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Versions: 00 01 03 04 05 06 07 08 RFC 5924
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, 2008
Using Extended Key Usage (EKU) for Session Initiation Protocol (SIP)
X.509 Certificates
draft-ietf-sip-eku-01
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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. Guidelines for a Certification Authority . . . . . . . . . . . 6
6. Security Considerations . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 6
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
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 [2] trapezoid shown in Figure 1.
proxyA.example.com ------------ proxyB.example.net
| |
| |
| |
| +---+
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 normal resolution rules cause
her INVITE to be sent to an outbound proxy proxyA.example.com, which
then uses RFC 3263 [7] resolution and finds that proxyB.example.net
is a valid proxy for example.net that uses TLS. proxyA.example.com
requests a TLS connection to proxyB.example.net, and each presents a
certificate to authenticate that connection. This is the basic
mutual 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
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equivalent in another domain. In Figure 1, proxyA.example.com
performs certain DNS queries to arrive at proxyB.example.net.
Because of the answers to the DNS queries, proxyA has a certain
expectation that proxyB is a valid proxy in the example.net domain
and is authorized to receive inbound requests targeted to that
domain.
However, the problem for proxyB is different; it is presented with a
connection from a specific host, but what it needs to determine is
whether or not that connection can be treated as coming from a
particular SIP domain. If it receives a certificate that contains
only the name proxyA.example.com, then it cannot determine that
proxyA is authorized to act as a SIP outbound proxy for example.com,
because example.com may use different systems for inbound messages so
SIP DNS resolution of example.com may not lead to proxyA.example.com
(if this is the case, proxyB should not reuse this connection if it
needs to send a request to example.com). The certificate usage in
SIP should not require that every outbound proxy for a domain must
also be an inbound proxy for that domain, but should provide for
certificate based binding of the SIP domain name to a particular
connection.
Thus, there is a need for an extra attribute that allows a proxy to
know that its peer is an authorized proxy for that domain. This memo
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
example.com 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
3280 [3] section 4.2.1.13 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 [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 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.
Inclusion of this KeyPurposeId in a certificate indicates that any
DNS 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 be
marked as critical so that implementations that do not understand it
will 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 }
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 SIP proxy 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:
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
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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.
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.
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. 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 include the 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.
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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] Housley, R., Polk, W., Ford, W., and D. Solo, "Internet X.509
Public Key Infrastructure Certificate and Certificate Revocation
List (CRL) Profile", RFC 3280, April 2002.
[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.
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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-00.txt (work in progress),
November 2007.
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 VALUE-TBD }
END
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Authors' Addresses
Scott Lawrence
Bluesocket Inc.
10 North Ave.
Burlington, MA 01803
USA
Phone: +1 781 229 0533
Email: slawrence@bluesocket.com
Vijay K. Gurbani
Bell Laboratories, Alcatel-Lucent
2701 Lucent Lane
Room 9F-546
Lisle, IL 60532
USA
Phone: +1 630 224-0216
Email: vkg@alcatel-lucent.com
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