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Versions: 00 01 02 03 04 05 06 RFC 6717

Network Working Group                                            H. Hotz
Internet-Draft                                Jet Propulsion Laboratory,
Intended status: Informational                  California Institute of
Expires: October 8, 2011                                      Technology
                                                           April 6, 2011

             KX509 Kerberized Certificate Issuance Protocol


   This rfc describes a protocol, called kx509, for using Kerberos
   tickets to acquire X.509 certificates.

   While not (previously) standardized, this protocol is already in use
   at several large organizations, and certificates issued with this
   protocol are recognized by the International Grid Trust Federation.

Status of this Memo

   This Internet-Draft is submitted in full conformance with the
   provisions of BCP 78 and BCP 79.

   Internet-Drafts are working documents of the Internet Engineering
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   Internet-Drafts are draft documents valid for a maximum of six months
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   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on October 8, 2011.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
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   include Simplified BSD License text as described in Section 4.e of

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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.  Requirements Language  . . . . . . . . . . . . . . . . . .  3
   2.  Protocol Data  . . . . . . . . . . . . . . . . . . . . . . . .  3
     2.1.  Request Packet . . . . . . . . . . . . . . . . . . . . . .  4
     2.2.  Reply Packet . . . . . . . . . . . . . . . . . . . . . . .  4
   3.  Protocol Operation . . . . . . . . . . . . . . . . . . . . . .  6
   4.  Acknowledgements . . . . . . . . . . . . . . . . . . . . . . .  7
   5.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  8
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  8
   7.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  9
     7.1.  Normative References . . . . . . . . . . . . . . . . . . .  9
     7.2.  Informative References . . . . . . . . . . . . . . . . . .  9
   Appendix A.  Certificate Cacheing and Deployment Considerations  . 10
   Appendix B.  Historically Used subjectAltName Extensions . . . . . 10
   Appendix C.  Issues and Changes from the Previous Draft  . . . . . 10
   Author's Address . . . . . . . . . . . . . . . . . . . . . . . . . 11

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1.  Introduction

   The two primary ways of providing cryptographically secure
   identification on the Internet are Kerberos tickets [RFC4120], and
   X.509 [RFC5280] and [X.509] certificates.

   In practical IT infrastructure where both are in use, it's highly
   desirable to deploy their support in a way which guarantees they both
   authoritatively refer to the same entities.  There is already a
   widely-adopted standard for using X.509 certificates to acquire
   corresponding Kerberos tickets called PKINIT [RFC4556].  This rfc
   describes the kx509 protocol for supporting the symmetric operation
   of acquiring X.509 certificates using Kerberos tickets.

   The International Grid Trust Federation [IGTF] supports the use of
   Short Lived Credential Services [SLCS] as a means to authenticate for
   resource usage based on other, native identity stores which an
   organization maintains.  X.509 certificates issued using the KX590
   protocol based on a Kerberos identity is one of the recognized
   Credential Services.  The certificate profile for that use is outside
   the scope of this RFC, but is described in [GRID-prof].

   In normal operation kx509 can be used after a Kerberos ticket-
   granting-ticket (TGT) is acquired, which is most likely during user
   login.  First, the client generates a RSA public/private key-pair.
   Next, using the Kerberos ticket-granting-ticket, it acquires a
   Kerberos service ticket for the KCA (Kerberized Certificate
   Authority), and uses this to send the public half of its key-pair.
   The KCA will decrypt the service ticket, verify the integrity of the
   incoming packet, determine the identity of the user, and use the
   session key to send back a corresponding X.509 certificate.

1.1.  Requirements Language

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   document are to be interpreted as described in RFC 2119 [RFC2119].

2.  Protocol Data

   The protocol consists of a single request/reply exchange using UDP.

   Both the request and the reply packet begin with four bytes of
   version ID information, followed by a DER encoded ASN.1 message.  The
   first two bytes of the version ID are reserved.  They MUST be set to
   zero when sent, and SHOULD be ignored when received.  The third and
   fourth bytes are the major and minor version numbers.  The version of

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   the protocol described in this document is designated 2.0, so the
   first four bytes of the packet are 0, 0, 2, 0.

   Incompatible variations of this protocol MUST use a different major
   version number.

