Internet Engineering Task Force (IETF)                       S. Lawrence
Request for Comments: 5924
Category: Experimental                                        V. Gurbani
ISSN: 2070-1721                        Bell Laboratories, Alcatel-Lucent
                                                               June 2010


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

Abstract

   This memo documents an extended key usage (EKU) X.509 certificate
   extension for restricting the applicability of a certificate to use
   with a Session Initiation Protocol (SIP) service.  As such, in
   addition to providing rules for SIP implementations, this memo also
   provides guidance to issuers of certificates for use with SIP.

Status of This Memo

   This document is not an Internet Standards Track specification; it is
   published for examination, experimental implementation, and
   evaluation.

   This document defines an Experimental Protocol for the Internet
   community.  This document is a product of the Internet Engineering
   Task Force (IETF).  It represents the consensus of the IETF
   community.  It has received public review and has been approved for
   publication by the Internet Engineering Steering Group (IESG).  Not
   all documents approved by the IESG are a candidate for any level of
   Internet Standard; see Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc5924.

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Table of Contents

   1. Introduction ....................................................3
   2. Terminology .....................................................3
      2.1. Key Words ..................................................3
      2.2. Abstract Syntax Notation ...................................3
   3. Problem Statement ...............................................3
   4. Restricting Usage to SIP ........................................4
      4.1. Extended Key Usage Values for SIP Domains ..................5
   5. Using the SIP EKU in a Certificate ..............................5
   6. Implications for a Certification Authority ......................6
   7. Security Considerations .........................................6
   8. IANA Considerations .............................................6
   9. Acknowledgments .................................................7
   10. Normative References ...........................................7
   Appendix A.  ASN.1 Module ..........................................8

1.  Introduction

   This memo documents an extended key usage (EKU) X.509 certificate
   extension for restricting the applicability of a certificate to use
   with a Session Initiation Protocol (SIP) service.  As such, in
   addition to providing rules for SIP implementations, this memo also
   provides guidance to issuers of certificates for use with SIP.

2.  Terminology

2.1.  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].

   Additionally, the following term is defined:

      SIP domain identity: A subject identity in the X.509 certificate
      that conveys to a recipient of the certificate that the
      certificate owner is authoritative for SIP services in the domain
      named by that subject identity.

2.2.  Abstract Syntax Notation

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

3.  Problem Statement

   Consider the SIP RFC 3261 [2] actors shown in Figure 1.

     Proxy-A.example.com           Proxy-B.example.net
        +-------+                    +-------+
        | Proxy |--------------------| Proxy |
        +----+--+                    +---+---+
             |                           |
             |                           |
             |                           |
             |                         +---+
           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 her INVITE arrived at an
   outbound proxy Proxy-A.example.com.  In order to route the request
   onward, Proxy-A uses RFC 3263 [7] resolution and finds that Proxy-
   B.example.net is a valid proxy for example.net that uses Transport
   Layer Security (TLS).  Proxy-A.example.com requests a TLS connection
   to Proxy-B.example.net, and in the TLS handshake each one presents a
   certificate to authenticate that connection.  The validation of these
   certificates by each proxy to determine whether or not their peer is
   authoritative for the appropriate SIP domain is defined in "Domain
   Certificates in the Session Initiation Protocol (SIP)" [8].

   A SIP domain name is frequently textually identical to the same DNS
   name used for other purposes.  For example, the DNS name example.com
   can serve as a SIP domain name, an email domain name, and a web
   service name.  Since these different services within a single
   organization might be administered independently and hosted
   separately, it is desirable that a certificate be able to bind the
   DNS name to its usage as a SIP domain name without creating the
   implication that the entity presenting the certificate is also
   authoritative for some other purpose.  A mechanism is needed to allow
   the certificate issued to a proxy to be restricted such that the
   subject name(s) that the certificate contains are valid only for use
   in SIP.  In our example, Proxy-B possesses a certificate making
   Proxy-B authoritative as a SIP server for the domain example.net;
   furthermore, Proxy-B has a policy that requires the client's SIP
   domain be authenticated through a similar certificate.  Proxy-A is
   authoritative as a SIP server for the domain example.com; when
   Proxy-A makes a TLS connection to Proxy-B, the latter accepts the
   connection based on its policy.

