INTERNET-DRAFT                                        Jonathan Trostle
draft-ietf-cat-kerberos-extra-tgt-02.txt              Cisco Systems
Updates: RFC 1510                                     Michael M. Swift
expires January 30, 2000                              University of WA


    Extension to Kerberos V5 For Additional Initial Encryption

0. Status Of This Memo

   This document is an Internet-Draft and is in full conformance
   with all provisions of Section 10 of RFC2026.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF), its areas, and its working groups.  Note that
   other groups may also distribute working documents as
   Internet-Drafts.

   Internet-Drafts are draft documents valid for a maximum of six
   months and may be updated, replaced, or obsoleted by other
   documents at any time.  It is inappropriate to use Internet-
   Drafts as reference material or to cite them other than as
   "work in progress."

   The list of current Internet-Drafts can be accessed at
   http://www.ietf.org/ietf/1id-abstracts.txt

   The list of Internet-Draft Shadow Directories can be accessed at
   http://www.ietf.org/shadow.html.

1. Abstract

   This document defines an extension to the Kerberos protocol
   specification (RFC 1510) [1] to enable a preauthentication field in
   the AS_REQ message to carry a ticket granting ticket. The session
   key from this ticket granting ticket will be used to
   cryptographically strengthen the initial exchange in either the
   conventional Kerberos V5 case or in the case the user stores their
   encrypted private key on the KDC [2].


2. Motivation

   In Kerberos V5, the initial exchange with the KDC consists of the
   AS_REQ and AS_REP messages. For users, the encrypted part of the
   AS_REP message is encrypted in a key derived from a password.
   Although a password policy may be in place to prevent dictionary
   attacks, brute force attacks may still be a concern due to
   insufficient key length.

   This draft specifies an extension to the Kerberos V5 protocol to
   allow a ticket granting ticket to be included in an AS_REQ message
   preauthentication field. The session key from this ticket granting
   ticket will be used to cryptographically strengthen the initial

   exchange in either the conventional Kerberos V5 case or in the case
   the user stores their encrypted private key on the KDC [2]. The
   session key from the ticket granting ticket is combined with the
   user password key (key K2 in the encrypted private key on KDC
   option) using HMAC to obtain a new triple des key that is used in
   place of the user key in the initial exchange. The ticket granting
   ticket could be obtained by the workstation using its host key.

3. The Extension

   The following new preauthentication type is proposed:

      PA-EXTRA-TGT                         22

   The preauthentication-data field contains a ticket granting ticket
   encoded as an ASN.1 octet string. The server realm of the ticket
   granting ticket must be equal to the realm in the KDC-REQ-BODY of
   the AS_REQ message. In the absence of a trust relationship, the
   local Kerberos client should send the AS_REQ message without this
   extension.

   In the conventional (non-pkinit) case, we require the RFC 1510
   PA-ENC-TIMESTAMP preauthentication field in the AS_REQ message.
   If neither it or the PA-PK-KEY-REQ preauthentication field is
   included in the AS_REQ message, the KDC will reply with a
   KDC_ERR_PREAUTH_FAILED error message.

   We propose the following new etypes:

     des3-cbc-md5-xor                              16
     des3-cbc-sha1-xor                             17

   The encryption key is obtained by:

   (1) Obtaining an output M from the HMAC-SHA1 function [3] using
       the user password key (the key K2 in the encrypted private
       key on KDC option of pkinit) as the text and the triple des
       session key as the K input in HMAC:

       M = H(K XOR opad, H(K XOR ipad, text)) where H = SHA1.

       The session key from the accompanying ticket granting ticket
       must be a triple des key when one of the triple des xor
       encryption types is used.
   (2) Concatenate the output M (20 bytes) with the first 8 non-parity
       bits of the triple-des ticket granting ticket session key to
       get 168 bits that will be used for the new triple-des encryption
       key.
   (3) Set the parity bits of the resulting key.

   The resulting triple des key is used to encrypt the timestamp
   for the PA-ENC-TIMESTAMP preauthentication value (or in the
   encrypted private key on KDC option of pkinit, it is used in
   place of the key K2 to both sign in the PA-PK-KEY-REQ and for
   encryption in the PA-PK-KEY-REP preauthentication types).

   If the KDC decrypts the encrypted timestamp and it is not within
   the appropriate clock skew period, the KDC will reply with the
   KDC_ERR_PREAUTH_FAILED error. The same error will also be sent if
   the above ticket granting ticket fails to decrypt properly, or if
   it is not a valid ticket.

   The KDC will create the shared triple des key from the ticket
   granting ticket session key and the user password key (the key K2
   in the encrypted private key on KDC case) using HMAC as specified
   above and use it to validate the AS_REQ message and then to
   encrypt the encrypted part of the AS_REP message (use it in place
   of the key K2 for encryption in the PA-PK-KEY-REP preauthentication
   field).

   Local workstation policy will determine the exact behaviour of
   the Kerberos client with respect to the extension protocol. For
   example, the client should consult policy to decide when to use
   use the extension. This policy could be dependent on the user
   identity, or whether the workstation is in the same realm as the
   user. One possibility is for the workstation logon to fail if
   the extension is not used. Another possibility is for the KDC
   to set a flag in tickets issued when this extension is used.

   A similar idea was proposed in OSF DCE RFC 26.0 [4]; there a
   preauthentication field containing a ticket granting ticket,
   a randomly generated subkey encrypted in the session key from
   the ticket, and a timestamp structure encrypted in the user
   password and then the randomly generated subkey was proposed.
   Some advantages of the current proposal are that the KDC has two
   fewer decryptions to perform per request and the client does not
   have to generate a random key.

4. Bibliography

   [1] J. Kohl, C. Neuman. The Kerberos Network Authentication
   Service (V5). Request for Comments 1510.

   [2] B. Tung, C. Neuman, J. Wray, A. Medvinsky, M. Hur, J. Trostle.
   Public Key Cryptography for Initial Authentication in Kerberos.
   ftp://ds.internic.net/internet-drafts/
   draft-ietf-cat-kerberos-pkinit-08.txt

   [3] H. Krawczyk, M. Bellare, R. Canetti. HMAC: Keyed-Hashing for
   Message Authentication. Request for Comments 2104.

   [4] J. Pato. Using Pre-authentication to Avoid Password Guessing
   Attacks. OSF DCE SIG Request for Comments 26.0.

5. Acknowledgement: We thank Ken Hornstein for some helpful comments.

6. Expires January 30, 2000.

7. Authors' Addresses

   Jonathan Trostle
   170 W. Tasman Dr.
   San Jose, CA 95134, U.S.A.

   Email: jtrostle@cisco.com
   Phone: (408) 527-6201

   Michael Swift
   Email: mikesw@cs.washington.edu