2.1.  Request Packet

   The request consists of a version ID, a Kerberos AP_REQ, integrity
   check data on the request, and a public key generated by the client.
   The ASN.1 encoding is:

   KX509Request ::= SEQUENCE {
           ap-req OCTET STRING,
           pk-hash OCTET STRING,
           pk-key OCTET STRING

   The ap-req is as described in [RFC4120] Section 5.5.1.

   The pk-hash is HMAC using SHA-1 as the underlying hash.  All 160 bits
   are sent.  The key used is the Kerberos session key.  The data is the
   4-byte version ID and the octet string for pk-key.

   The pk-key contains a public key.  This key and its corresponding
   private key are generated by the client before contacting the server.
   Implementations of this protocol MUST support RSA keys, in which case
   the key is a DER encoded RSAPublicKey as defined in [RFC3447],
   section A.1.1, and then stored in this octet string in the request.
   Use of other public-key types is not defined.

2.2.  Reply Packet

   The reply consists of a version ID, an error code, and an optional
   authentication hash, optional certificate, and optional error text.
   The service SHOULD return replies of the same version as the request
   where possible.

   KX509Response ::= SEQUENCE {
           error-code[0] INTEGER DEFAULT 0,
           hash[1] OCTET STRING OPTIONAL,
           certificate[2] OCTET STRING OPTIONAL,
           e-text[3] VisibleString OPTIONAL

   Although the format of the reply contains independently optional
   objects, the server MUST only generate replies with one of the
   following allowed combinations.

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               |            | hash | certificate |        |
               | error-code | hash |             | e-text |
               | error-code |      |             | e-text |

   The first case is returned when the server successfully generates a
   certificate for the user.  The certificate is a DER encoded
   Certificate as defined in [RFC5280] Section A, page 116.

   The second case is returned when the server successfully
   authenticates the user and their key, but is unable for some other
   reason to generate a certificate.

   The third case MAY be returned if the server is unable to
   successfully authenticate the user and intends to return some
   unauthenticated information to the client.

   The hash on a response is computed using SHA-1 HMAC as for the
   request.  The data that is hashed is the concatenation of the 4-byte
   version ID at the beginning of the packet, the error-code (if
   present), and all other optional fields which are present except the
   hash itself.  In other words, the hash is computed on the fields
   which are present exclusive of the overall ASN.1 wrapping.  The
   e-text MAY be translated into other character sets for display
   purposes, but the hash is computed on the e-text in its VisibleString

   If the e-text contains NUL characters, the client MAY ignore any part
   of the error message after the first NUL character for display

   As implied by the above table, if the reply does not contain a
   certificate it MUST contain an error message and a non-zero error
   code.  Conversely, if a certificate is returned then the error code
   MUST be zero.  The server SHOULD NOT send a zero error-code.  The
   client MUST treat a missing error-code as if it were zero.

   The defined error codes are as follows:

   | error-code | Condition                   | Example                |
   | 1          | Permanent problem with      | Incompatible version   |
   |            | client request              |                        |
   | 2          | Solvable problem with       | Expired Kerberos       |
   |            | client request              | credentials            |

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   | 3          | Temporary problem with      | Packet loss            |
   |            | client request              |                        |
   | 4          | Permanent problem with the  | Internal               |
   |            | server                      | misconfiguration       |
   | 5          | Temporary problem with the  | Server overloaded      |
   |            | server                      |                        |

   If a client error is returned, the client SHOULD NOT retry the
   request unless some remedial action is first taken.  If error-code 3
   is returned, the client MAY retry with other servers before giving

   If a server error is returned, it is RECOMMENDED that the client
   retry the request with a different server if one is known.

   Since all KCAs serving a Kerberos realm are intended to be
   equivalent, in accordance with [RFC5280] Section, the
   certificates returned from different KCAs serving the same Kerberos
   realm MUST NOT contain duplicate serial numbers.

   The returned certificate MUST identify the Kerberos client principal
   from the ap-req in the original KX509Request in the subject of the
   cert, or in a subjectAltName extension.  The identification MUST be
   unique within the organization's deployed infrastructure.  It is
   RECOMMENDED that a subjectAltName extension be included of type id-
   pkinit-san as described in [RFC4556] Section 3.2.2.  Note that the
   id-pkinit-san is simply a standard representation of a Kerberos
   principal, and has no other implications with respect to PKINIT.

