Internet DRAFT - draft-gurbani-sip-domain-certs
draft-gurbani-sip-domain-certs
SIP WG V. Gurbani
Internet-Draft Bell Laboratories, Alcatel-Lucent
Updates: 3261 (if approved) S. Lawrence
Intended status: Best Current Pingtel Corp.
Practice A. Jeffrey
Expires: January 10, 2008 Bell Laboratories, Alcatel-Lucent
July 9, 2007
Domain Certificates in the Session Initiation Protocol (SIP)
draft-gurbani-sip-domain-certs-06
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Copyright Notice
Copyright (C) The IETF Trust (2007).
Abstract
This document provides a profile of PKIX-compliant certificates for
the purpose of domain authentication in the Session Initiation
Protocol (SIP).
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Table of Contents
1. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.1. Key Words . . . . . . . . . . . . . . . . . . . . . . . . 3
1.2. Abstract syntax notation . . . . . . . . . . . . . . . . . 3
2. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
3. Problem statement . . . . . . . . . . . . . . . . . . . . . . 3
4. SIP domain to host resolution . . . . . . . . . . . . . . . . 4
5. The need for mutual interdomain authentication . . . . . . . . 5
5.1. Restricting usage to SIP . . . . . . . . . . . . . . . . . 6
5.1.1. Extended Key Usage values for SIP domains . . . . . . 6
6. Guidelines for a Certification Authority . . . . . . . . . . . 7
7. Guidelines for a service provider . . . . . . . . . . . . . . 7
8. Behavior of SIP entities . . . . . . . . . . . . . . . . . . . 8
8.1. Finding SIP Identities in a Certificate . . . . . . . . . 8
8.2. Comparing SIP Identities . . . . . . . . . . . . . . . . . 9
8.3. Client behavior . . . . . . . . . . . . . . . . . . . . . 10
8.4. Server behavior . . . . . . . . . . . . . . . . . . . . . 10
8.5. Proxy behavior . . . . . . . . . . . . . . . . . . . . . . 11
8.6. Registrar behavior . . . . . . . . . . . . . . . . . . . . 11
8.7. Redirect server behavior . . . . . . . . . . . . . . . . . 11
8.8. Virtual SIP Servers and Certificate Content . . . . . . . 12
9. Security Considerations . . . . . . . . . . . . . . . . . . . 12
9.1. Connection authentication using Digest . . . . . . . . . . 13
10. Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . 13
11. References . . . . . . . . . . . . . . . . . . . . . . . . . . 14
11.1. Normative References . . . . . . . . . . . . . . . . . . . 14
11.2. Informative References . . . . . . . . . . . . . . . . . . 14
Editorial Comments . . . . . . . . . . . . . . . . . . . . . . . .
Appendix A. ASN.1 Module . . . . . . . . . . . . . . . . . . . . 15
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 16
Intellectual Property and Copyright Statements . . . . . . . . . . 17
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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 [5] extensions are defined using ASN.1
X.680 [6],X.690 [7].
2. Introduction
Transport Layer Security (TLS) [3] has started to appear in an
increasing number of Session Initiation Protocol (SIP) [2]
implementations. In order to use the authentication capabilities of
TLS, certificates as defined by the Internet X.509 Public Key
Infrastructure RFC 3280 [4] are required.
Existing SIP specifications do no sufficiently specify how to use
certificates for domain (as opposed to host) authentication. This
document attempts to provide sufficient guidance to ensure
interoperability and uniform conventions for the construction of SIP
domain certificates.
The discussion in this document is pertinent to an X.509 PKIX-
compliant certificate used for a TLS connection; it may not apply to
use of such certificates with S/MIME, for instance.
3. Problem statement
TLS uses X.509 Public Key Infrastructure [4] to bind an identity, or
a set of identities, to the subject of a X.509 certificate.
Accordingly, the recommendations of the SIP working group have been
to populate the X.509v3 subjectAltName extension with an identity.
However, this is under-specified in RFC 3261, which mentions
subjectAltName in conjunction with S/MIME only and not TLS. The
security properties of TLS and S/MIME as used in SIP are different:
X.509 certificates used for S/MIME are generally used for end-to-end
authentication and encryption, thus they serve to bind the identity
of a user to the certificate. On the other hand, X.509 certificates
used for TLS serve to bind the identities of the domain sending or
receiving the SIP messages.
