Kerberos etype negotiate: Kerberos Cryptosystem Negotiation Extension

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).

Abstract

   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

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",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   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
   enctypes.

   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

   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
   subkey is sent in the subkey field of AP-REP message, and it is then
   used as the protocol key or base key [RFC3961] for subsequent
   communication.

   If the enctype of the ticket session key is included in the enctype
   list sent by the client, it SHOULD be the last on the list;
   otherwise, this enctype MUST NOT be negotiated if it was not included
   in the list.

   This negotiation extension SHOULD NOT be used when the client does
   not expect the subkey in the AP-REP message from the server.

   A note on key generation: The KDC has a strong Pseudo-Random Number
   Generator (PRNG); as such, the client can take advantage of the
   randomness provided by the KDC by reusing the KDC key data when
   generating keys.  Implementations SHOULD use the service ticket
   session key value as a source of additional entropy when generating
   the negotiated subkey.  If the AP-REQ authenticator subkey is
   present, it MAY also be used as a source of entropy.

   The server MAY ignore the preference order indicated by the client.
   The policy by which the client or the server chooses an enctype
   (i.e., how the preference order for the supported enctypes is
   selected) is a local matter.

4.  Security Considerations

   The client's enctype list and the server's reply enctype are part of
   encrypted data; thus, the security considerations are the same as
   those of the Kerberos encrypted data.

   Both the EtypeList and the server's sub-session key are protected by
   the session key or sub-session key used for the AP-REQ, and as a
   result, if a key for a stronger enctype is negotiated underneath a
   key for a weaker enctype, an attacker capable of breaking the weaker
   enctype can also discover the key for the stronger enctype.  The
   advantage of this extension is to minimize the amount of cipher text
   encrypted under a weak enctype to which an attacker has access.

5.  Acknowledgements

   The authors would like to thank the following individuals for their
   comments and suggestions: Ken Raeburn, Luke Howard, Tom Yu, Love
   Hornquist Astrand, Sam Hartman, and Martin Rex.

6.  Normative References

   [RFC2119]  Bradner, S., "Key words for use in RFCs to Indicate
              Requirement Levels", BCP 14, RFC 2119, March 1997.

   [RFC3961]  Raeburn, K., "Encryption and Checksum Specifications for
              Kerberos 5", RFC 3961, February 2005.

   [RFC4120]  Neuman, C., Yu, T., Hartman, S., and K. Raeburn, "The
              Kerberos Network Authentication Service (V5)", RFC 4120,
              July 2005.

   [X680]     ITU-T Recommendation X.680 (2002) | ISO/IEC 8824-1:2002,
              Information technology - Abstract Syntax Notation One
              (ASN.1): Specification of basic notation.

   [X690]     ITU-T Recommendation X.690 (2002) | ISO/IEC 8825-1:2002,
              Information technology - ASN.1 encoding Rules:
              Specification of Basic Encoding Rules (BER), Canonical
              Encoding Rules (CER) and Distinguished Encoding Rules
              (DER).

Authors' Addresses

   Larry Zhu
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   EMail: lzhu@microsoft.com


   Paul Leach
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   EMail: paulle@microsoft.com


   Karthik Jaganathan
   Microsoft Corporation
   One Microsoft Way
   Redmond, WA  98052
   US

   EMail: karthikj@microsoft.com

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