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Internet Engineering Task Force (IETF)                       S. LawrenceRequest for Comments: 5924Category: Experimental                                        V. GurbaniISSN: 2070-1721                        Bell Laboratories, Alcatel-Lucent                                                               June 2010Extended Key Usage (EKU) for Session Initiation Protocol (SIP)X.509 CertificatesAbstract   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; seeSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc5924.Lawrence & Gurbani            Experimental                      [Page 1]

RFC 5924                         SIP EKU                       June 2010Copyright Notice   Copyright (c) 2010 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) in effect on the date of   publication of this document.  Please review these documents   carefully, as they describe your rights and restrictions with respect   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 Simplified BSD License.   This document may contain material from IETF Documents or IETF   Contributions published or made publicly available before November   10, 2008.  The person(s) controlling the copyright in some of this   material may not have granted the IETF Trust the right to allow   modifications of such material outside the IETF Standards Process.   Without obtaining an adequate license from the person(s) controlling   the copyright in such materials, this document may not be modified   outside the IETF Standards Process, and derivative works of it may   not be created outside the IETF Standards Process, except to format   it for publication as an RFC or to translate it into languages other   than English.Table of Contents1. Introduction ....................................................32. Terminology .....................................................32.1. Key Words ..................................................32.2. Abstract Syntax Notation ...................................33. Problem Statement ...............................................34. Restricting Usage to SIP ........................................44.1. Extended Key Usage Values for SIP Domains ..................55. Using the SIP EKU in a Certificate ..............................56. Implications for a Certification Authority ......................67. Security Considerations .........................................68. IANA Considerations .............................................69. Acknowledgments .................................................710. Normative References ...........................................7Appendix A.  ASN.1 Module ..........................................8Lawrence & Gurbani            Experimental                      [Page 2]

RFC 5924                         SIP EKU                       June 20101.  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.  Terminology2.1.  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 inRFC 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 SIPRFC 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 TrapezoidLawrence & Gurbani            Experimental                      [Page 3]

RFC 5924                         SIP EKU                       June 2010   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 usesRFC 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 keyLawrence & Gurbani            Experimental                      [Page 4]

RFC 5924                         SIP EKU                       June 2010      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 (seeSection 4.1).4.1.  Extended Key Usage Values for SIP DomainsRFC 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 CertificateSection 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:Lawrence & Gurbani            Experimental                      [Page 5]

RFC 5924                         SIP EKU                       June 2010   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.Lawrence & Gurbani            Experimental                      [Page 6]

RFC 5924                         SIP EKU                       June 20109.  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.Lawrence & Gurbani            Experimental                      [Page 7]

RFC 5924                         SIP EKU                       June 2010Appendix 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 }   ENDAuthors' 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.comLawrence & Gurbani            Experimental                      [Page 8]