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Versions: 00 01 02 03 04 05 06 07 08                                    
INTERNET-DRAFT                                              Brian Tung
draft-ietf-cat-kerberos-pk-cross-04.txt                 Tatyana Ryutov
Updates: RFC 1510                                      Clifford Neuman
expires September 15, 1998                                 Gene Tsudik
                                                       Bill Sommerfeld
                                                         Ari Medvinsky
                                                           Matthew Hur
                                                 CyberSafe Corporation

    Public Key Cryptography for Cross-Realm Authentication in Kerberos

0.  Status Of this Memo

    This document is an Internet-Draft.  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

    To learn the current status of any Internet-Draft, please check
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    Shadow Directories on ds.internic.net (US East Coast),
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    munnari.oz.au (Pacific Rim).

    The distribution of this memo is unlimited.  It is filed as
    draft-ietf-cat-kerberos-pk-cross-04.txt, and expires September 15,
    1998.  Please send comments to the authors.

1.  Abstract

    This document defines extensions to the Kerberos protocol
    specification (RFC 1510, ''The Kerberos Network Authentication
    Service (V5)'', September 1993) to provide a method for using
    public key cryptography during cross-realm authentication.  The
    methods defined here specify the way in which message exchanges
    are to be used to transport cross-realm secret keys protected by
    encryption under public keys certified as belonging to KDCs.

2.  Introduction

    The advantages provided by public key cryptography have
    produced a demand for use by the Kerberos authentication protocol.
    A draft describing the use of public key cryptography in the
    initial authentication exchange in Kerberos has already been
    submitted.  This draft describes its use in cross-realm

    The principal advantage provided by public key cryptography in
    cross-realm authentication lies in the ability to leverage the
    existing public key infrastructure.  It frees the Kerberos realm
    administrator from having to maintain separate keys for each other
    realm with which it wishes to exchange authentication information,
    or from having to utilize a hierarchical arrangement, which may
    pose problems of trust.

    Even with the multi-hop cross-realm authentication, there must be
    some way to locate the path by which separate realms are to be
    transited.  The current method, which makes use of the DNS-like
    realm names typical to Kerberos, requires trust of the intermediate

    The methods described in this draft allow a realm to specify, at
    the time of authentication, which certification paths it will
    trust.  A shared key for cross-realm authentication can be
    established for a set period of time.  Furthermore, these methods
    are transparent to the client; therefore, only the KDCs need to be
    modified to use them.

    It is not necessary to implement the changes described in the
    "Public Key Cryptography for Initial Authentication" draft to make
    use of the changes in this draft.  We solicit comments about the
    interaction between the two protocol changes, but as of this
    writing, the authors do not perceive any obstacles to using both.

3.  Protocol Specification

    We assume that the client has already obtained a TGT.  To perform
    cross-realm authentication, the client does exactly what it does
    with ordinary (i.e., non-public-key-enabled) Kerberos; the only
    changes are in the KDC; although the ticket which the client
    forwards to the remote realm may be changed.  This is acceptable
    since the client treats the ticket as opaque.

    The revised PKCROSS protocol makes use of a proposed field in the
    Kerberos response, namely a TicketExtension.  This field is not
    part of the encrypted part of the ticket (although it may have
    been encrypted earlier), but it accompanies the ticket and should
    be passed along by unaware clients with the rest of the ticket in
    an opaque fashion.  Using this field allows us to achieve the
    following objectives:

        *   Avoid modification of clients and application servers.

        *   Allow remote KDC to control its policy on cross-realm
            keys shared between KDCs, and on cross-realm tickets
            presented by clients.

        *   Remove any need for KDCs to maintain state about keys
            shared with other KDCs.

        *   Leverage work already done for PKINIT.

3.1.  Overview of Protocol Changes

    The basic operation of the revised PKCROSS protocol is as follows:

        1.  The client submits a request to the local KDC for
            credentials for the remote realm.

        2.  The local KDC submits a PKINIT request to the remote
            KDC.  This request has a flag set to indicate that
            the request is a special cross-realm key request.

        3.  The remote KDC responds as per PKINIT, except that
            the ticket contains a TicketExtension which indicates
            that this is a cross-realm key response.  The
            TicketExtension may also contain policy information,
            which the local KDC must reflect in the credentials
            it forwards to the client.  Call this ticket TGT_{L,R}.

        4.  The local KDC passes a ticket, TGT_{C,R}, to the client.
            This ticket contains in its TicketExtension field the
            cross-realm key ticket, TGT_{L,R}.  This ticket is
            encrypted using the key sealed in TGT_{L,R}.  (The
            TicketExtension field is not encrypted.)  The local KDC
            may optionally include another TicketExtension type that
            indicates the hostname and/or IP address for the remote KDC.

        5.  The client submits the request directly to the remote
            KDC, as before.

    Sections 3.2 through 3.4 describe in detail steps 2 through 4
    above.  Section 3.5 describes the conditions under which steps
    2 and 3 may be skipped.

    A comment about the KDC-to-KDC communication present in these
    protocol changes.  If there is no such exchange between the KDCs,
    then the local KDC will have to issue a ticket with the expectation
    that the remote KDC will accept it, but no guarantee.  This is a
    simple trade-off: non-repudiability of the ticket, so to speak, or
    avoiding KDC-to-KDC communication.  As a matter of principle, we
    choose the former, so that operation from the client's perspective
    is unchanged.

