Internet Engineering Task Force (IETF) S. Sakane
Request for Comments: 5868 K. Kamada
Category: Informational S. Zrelli
ISSN: 2070-1721 Yokogawa Electric Corp.
M. Ishiyama
Toshiba Corp.
May 2010
Problem Statement on the Cross-Realm Operation of Kerberos
Abstract
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 RFC 4120.
This document describes some examples of actual large-scale
industrial systems, and lists requirements and restrictions regarding
authentication operations in such environments. It 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.
Status of This Memo
This document is not an Internet Standards Track specification; it is
published for informational purposes.
This document is a product of the Internet Engineering Task Force
(IETF). It represents the consensus of the IETF community. It has
received public review and has been approved for publication by the
Internet Engineering Steering Group (IESG). Not all documents
approved by the IESG are a candidate for any level of Internet
Standard; see Section 2 of RFC 5741.
Information about the current status of this document, any errata,
and how to provide feedback on it may be obtained at
http://www.rfc-editor.org/info/rfc5868.
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RFC 5868 Kerberos Cross-Realm Operation Problem Statement May 2010
Copyright Notice
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described in the Simplified BSD License.
Table of Contents
1. Introduction ....................................................3
2. Kerberos System .................................................4
2.1. Kerberos Basic Operation ...................................4
2.2. Cross-Realm Operation ......................................4
3. Applying Cross-Realm Kerberos in Complex Environments ...........5
4. Requirements ....................................................7
5. Issues ..........................................................8
5.1. Unreliability of Authentication Chain ......................8
5.2. Possibility of MITM in 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 ......................................10
5.6. Kerberos Pre-Authentication Problem in Roaming Scenarios ..10
6. Implementation Considerations ..................................11
7. Security Considerations ........................................11
8. Acknowledgements ...............................................11
9. References .....................................................11
9.1. Normative References ......................................11
9.2. Informative References ....................................11
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RFC 5868 Kerberos Cross-Realm Operation Problem Statement May 2010
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 a
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.
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 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 of both management and operations) that
outline various constraints that 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 the issues
described in this document. Furthermore, publication of this
document does not mean that each of the issues has 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 assumes that the readers are familiar with the terms
and concepts described in "The Kerberos Network Authentication
Service (V5)" ([RFC4120]).
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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.
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 is called "tickets". There are
two kinds of tickets: the Ticket-Granting Ticket (TGT) and the
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 consist of the TGT along
with a random key called the "TGS session key". The TGT contains
information encrypted using a secret key associated with a special
service, referred to as the "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 is then used to obtain a service ticket
from the 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
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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
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 her 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 of 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
the 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 the reader understand 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
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 under consideration
possesses 43 plants all over the world managed by operation sites in
35 countries. This section shows two examples of these plants.
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The first example is a plant deploying a centralized system [CSPC].
CSPC, also referred to as China National Offshore Oil Corporation
(CNOOC) and Shell Petrochemicals Company, is operated by a joint
enterprise of these two companies. This system is one of the largest
of its type 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 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 Nederlandse
Aardolie Maatschappij (NAM) 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 the
Netherlands. 26 plants, each one called a "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 a local control center in each cluster. In
the entire system of the NAM, there are one million control points.
In both examples, the end devices are basically connected to a local
network by a twisted-pair cable, with a low bandwidth 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. These data are then used to make
decisions on how to control other devices via instructions
transmitted over the network. If it takes time for data to travel
through the network, normal operations cannot be ensured. The travel
time of data from a device to another device in both examples must be
within 1 second. 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 plant operations, or to watch the plant
from a remote site.
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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
future developments of cross-realm framework or architecture for
Kerberos.
Requirements R-1, R-2, R-3, and R-4 are related to the management of
the divided system. Requirements R-5, R-6, and R-7 are related to
restrictions in such large-scale industrial networks as those
discussed in Section 3.
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 management 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.
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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
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 into consideration the requirements described in
Section 4.
5.1. Unreliability of Authentication Chain
When the trust relationship between realms follows a chain or
hierarchical model, the cross-realm authentication operations are not
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 cannot perform cross-realm operations.
The end-point realms do not have full control of and responsibility
for 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 man-in-the-middle (MITM) attacks on
communications between the client and any KDC in the remaining
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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
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's
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 direct 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 worldwide), then
the administrator of the local KDC must collect the list of prefixes
that belong to these organizations. 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. 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
targets of 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 denial-of-service
(DoS) attacks.
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RFC 5868 Kerberos Cross-Realm Operation Problem Statement May 2010
5.5. Client's Performance
In Kerberos cross-realm operations, clients have to perform TGS
exchanges with all of 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 cause
unacceptable delays in processing.
We ported the MIT Kerberos library (version 1.2.4), implemented a
Kerberos client on our original board with H8 (16-bit, 20 MHz), 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, given 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 an actual environment, because
H8 is used with a lowered clock speed in such a system. With such
devices, the delays can become unacceptable 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 cannot be used as it is for authenticating roaming
clients requesting network access. Typically, a VPN approach is
applied to solve this problem. However, we cannot 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.
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RFC 5868 Kerberos Cross-Realm Operation Problem Statement May 2010
6. Implementation Considerations
This document describes issues of Kerberos cross-realm operations.
There are important matters to be considered when designing and
implementing solutions for these issues. Such solutions must not
introduce new problems. Any solution should use existing components
or protocols as much as possible, and it should avoid introducing
definitions of new components. It should not require new changes to
existing deployed clients and as much as possible should not
influence the client code-base. Because a KDC is a significant
server in an information system based on Kerberos, any new burden
placed on the KDC should be minimal. Solutions must take these
tradeoffs and requirements into consideration. On the other hand,
solutions are not required to solve all of the issues listed in this
document at once.
7. 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 Sections 5.1, 5.2, 5.3, and 5.4 for further details.
8. Acknowledgements
The authors are grateful to Nobuo Okabe, Kazunori Miyazawa, and
Atsushi Inoue. They gave us lots of comments and suggestions for
this document from its earliest stages. 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 Jeffrey Hutzelman. He
gave us a lot of suggestions for completion of this document.
9. References
9.1. Normative References
[RFC4120] Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
Kerberos Network Authentication Service (V5)", RFC 4120,
July 2005.
9.2. Informative References
[CSPC] "CSPC Nanhai - Shell Global Solutions",
.
[RNSS-M16C] "M16C Family (R32C/M32C/M16C) | Renesas Electronics",
.
[SHELLCHEM] "Shell Chemicals",
.
[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, January 2005.
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Authors' Addresses
Shoichi Sakane
Yokogawa Electric Corporation
2-9-32 Nakacho, Musashino-shi
Tokyo 180-8750 Japan
EMail: Shouichi.Sakane@jp.yokogawa.com
Ken'ichi Kamada
Yokogawa Electric Corporation
2-9-32 Nakacho, Musashino-shi
Tokyo 180-8750 Japan
EMail: Ken-ichi.Kamada@jp.yokogawa.com
Saber Zrelli
Yokogawa Electric Corporation
2-9-32 Nakacho, Musashino-shi
Tokyo 180-8750 Japan
EMail: Saber.Zrelli@jp.yokogawa.com
Masahiro Ishiyama
Toshiba Corporation
1, Komukai Toshiba-cho, Saiwai-ku
Kawasaki 212-8582 Japan
EMail: masahiro@isl.rdc.toshiba.co.jp
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RFC, FYI, BCP