One document matched: draft-ietf-mmusic-comedia-tls-01.txt
Differences from draft-ietf-mmusic-comedia-tls-00.txt
Multiparty Multimedia Session J. Lennox
Control Columbia U.
Internet-Draft July 16, 2004
Expires: January 14, 2005
Connection-Oriented Media Transport over the Transport Layer Security
(TLS) Protocol in the Session Description Protocol (SDP)
draft-ietf-mmusic-comedia-tls-01
Status of this Memo
By submitting this Internet-Draft, I certify that any applicable
patent or other IPR claims of which I am aware have been disclosed,
and any of which I become aware will be disclosed, in accordance with
RFC 3668.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that
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Internet-Drafts are draft documents valid for a maximum of six months
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The list of current Internet-Drafts can be accessed at
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This Internet-Draft will expire on January 14, 2005.
Copyright Notice
Copyright (C) The Internet Society (2004). All Rights Reserved.
Abstract
This document specifies how to establish secure connection-oriented
media transport sessions over the Transport Layer Security (TLS)
protocol using the Session Description Protocol (SDP). It defines a
new protocol identifier, TCP/TLS. It also defines the syntax and
semantics for an SDP "fingerprint" attribute that identifies the
certificate which will be presented for the TLS session. This
mechanism allows media transport over TLS connections to be
established securely, so long as the integrity of session
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descriptions is assured.
Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Terminology . . . . . . . . . . . . . . . . . . . . . . . . . 4
3. Overview . . . . . . . . . . . . . . . . . . . . . . . . . . . 4
3.1 SDP Operational Modes . . . . . . . . . . . . . . . . . . 4
3.2 Threat Model . . . . . . . . . . . . . . . . . . . . . . . 4
3.3 The Need For Self-Signed Certificates . . . . . . . . . . 5
3.4 Example SDP Description For TLS Connection . . . . . . . . 6
4. Protocol Identifiers . . . . . . . . . . . . . . . . . . . . . 6
5. Fingerprint Attribute . . . . . . . . . . . . . . . . . . . . 7
6. Endpoint Identification . . . . . . . . . . . . . . . . . . . 8
6.1 Certificate Choice . . . . . . . . . . . . . . . . . . . . 8
6.2 Certificate Presentation . . . . . . . . . . . . . . . . . 9
7. Security Considerations . . . . . . . . . . . . . . . . . . . 10
8. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 10
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 11
9.1 Normative References . . . . . . . . . . . . . . . . . . . . 11
9.2 Informative References . . . . . . . . . . . . . . . . . . . 12
Author's Address . . . . . . . . . . . . . . . . . . . . . . . 13
Intellectual Property and Copyright Statements . . . . . . . . 14
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1. Introduction
The Session Description Protocol (SDP) [1] provides a general purpose
format for describing multimedia sessions in announcements or
invitations. For many applications, it is desirable to establish, as
part of a multimedia session, a media stream which uses a
connection-oriented transport. The document Connection-Oriented
Media Transport in the Session Description Protocol (SDP) [2]
specifies a general mechanism for describing and establishing such
connection-oriented streams; however, the only transport protocol it
directly supports is TCP. In many cases, session participants wish
to provide confidentiality, data integrity, and authentication for
their media sessions. This document therefore extends the
Connection-Oriented Media specification to allow session descriptions
to describe media sessions that use the Transport Layer Security
(TLS) protocol [3].
The TLS protocol allows applications to communicate over a channel
which provides privacy and data integrity. The TLS specification,
however, does not specify how specific protocols establish and use
this secure channel; particularly, TLS leaves the question of how to
interpret and validate authentication certificates as an issue for
the protocols which run over TLS. This document specifies such usage
for the case of connection-oriented media transport.
Complicating this issue, endpoints exchanging media will often be
unable to obtain authentication certificates signed by a well-known
root certificate authority (CA). Most certificate authorities charge
for signed certificates, particularly host-based certificates;
additionally, there is a substantial administrative overhead to
obtaining signed certificates, as certificate authorities must be
able to confirm that they are issuing the signed certificates to the
correct party. Furthermore, in many cases endpoints' IP addresses
and host names are dynamic: they may be obtained from DHCP, for
example. It is impractical to obtain a CA-signed certificate valid
for the duration of a DHCP lease. For such hosts, self-signed
certificates are usually the only option. This specification defines
a mechanism which allows self-signed certificates can be used
securely, provided that the integrity of the SDP description is
assured. It provides for endpoints to include a secure hash of their
certificate, known as the "certificate fingerprint", within the
session description. Provided the fingerprint of the offered
certificate matches the one in the session description, end hosts can
trust even self-signed certificates.