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While RFC3261 provides adequate guidance on the use of X.509
certificates used for S/MIME, it is relatively silent on the use of
such certificates for TLS. The concept of what should be contained
in a site (or domain) certificate in RFC3261 is quoted below (Section
26.3.1):
Proxy servers, redirect servers and registrars SHOULD possess a
site certificate whose subject corresponds to their canonical
hostname.
The lack of specifications leads to problems when attempting to
interpret the certificate contents for TLS connections in a uniform
manner.
This document addresses two concerns related to X.509 certificates
used in SIP. First, it shows how the certificates are to be used for
mutual authentication when both the client and server possess
appropriate certificates; and second, it provides normative behavior
for matching the DNS query string with an identity stored in the
X.509 certificate (following the accepted practice of the time,
legacy X.509 certificates may store the identity in the Common Name
(CN) field of the certificate [Comment.1] instead of the currently
used subjectAltName extension. Furthermore, it is permissible for a
certificate to contain multiple identifiers for the Subject. As
such, this document specifies the appropriate matching rules.) And
finally, this document also provides guidelines for a Certification
Authority (CA) for issuing certificates to be used with SIP and to
service providers for assigning certificates to SIP servers.
The rest of this document is organized as follows: the next section
provides an overview of the most primitive case of a client using DNS
to access a SIP server and the resulting authentication steps.
Section 5 looks at the reason why mutual inter-domain authentication
is desired in SIP, and the lack of normative text and behavior in
RFC3261 for doing so. Section 6 outlines general guidelines for the
CA. Section 8 provides normative behavior of the SIP entities (user
agent clients, user agent servers, registrars, redirect servers, and
proxies) that need perform authentication based on X.509
certificates. Section 9 includes the security considerations.
4. SIP domain to host resolution
Routing in SIP is performed by having the client execute RFC 3263 [8]
procedures on a URI, called the "Application Unique String (AUS)
(c.f. Section 8 of RFC 3263 [8]). These procedures take as input a
SIP AUS (the SIP domain) and return an ordered set containing one or
more IP addresses, and a port number and transport corresponding to
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each IP address in the set (the "Expected Output") by querying an
Domain Name Service (DNS). If the transport indicates the use of
TLS, then a TLS connection is opened towards the server on a specific
IP address and port. The server presents an X.509 certificate to the
client for verification as part of the initial TLS handshake.
The client should determine the subjects of the certificate (see
Section 8.1) and compare these values to the AUS. If any subject
match is found, the server is considered to be authenticated and
subsequent signaling can now proceed over the TLS connection.
Matching rules for X.509 certificates and the normative behavior for
clients is specified in Section 8.3.
As an example: a request is to be routed to the SIP address
"sips:alice@example.com". This address requires a secure connection
to the SIP domain "example.com", which is used as the SIP AUS value.
Through a series of untrusted DNS manipulations, that AUS is mapped
to a set of host addresses and transports, from which an address
appropriate for use with TLS is selected. A connection is
established to that server, which presents a certificate with the
subject "sip:example.com". Since the SIP AUS matches the subject of
the certificate, the server is considered authenticated.
This is the way HTTPS operates, and SIPS simply borrows this
behavior from HTTP.
A domain name in an X.509 certificates is properly interpreted
only as a sequence of octets to be compared to the URI used to
reach the host. No inference should be made based on the DNS name
hierarchy.
5. The need for mutual interdomain authentication
RFC 3261 [2] section 26.3.2.2 "Interdomain Requests" discusses the
requirement that when a TLS connection is created between two
proxies, those proxies should each authenticate the other by
validating the certificate presented by the other during the TLS
handshake and comparing the subject of those certificates to the
expected domain name.
For example, suppose 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 [8] resolution and finds
that proxyB.example.net is a valid proxy for example.net using TLS.
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proxyA.example.com requests a TLS connection to proxyB.example.net,
and each presents a certificate to authenticate that connection.
The authentication problem for proxyA is straightforward - if we
assume secure DNS, then proxyA already knows that proxyB is a valid
proxy for the SIP domain example.net, so it only needs a valid
certificate from proxyB that contains the fully qualified host name
proxyB.example.net, or a SIP URI that asserts proxy B's authority
over example.net domain, i.e., a certificate that asserts the
identity "sip:example.net". [Comment.2]
The problem for proxyB is different, however; 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.
5.1. Restricting usage to SIP
The intent of this draft is to define certificate usage for binding a
SIP domain name to a connection. 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 may well 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 [4] 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.
5.1.1. Extended Key Usage values for SIP domains
RFC 3280 [4] specifies the extended key usage X.509 certificate
Extension for use in the Internet. The extension indicates one or
more purposes for which the certified public key may be used. The
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extended key usage 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 crytographic way.
