SIP WG                                                       S. Lawrence
Internet-Draft                                           Bluesocket Inc.
Updates:  3261 (if approved)                                  V. Gurbani
Intended status:  Standards Track      Bell Laboratories, Alcatel-Lucent
Expires:  August 21, 2008                              February 18,  January 12, 2009                                 July 11, 2008

  Using Extended Key Usage (EKU) for Session Initiation Protocol (SIP)
                           X.509 Certificates

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Copyright Notice

   Copyright (C) The IETF Trust (2008).


   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.

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.  IANA Considerations  . . . . . . . . . . . . . . . . . . . . .  6  7
   8.  Acknowledgments  . . . . . . . . . . . . . . . . . . . . . . .  7
   9.  References . . . . . . . . . . . . . . . . . . . . . . . . . .  7
     9.1.  Normative References . . . . . . . . . . . . . . . . . . .  7
     9.2.  Informative References . . . . . . . . . . . . . . . . . .  8
   Appendix A.  ASN.1 Module  . . . . . . . . . . . . . . . . . . . .  8
   Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . .  8  9
   Intellectual Property and Copyright Statements . . . . . . . . . . 10

1.  Terminology

1.1.  Key Words

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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 [5], X.690 [6].

2.  Problem statement

   Consider the SIP [2] trapezoid shown in Figure 1. ------------ 
        +-------+                    +-------+
        | Proxy |--------------------| Proxy |
        +----+--+                    +---+---+
             |                           |
             |                           |
             |                           |
             |                         +---+
           0---0                       |   |
            /-\                        |___|
           +---+                      /    /

                          Figure 1: SIP Trapezoid

   Assume that creates an INVITE

   An user,, invites for;
   her a multimedia
   communication session.  Her user agent routes the request to some proxy a proxy,
   in her domain,  Suppose domain.  Assume also that is a large organization
   that maintains several SIP proxies, and normal resolution request forwarding
   rules cause her INVITE invitation to be sent to an outbound proxy, Proxy-  This proxy, which
   then uses RFC 3263 [7] resolution and finds to discover
   is a valid proxy for offers TLS support and can accept the
   request on behalf of the that uses TLS.
   requests domain.  Proxy-A establishes a
   TLS connection to, and Proxy-B, each proxy presents a X.509 certificate to
   authenticate that and protect the confidentiality of the connection.  This
   is the basic
   mutual mututal 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
   equivalent in another domain.  In Figure 1,
   performs certain DNS queries to arrive at
   Because of the answers to the DNS queries, proxyA Proxy-A has a certain
   expectation that proxyB Proxy-B is a valid proxy in the domain
   and is authorized to receive inbound requests targeted to that

   However, the

   The problem for proxyB Proxy-B is different; different:  it is presented with accepts a connection from a
   specific host, host ( but what it needs to determine is
   whether or not that connection can be treated as coming
   authoritatively from a particular SIP domain.  If it Proxy-B receives a
   certificate that contains tha only contains the name, then identity "",
   it cannot determine is unable to make a determination that
   proxyA Proxy-A is authorized to
   act as a SIP outbound proxy for,
   because the domain (note that domain may use different systems for inbound messages proxies, so SIP DNS
   resolution of "" may not lead to
   (if this is the case, proxyB should not reuse this connection if it
   needs to send a request to  The certificate "Proxy-".)  Certificate usage in SIP should not cannot require that every
   outbound proxy for a domain must also be an serve as its inbound proxy for that domain, but should provide for
   certificate based binding of the SIP domain name to a particular
   connection. proxy.  Thus,
   there is a need for an extra attribute that allows a an inbound proxy
   to know that its peer is an authorized proxy for that domain.  This memo
   document 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

   This memo defines a certificate profile for binding a SIP domain name
   to an entity.  A SIP domain name is frequently textually identical to
   the same DNS name used for other purposes.  For example, the DNS name may serve as a SIP domain name, an email domain name, and
   web service name.  Since these different services within a single
   organization might 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
   RFC5280 [3] section 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 5280 [3] specifies the EKU X.509 certificate Extension for use in
   the Internet.  The extension indicates one or more purposes for which
   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.

         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 }

   Inclusion of this KeyPurposeId in a certificate indicates that any
   DNS Subject subject names in the certificate are intended to identify the
   holder as authoritative for a SIP service in the domain named by the
   subjectAltName values.  Whether or not to include this restriction is
   up to the certificate issuer, but if it is included, it MUST SHOULD be
   marked as critical critical.

      This is so that implementations that do not understand it this extension
      will honor it while those that do 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 } are not impeded by its

   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 the SIP proxy entity presenting the certificate 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 Extended Key Usage value(s), if any, MUST be examined to
   determine whether or not the certificate is valid for use in SIP: SIP
   (note that in the steps below, we assume that the certificate is
   otherwise valid following the checks in RFC5280 [3]):

   o  If the certificate does not contain any EKU values (the Extended
      Key Usage extension does not exist), it is a matter of local
      policy whether or not to accept the certificate for use as a SIP

   o  If the certificate contains the id-kp-sipDomain EKU extension,
      then the certificate MUST be accepted acceptable as valid for use as a SIP

   o  If the certificate does not contain the id-kp-sipDomain EKU value,
      but does contain the id-kp-anyExtendedKeyUsage EKU value, it is a
      matter of local policy whether or not to accept it for use as a
      SIP certificate.

   o  If the certificate does not contain the id-kp-sipDomain EKU value,
      but does contain either the id-kp-serverAuth or id-kp-clientAuth
      EKU values, it is a matter of local policy whether or not to
      accept it for use as a SIP certificate.

         id-kp-serverAuth and id-kp-clientAuth EKU values are defined in
         Section of RFC 5280 [3].

   o  If EKU extension exists but does not contain any of the id-kp-
      sipDomain, id-kp-anyExtendedKeyUsage, id-kp-serverAuth, or id-kp-
      clientAuth EKU values, then the certificate MUST NOT be accepted considered
      acceptable as valid being authoritative for use as a the domain portion of the
      SIP domain identities contained in the certificate.

         The term "SIP domain identity" is defined in RFC XXXX [8].

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 SHOULD include the id-
   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.

7.  IANA Considerations

   The id-kp-sipDomain purpose requires an object idenitifier (OID).
   The objects are defined in an arc delegated by IANA to the PKIX
   working group.  No further action is necessary by IANA.

8.  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.

   Sharon Boyen and Trevor Freeman reviewed the document and facilitated
   the discussion on id-kp-anyExtendedKeyUsage, id-kpServerAuth and id-
   kp-ClientAuth purposes in certificates.

9.  References

9.1.  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]  Cooper, D., Santesson, S., Farrell, S., Boyen, S., Housley, R., Polk, W., Ford, W.,
        and D. Solo, W. Polk, "Internet X.509 Public Key Infrastructure
        Certificate and Certificate Revocation List (CRL) Profile",
        RFC 3280, April 2002. 5280, May 2008.

   [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.

9.2.  Informative References

   [8]  Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain Certificates
        in the Session Initiation Protocol (SIP)",
        draft-ietf-sip-domain-certs-01.txt (work in progress),
        November 2007.
        July 2008.

Appendix A.  ASN.1 Module

        { 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) }


      -- 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 20 }

Authors' Addresses

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

   Phone:  +1 781 229 0533

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

   Phone:  +1 630 224-0216

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