The rest of this document is laid out as follows. An overview of the
problem and threat model is given in Section 3. Section 4 gives the
basic use of SDP. Section 5 describes the SDP fingerprint attribute,
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which, assuming the integrity of SDP content is assured, allows the
secure use of self-signed certificates. Section 6 describes which
X.509 certificates are presented, and how they are used in TLS.
Section 7 discusses additional security considerations.
2. Terminology
In this document, the key words "MUST", "MUST NOT", "REQUIRED",
"SHALL", "SHALL NOT", "SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY",
and "OPTIONAL" are to be interpreted as described in RFC 2119 [4] and
indicate requirement levels for compliant implementations.
3. Overview
This section discusses the threat model which motivates TLS transport
for connection-oriented media streams. It also discusses in more
detail the need for end systems to use self-signed certificates.
3.1 SDP Operational Modes
There are two principal operational modes for multimedia sessions:
advertised and offer-answer. Advertised sessions are the simpler
mode. In this mode, a server publishes, in some manner, an SDP
session description describing a multimedia session it is making
available. The classic example of this mode of operation is the
Session Announcment Protocol (SAP) [13], in which SDP session
descriptions are periodically transmitted to a well-known multicast
group. Traditionally, these descriptions involve multicast
conferences, but unicast sessions are also possible.
(Connection-oriented media, obviously, cannot use multicast.)
Recipients of a session description connect to the addresses
published in the session description. These recipients may not
previously have been known to the advertiser of the session
description.
Alternatively, SDP conferences can operate in offer-answer mode [5].
This mode allows two participants in a multimedia session to
negotiate the multimedia session between them. In this model, one
participant offers the other a description of the desired session
from its perspective, and the other participant answers with the
desired session from its own perspective. In this mode, each of the
participants in the session has knowledge of the other one. This is
the mode of operation used by the Session Initiation Protocol (SIP)
[14].
3.2 Threat Model
Participants in multimedia conferences often wish to guarantee
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confidentiality, data integrity, and authentication for their media
sessions. This section describes various types of attackers and the
ways they attempt to violate these guarantees. It then describes how
the TLS protocol can be used to thwart the attackers.
The simplest type of attacker is one who listens passively to the
traffic associated with a multimedia session. This attacker might,
for example, be on the same local-area or wireless network as one of
the participants in a conference. This sort of attacker does not
threaten a connection's data integrity or authentication, and almost
any operational mode of TLS can provide media stream confidentiality.
More sophisticated is an attacker who can send his own data traffic
over the network, but who cannot modify or redirect valid traffic.
In SDP's 'advertised' operational mode, this can barely be considered
an attack; media sessions are expected to be initiated from anywhere
on the network. In SDP's offer-answer mode, however, this type of
attack is more serious. An attacker could initiate a connection to
one or both of the endpoints of a session, thus impersonating an
endpoint, or acting as a man in the middle to listen in on their
communications. To thwart these attacks, TLS uses endpoint
certificates. So long as the certificates' private keys have not
been compromised, the endpoints have an external trusted mechanism
(most commonly, a mutually-trusted certificate authority) to validate
certificates, and the endpoints know what certificate identity to
expect, endpoints can be certain that such an attack has not taken
place.
Finally, the most serious type of attacker is one who can modify or
redirect session descriptions: for example, a compromised or
malicious SIP proxy server. Neither TLS itself, nor any mechanisms
which use it, can protect an SDP session against such an attacker.
Instead, the SDP description itself must be secured through some
mechanism; SIP, for example, defines how S/MIME [15] can be used to
secure session descriptions.
3.3 The Need For Self-Signed Certificates
SDP session descriptions are created by any endpoint that needs to
participate in a multimedia session. In many cases, such as SIP
phones, such endpoints have dynamically-configured IP addresses and
host names, and must be deployed with nearly zero configuration. For
such an endpoint, it is for practical purposes impossible to obtain a
certificate signed by a well-known certificate authority.
