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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   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

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]).

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

   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.

   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.

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.

   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

   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.

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.

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",
               <http://www.shell.com/home/content/global_solutions/
               aboutshell/key_projects/nanhai/>.

   [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]       Nederlandse Aardolie Maatschappij BV,
               <http://www.nam.nl/>.

   [RNSS-H8]   "H8 Family | Renesas Electronics",
               <http://www.renesas.com/products/mpumcu/h8/
               h8_landing.jsp>.

   [RNSS-M16C] "M16C Family (R32C/M32C/M16C) | Renesas Electronics",
               <http://www.renesas.com/products/mpumcu/m16c/
               m16c_landing.jsp>.

   [SHELLCHEM] "Shell Chemicals",
               <http://www.shell.com/home/content/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.

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