INTERNET-DRAFT S. Sakane
Intended Status: Informational Ken'ichi Kamada
Expires: July 9, 2010 S. Zrelli
Yokogawa Electric Corp.
M. Ishiyama
Toshiba Corp.
January 5, 2010
Problem statement on the cross-realm operation of Kerberos
draft-ietf-krb-wg-cross-problem-statement-06.txt
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Abstract
The Kerberos protocol is today one of the most widely deployed
authentication protocols in the Internet. In order for a Kerberos
deployment to operate in a scalable manner, different Kerberos realms
must interoperate in such a way that cross-realm operations can be
performed efficiently and securely.
This document provides background information regarding large scale
Kerberos deployments in the industrial sector, with the aim of
identifying issues in the current Kerberos cross-realm authentication
model as defined in RFC4120.
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 current document also identifies a number of requirements derived
from the industrial automation field. Although they are found in the
field of industrial automation, these requirements are general enough
and are applicable to the problem of Kerberos cross-realm operations.
Conventions used in this document
The reader is assumed to be familiar with the terms and concepts
described in the Kerberos Version 5 [RFC4120].
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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 ....................................................... 9
5.1. Unreliability of authentication chain ................... 9
5.2. Possibility of MITM in the indirect trust model ......... 9
5.3. Scalability of the direct trust model ................... 10
5.4. Exposure to DoS Attacks ................................. 10
5.5. Client's performance .................................... 10
5.6. Kerberos Pre-authentication problem in roaming scenarios 11
6. Implementation considerations ................................ 11
7. IANA Considerations .......................................... 12
8. Security Considerations ...................................... 12
9. Acknowledgements ............................................. 12
10. References ................................................... 12
10.1. Normative References ................................... 12
10.2. Informative References ................................. 12
Authors' Addresses ............................................... 13
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1. Introduction
Kerberos Version 5 is a widely deployed mechanism that enables a
server to authenticate a client before granting it access to a
certain service. 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
authentication.
There exist several ways for using Kerberos in large scale
distributed systems. Such infrastructures are typically split into
several managed domains for geographical reasons, and to implement
different management policies. In order to ensure smooth network
operations in such systems, a common authentication mechanism for the
different managed domains is required. When using the Kerberos
cross-realm operation in large scale distributed systems some issues
arise.
This document briefly describes the Kerberos Version 5 system and its
cross-realm operation mode. Then it describes two case-study systems
that Kerberos could be applied to, and describes seven requirements
in those systems in terms both of management and operations that
outline various constraints which Kerberos operations might be
subjected to. Finally, it lists six issues related to Kerberos
cross-realm operations when applied to those systems.
Note that this document might not describe all issues related to
Kerberos cross-realm operations. 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. Detailed analysis
of the issues, problem definitions and exploration of possible
solutions may be carried out as separate work items.
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.
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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 kinds of tickets: Ticket Granting Ticket (TGT) and Service
Ticket. The TGT is obtained periodically from the KDC and has a
limited lifetime 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 request for a TGT is issued by
the 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 she shares with the KDC. The TGT
then is used to obtain a 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 of 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 received from the user, the TGS extracts the TGS session key.
Using that session key, it decrypts the Authenticator and
authenticates the user. Then, the TGS issues the credentials
requested by the user. These credentials consist of a service ticket
and a session key that will be used to authenticate the user to 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
(practically materialized in shared 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, we are in a so-called indirect trust model situation.
In this situation, the home KDC will provide a TGT that can be used
with an intermediary foreign realm that is likely to be sharing
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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
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 accesses the service using the
service ticket.
When the realms belong to the same institution, a chain of trust can
be automatically determined by the client or the KDC by following the
DNS domain hierarchy and assuming that a parent domain shares keys
with all its child sub-domains. However, since this assumption is
not always true, in many situations, the trust path might have to be
specified manually. Since the Kerberos cross-realm operations with
indirect inter-realm trust model rely on intermediary realms, 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 requirements and restrictions for
cross-realm authentication operations, this section describes the
scale and operations of two actual systems that could be supported by
cross-realm Kerberos. The two systems would be most naturally be
implemented using different trust models, which will imply different
requirements for cross-realm Kerberos.
