


Network Working Group                               L. Hornquist Astrand
Internet-Draft                                      Stockholm University
Expires: September 2, 2006                                        L. Zhu
                                                   Microsoft Corporation
                                                              March 2006


                       PK-INIT algorithm agility
                draft-ietf-krb-wg-pkinit-alg-agility-00

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Copyright Notice

   Copyright (C) The Internet Society (2006).

Abstract

   The PK-INIT protocol have in several places hard coded crypto
   algorithms.  The protocol specification needs to be updated so it can
   support negotiation to upgrading to newer versions of crypto
   algorithms.  This document addresses this issue.






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Table of Contents

   1.  Introduction . . . . . . . . . . . . . . . . . . . . . . . . .  3
   2.  Requirements notation  . . . . . . . . . . . . . . . . . . . .  4
   3.  paChecksum agility . . . . . . . . . . . . . . . . . . . . . .  5
   4.  CMS Digest Algorithm agility . . . . . . . . . . . . . . . . .  6
   5.  Certificate Signer Algorithm Identifier agility  . . . . . . .  7
   6.  octetstring2key function agility . . . . . . . . . . . . . . .  8
   7.  Security Considerations  . . . . . . . . . . . . . . . . . . .  9
   8.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . . 10
   9.  Normative References . . . . . . . . . . . . . . . . . . . . . 10
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 11
   Intellectual Property and Copyright Statements . . . . . . . . . . 12






































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1.  Introduction

   The Kerberos PK-INIT document contains several hardcoded algorithms
   that was know designed at design time that they had to be replaced by
   something else at a later time, this document described how to use
   other algorithms other then those that are hard-coded.













































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2.  Requirements notation

   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].














































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3.  paChecksum agility

   The paChecksum binds the PK-INIT part of the request to main body of
   the Kerberos request (KDC-REQ-BODY).  This is to makes sure an
   attacker can not change the request from the client to the server.
   The problem is that paChecksum is hardcoded to use SHA1-1, however,
   there is a mechaism to provide algorithm agility for the paChecksum
   within the PK-INIT prototcol.  Newer clients can choose not send the
   paChecksum field, but rather add some new fields after the existing
   fields, older KDC will send back know failure-code so that newer
   clients can fall back to the old protocol if local policy allows
   that.

   If the attacker can preserve the checksum in paChecksum, an attacker
   can, for example, change the KDC-REQ-BODY is to downgrade the
   encryption types used, expend the expiration time, etc, and then try
   to brute-force the request.

   In the Public Key Encryption case of PK-INIT the reply contains a
   checksum over the whole request in the asChecksum field, in this case
   the client will detect any modifications to the request.  Since the
   asChecksum is using the associated checksum of the session key
   encryption type, asChecksum field is algorithm agile.

   One way to solve this problem is to add the asChecksum to the Diffie-
   Hellman case reply too, and just ignore the paCheckSum field.  The
   KDC should still not issue tickets that are too weak, since that
   exposes the problem.  This is regardless of the using PK-INIT or not.

   Questions for wg: Wait for Kerberos Extensions that will solve this
   problem (ignore the problem for how), or use add asChecksum to DH
   case.



















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4.  CMS Digest Algorithm agility

   The client can tell KDC what the supported CMS types are in the
   requset packet, but there are no equivalent for KDC to the the client
   what the digest algorithm are support in an reply.

   Have KDC send the CMS list of supported encryption types in the
   e-data field of KRB-ERROR when returning the
   KDC_ERR_DIGEST_IN_SIGNED_DATA_NOT_ACCEPTED error.

   DER encoded TS-SD-PARAMETERS specifies supported digest algorithms.
   The list is in decreasing preference order.



   TD-SD-PARAMETERS ::= SEQUENCE OF AlgorithmIdentifier



































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5.  Certificate Signer Algorithm Identifier agility

   The KDC can reject a certificate based on the signers hash algorithm
   with the error KDC_ERR_DIGEST_IN_CERT_NOT_ACCEPTED, but doesn't tell
   the client what algorithm are supported.

   DER encoded TS-DC-PARAMETERS specifies supported certificate digest
   algorithms.  The AllowedAlgorithms is in decreasing preference order.
   RejectedAlgorithm may be include my the KDC to tell what algorithm
   was rejected in case the rejected certificate was part of a computed
   chain.



   TD-DC-PARAMETERS ::= SEQUENCE {
       AllowedAlgorithms [0] SEQUENCE OF AlgorithmIdentifier,
       RejectedAlgorithm [1] AlgorithmIdentifier OPTIONAL
   }

































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6.  octetstring2key function agility

   The PK-INIT standard uses a home-grown string 2 key function in the
   DH case.  The function uses SHA-1 to mix and stretch the DH shared
   key.

   Describe how the client announces that is supports the new String to
   key function.  Probably by stuffing it into the supportCMSTypes field
   in the request.

   Use NIST SP 800 56B when its published.








































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7.  Security Considerations

   This document describes negotiation of checksum types and other
   cryptographic functions.  Most of this negotiation is done
   unauthenticated with no way to very














































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8.  IANA Considerations

   No IANA considerations.

9.  Normative References

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











































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Authors' Addresses

   Love Hornquist Astrand
   Stockholm University
   SE-106 91  STOCKHOLM
   SWEDEN

   Email: lha@it.su.se


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

   Email: lzhu@microsoft.com


































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