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Versions: 00 01 03 04 05 06 07 08 RFC 5924

SIP WG                                                       S. Lawrence
Internet-Draft                                           Bluesocket Inc.
Updates:  3261 (if approved)                                  V. Gurbani
Intended status:  Standards Track      Bell Laboratories, Alcatel-Lucent
Expires:  May 11, 2008                                  November 8, 2007


  Using Extended Key Usage (EKU) for Session Initiation Protocol (SIP)
                           X.509 Certificates
                         draft-ietf-sip-eku-00

Status of this Memo

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   This Internet-Draft will expire on May 11, 2008.

Copyright Notice

   Copyright (C) The IETF Trust (2007).

Abstract

   This memo documents an extended key usage (EKU) X.509 certificate
   extension for identifying the holder of a certificate as
   authoritative for a Session Initiation Protocol (SIP) service in the
   domain named by the DNS name in the certificate.





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Table of Contents

   1.  Terminology  . . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.1.  Key Words  . . . . . . . . . . . . . . . . . . . . . . . .  3
     1.2.  Abstract syntax notation . . . . . . . . . . . . . . . . .  3
   2.  Problem statement  . . . . . . . . . . . . . . . . . . . . . .  3
   3.  Restricting usage to SIP . . . . . . . . . . . . . . . . . . .  4
     3.1.  Extended Key Usage values for SIP domains  . . . . . . . .  4
   4.  Using the SIP EKU in a certificate . . . . . . . . . . . . . .  5
   5.  Guidelines for a Certification Authority . . . . . . . . . . .  6
   6.  Security Considerations  . . . . . . . . . . . . . . . . . . .  6
   7.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  7
   8.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     8.1.  Normative References . . . . . . . . . . . . . . . . . . .  7
     8.2.  Informative References . . . . . . . . . . . . . . . . . .  7
   Appendix A.  ASN.1 Module  . . . . . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  8
   Intellectual Property and Copyright Statements . . . . . . . . . . 10

































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1.  Terminology

1.1.  Key Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
   document are to be interpreted as described in RFC 2119 [1].

1.2.  Abstract syntax notation

   All X.509 certificate X.509 [4] extensions are defined using ASN.1
   X.680 [5],X.690 [6].


2.  Problem statement

   Consider the SIP [2] trapezoid shown in Figure 1.


      proxyA.example.com ------------ proxyB.example.net
             |                           |
             |                           |
             |                           |
             |                         +---+
           0---0                       |   |
            /-\                        |___|
           +---+                      /    /
                                     +----+
      alice@example.com          bob@example.net


                          Figure 1: SIP Trapezoid

   Assume that alice@example.com creates an INVITE for bob@example.net;
   her user agent routes the request to some proxy in her domain,
   example.com.  Suppose also that example.com is a large organization
   that maintains several SIP proxies, and normal resolution rules cause
   her INVITE to be sent to an outbound proxy proxyA.example.com, which
   then uses RFC 3263 [7] resolution and finds that proxyB.example.net
   is a valid proxy for example.net that uses TLS. proxyA.example.com
   requests a TLS connection to proxyB.example.net, and each presents a
   certificate to authenticate that connection.  This is the basic
   mutual authentication model explored in depth in [8].

   However, there arise certain cases where one SIP proxy needs to know
   whether it has reached an authoritative proxy in target SIP domain.
   For instance, billing transactions may be triggered when an
   authoritative SIP proxy in one domain sends messages to its



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   equivalent in another domain.  In Figure 1, proxyA.example.com
   performs certain DNS manipulations to arrive at proxyB.example.net.
   Because of these DNS machinations, proxyA has a certain expectation
   that proxyB is a valid proxy in the example.net domain and is
   authorized to receive inbound requests targeted to that domain.

   However, the problem for proxyB is different; it is presented with a
   connection from a specific host, but what it needs to determine is
   whether or not that connection can be treated as coming from a
   particular SIP domain.  If it receives a certificate that contains
   only the name proxyA.example.com, then it cannot determine that
   proxyA is authorized to act as a SIP outbound proxy for example.com,
   because example.com may use different systems for inbound messages so
   SIP DNS resolution of example.com may not lead to proxyA.example.com
   (if this is the case, proxyB should not reuse this connection if it
   needs to send a request to example.com).  The certificate usage in
   SIP should not require that every outbound proxy for a domain must
   also be an inbound proxy for that domain, but should provide for
   certificate based binding of the SIP domain name to a particular
   connection.

