Network Working Group                                        N. Williams
Internet-Draft                                              Cryptonector
Intended status: Standards Track                           July 11, 2013
Expires: January 12, 2014

     Public Key-Based Kerberos Cross Realm Path Traversal Protocol


   This document specifies a protocol for obtaining cross-realm Kerberos
   tickets using existing, related protocols.  The resulting protocol
   has a number of desirable security properties, including privacy
   protection for the user relative to their home realm's
   infrastructure, as well a support for leap-of-faith trust
   establishment, and automated cross-realm keying.  This protocol
   allows Kerberos to scale to large numbers of realms.

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   This Internet-Draft will expire on January 12, 2014.

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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

Table of Contents

   1.    Introduction . . . . . . . . . . . . . . . . . . . . . . . .  3
   1.1.  Conventions used in this document  . . . . . . . . . . . . .  3
   2.    The Protocol . . . . . . . . . . . . . . . . . . . . . . . .  4
   2.1.  Exchange of Long-Term Cross-Realm Symmetric Keys . . . . . .  4
   3.    Privacy Protection . . . . . . . . . . . . . . . . . . . . .  6
   4.    Leap-of-Faith / TOFU . . . . . . . . . . . . . . . . . . . .  7
   5.    Using DNSSEC for Realm Certificate Validation  . . . . . . .  8
   6.    Security Considerations  . . . . . . . . . . . . . . . . . .  9
   7.    IANA Considerations  . . . . . . . . . . . . . . . . . . . . 10
   8.    References . . . . . . . . . . . . . . . . . . . . . . . . . 11
   8.1.  Normative References . . . . . . . . . . . . . . . . . . . . 11
   8.2.  Informative References . . . . . . . . . . . . . . . . . . . 11
         Author's Address . . . . . . . . . . . . . . . . . . . . . . 12

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1.  Introduction

   Kerberos [RFC4120] supports meshes of many realms.  The individual
   relationships between realms must be manually keyed, usually with
   keys derived from passwords.  These keys are very difficult to
   rollover, and when they are changed the result is often outages --
   controlled outages where foreseen, but outages nonetheless.  This
   method of cross-realm keying does not scale, and has very poor
   security properties.  We seek to remediate this.

   Many years ago there was a proposal for exchanging cross-realm keys
   using a public key infrastructure (PKI) [RFC5280]; that proposal went
   by the name "PKCROSS".  We appropriate that long-dead proposal's
   name, but the protocol specified here is very different from the
   original proposal.

1.1.  Conventions used in this document

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

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2.  The Protocol

   A Kerberos client in with a ticket-granting ticket (TGT) for any one
   source realm (usually but not necessarily the client's own realm)
   wishing to acquire a TGT for a destination realm may use this
   protocol instead of the traditional cross-realm ticket-granting
   service (TGS) exchanges as follows:

   1.  Generate private key to a public key cryptosystem;

   2.  Generate a certificate signing request (CSR) [RFC2986], such that
       the resulting certificate has an id-pkinit-san subject
       alternative name (SAN) corresponding to the client's principal
       name and realm;

   3.  Request a certificate from the kx509 [I-D.hotz-kx509] service run
       by the source realm;

   4.  Request a TGT from the destination realm using PKINIT [RFC4556].

   If the destination realm issues the requested Ticket then it SHOULD
   include the client's certificate in an AD-CLIENT-CERTIFICATE
   authorization-data element, and it MUST do so if it does not validate
   the client's certificate to an acceptable trust anchor.

   The destination realm MUST NOT set the TRANSIT-POLICY-CHECKED flag on
   the tickets they issue to clients whose foreign realm certificates
   are not validated by the KDC.  Destination realm administrators may
   configure their realms to know specific foreign realm clients'

   The destination MUST include the trust path of the client's
   certificate, if validated, in the 'transited' field of the issued
   Ticket, using a mapping of the issuer names to the X.500 realm naming
   style [XXX must specify this mapping; hopefully it can be the
   identity function or close enough].

2.1.  Exchange of Long-Term Cross-Realm Symmetric Keys

   When the client principal is a TGS principal and its PKINIT AS-REQ
   protocol data unit (PDU) has the USE-SESSION-KEY-AS-REALM-KEY
   KDCOptions flag set then the client is requesting that the session
   key of the ticket issued by the destination realm become the long-
   term key for the corresponding krbtgt/DESTINATION@SOURCE principal.
   The destination realm MUST validate the client principal's
   certificate, building a trust path if need be, and validating it to a
   trust anchor.  The source and destination realm MAY have previously
   exchange fingerprints of their respective key distribution service

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   (KDC) public keys and/or certificates and/or the source realm's kx509
   root or intermediate certification authority (CA), and such
   previously exchanged material, if any, MUST be used for certificate
   trust validation.