Hereafter, we will consider an actual petrochemical company
[SHELLCHEM], and overview two examples among its plants.
Petrochemical companies produce bulk petrochemicals and deliver them
to large industrial customers. The company in consideration
possesses 43 plants all over the world managed by operation sites in
35 countries. This section shows two examples of these plants.
The first example is a plant deploying a centralized system [CSPC].
CSPC is operated by a joint enterprise of two companies. This system
is one of the largest systems of this company in the world. It is
located in an area of 3.4 square kilometers in the north coast of
Daya Bay, Guangdong, in southeast China. 3,000 network segments are
deployed in the system and 16,000 control devices are connected to
local area networks. These devices belong to 9 different subsystems.
A control device can have many control and monitoring points. In the
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plant considered in this example, there are 200,000 control points in
all. They are controlled by 3 different control centers.
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 composed
of some plants that are geographically distributed within the range
of 863 square kilometers in the northern part of Netherlands. 26
plants, each one called "cluster", are scattered in the area. They
are connected to each other by a private ATM WAN. Each cluster has
approximately 500-1,000 control devices. These devices are managed
by local control center in each cluster. In the entire system of the
NAM, there are one million control points.
In the both examples, the end devices are basically connected to a
local network by a twisted pair cable, with a low band-width of 32
kbps. End devices use a low clock CPU, for example the H8 [RNSS-H8]
and M16C [RNSS-M16C]. Furthermore, to reduce power consumption, the
clock on the CPU may be lowered. This adjustment restricts the
amount of total energy in the device, thereby reducing the risk of
explosions.
A device on the network collects data from other devices monitoring
the condition of the system. This date is then used to make
decisions on how to control other devices with instructions
transmitted over the network. If it takes time for data to travel
through the network, normal operations can not be ensured. The
travel time of data from a device to another device in the both
examples must be within 1 second at most. Other control system
applications may have shorter or longer timescales.
Some parts of the operations such as control, maintenance, and
environmental monitoring can be consigned to an external
organization. Also, agents may be consigned to walk around the plant
and collect information about the plant operations, or watch the
plant from a remote site.
4. Requirements
This section provides a list of requirements derived from the
industrial automation use-case. The requirements are written in a
generic fashion, and are addressed towards frameworks and
architectures that underlie Kerberos cross-realm operations. The aim
of these requirements is to provide some foundational guidelines to
the future developments of cross-realm framework or architecture for
Kerberos.
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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
large scale industrial network.
R-1 For organizational reasons and scalability needs, a management
domain typically must be partitioned into two or more sub-
domains of management. Therefore, any architecture and
implementation solution to the Kerberos cross-realm problem must
(i) support the case of cross-realm operations across multiple
management domains and (ii) support delegation of management
authority from one domain to another management domain. This
must be performed without any decrease in the security level or
quality of those cross-realm operations and must not expose
Kerberos entities to new types of attacks.
R-2 Any architecture and implementation solution to the Kerberos
cross-realm problem must support the co-existence of multiple
independent management domains on the same network.
Furthermore, it must allow organizations (corresponding to
different management domains) to delegate the management of a
part of or the totality of their system at any one time.
R-3 Any architecture and implementation solution to the Kerberos
cross-realm problem must allow the use-case in which one device
operationally controls another device, but each belongs to
different management domains respectively.
R-4 Any architecture and implementation solution to the Kerberos
cross-realm problem must address the fundamental deployment use-
case in which the management domain traverses geographic
boundaries and network topological boundaries. In particular,
it must address the case where devices are geographically (or
topologically) remote, even though they belong to the same
management domain.
R-5 Any architecture and implementation solution to the Kerberos
cross-realm problem must be aimed at reducing operational and
management costs as much as possible.
R-6 Any architecture and implementation solution to the Kerberos
cross-realm problem must address the (limited) processing
capabilities of devices, and implementations of solutions must
be considered to aim at limiting or suppressing power
consumption of such devices.
R-7 Any architecture and implementation solution to the Kerberos
cross-realm problem must address the possibility of limited
availability of communications bandwidth between devices within
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one domain, and also across domains.
