Kerberos Cryptosystem Negotiation Extension
RFC 4537

Document Type RFC - Proposed Standard (June 2006; No errata)
Updates RFC 4120
Last updated 2013-03-02
Stream IETF
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IESG IESG state RFC 4537 (Proposed Standard)
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Network Working Group                                             L. Zhu
Request for Comments: 4537                                      P. Leach
Updates: 4120                                              K. Jaganathan
Category: Standards Track                          Microsoft Corporation
                                                               June 2006

              Kerberos Cryptosystem Negotiation Extension

Status of This Memo

   This document specifies an Internet standards track protocol for the
   Internet community, and requests discussion and suggestions for
   improvements.  Please refer to the current edition of the "Internet
   Official Protocol Standards" (STD 1) for the standardization state
   and status of this protocol.  Distribution of this memo is unlimited.

Copyright Notice

   Copyright (C) The Internet Society (2006).


   This document specifies an extension to the Kerberos protocol as
   defined in RFC 4120, in which the client can send a list of supported
   encryption types in decreasing preference order, and the server then
   selects an encryption type that is supported by both the client and
   the server.

Table of Contents

   1.  Introduction  . . . . . . . . . . . . . . . . . . . . . . . . . 2
   2.  Conventions Used in This Document . . . . . . . . . . . . . . . 2
   3.  Negotiation Extension . . . . . . . . . . . . . . . . . . . . . 2
   4.  Security Considerations . . . . . . . . . . . . . . . . . . . . 4
   5.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 4
   6.  Normative References  . . . . . . . . . . . . . . . . . . . . . 4

Zhu, et al.                 Standards Track                     [Page 1]
RFC 4537                  Enctype Negotiation                  June 2006

1.  Introduction

   Under the current mechanism [RFC4120], the Kerberos Distribution
   Center (KDC) must limit the ticket session key encryption type
   (enctype) chosen for a given server to one it believes is supported
   by both the client and the server.  If both the client and server
   understand a stronger enctype than the one selected by the KDC, they
   cannot negotiate it.  As the result, the protection of application
   traffic is often weaker than necessary when the server can support
   different sets of enctypes depending on the server application
   software being used.

   This document specifies an extension to the Kerberos protocol to
   allow clients and servers to negotiate use of a different and
   possibly stronger cryptosystem in subsequent communication.

   This extension utilizes an authorization data element in the
   authenticator of the AP-REQ message [RFC4120].  The client sends the
   list of enctypes that it supports to the server; the server then
   informs the client of its choice.  The negotiated subkey is sent in
   the AP-REP message [RFC4120].

2.  Conventions Used in This Document

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in [RFC2119].

3.  Negotiation Extension

   If the client prefers an enctype over that of the service ticket
   session key, then it SHOULD send a list of enctypes in decreasing
   preference order to the server.  Based on local policy, the client
   selects enctypes out of all the enctypes available locally to be
   included in this list, and it SHOULD NOT include enctypes that are
   less preferable than that of the ticket session key in the service
   ticket.  In addition, the client SHOULD NOT include negative (local-
   use) enctype numbers unless it knows a priori that the server has
   been configured to use the same negative enctype numbers for the same

   The client sends the enctype list via the authorization-data of the
   authenticator in the AP-REQ [RFC4120].  A new authorization data
   element type AD-ETYPE-NEGOTIATION is defined.

           AD-ETYPE-NEGOTIATION              129

Zhu, et al.                 Standards Track                     [Page 2]
RFC 4537                  Enctype Negotiation                  June 2006

   This authorization data element itself is enclosed in the AD-IF-
   RELEVANT container; thus, a correctly implemented server that does
   not understand this element should ignore it [RFC4120].  The value of
   this authorization element contains the DER [X680] [X690] encoding of
   the following ASN.1 type:

           EtypeList ::= SEQUENCE OF Int32
              -- Specifies the enctypes supported by the client.
              -- This enctype list is in decreasing preference order
              -- (favorite choice first).
              -- Int32 is defined in [RFC4120].

   If the EtypeList is present and the server prefers an enctype from
   the client's enctype list over that of the AP-REQ authenticator
   subkey (if that is present) or the service ticket session key, the
   server MUST create a subkey using that enctype.  This negotiated
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