SIP WG S. Lawrence
Internet-Draft Nortel Networks, Inc.
Intended status: Standards Track V. Gurbani
Expires: April 9, 2009 Bell Laboratories, Alcatel-Lucent
October 06, 2008
Using Extended Key Usage (EKU) for Session Initiation Protocol (SIP)
X.509 Certificates
draft-ietf-sip-eku-03
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Copyright Notice
Copyright (C) The IETF Trust (2008).
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. Implications for a Certification Authority . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 7
9.1. Normative References . . . . . . . . . . . . . . . . . . . 7
9.2. 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 RFC 3261 [2] trapezoid 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 TLS. Proxy-
A.example.com requests a TLS connection to Proxy-B.example.net, and
in the TLS handshake each 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
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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) it contains are valid only for use in SIP. In our
example, Proxy-B possesses a certificate making it authoritative as a
SIP server for the domain example.net; furthermore, it 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.
3. 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 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 3.1).
3.1. 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
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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 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 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 }
4. 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, implementations
MUST perform the step given below as a part of the certificate
validation:
The Extended Key Usage value(s), if any, MUST be examined:
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.
o If the certificate contains the id-kp-sipDomain EKU extension,
then the certificate MUST be accepted as valid 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 accept it for use as a
SIP certificate.
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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 4.2.1.12 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
as valid for use as a SIP certificate.
5. 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.
6. Security Considerations
This memo defines an EKU X.509 certificate extension that restricts
the the usage of a certificate to 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.
7. 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.
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, Paul Hoffman, and Stephen Kent.
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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.,
and W. Polk, "Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL) Profile",
RFC 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-03.txt (work in progress),
July 2008.
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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(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 20 }
END
Authors' Addresses
Scott Lawrence
Nortel Networks, Inc.
600 Technology Park
Billerica, MA 01821
USA
Phone: +1 978 248 5508
Email: scott.lawrence@nortel.com
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Vijay K. Gurbani
Bell Laboratories, Alcatel-Lucent
1960 Lucent Lane
Room 9C-533
Naperville, IL 60566
USA
Phone: +1 630 224-0216
Email: vkg@alcatel-lucent.com
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