4.  Restricting Usage to SIP

   This memo defines a certificate profile for restricting the usage of
   a domain name binding to usage as a SIP domain name.  RFC 5280 [3],
   Section 4.2.1.12, defines a mechanism for this purpose: an "Extended
   Key Usage" (EKU) attribute, where the purpose of the EKU extension is
   described as:

      If the extension is present, then the certificate MUST only be
      used for one of the purposes indicated.  If multiple purposes are
      indicated the application need not recognize all purposes
      indicated, as long as the intended purpose is present.
      Certificate using applications MAY require that the extended key

      usage extension be present and that a particular purpose be
      indicated in order for the certificate to be acceptable to that
      application.

   A Certificate Authority issuing a certificate whose purpose is to
   bind a SIP domain identity without binding other non-SIP identities
   MUST include an id-kp-sipDomain attribute in the Extended Key Usage
   extension value (see Section 4.1).

4.1.  Extended Key Usage Values for SIP Domains

   RFC 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 is valid.  The EKU extension can be used in
   conjunction with the key usage extension, which indicates how the
   public key in the certificate is 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 the
   use of any Subject names in the certificate is restricted to use by a
   SIP service (along with any usages allowed by other EKU values).

         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 20 }

5.  Using the SIP EKU in a Certificate

   Section 7.1 of Domain Certificates in the Session Initiation Protocol
   [8] contains the steps for finding an identity (or a set of
   identities) in an X.509 certificate for SIP.  In order to determine
   whether the usage of a certificate is restricted to serve as a SIP
   certificate only, implementations MUST perform the steps given below
   as a part of the certificate validation:

   The implementation MUST examine the Extended Key Usage value(s):

   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
      certificate.  Note that since certificates not following this
      specification will not have the id-kp-sipDomain EKU value, and
      many do not have any EKU values, the more interoperable local
      policy would be to accept the certificate.

   o  If the certificate contains the id-kp-sipDomain EKU extension,
      then implementations of this specification MUST consider the
      certificate acceptable for use as a SIP certificate.

   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 consider the certificate
      acceptable for use as a SIP certificate.

   o  If the EKU extension exists, but does not contain any of the id-
      kp-sipDomain or id-kp-anyExtendedKeyUsage EKU values, then the
      certificate MUST NOT be accepted as valid for use as a SIP
      certificate.

6.  Implications for a Certification Authority

   The procedures and practices employed by a 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.

7.  Security Considerations

   This memo defines an EKU X.509 certificate extension that restricts
   the usage of a certificate to a SIP service belonging to an
   autonomous domain.  Relying parties can 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.

8.  IANA Considerations

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

9.  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, Paul Hoffman, 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.

10.  Normative References

   [1]  Bradner, S., "Key words for use in RFCs to Indicate Requirement
        Levels", BCP 14, 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., Boeyen, S., Housley, R.,
        and W. Polk, "Internet X.509 Public Key Infrastructure
        Certificate and Certificate Revocation List (CRL) Profile",
        RFC 5280, May 2008.

   [4]  International Telecommunications Union, "Information technology
        - Open Systems Interconnection - The Directory: Public-key and
        attribute certificate frameworks", ITU-T Recommendation X.509,
        ISO Standard 9594-8, March 2000.

   [5]  International International Telephone and Telegraph Consultative
        Committee, "Abstract Syntax Notation One (ASN.1): Specification
        of basic notation", CCITT Recommendation X.680, July 2002.

   [6]  International International Telephone and Telegraph Consultative
        Committee, "ASN.1 encoding rules: Specification of basic
        encoding Rules (BER), Canonical encoding rules (CER) and
        Distinguished encoding rules (DER)", CCITT Recommendation X.690,
        July 2002.

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

   [8]  Gurbani, V., Lawrence, S., and A. Jeffrey, "Domain Certificates
        in the Session Initiation Protocol (SIP)", RFC 5922, June 2010.

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(62) }

   DEFINITIONS IMPLICIT TAGS ::=
   BEGIN

   -- OID Arcs

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

   -- Extended Key Usage Values

   id-kp-sipDomain  OBJECT IDENTIFIER  ::=  { id-kp 20 }

   END

Authors' Addresses

   Scott Lawrence

   EMail: scott-ietf@skrb.org


   Vijay K. Gurbani
   Bell Laboratories, Alcatel-Lucent
   1960 Lucent Lane
   Room 9C-533
   Naperville, IL  60566
   USA

   Phone: +1 630 224-0216
   EMail: vkg@bell-labs.com