If two endpoints have no prior relationship, self-signed certificates
cannot generally be trusted, as there is no guarantee that an
attacker is not launching a man-in-the-middle attack. Fortunately,
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however, if the integrity of SDP session descriptions can be assured,
it is possible to consider those SDP descriptions themselves as a
prior relationship: certificates can be securely described in the
session description itself. This is done by providing a secure hash
of a certificate, or "certificate fingerprint", as an SDP attribute;
this mechanism is described in Section 5.
3.4 Example SDP Description For TLS Connection
Figure 1 illustrates an SDP offer which signals the availability of a
T.38 fax session over TLS. For the purpose of brevity, the main
portion of the session description is omitted in the example, showing
only the m= line and its attributes. (This example is the same as
the first one in [2], except for the proto parameter and the
fingerprint attribute.) See the subsequent sections for explanations
of the example's TLS-specific attributes.
(Note that the example uses MD5 as its one-way hash function, even
though SHA-1 is preferred. This has been done only because the
longer SHA-1 fingerprint would cause that line of the example to be
wider than the number of characters allowed in an Internet-Draft.)
m=image 54111 TCP/TLS t38
c=IN IP4 10.1.1.2
a=setup:passive
a=connid:1
a=fingerprint:MD5 48:AA:D8:BA:36:7C:6D:70:7F:81:BB:BA:ED:6D:B8:C7
Figure 1: Example SDP Description Offering a TLS Media Stream
4. Protocol Identifiers
The m= line in SDP specifies, among other items, the transport
protocol to be used for the media in the session. See the "Media
Descriptions" section of SDP [1] for a discussion on transport
protocol identifiers.
This specification defines a new protocol identifier, TCP/TLS, which
indicates that the media described will use the Transport Layer
Security protocol [3] over TCP. (Using TLS over other transport
protocols is not discussed by this document.) An m= line that
specifies TCP/TLS MUST further qualify the protocol using a fmt
identifier, to indicate the application being run over TLS.
As TLS sessions are connection-oriented, media sessions described in
this manner follow the procedures defined in the connection-oriented
media specification [2]. They also use the attributes defined in
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that specification, "a=setup" and "a=connid".
5. Fingerprint Attribute
Parties to a TLS session indicate their identities by presenting
authentication certificates as part of the TLS handshake procedure.
Authentication certificates are X.509 [6] certificates, as profiled
by RFC 3279 [7] and RFC 3280 [8].
In order to associate media streams with connections, and to prevent
unauthorized barge-in attacks on the media streams, endpoints MAY
provide a certificate fingerprint. If the X.509 certificate
presented for the TLS connection matches the fingerprint presented in
the SDP, the endpoint can be confident that the author of the SDP is
indeed the initiator of the connection.
A certificate fingerprint is a secure one-way hash of the DER
(distinguished encoding rules) form of the certificate. (Certificate
fingerprints are widely supported by tools which manipulate X.509
certificates; for instance, the command "openssl x509 -fingerprint"
causes the command-line tool of the openssl package to print a
certificate fingerprint, and the certificate managers for Mozilla and
Internet Explorer display them when viewing the details of a
certificate.)
A fingerprint is represented in SDP as an attribute (an "a=" line).
It consists of the name of the hash function used, followed by the
hash value itself. The hash value is represented as a sequence of
upper-case hexadecimal bytes, separated by colons. The number of
bytes is defined by the hash function. (This is the syntax used by
openssl and by the browsers' certificate managers. It is different
from the syntax used to represent hash values in, e.g., HTTP digest
authentication [16], which uses unseparated lower-case hexadecimal
bytes. It was felt that consistency with other applications of
fingerprints was more important.)
The formal syntax of the fingerprint attribute is given in Augmented
Backus-Naur Form [9] in Figure 2. This syntax extends the BNF syntax
of SDP [1].
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attribute =/ fingerprint-attribute
fingerprint-attribute = "fingerprint" ":" hash-func SP fingerprint
hash-func = "sha-1" / "md5" / "md2" / token
; Additional hash functions can only come
; from updates to RFC 3279
fingerprint = 2UHEX *(":" 2UHEX)
; Each byte in upper-case hex, separated
; by colons.
UHEX = DIGIT / %x41-46 ; A-F uppercase
Figure 2: Abstract Backus-Naur Syntax for the Fingerprint Attribute
A certificate fingerprint SHOULD be computed using the same one-way
hash function as is used in the certificate's signature algorithm.