The extended key usage 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
DNS name(s) in question. 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 8.1 for how the presence of an id-kp-sipDomain value
affects the interpretation of the certificate.
6. Guidelines for a Certification Authority
The procedures and practices employed by the certification authority
(CA) MUST ensure that the correct values for the extended key usage
extension and subjectAltName are inserted in each certificate that is
issued.
7. Guidelines for a service provider
When assigning certificates to proxy servers, registrars, and
redirect servers, a service provider MUST ensure that the SIP AUS
used to address the server is present as an identity in the
subjectAltName field of the certificate.
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8. Behavior of SIP entities
This section is normative; it specifies the behavior of SIP entities
when using X.509 certificates to determine an authenticated SIP
domain identity.
8.1. Finding SIP Identities in a Certificate
Procedures for determining a certificate's validity period, its
certification path, its presence on a certificate revocation list,
and other checks are described in RFC 3280 [4]; implementations must
follow checks as prescribed in RFC3280. This document adds rules for
interpreting an X.509 certificate for use in SIP.
Given an X.509 certificate that the above checks have found to be
acceptable, the following describes how to determine what SIP
identity or identities it contains. Note that a single certificate
MAY serve more than one purpose - that is, it MAY contain identities
not valid for use in SIP, and/or MAY contain more than one identity
for use in SIP.
1. The extendend key usage value(s), if any, MUST be examined to
determine whether or not the certificate is valid for use in SIP:
* If the certificate contains any extended key usage (EKU) value
other than id-kp-sipDomain, and does not contain the id-kp-
sipDomain value, then the certificate MUST NOT be accepted as
valid for use as a SIP certificate, and none of the identities
it contains are acceptable for SIP domain authentication.
* If the certificate does not contain any EKU values, it is a
matter of local policy whether or not to accept it for use as
a SIP certificate.
2. Examine the values in the subjectAltName field. The contents of
subjectAltName field and the constraints that may be imposed on
them are defined in Section 4.2.1.7 of RFC 3280 [4]. The
subjectAltName field may be empty, or may not exist at all, or it
may contain more than one identity. Each value in the
subjectAltName has a type; the only types acceptable for encoding
a SIP domain identity are:
URI If the scheme of the URI value is 'sip' (URI scheme tokens
are always case insensitive), and there is no userinfo
component in the URI (there is no '@'), then the hostpart is a
SIP domain identity. A URI value that does contain a userpart
MUST NOT be used as a domain identity (such a certificate
identifies an individual user, not a server for the domain).
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DNS A domain name system identifier MAY be accepted as a SIP
domain identity. An implementation MAY choose to accept a DNS
name as a domain identity, but only when no identity is found
using the URI type above.
3. If and only if the subjectAltName is empty or does not exist, the
client MAY examine the Subject Common Name (CN) field of the
certificate. If a valid DNS name is found there, the
implementation MAY use this value as a SIP domain identity. The
use of the CN value is allowed for backward compatibility, but is
NOT RECOMMENDED.
The above procedure yields a set containing zero or more identities
from the certificate. A client uses these identities to authenticate
a server (see Section 8.3) and a server uses them to authenticate a
client (see Section 8.4).
8.2. Comparing SIP Identities
When comparing two values as SIP identities:
Implementations MUST compare only that part of each identifier
(from the procedure defined in Section 8.1 that is a DNS name.
Any scheme or parameters extracted from an identifier MUST NOT be
used in the comparison procedure described below.
The values MUST be compared as DNS names, which means that the
comparison is case insensitive.
The match MUST be exact:
A suffix match MUST NOT be considered a match. For example,
"foo.example.com" does not match "example.com".
Any form of wildcard, such as a leading "." or "*.", MUST NOT
be considered a match. For example, "foo.example.com" does not
match ".example.com" or "*.example.com".
Note: RFC 2818 [9] (HTTP over TLS) allows the dNSName
component to contain a wildcard; e.g., "DNS:*.example.com".
RFC 3280 [4], while not disallowing this explicitly, leaves
the interpretation of wildcards to the individual
specification.
RFC 3261 does not provide any guidelines on the presence of
wildcards in certificates. The consensus from the working
group discussion leans in the favor of not using them in
SIP.
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8.3. Client behavior
A client uses the SIP AUS (the SIP domain name) to query a (possibly
untrusted) DNS to obtain a result set, which is a one or more SRV and
A records identifying the server for the domain (see Section 4 for an
overview.)