   Other extensions MAY be added according to local policy.

3.  Protocol Operation

   Absent errors, the protocol consists of a single request, sent via
   UDP, and a single reply, also sent via UDP.

   There is no provision for requests or replies which exceed the
   allowable size of a UDP packet.  Furthermore, if the request or reply
   exceeds the MTU size of a UDP packet for the infrastructure in use,
   then the reliability of the exchange will decrease significantly.
   For "normal" Kerberos ap-req structures, and "normal" X.509
   certificates, this is unlikely unless the Kerberos service ticket
   contains large amounts of authorization data.  For this reason, it is
   RECOMMENDED that service tickets for the KCA be issued without
   authorization data.  If the KCA performs authorization, it should do
   so by other means.

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   Before constructing the request, the client must know the canonical
   name(s) and port(s) of the server(s) to contact.  It MAY determine
   them by looking up the service's SRV record as described in[RFC2782].
   The entry to be used is _kca._udp._realm_, where _realm_ is the
   Kerberos realm, used as part of the DNS name.

   The client must then acquire a service ticket in order to construct
   the ap-req for the service.  The Kerberos service principal name to
   use for this service has a first component of "kca_service".  The
   second component and the realm of the principal follow normal
   Kerberos conventions.

   When the server receives a request, it MUST make sanity checks
   including at least the following:

   o  The AP-REQ can be decoded and is not expired.

   o  If the request uses cross-realm authentication, then it satisfies
      the requirements of local policy and [RFC4120] Sections 1.2 and

   o  The request's hash is valid.

   The server SHOULD make other sanity checks, such as a minimum public
   key length, to the extent feasible.

   The server MAY decline to respond to an erroneous request.  If it
   does not receive a response a client MAY retry its request, but the
   client SHOULD wait at least one second before doing so.

   The client MUST verify any hash in the reply, and MUST NOT use any
   certificate in a reply whose hash does not verify.  The client MAY
   display the e-text if the hash is absent or does not verify, but
   SHOULD indicate the message is not authenticated.

4.  Acknowledgements

   The original version of kx509 was implemented using Kerberos 4 at the
   University of Michigan, and was nicely documented in [KX509].  Many
   thanks to them for their original work.

   While developing this document I received important corrections and
   comments from Jeffrey Altman, and Love Hornquist Astrand.  I also
   received many helpful comments and corrections from Doug Engert,
   Jeffrey Hutzelman, Sam Hartman, and Timothy J. Miller.  Alan Sill
   provided the references to the International Grid Trust Federation
   and its acceptable credential services.  Example network traffic was

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   provided by Doug Engert, Marcus Watts, and Matt Crawford from their
   deployments, and was extremely useful for verifying the reality of
   this specification.

5.  IANA Considerations

   This service is conventionally run on UDP port 9878, but this memo
   does not request that IANA standardize the port number.

   IANA is requested to add "kca_service" as a GSSAPI/Kerberos/SASL
   service name for a "Kerberized Certificate Authority".

6.  Security Considerations

   The only encrypted information in the protocol is that used by
   Kerberos itself.  The considerations for any Kerberized service apply

   The public key in the request is sent in the clear, and without any
   guarantees that the requestor actually possesses the corresponding
   private key.  Therefore the only appropriate uses of the returned
   certificate are those where the subsequent use independently
   guarantees that the user possesses the private key.  In particular
   the returned certificate MUST NOT be used for digital signatures
   unless the application's use is appropriately structured, or the
   identity of the signer is not important.

   Some information, such as the public key and certificate, is
   transmitted in the clear but (as the name implies) were generally
   intended to be publicly available.  However their visibility could
   still raise privacy concerns.  The hash is used to protect their

   The policies for issuing Kerberos tickets and X.509 certificates are
   usually expressed very differently.  An implementation of this
   protocol should not provide a mechanism for bypassing ticket or
   certificate policies.

   In particular, if the issued certificate can be used with PKINIT,
   this authentication loop should not bypass policy limits for either
   X.509 certificates or Kerberos tickets.  Since a PKINIT request must
   be signed and digitalSignature should not be an allowed usage for the
   issued certificate, this loop should not occur.