(This guarantees that the fingerprint will be usable by the other
endpoint so long as the certificate itself is.) Following RFC 3279
[7], therefore, the defined hash functions are SHA-1 [10][17], MD5
[11], and MD2 [12], with SHA-1 preferred. Additional hash functions
can be defined only by standards-track RFCs which update or obsolete
RFC 3279 [7].
The fingerprint attribute may be either a session-level or a
media-level SDP attribute. If it is a session-level attribute, it
applies to all TLS sessions for which no media-level fingerprint
attribute is defined.
6. Endpoint Identification
6.1 Certificate Choice
X.509 certificates certify identities. The certificate provided for
a TLS connection needs to certify an appropriate identity for the
connection. Identity matching is performed using the matching rules
specified by RFC 3280 [8]. If more than one identity of a given type
is present in the certificate (e.g., more than one dNSName name), a
match in any one of the set is considered acceptable.
If an endpoint does not provide a certificate fingerprint in its SDP,
its certificate MUST correspond to one of the following identities,
and MUST be signed by a certificate authority known to the other
endpoint.
o If the connection address for the media description is specified
as an IP address, the endpoint MAY use a certificate with an
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iPAddress subjectAltName which exactly matches the IP in the
connection-address in the session description's c= line.
o If the connection address for the media description is specified
as a fully-qualified domain name, the endpoint MAY use a
certificate with a dNSName subjectAltName matching the specified
c= line connection-address. Names may contain the wildcard
character * which is considered to match any single domain name
component or component fragment. E.g., *.a.com matches foo.a.com
but not bar.foo.a.com. f*.com matches foo.com but not bar.com.
(This last pattern is not often meaningful, but is supported by
https [18]; thus, it is allowed here as well.)
o If the SDP session description describing the session was
transmitted over an end-to-end secure protocol which uses X.509
certificates, the endpoint MAY use the same certificate to certify
the media connection. For example, an SDP description sent over
HTTP/TLS [18] or secured by S/MIME [15] MAY use the same
certificate to secure the media connection. (Note, however, that
the sips protocol [14] (SIP over TLS) provides only hop-by-hop
security, so its TLS certificates do not satisfy this criterion.)
In this case, the certificate must be one that is allowed in this
context by the transmitting protocol.
In those cases where an endpoint does provide a certificate
fingerprint, the certificate MAY be self-signed, but MUST indicate
some identity which has a meaningful relationship to the end point.
This identity MAY be one of the identities allowed above for
non-fingerprinted certificates, or MAY correspond to the protocol
over which the SDP was transmitted. For example, protocols which use
URIs could include a certificate with a subjectAltName field of type
uniformResourceIdentifier with a value matching the endpoint's URI.
6.2 Certificate Presentation
In all cases, an endpoint acting as the TLS server, i.e., one taking
the a=setup:passive role, in the terminology of connection-oriented
media, MUST present a certificate during TLS initiation, following
the rules presented in Section 6.1. If the certificate does not
match the original fingerprint, or, if there is no fingerprint, the
certificate identity is incorrect, the client endpoint MUST either
notify the user, if possible, or terminate the media connection with
a bad certificate error.
If the SDP offer/answer model [5] is being used, the client (the
endpoint with the setup:active role) MUST also present a certificate
following the rules of Section 6.1. The server MUST request a
certificate, and if the client does not provide one, if the
certificate does not match the provided fingerprint, or, if there was
no fingerprint, the certificate identity is incorrect, the server
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endpoint MUST either notify the user or terminate the media
connection with a bad certificate error.
Note that when the offer/answer model is being used, it is possible
for a media connection to outrace the answer back to the offerer.
Thus, if the offerer has offered a setup:passive or setup:actpass
role, it MUST (as specified in the Connection-Oriented Media
specification [2]) begin listening for an incoming connection as soon
as it sends its offer. However, because its peer's media connection
may outrace its answer, it SHOULD NOT definitively accept or reject
the peer's certificate until it has received and processed the SDP
answer.
If offer/answer is not being used (e.g., if the SDP was sent over the
Session Announcement Protocol [13]), the TLS server typically has no
external knowledge of what the TLS client's identity ought to be. In
this case, no client certificate need be presented, and no
certificate validation can be performed, unless the server has
knowledge of valid clients through some external means.
7. Security Considerations
This entire document concerns itself with security. The problem to
be solved is addressed in Section 1, and a high-level overview is
presented in Section 3.