   Realm administrators should use the procedure to setup symmetric
   cross-realm keys as necessary to save clients from having to
   frequently use kx509 and PKINIT as described in the preceding

   Where public key infrastructure (PKI) exists allowing this to happen
   automatically, realms' KDCs MAY be configured to automatically key
   cross-realm principals for any realms that their source realms'
   clients request cross-realm TGTs for, but note that this presents a
   denial of service (DoS) opportunity to the source realm's clients.
   Source realm KDCs SHOULD only do this when a) they are configured to
   do so, b) the requesting client principal is in the same realm, c)
   the KDC has not spent too much effort recently providing this service
   (i.e., KDCs should throttle attempts to establish symmetric cross-
   realm keys in this manner), and d) up to some maximum number of
   cross-realm principals.

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3.  Privacy Protection

   This protocol protects the privacy of client principals vis-a-vis
   their home realms: client principals' home realms need not know what
   destination realms the clients are speaking to because client
   principals need not ask their home realms.

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4.  Leap-of-Faith / TOFU

   Clients need not validate the certificate trust path of destination
   realms.  When they do not, the services used through those
   destination realms are as good as anonymous authentication.  If the
   client saves the root or intermediate or end entity certificates of
   the destination realms that it cannot or does not validate, then the
   client can check that on future occasions the destination realm's
   certificate has not changed, and it may warn the user if it has.
   This quite similar to how clients using the secure shell (SSH)
   protocol [RFC4251] handle server authentication, and is commonly
   known as "leap-of-faith" (LoF) or trust-on-first-use (TOFU).  The
   result is as good as pseudonymous authentication.

   Destination services too may apply apply LoF/TOFU: by not validating
   the transit path of the client (e.g., if it's not in a white-list of
   realms whose clients must have valid transit paths) and accepting
   tickets without the TRANSITED-POLICY-CHECKED ticket flag set.  The
   destination service can save the client's certificate, if found in an
   AD-CLIENT-CERTIFICATE authorization-data element in the client's
   Ticket, and may use it later to ensure that it is talking to the same

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5.  Using DNSSEC for Realm Certificate Validation

   [Specify how to use DNS-Based Authentication of Named Entities (DANE)
   [RFC6698] to authenticate the KDC certificates of realms with domain-
   style names.  Roughly: format the realm's name as a domainname, then
   format the DANE TLSA resource record set's (RRset) domainname per-
   DANE, using the KDC's port number.  Note that the KDCs will usually
   not speak TLS, though there is an extension for using TLS in the KDC
   over TCP protocol.  For example, the TLSA RRset for any KDC for the
   DESTINATION.EXAMPLE realm might be named

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6.  Security Considerations

   [[anchor1: ...]]

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7.  IANA Considerations

   [[anchor2: Allocate the new KDCOptions flag (USE-SESSION-KEY-AS-
   REALM-KEY) and authorization-data element (AD-CLIENT-CERTIFICATE).]]

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8.  References

8.1.  Normative References

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

   [RFC2986]  Nystrom, M. and B. Kaliski, "PKCS #10: Certification
              Request Syntax Specification Version 1.7", RFC 2986,
              November 2000.

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

   [RFC4556]  Zhu, L. and B. Tung, "Public Key Cryptography for Initial
              Authentication in Kerberos (PKINIT)", RFC 4556, June 2006.

   [RFC5280]  Cooper, D., Santesson, S., Farrell, S., Boeyen, S.,
              Housley, R., and W. Polk, "Internet X.509 Public Key
              Infrastructure Certificate and Certificate Revocation List
              (CRL) Profile", RFC 5280, May 2008.

   [RFC6698]  Hoffman, P. and J. Schlyter, "The DNS-Based Authentication
              of Named Entities (DANE) Transport Layer Security (TLS)
              Protocol: TLSA", RFC 6698, August 2012.

              Hotz, H. and R. Allbery, "KX509 Kerberized Certificate
              Issuance Protocol in Use in 2012", draft-hotz-kx509-06
              (work in progress), July 2012.

8.2.  Informative References

   [RFC4251]  Ylonen, T. and C. Lonvick, "The Secure Shell (SSH)
              Protocol Architecture", RFC 4251, January 2006.

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Author's Address

   Nicolas Williams
   Cryptonector, LLC


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