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INTERNET-DRAFT S. Sakane
Intended Status: Informational Yokogawa Electric Corp.
Expires: January 10, 2008 S. Zrelli
JAIST
M. Ishiyama
Toshiba Corp.
July 9, 2007
Problem statement on the cross-realm operation
of Kerberos in a specific system
draft-sakane-krb-cross-problem-statement-03.txt
Status of this Memo
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This Internet-Draft expires in January 10, 2008.
Copyright Notice
Copyright (C) The IETF Trust (2007).
S.Sakane, et al. [Page 1]
Internet-Draft July 2007
Abstract
There are some issues when the cross-realm operation of the Kerberos
Version 5 [RFC4120] is employed into actual specific systems. This
document describes some examples of actual systems, and lists
requirements and restriction of the operation in such system. Then
it describes issues when we apply the cross-realm operation to such
system.
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 2007
Table of Contents
1. Introduction ................................................. 4
2. Kerberos system .............................................. 4
2.1. Kerberos basic operation ................................ 4
2.2. Cross-realm operation ................................... 5
3. Example of actual environment ................................ 6
4. Requirements ................................................. 7
5. Issues ....................................................... 8
5.1. Unreliability of authentication chain ................... 8
5.2. No PFS in case of the indirect trust model .............. 8
5.3. Scalability of the direct trust model ................... 9
5.4. Exposure to DoS Attacks ................................. 9
5.5. Client's performance .................................... 9
5.6. Pre-authentication problem in roaming scenarios ......... 10
6. Implementation consideration ................................. 10
7. IANA Considerations .......................................... 11
8. Security Considerations ...................................... 11
9. Acknowledgments .............................................. 11
10. References ................................................... 11
10.1. Normative References ................................... 11
10.2. Informative References ................................. 11
Authors' Addresses ............................................... 12
Full Copyright Statement ......................................... 12
Intellectual Property Statement .................................. 13
S.Sakane, et al. [Page 3]
Internet-Draft July 2007
1. Introduction
The Kerberos Version 5 is a widely deployed mechanism that a server
can authenticate a client access. Each client belongs to a managed
domain called realm. Kerberos supports the authentication in case of
situation that a client and a server belong to different realms.
This is called the cross-realm operation.
Meanwhile, there are lots of manners of operation in actual systems,
where Kerberos could be applied. Large system or distributed system
are typically split into several managed domain. The reason is, for
example, geographical reason or different management policy. Even in
such system, an authentication mechanism over 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 the
cross-realm operation. Then, it describes two actual systems that
could be applied the Kerberos system, 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 whole of issues of the
cross-realm operation. It also does not propose any solution to
solve issues which described in this document. In further step, we
have to analyze the issues, define problems and explore the
solutions. This work will be 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 principals database and shares a secret key with
each registered principal.
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
S.Sakane, et al. [Page 4]
Internet-Draft July 2007
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 the 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 the home realm and the foreign
realm share keys and have an established trust relationship, 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
'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
S.Sakane, et al. [Page 5]
Internet-Draft July 2007
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. 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. Example of actual environment
In order to help understanding both requirements and restriction,
this section describes scale and operation of actual systems, where
it is possible to apply 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.
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
S.Sakane, et al. [Page 6]
Internet-Draft July 2007
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
it.
R-3 It is necessary that a device controls other devices that belong
to a different domain.
S.Sakane, et al. [Page 7]
Internet-Draft July 2007
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.
5.2. No PFS in case of the indirect trust model
In [SPECCROSS], any KDC in the authentication path can learn the
session key that will be used between the client and the desired
service. This means that any intermediary realm is able to spoof the
S.Sakane, et al. [Page 8]
Internet-Draft July 2007
identity either of the service or the client as well as to eavesdrop
on the communication between the client and the server.
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
becoming target to attacks from remote malicious users increases.
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.
S.Sakane, et al. [Page 9]
Internet-Draft July 2007
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.
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. Solution should use existing components or protocols as
much as possible, should not introduce any definition of new
component. Solution must not require a KDC to have any additional
process. 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.
S.Sakane, et al. [Page 10]
Internet-Draft July 2007
7. IANA Considerations
This document makes no request of IANA.
8. Security Considerations
This document just clarifies some issues of the cross-realm operation
of the Kerberos V system. There is especially not describing
security. Some troubles might be caused to your system by malicious
user who misuses the description of this document if it dares to say.
9. Acknowledgments
The authors are very grateful to Nobuo Okabe, Kazunori Miyazawa,
Ken'ichi Kamada and Atsushi Inoue. They gave us lots of comments and
input for 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.
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/
S.Sakane, et al. [Page 11]
Internet-Draft July 2007
[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
Yokogawa Electric Corporation
2-9-32 Nakacho, Musashino-shi,
Tokyo 180-8750 Japan
E-mail: Shouichi.Sakane@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
Masahiro Ishiyama
Toshiba Corporation
1, komukai-toshiba-cho, Saiwai-ku,
Kawasaki 212-8582 Japan
E-mail: masahiro@isl.rdc.toshiba.co.jp
Full Copyright Statement
Copyright (C) The IETF Trust (2007).
This document is subject to the rights, licenses and restrictions
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This document and the information contained herein are provided on an
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THE INFORMATION HEREIN WILL NOT INFRINGE ANY RIGHTS OR ANY IMPLIED
WARRANTIES OF MERCHANTABILITY OR FITNESS FOR A PARTICULAR PURPOSE.
S.Sakane, et al. [Page 12]
Internet-Draft July 2007
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S.Sakane, et al. [Page 13]