Keywords: PKIX, SIP Domain, X.509 Certificate
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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RFC 5924 SIP EKU June 2010
Copyright Notice
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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
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RFC 5924 SIP EKU June 2010
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
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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
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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:
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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.
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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.
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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
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