   Thus, there is a need for an extra attribute that allows a proxy to
   know that its peer is an authorized proxy for that domain.  This memo
   discusses such an attribute as part of the X.509 certificate
   exchanged by the proxies when a TLS connection is first established.


3.  Restricting usage to SIP

   The intent of this draft is to define certificate profile for binding
   a SIP domain name to a connection.  A SIP domain name is frequently
   textually identical to the same DNS name used for other purposes.
   For example, the DNS name example.com may serve as a SIP domain name,
   an email domain name, and web service name.  Since these different
   services within a single organization may well be administered
   independently and hosted separately, it should be possible to create
   a certificate that binds the DNS name to its usage as a SIP domain
   name without creating the implication that the usage is also valid
   for some other purpose.  RFC 3280 [3] section 4.2.1.13 defines a
   mechanism for this purpose:  an "Extended Key Usage" attribute.
   Certificates whose purpose is to bind a SIP domain identity without
   binding other non-SIP identities MUST include an id-kp-SIPdomain
   attribute.

3.1.  Extended Key Usage values for SIP domains

   RFC 3280 [3] specifies the EKU X.509 certificate Extension for use in
   the Internet.  The extension indicates one or more purposes for which



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   the certified public key may be used.  The EKU extension can be used
   in conjunction with the key usage extension, which indicates how the
   public key in the certificate may be used, in a more basic
   cryptographic way.

   The EKU extension syntax is repeated here for convenience:

         ExtKeyUsageSyntax  ::=  SEQUENCE SIZE (1..MAX) OF KeyPurposeId

         KeyPurposeId  ::=  OBJECT IDENTIFIER

   This specification defines the KeyPurposeId id-kp-sipDomain.
   Inclusion of this KeyPurposeId in a certificate indicates that any
   DNS Subject names in the certificate are intended to identify the
   holder as authoritative for a SIP service in the domain named by the
   DNS name(s) in question.  Whether or not to include this restriction
   is up to the certificate issuer, but if it is included, it MUST be
   marked as critical so that implementations that do not understand it
   will not accept the certificate for any other purpose.

         id-kp  OBJECT IDENTIFIER  ::=
            { iso(1) identified-organization(3) dod(6) internet(1)
              security(5) mechanisms(5) pkix(7) 3 }

         id-kp-sipDomain  OBJECT IDENTIFIER  ::=  { id-kp VALUE-TBD }

   See Section 4 for how the presence of an id-kp-sipDomain value
   affects the interpretation of the certificate.


4.  Using the SIP EKU in a certificate

   Section 7.1 of [8] contains two steps for finding an identity (or a
   set of identities) in an X.509 certificate.  In order to determine
   whether a SIP proxy is authoritative for its domain, implementations
   MUST perform the step given below first, and then proceed with the
   steps in Section 7.1 of [8].

   The key usage value(s), if any, MUST be examined to determine whether
   or not the certificate is valid for use in SIP:

   o  If the certificate contains any EKU extension other than id-kp-
      sipDomain, and does not contain the id-kp-sipDomain extension,
      then the certificate MUST NOT be accepted as valid for use as a
      SIP certificate, and none of the identities it contains are
      acceptable for SIP domain authentication.





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   o  If the certificate contains the id-kp-anyExtendedKeyUsage
      extension and also contains the id-kp-sipDomain extension, then
      the certificate can be used as a SIP certificate.  Furthermore, it
      can also be used for any other application that the key usage
      extension permits.

   o  If the certificate does not contain any EKU values, it is a matter
      of local policy whether or not to accept it for use as a SIP
      certificate.

   A summary of the logic flow for peer certificate validation follows:

   1.  If no EKU extension, apply local policy and accept the
       certificate.

   2.  If EKU is present and contains id-kp-sipDomain, accept the
       certificate.

   3.  If any EKU is present and contains both id-kp-anyExtendedKeyUsage
       and id-kp-sipDomain, accept the certificate.