The SIP server, when accepting a TLS connection, presents its
certificate to the client for authentication. The client MUST
determine the SIP identities in the server certificate using the
procedure in Section 8.1. Then, the client MUST compare the original
SIP domain name (the AUS) used as input to the server location
procedures [8] to the set of SIP domain identities obtained from the
certificate.
o If the set of SIP identities is empty, the server is not
authenticated.
o If the AUS matches any SIP domain identity in the set, the server
is authenticated for the domain (SIP identity matching rules are
described in Section 8.2.)
If the server is not authenticated, the client MUST close the
connection immediately.
8.4. Server behavior
When a server accepts a TLS connection, it presents its own X.509
certificate to the client. To authenticate the client, the server
asks the client for a certificate. If the client possesses a
certificate, it is presented to the server. If the client does not
present a certificate, it MUST NOT be considered authenticated.
Whether or not to close a connection if the client cannot present
a certificate is a matter of local policy, and depends on the
authentication needs of the server for the connection. Some
currently deployed servers use Digest authentication to
authenticate individual requests on the connection, and choose to
treat the connection as authenticated by those requests for some
purposes (but see Section 9.1).
If the server requires client authentication for some local
purpose, then it MAY implement a policy of allowing the connection
only if the client is authenticated. For example, if the server
is an inbound proxy that has peering relationships with the
outbound proxies of other specific domains, it might only allow
connections authenticated as coming from those domains.
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The server MUST obtain the set of SIP domain identities from the
client certificate as described in Section 8.1. Because the server
accepted the TLS connection passively, unlike a client, it does not
possess an AUS for comparison. Instead, server policies can use the
authenticated SIP domain identity to make authorization decisions.
For example, a very open policy could be to accept any X.509
certificates and validate them using the procedures in RFC 3280; if
they validate, the identity is accepted and logged. Alternatively,
the server could have a list of all SIP domain names is allowed to
accept connections from; when a client presents its certificate, for
each identity in the client certificate, the server searches for it
in the list of acceptable domains to decide whether or not to accept
the connection. Other policies that make finer distinctions are
possible.
Note that the decision of whether or not the authenticated connection
to the client is appropriate for use to route new requests to the
authenticated domain is independent of whether or not the connection
is authenticated; the normal routing rules for SIP as defined
elsewhere MUST be used.
8.5. Proxy behavior
A proxy MUST use the procedures defined for a User Agent Server (UAS)
in Section 8.4 when authenticating a connection from a client.
A proxy MUST use the procedures defined for a User Agent Client (UAC)
in Section 8.3 when requesting an authenticated connection to a UAS.
If a proxy adds a Record-Route when forwarding a request with the
expectation that the route is to use secure connections, it MUST
insert into the Record-Route header a URI that corresponds to an
identity for which it has a certificate; if it does not, then it will
not be possible to create a secure connection using the value from
the Record-Route as the AUS.
8.6. Registrar behavior
A SIP registrar, acting as a server, follows the normative behavior
of Section 8.4. It may accept a TLS connection from the client,
present its certificate, and then challenge the client with HTTP
Digest.
8.7. Redirect server behavior
A SIP redirect server follows the normative behavior of Section 8.4.
It may accept a TLS connection from the client, present its
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certificate, and then challenge the client with HTTP Digest.
8.8. Virtual SIP Servers and Certificate Content
The closest guidance in SIP today regarding certificates and virtual
SIP servers occurs in SIP Identity ([11], Section 13.4). The quoted
section states that, "... certificates have varying ways of
describing their subjects, and may indeed have multiple subjects,
especially in the 'virtual hosting' cases where multiple domains are
managed by a single application."
The above quote appears to imply that a certificate that is shared
among virtual servers will have multiple identifiers in the
subjectAltName field, each corresponding to a discrete virtual server
that represents a single domain (PKIX-compliant certificates have
exactly one Subject field and at most one subjectAltName field, which
may contain multiple identifiers for the Subject.)
Since only one certificate is needed for multiple domains, the keying
material management is straightforward, but such a certificate MUST
be revoked if ANY identifier in the certificate is no longer
associated with the holder of the private key for the the
certificate.
The TLS extended client hello [10] allows a TLS client to provide to
the TLS server the name of the server to which a connection is
desired. Thus, the server can present the correct certificate to
establish the TLS connection.
9. Security Considerations
The goals of TLS (when used with X.509 certificates) include the
following security guarantees at the transport layer:
Confidentiality: packets tunneled through TLS can only be read by
the sender and receiver.