5. Issues
This section lists issues in Kerberos cross-realm operations when
used in large scale systems such as the ones described in section 3,
and taking in consideration the requirements described in section 4.
5.1. Unreliability of authentication chain
When the trust relationship between realms follows chain or
hierarchical model, the cross-realm authentication operations are no
dependable since they strongly depend 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 cross-realm operations.
The end-point realms do not have full control and responsibility of
the success of the cross-realm operations even if their own
respective KDCs are fully functional. Dependability of a system
decreases if the system relies on uncontrolled components. End-point
realms have no way of knowing the authentication result occurring
within intermediary realms.
Satisfying requirements R-1 and R-2 will eliminate (or considerably
diminish) this issue of the unreliability of the authentication
chain.
5.2. Possibility of MITM in 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. It can then use this key to perform a MITM
attack.
In [SPECCROSS], the authors have analyzed the cross-realm operations
in Kerberos and provided formal proof of the issue discussed in this
section.
Satisfying requirements R-1 and R-2 will eliminate (or considerably
diminish) this issue of MITM attacks by intermediate KDCs in the
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indirect trust model.
5.3. Scalability of the direct trust model
In the direct trust relationship model, the realms involved in the
cross-realm operations share keys and their respective TGS principals
are registered in each other's KDC. Each realm must maintain keys
with all foreign realms that it interacts with. This can become a
cumbersome task and may increase maintenance costs when the number of
realms increases.
Satisfying requirements R-1, R-2 and R-5 will eliminate (or
considerably diminish) this issue of scalability of the indirect
trust model.
5.4. Exposure to DoS Attacks
One of the assumptions 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 a range of external IP
addresses are allowed to communicate with the KDC then the risk of
becoming target to attacks from remote malicious users increases.
Satisfying requirements R-1, R-3, R-4 and R-5 will eliminate (or
considerably diminish) this issue of exposure to DoS attacks.
5.5. Client's performance
In Kerberos cross-realm operations, clients have to perform TGS
exchanges with all the KDCs in the trust path, including the home KDC
and the target KDC. A TGS exchange requires cryptographic operations
and may consume a large amount of processing time especially when the
client has limited computational capabilities. As a result, the
overhead of Kerberos cross-realm exchanges may grows into
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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. With such devices,
the delays can grow to unacceptable delays when the number of
intermediary realms increases.
Satisfying requirements R-1, R-2, R-6 and R-7 will eliminate (or
considerably diminish) this issue relating to the client's
performance.
5.6. Kerberos 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. Typically, a VPN approach is
applied to solve this problem. However, we can not always establish
VPNs between different sites.
Satisfying requirements R-3, R-4 and R-5 will eliminate (or
considerably diminish) this roaming-related issue pertaining to
Kerberos pre-authentication.
6. Implementation considerations
This document describes issues of the cross-realm operation. There
are important matters to be considered, when designing and
implementing solutions for these issues. Solution must not introduce
new problems. Any solution should use existing components or
protocols as much as possible, and it should avoid introducing
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definitions of new components. 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
in an information system based on Kerberos. New burden on the KDC
should be minimal. Solutions must take these tradeoffs and the
requirements into consideration. On the other hand, solutions are
not required to solve all the issues listed in this document at once.
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. Acknowledgements
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.
Thomas Hardjono devoted much work and helped to improve this
document. 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.
10.2. Informative References
[CSPC] http://www.shellchemicals.com/news/1,1098,72-news_id=
531,00.html
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[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
Yokogawa Electric Corporation
2-9-32 Nakacho, Musashino-shi,
Tokyo 180-8750 Japan
E-mail: Shouichi.Sakane@jp.yokogawa.com
Ken'ichi Kamada
Yokogawa Electric Corporation
2-9-32 Nakacho, Musashino-shi,
Tokyo 180-8750 Japan
E-mail: Ken-ichi.Kamada@jp.yokogawa.com
Saber Zrelli
Yokogawa Electric Corporation
2-9-32 Nakacho, Musashino-shi,
Tokyo 180-8750 Japan
E-mail: Saber.Zrelli@jp.yokogawa.com
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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
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