   4.  If any EKU is present but does not include id-kp-sipDomain,
       reject the certificate.


5.  Guidelines for a Certification Authority

   The procedures and practices employed by the certification authority
   MUST ensure that the correct values for the EKU extension and
   subjectAltName are inserted in each certificate that is issued.  For
   certificates that indicate authority over a SIP domain, but not over
   services other than SIP, certificate authorities MUST include the id-
   kp-sipDomain EKU extension.


6.  Security Considerations

   This memo defines an EKU X.509 certificate extension that enables the
   holder of a certificate to be authoritative for a SIP service
   belonging to an autonomous domain.  Relying parties may execute
   applicable policies (such as those related to billing) on receiving a
   certificate with the id-kp-sipDomain EKU value.  An id-kp-sipDomain
   EKU value does not introduce any new security or privacy concerns.
   At the very most, it simply allows the relying party to know that the
   holder of the certificate is authoritative for the SIP service in a
   certain domain.  In the absence of the id-kp-sipDomain EKU value,
   this information can be collected over time by a peer in any case.




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7.  Acknowledgments

   The following IETF contributors provided substantive input to this
   document:  Jeroen van Bemmel, Michael Hammer, Cullen Jennings, Paul
   Kyzivat, Derek MacDonald, Dave Oran, Jon Peterson, Eric Rescorla,
   Jonathan Rosenberg, Russ Housley, and Stephen Kent.  Special
   acknowledgement is due to Sharon Boyen for reviewing the document and
   pointing out the role of id-kp-anyExtendedKeyUsage in certificates.


8.  References

8.1.  Normative References

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

   [2]  Rosenberg, J., Schulzrinne, H., Camarillo, G., Johnston, A.,
        Peterson, J., Sparks, R., Handley, M., and E. Schooler, "SIP:
        Session Initiation Protocol", RFC 3261, June 2002.

   [3]  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.

   [4]  International International Telephone and Telegraph Consultative
        Committee, "Information Technology - Open Systems
        Interconnection - The Directory: Authentication Framework",
        CCITT Recommendation X.509, November 1988.

   [5]  International International Telephone and Telegraph Consultative
        Committee, "Specification of Abstract Syntax Notation One
        (ASN.1): Specification of Basic Notation", CCITT Recommendation
        X.680, July 1994.

   [6]  International Telecommunications Union, "Information Technology
        - ASN.1 encoding rules: Specification of Basic Encoding Rules
        (BER), Canonical Encoding Rules (CER) and Distinguished Encoding
        Rules (DER)", ITU-T Recommendation X.690, 1994.

   [7]  Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
        (SIP): Location SIP Servers", RFC 3263, June 2002.

8.2.  Informative References

   [8]  Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain Certificates
        in the Session Initiation Protocol (SIP)",
        draft-ietf-sip-domain-certs-00.txt (work in progress),



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        November 2007.


Appendix A.  ASN.1 Module

      SIPDomainCertExtn
        { iso(1) identified-organization(3) dod(6) internet(1)
          security(5) mechanisms(5) pkix(7) id-mod(0)
          id-mod-sip-domain-extns2007(VALUE-TBD) }

      DEFINITIONS IMPLICIT TAGS ::=
      BEGIN

      -- OID Arcs

      id-pe  OBJECT IDENTIFIER  ::=
         { iso(1) identified-organization(3) dod(6) internet(1)
           security(5) mechanisms(5) pkix(7) 1 }

      id-kp  OBJECT IDENTIFIER  ::=
         { iso(1) identified-organization(3) dod(6) internet(1)
           security(5) mechanisms(5) pkix(7) 3 }

      id-aca  OBJECT IDENTIFIER  ::=
         { iso(1) identified-organization(3) dod(6) internet(1)
           security(5) mechanisms(5) pkix(7) 10 }

      -- Extended Key Usage Values

      id-kp-sipDomain  OBJECT IDENTIFIER  ::=  { id-kp VALUE-TBD }

      END


Authors' Addresses

   Scott Lawrence
   Bluesocket Inc.
   10 North Ave.
   Burlington, MA  01803
   USA

   Phone:  +1 781 229 0533
   Email:  slawrence@bluesocket.com







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   Vijay K. Gurbani
   Bell Laboratories, Alcatel-Lucent
   2701 Lucent Lane
   Room 9F-546
   Lisle, IL  60532
   USA

   Phone:  +1 630 224-0216
   Email:  vkg@alcatel-lucent.com










































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