   X.509 certificates are usually issued with considerably longer
   validity times than Kerberos tickets.  Care should be taken that the

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   issued certificate is not valid for longer than the intended policy
   should allow.  Note that[RFC4556] Section REQUIRES that the
   lifetime of an issued ticket not exceed the lifetime of the
   predecessor certificate.  By analogy it is RECOMMENDED that the
   lifetime of an issued certificate not exceed the lifetime of the
   predecessor Kerberos ticket unless the implications with respect to
   local policy are clearly understood and allow it.

7.  References

7.1.  Normative References

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

   [RFC2782]  Gulbrandsen, A., Vixie, P., and L. Esibov, "A DNS RR for
              specifying the location of services (DNS SRV)", RFC 2782,
              February 2000.

   [RFC3447]  Jonsson, J. and B. Kaliski, "Public-Key Cryptography
              Standards (PKCS) #1: RSA Cryptography Specifications
              Version 2.1", RFC 3447, February 2003.

   [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.

7.2.  Informative References

              "GRID Certificate Profile", March 2008,

   [IGTF]     "The International Grid Trust Federation",

   [KX509]    Doster, W., Watts, M., and D. Hyde, "The KX509 Protocol",
              September 2001, <http://www.citi.umich.edu/techreports/

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   [SLCS]     "Short Lived Credential Services", February 2009,

   [X.509]    International Telecommunications Union, "Recommendation
              X.509: The Directory: Public-key and attribute certificate
              framework", November 2008.

Appendix A.  Certificate Cacheing and Deployment Considerations

   As noted in the Security Considerations section, the functional
   lifetime of the acquired X.509 certificate should usually match the
   lifetime of its predecessor Kerberos ticket.  Therefore, it is likely
   that X.509 certificates issued with this protocol should be deleted
   when the supporting Kerberos tickets are deleted.  That makes the
   Kerberos ticket cache a reasonable location to store the certificate
   (and its private key).

   On the other hand applications, such as web browsers, probably expect
   certificates in different stores.

   A widely used solution to this dichotomy is to implement a PKCS11
   library which supports the KX509-acquired credentials.  The
   credentials remain stored in the kerberos credentials cache, but full
   PKI functionality is still available via a standard interface for PKI

Appendix B.  Historically Used subjectAltName Extensions

   A subjectAltName othername extension of type kcaAuthRealm (OID value is frequently used to include the client's
   realm as an ASN.1 octet string.

   The Microsoft-defined userPrincipalName has sometimes been used for
   the same purpose as the id-pkinit-san.

Appendix C.  Issues and Changes from the Previous Draft

   RFC Editor Note: Delete this appendix before final publication.

   Possible Issues:

   1.  This draft only describes the existing in-the-wild protocol,
       which has some warts.  We need an updated, standards-track

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   2.  Should IANA registration of the service port be requested?

   3.  Should timeouts be more fully specified?  Any other issues with

   4.  Several people suggested more information be given on UDP size
       restrictions and their effect on reliability.  Is the text in
       section 3 sufficient?

   5.  The original UMICH code has provisions for DSA private keys in
       addition to RSA.  Nobody seems to use DSA.  Everyone seems to use
       RSA.  Is it worth specifying how DSA would work?

   Changes from the Previous Draft:

   1.  The retry behavior was made slightly less specific.

   2.  The traditionally used SAN extensions were moved to a new
       appendix, leaving only the id-pkinit-san as the RECOMMENDED SAN.

   3.  The absolute prohibition against digital signatures in the
       Security Considerations section was relaxed since there are
       legitimate situations where a signature based on the KX509
       certificate is still useful.  (E.g. integrity protection where
       the actual signing identity is not important.)

   4.  Reference to TAGPMA in the abstract was replaced with a reference
       to its parent, the International Grid Trust Federation, and more
       detailed informative references were expanded in the

   5.  Assorted other wording changes were made for clarity, but are not
       believed to have changed the meaning.

Author's Address

   Henry B. Hotz
   Jet Propulsion Laboratory, California Institute of Technology
   4800 Oak Grove Dr.
   Pasadena, CA  91109

   Phone: +01 818 354-4880
   Email: hotz@jpl.nasa.gov

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