any KDC in the remaining authentication chain.
A malicious KDC also may learn the service session key that is used
to protect the communication between the client and the actual
application service, and performs a MITM attack between them.
In [SPECCROSS], the authors have analyzed the cross-realm operations
in Kerberos and provided formal proof of the issue discussed in this
section.
This issue will happen as a by-product of a result meeting the
requirements R-1 and R-2.
5.3. Scalability of the direct trust model
In the direct relationship of trust between each realm, the realms
involved in the cross-realm operation share keys and their respective
TGS principals are registered in each other's KDC. When direct trust
relationships are used, the KDC of each realm must maintain keys with
all foreign realms. This can become a cumbersome task when the
number of realms increase. This also increases maintenance cost.
This issue will happen as a by-product of a result meeting the
requirements R-1, R-2 and R-5.
5.4. Exposure to DoS Attacks
One of the assumption made when allowing the cross-realm operation in
Kerberos is that users can communicate with KDCs located in remote
realms. This practice introduces security threats because KDCs are
open to the public network. Administrators may think of restricting
the access to the KDC to the trusted realms only. However, this
approach is not scalable and does not really protect the KDC.
Indeed, when the remote realms have several IP prefixes (e.g. control
centers or outsourcing companies, located world wide), then the
administrator of the local KDC must collect the list of prefixes that
belong to these organization. The filtering rules must then
explicitly allow the incoming traffic from any host that belongs to
one of these prefixes. This makes the administrator's tasks more
complicated and prone to human errors. And also, the maintenance
cost increases. On the other hand, when ranges of external IP
addresses are allowed to communicate with the KDC, the risk of
S.Sakane, et al. [Page 9]
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becoming target to attacks from remote malicious users increases.
This issue will happen as a by-product of a result meeting the
requirements R-3, R-4 and R-5.
5.5. Client's performance
In the cross-realm operation, Kerberos clients have to perform TGS
exchanges with all the KDCs in the trust path, including the home KDC
and the target KDC. TGS exchange requires cryptographic operations.
This exchange demands important processing time especially when the
client has limited computational capabilities. The overhead of these
cross-realm exchanges grows into unacceptable delays.
We ported the MIT Kerberos library (version 1.2.4), implemented a
Kerberos client on our original board with H8 (16-bit, 20MHz), and
measured the process time of each Kerberos message [KRBIMPL]. It
takes 195 milliseconds to perform a TGS exchange with the on-board
H/W crypto engine. Indeed, this result seems reasonable to the
requirement of the response time for the control network. However,
we did not modify the clock speed of the H8 during our measurement.
The processing time must be slower in a actual environment because H8
is used with lowered clock speed in such system. Also, the delays
can grow to unacceptable delays when the number of intermediary
realms increases.
This issue will happen as a by-product of a result meeting the
requirements R-1, R-2, R-6 and R-7.
5.6. Pre-authentication problem in roaming scenarios
In roaming scenarios, the client needs to contact her home KDC to
obtain a cross-realm TGT for the local (or visited) realm. However,
the policy of the network access providers or the gateway in the
local network usually does not allow clients to communicate with
hosts in the Internet unless they provide valid authentication
credentials. In this manner, the client encounters a chicken-and-egg
problem where two resources are interdependent; the Internet
connection is needed to contact the home KDC and for obtaining
credentials, and on the other hand, the Internet connection is only
granted for clients who have valid credentials. As a result, the
Kerberos protocol can not be used as it is for authenticating roaming
clients requesting network access.
This issue will happen as a result meeting the requirements R-3 and
R-4.
S.Sakane, et al. [Page 10]
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6. Implementation consideration
This document just describes issues of the cross-realm operation.
However, there are important matters to be considered, when we solve
these issues and implement solution. Solution must not introduce new
problem. It should use existing components or protocols as much as
possible, and it should not introduce any definition of new
component. It should not require new changes to existing deployed
clients, and it should not influence the client code-base as much as
possible. Because a KDC is a significant server of the Kerberos
system. New burden should not be introduced into a KDC as much as
possible. You must not forget that there would be a trade-off matter
anytime. So an implementation may not solve all of the problems
stated in this document.
7. IANA Considerations
This document makes no request of IANA.
8. Security Considerations
This document clarifies the issues of the cross-realm operation of
the Kerberos V system, which include security issues to be
considered. See Section 5.1, 5.2, 5.3 and 5.4 for further details.
9. Acknowledgments
The authors are grateful to Nobuo Okabe, Kazunori Miyazawa, and
Atsushi Inoue. They gave us lots of comments and suggestions to this
document from the early stage. Nicolas Williams, Chaskiel Grundman
and Love Hornquist Astrand gave valuable suggestions and corrections.
Finally, the authors thank to Jeffrey Hutzelman. He gave us a lot of
suggestions for completion of this document.
10. References
10.1. Normative References
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC
4120, July 2005.
S.Sakane, et al. [Page 11]
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10.2. Informative References
[CSPC] http://www.shellchemicals.com/news/1,1098,72-news_id=
531,00.html
[KRBIMPL] "A Prototype of a Secure Autonomous Bootstrap Mechanism
for Control Networks", Nobuo Okabe, Shoichi Sakane,
Masahiro Ishiyama, Atsushi Inoue and Hiroshi Esaki,
SAINT, pp. 56-62, IEEE Computer Society, 2006.
[NAM] http://www.nam.nl/
[RNSS-H8] http://www.renesas.com/fmwk.jsp?cnt=h8_family_landing.
jsp&fp=/products/mpumcu/h8_family/
[RNSS-M16C] http://www.renesas.com/fmwk.jsp?cnt=m16c_family_landi
ng.jsp&fp=/products/mpumcu/m16c_family/
[SHELLCHEM] http://www.shellchemicals.com/home/1,1098,-1,00.html
[SPECCROSS] I. Cervesato and A. Jaggard and A. Scedrov and C.
Walstad, "Specifying Kerberos 5 Cross-Realm
Authentication", Fifth Workshop on Issues in the Theory
of Security, Jan 2005.
Authors' Addresses
Shoichi Sakane
Ken'ichi Kamada
Yokogawa Electric Corporation
2-9-32 Nakacho, Musashino-shi,
Tokyo 180-8750 Japan
E-mail: Shouichi.Sakane@jp.yokogawa.com,
Ken-ichi.Kamada@jp.yokogawa.com
Saber Zrelli
Japan Advanced Institute of Science and Technology
1-1 Asahidai, Nomi,
Ishikawa 923-1292 Japan
E-mail: zrelli@jaist.ac.jp
S.Sakane, et al. [Page 12]
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Masahiro Ishiyama
Toshiba Corporation
1, komukai-toshiba-cho, Saiwai-ku,
Kawasaki 212-8582 Japan
E-mail: masahiro@isl.rdc.toshiba.co.jp
S.Sakane, et al. [Page 13]