Like all SDP messages, SDP messages describing TLS streams are
conveyed in an encapsulating application protocol (e.g., SIP, MGCP,
etc.). It is the responsibility of the encapsulating protocol to
ensure the integrity and confidentiality of the SDP security
descriptions. Therefore, the application protocol SHOULD either
invoke its own security mechanisms (e.g., secure multiparts) or
alternatively utilize a lower-layer security service (e.g., TLS or
IPSec). This security service SHOULD provide strong message
authentication and packet-payload encryption as well as effective
replay protection.
TLS is not always the most appropriate choice for secure
connection-oriented media; in some cases, a higher-level security
protocol may be appropriate. For example, RTP and RTCP packets may
be sent over a connection-oriented transport [19]. In this case, it
may be more appropriate to use the Secure RTP protocol [20] with
appropriate SDP descriptions [21].
8. IANA Considerations
This document defines an SDP proto value: TCP/TLS. Its format is
defined in Section 4. This proto value should be registered by IANA
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on http://www.iana.org/assignments/sdp-parameters under "proto".
This document defines an SDP session and media level attribute:
fingerprint. Its format is defined in Section 5. This attribute
should be registered by IANA on
http://www.iana.org/assignments/sdp-parameters under "att-field (both
session and media level)".
9. References
9.1 Normative References
[1] Handley, M., Jacobson, V. and C. Perkins, "SDP: Session
Description Protocol", draft-ietf-mmusic-sdp-new-18 (work in
progress), June 2004.
[2] Yon, D., "Connection-Oriented Media Transport in the Session
Description Protocol (SDP)", draft-ietf-mmusic-sdp-comedia-07
(work in progress), June 2004.
[3] Dierks, T. and C. Allen, "The TLS Protocol Version 1.0", RFC
2246, January 1999.
[4] Bradner, S., "Key words for use in RFCs to Indicate Requirement
Levels", BCP 14, RFC 2119, March 1997.
[5] Rosenberg, J. and H. Schulzrinne, "An Offer/Answer Model with
Session Description Protocol (SDP)", RFC 3264, June 2002.
[6] 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.
[7] Bassham, L., Polk, W. and R. Housley, "Algorithms and
Identifiers for the Internet X.509 Public Key Infrastructure
Certificate and Certificate Revocation List (CRL) Profile", RFC
3279, April 2002.
[8] 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.
[9] Crocker, D. and P. Overell, "Augmented BNF for Syntax
Specifications: ABNF", RFC 2234, November 1997.
[10] National Institute of Standards and Technology, "Secure Hash
Standard", FIPS PUB 180-1, April 1995,
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<http://www.itl.nist.gov/fipspubs/fip180-1.htm>.
[11] Rivest, R., "The MD5 Message-Digest Algorithm", RFC 1321, April
1992.
[12] Kaliski, B., "The MD2 Message-Digest Algorithm", RFC 1319,
April 1992.
9.2 Informative References
[13] Handley, M., Perkins, C. and E. Whelan, "Session Announcement
Protocol", RFC 2974, October 2000.
[14] 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.
[15] Ramsdell, B., "S/MIME Version 3 Message Specification", RFC
2633, June 1999.
[16] Franks, J., Hallam-Baker, P., Hostetler, J., Lawrence, S.,
Leach, P., Luotonen, A. and L. Stewart, "HTTP Authentication:
Basic and Digest Access Authentication", RFC 2617, June 1999.
[17] Eastlake, D. and P. Jones, "US Secure Hash Algorithm 1 (SHA1)",
RFC 3174, September 2001.
[18] Rescorla, E., "HTTP Over TLS", RFC 2818, May 2000.
[19] Lazzaro, J., "Framing RTP and RTCP Packets over
Connection-Oriented Transport",
draft-ietf-avt-rtp-framing-contrans-01 (work in progress),
March 2004.
[20] Baugher, M., "The Secure Real-time Transport Protocol",
draft-ietf-avt-srtp-09 (work in progress), July 2003.
[21] Andreasen, F., Baugher, M. and D. Wing, "Session Description
Protocol Security Descriptions for Media Streams",
draft-ietf-mmusic-sdescriptions-04 (work in progress), May
2004.
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Author's Address
Jonathan Lennox
Columbia University Department of Computer Science
450 Computer Science
1214 Amsterdam Ave., M.C. 0401
New York, NY 10027
US
Phone: +1 212 939 7018
EMail: lennox@cs.columbia.edu
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