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Versions: 00 01 02 03 04 05 06 RFC 6717

Kerberos Working Group                                           H. Hotz
Internet-Draft                                Jet Propulsion Laboratory,
Intended status: Standards Track                California Institute of
Expires: November 14, 2010                                    Technology
                                                            May 13, 2010

             KX509 Kerberized Certificate Issuance Protocol


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

Status of this Memo

   This Internet-Draft is submitted to IETF 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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   This Internet-Draft will expire on November 14, 2010.

Copyright Notice

   Copyright (c) 2010 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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   carefully, as they describe your rights and restrictions with respect

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   to this document.  Code Components extracted from this document must
   include Simplified BSD License text as described in Section 4.e of
   the Trust Legal Provisions and are provided without warranty as
   described in the 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  . . . . . . . . . . . . . . . . . . . . . . 5
   4.  Acknowledgements  . . . . . . . . . . . . . . . . . . . . . . . 6
   5.  IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
   6.  Security Considerations . . . . . . . . . . . . . . . . . . . . 6
   7.  References  . . . . . . . . . . . . . . . . . . . . . . . . . . 7
     7.1.  Normative References  . . . . . . . . . . . . . . . . . . . 7
     7.2.  Informative References  . . . . . . . . . . . . . . . . . . 7
   Appendix A.  Certificate Cacheing and Deployment Considerations . . 7
   Appendix B.  Known Issues with This Draft . . . . . . . . . . . . . 8
   Author's Address  . . . . . . . . . . . . . . . . . . . . . . . . . 8

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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 [RFC3280] 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.

   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
   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.

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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.
   The key is DER encoded and then stored in this octet string in the

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
   string.  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 optional objects, the
   server MUST only generate replies with one of the following allowed

                  | certificate |               | hash |
                  | error-code  | error-message | hash |
                  | error-code  | error-message |      |

   The first case is returned when the server successfully generates a
   certificate for the user.

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   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 consists of the 4-byte version ID
   at the beginning of the packet, the error-code, and all other
   optional fields except the hash itself.  The error-message MAY be
   translated into other character sets for display purposes, but the
   hash is computed on the error-message in its VisibleString

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

   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.

3.  Protocol Operation

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

   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.  (KCA stands for
   Kerberized Certificate Authority.)

   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:

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

   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 an error-message 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.

5.  IANA Considerations

   This service is conventionally run on UDP port 9878, but this memo
   includes no request to IANA.

6.  Security Considerations

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

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

   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.  Furthermore, 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.

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   X.509 certificates are usually issued with considerably longer
   validity times than Kerberos tickets.  If an implementation of this
   protocol does not limit the validity time of the issued certificates
   to the Kerberos ticket lifetime, then care should be taken that the
   issued certificate is not valid for longer than the intended policy
   should allow.

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.

   [RFC3280]  Housley, R., Polk, W., Ford, W., and D. Solo, "Internet
              X.509 Public Key Infrastructure Certificate and
              Certificate Revocation List (CRL) Profile", RFC 3280,
              April 2002.

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

7.2.  Informative References

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

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

   [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 function
   lifetime of the acquired X.509 certificate should probably match the
   lifetime of its predecessor Kerberos ticket.  It is also likely that
   X.509 certificates issued with this protocol should be deleted when

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   the supporting Kerberos tickets are deleted.  That makes the Kerberos
   ticket cache a reasonable location to store the certificate.

   On the other hand applications, such as web browsers, probably expect
   certificates in different stores.  A widely used solution to this
   problem is to implement the KX509 client within a PKCS11 library.

Appendix B.  Known Issues with This Draft

   1.  The hashing should be better described.

   2.  Should there be provision for SHA-2 or some other hash?

   3.  Should there be provision for DSA keys?

   4.  Should there be IANA registration of a service port requested?

   5.  Is there a better way to specify how to find the service that
       allows for how Windows/AD does it? (section 3)

   6.  Should timeouts, etc. be more fully specified?

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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