Integrity: packets tunneled through TLS cannot be undetectably
modified on the connection between the sender and receiver.
Authentication: each principal is authenticated to the other as
possessing a private key for which a certificate has been issued.
Moreover, this certificate has not been revoked, and is backed by
a certificate chain leading to a trusted certification authority.
We expect appropriate processing of domain certificates to provide
the following security guarantees at the application level:
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Confidentiality: SIPS messages from alice@example.com to
bob@example.edu can be read only by alice@example.com,
bob@example.edu, and SIP proxies issued with domain certificates
for example.com or example.edu.
Integrity: SIPS messages from alice@example.com to bob@example.edu
cannot be undetectably modified on the links between
alice@example.com, bob@example.edu, and SIP proxies issued with
domain certificates for example.com or example.edu.
Authentication: alice@example.com and proxy.example.com are mutually
authenticated, and moreover proxy.example.com is authenticated to
alice@example.com as an authoritative proxy for domain
example.com. Similar mutual authentication guarantees are given
between proxy.example.com and proxy.example.edu and between
proxy.example.edu and bob@example.edu. As a result,
alice@example.com is transitively mutually authenticated to
bob@example.edu (assuming trust in the authoritative proxies for
example.com and example.edu).
9.1. Connection authentication using Digest
Digest authentication in SIP provides for authentication of the
message sender to the challenging UAS. As commonly deployed, it
provides only very limited integrity protection of the authenticated
message. Many existing deployments have chosen to use the Digest
authentication of one or more messages on a particular connection as
a way to authenticate the connection itself - and by implication,
authenticating other (unchallenged) messages on that connection.
Some even choose to similarly authenticate a UDP source address and
port based on the Digest authentication of a message received from
that address and port. This use of Digest goes beyond the assurances
it was designed to provide, and is NOT RECOMMENDED. Authentication
of the domain at the other end of a connection SHOULD be accomplished
using TLS and the certificate validation rules described by this
specification instead.
10. 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.
11. References
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11.1. 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] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0",
RFC 2246, January 1999.
[4] 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.
[5] International International Telephone and Telegraph
Consultative Committee, "Information Technology - Open Systems
Interconnection - The Directory: Authentication Framework",
CCITT Recommendation X.509, November 1988.
[6] 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.
[7] 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.
11.2. Informative References
[8] Rosenberg, J. and H. Schulzrinne, "Session Initiation Protocol
(SIP): Location SIP Servers", RFC 3263, June 2002.
[9] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[10] Blake-Wilson, S., Nystrom, M., Hopwood, D., Mikkelsen, J., and
T. Wright, "Transport Layer Security (TLS) Extensions",
RFC 4366, April 2006.
[11] Peterson, J. and C. Jennings, "Enhancements for Authenticated
Identity Management in the Session Initiation Protocol (SIP)",
draft-ietf-sip-identity-06.txt (work in progress),
October 2005.
Editorial Comments
Gurbani, et al. Expires January 10, 2008 [Page 14]
Internet-Draft Domain Certs July 2007
[Comment.1] Stephen Kent: PKIX standards made an exception for RFC
822 names in legacy certificates, but not for DNS names
or URIs! There is a private extension, developed by
Netscape for representing a DNS name in a certificate
prior to the advent of SAN. I think it's rather late to
be accomodating certificates that are not compiant with
RFC 3280, a spec that is 5 years old.
[Comment.2] (authors) and Stephen Kent: Actually, even if DNSSEC
provides a trusted host name, it is sufficient for
proxyB to have presented a certificate that contains a
SIP identity for example.net, so authentication of just
the proxyB hostname has little value since it would not
be sufficient without DNSSEC.
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
Vijay K. Gurbani
Bell Laboratories, Alcatel-Lucent
2701 Lucent Lane
Room 9F-546
Lisle, IL 60532
USA
Phone: +1 630 224-0216
Email: vkg at bell hyphen labs dot com
Scott Lawrence
Pingtel Corp.
400 West Cummings Park
Suite 2200
Woburn, MA 01801
USA
Phone: +1 781 938 5306
Email: slawrence@pingtel.com
Alan S.A. Jeffrey
Bell Laboratories, Alcatel-Lucent
2701 Lucent Lane
Room 9F-534
Lisle, IL 60532
USA
Email: ajeffrey at bell hyphen labs dot com
Gurbani, et al. Expires January 10, 2008 [Page 16]
Internet-Draft Domain Certs July 2007
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