3.2.  Local KDC's PKINIT Request to Remote KDC

    When the local KDC receives a request for cross-realm
    authentication, it sends a request to the remote KDC for a
    ticket, denoted by TGT_{L,R}.  This request is in fact a PKINIT
    request as described in Section 3.2 of the PKINIT draft; i.e.,
    it consists of an AS-REQ, with a PA-PK-AS-REQ included as a
    preauthentication field.

    In addition, the local KDC indicates that this request is for
    a PKCROSS cross-realm key, by setting bit 9 in the kdc-options
    field of the AS-REQ.  We propose to call this bit the PKCROSSKEY
    flag.  Otherwise, this exchange exactly follows the description
    given by Section 3.2 of the PKINIT draft.

3.3.  Remote KDC's PKINIT Response to Local KDC

    When the remote KDC receives the PKINIT/CROSS request from the
    local KDC, it sends back a PKINIT response as described in
    Section 3.2 of the PKINIT draft, with the following changes:
    The remote KDC does not encrypt the encryptedTktPart with a
    random key, as described in the PKINIT draft, but instead encrypts
    it with a special symmetric key it uses for validating PKCROSS
    requests.  This key, instead of a random key, is then placed in an
    envelope as described in the PKINIT draft.

    In addition, a TicketExtension field of type 2 (TE-TYPE-PKCROSS) is
    included with the ticket (as a non-encrypted part):

    TicketExtension ::= SEQUENCE OF SEQUENCE {
            te-type[0]       INTEGER,
            te-data[1]       OCTET STRING
    } -- per the revised Kerberos specification

       te-type equals 2 for TE-TYPE-PKCROSS-KDC
       te-data is ASN.1 encoding of CrossRealmTktData.

    CrossRealmTktData ::= SEQUENCE OF SEQUENCE {
            type                [0] INTEGER,
            data                [1] OCTET STRING

    where types and the corresponding data are to be interpreted as

        type        interpretation          data is ASN.1 encoding of
        ----        --------------          -------------------------
          1         lifetime (in seconds)           INTEGER

    Further types may be added to this table.

3.4.  Local KDC's Response to Client

    Upon receipt of the PKINIT/CROSS response from the remote KDC,
    the local KDC formulates a response to the client.  This reply
    is constructed exactly as in the Kerberos specification, except
    for the following:

    * The local KDC places TGT_{L,R} in the TicketExtension field of
      the client's ticket for the remote realm (denoted here by
    * The local KDC adds the name of its CA to the transited field of

    TicketExtension ::= SEQUENCE OF SEQUENCE {
            te-type[0]       INTEGER,
            te-data[1]       OCTET STRING
    } -- per the revised Kerberos specification

       te-type equals 3 for TE-TYPE-PKCROSS-CLIENT
       te-data is ASN.1 encoding of TGT_(L,R).

    Optionally, the local KDC may add another TicketExtention where
       te-type equals 5 for TE-TYPE-DEST-HOST
       te-data is ASN.1 encoding of DestHost.

    A client could then rely on the local KDC to provide a referral to
    the remote KDC, thus removing the need for the client to maintain
    local host/realm mapping information.

    DestHost ::= SEQUENCE {
           DNSHostname [0]    OCTET STRING,
                              -- authoritative DNS hostname
           HostAddr    [1]    HostAddress OPTIONAL
                              -- as defined by RFC 1510

3.5.  Short-Circuiting the KDC-to-KDC Exchange

    Under certain circumstances, the KDC-to-KDC exchange described
    in Sections 3.2 and 3.3 may be skipped.  The local KDC has a
    known lifetime for TGT_{C,R} (which may in part be determined by
    the policy sent along with TGT_{L,R}).  If the local KDC already
    has a ticket TGT_{L,R}, and the start time plus the lifetime for
    TGT_{C,R} does not exceed the expiration time for TGT_{L,R}, then
    the local KDC may skip the exchange with the remote KDC, and
    issue a cross-realm ticket to the client as described in Section

    Since the remote KDC may change the special cross-realm symmetric
    key (referred to in Section 3.2) while there are cross-realm key
    tickets (the TGT_{L,R}) still active, it is obligated to cache
    those tickets until they expire.

4.  Transport Issues

    Because the messages between the KDCs involve PKINIT exchanges,
    and PKINIT recommends TCP as a transport mechanism (due to the
    length of the messages and the likelihood that they will fragment),
    the same recommendation for TCP applies to PKCROSS as well.

5.  Expiration Date

    This Internet-Draft will expire on September 15, 1998.

6.  Authors' Addresses

    Brian Tung
    Tatyana Ryutov
    Clifford Neuman
    Gene Tsudik
    USC/Information Sciences Institute
    4676 Admiralty Way Suite 1001
    Marina del Rey, CA 90292-6695
    Phone: +1 310 822 1511
    E-Mail: {brian, tryutov, bcn, gts}@isi.edu

    Bill Sommerfeld
    Hewlett Packard
    300 Apollo Drive
    Chelmsford MA 01824
    Phone: +1 508 436 4352
    E-Mail: sommerfeld@apollo.hp.com

    Ari Medvinsky
    Matthew Hur
    CyberSafe Corporation
    1605 NW Sammamish Road Suite 310
    Issaquah WA 98027-5378
    Phone: +1 206 391 6000
    E-mail: {ari.medvinsky, matt.hur}@cybersafe.com