Network Working Group P. Jones (Ed.)
Internet Draft N. Ismail
Intended status: Informational D. Benham
Expires: April 27, 2015 N. Buckles
Cisco Systems
J. Mattsson
Y. Cheng
Ericsson
R. Barnes
Mozilla
October 27, 2014
Requirements for Private Media in a Switched Conferencing Environment
draft-jones-avtcore-private-media-reqts-00
Abstract
This document specifies the requirements for ensuring the privacy and
integrity of real-time media flows between two or more endpoints
communicating in a switched conferencing environment. This document
also provides a high-level overview of switched conferencing in order
to establish a common understanding of the goals and objectives of
this work.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on April 27, 2015.
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This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
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Table of Contents
1. Introduction...................................................2
2. Requirements Language..........................................3
3. Terminology....................................................3
4. Background.....................................................4
5. Motivation for Private Media in Switched Conferencing..........5
5.1. Switched Conferencing in Cloud Services...................5
5.2. Private Media Security through Switching..................7
6. Goals and Non-Goals............................................8
6.1. Goals.....................................................8
6.1.1. Ensure End-To-End Confidentiality....................8
6.1.2. Ensure End-To-End Source Authentication of Media.....9
6.1.3. Provide a More Efficient Service than "Full-Mesh"....9
6.1.4. Support Cloud-Based Conferencing.....................9
6.1.5. Limiting a User's Access to Content..................9
6.1.6. Compatibility with the WebRTC Security Architecture.10
6.2. Non-Goals................................................10
6.2.1. Securing the Endpoints..............................10
6.2.2. Concealing that Communication Occurs................10
6.2.3. Individual Media Source Authentication..............11
6.2.4. Support for Multicast in Switched Conferencing......11
7. Requirements..................................................11
8. IANA Considerations...........................................12
9. Security Considerations.......................................12
10. References...................................................12
10.1. Normative References....................................12
10.2. Informative References..................................13
11. Acknowledgments..............................................13
12. Contributors.................................................13
Authors' Addresses...............................................14
1. Introduction
Users of multimedia communication products and services have privacy
expectations that are largely satisfied with the use of SRTP
[RFC3711] and related technologies when communicating point-to-point
over the Internet. When communicating in a conferencing environment
with two or more participants, though, it is necessary for an
endpoint to share the SRTP master key and salt with the conference
server so that it can authenticate and decrypt received RTP and RTCP
packets. The conference server also needs the master key and salt in
order to transmit media packets it receives to other participants in
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the conference. The need for conferencing servers to have the master
key is a security risk for users.
Within a corporate or other isolated environment where conferencing
servers are tightly controlled, this security risk can be effectively
managed. However, managing this risk is becoming increasing
difficult as conferencing resources are being deployed in networks
that are less than fully trusted, including virtualized conferencing
servers deployed in cloud environments.
There are also public voice and video conferencing service providers
in which users must place full trust in order to use those services,
as it is necessary for an endpoint to share the SRTP master key with
those conferencing servers. This exposes corporations, for example,
to a higher risk of being subjected to corporate espionage. While it
is not the intent of this draft to suggest that any existing service
provider would permit or condone any illicit use of its service, the
fact is that security threats can come from external sources and
remain undiscovered for long periods of time.
It is possible to ensure communication privacy within the context of
a switched conferencing environment with limited changes in the
security mechanisms used today. This document discusses this
possibility in more detail and presents a set of requirements for
meeting this objective.
2. Requirements Language
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 [RFC2119]
when they appear in ALL CAPS. These words may also appear in this
document in lower case as plain English words, absent their normative
meanings.
3. Terminology
[Editor's Note: we may want to refine these or add/remove terms]
Adversary - An unauthorized entity that may attempt to compromise the
performance of a conference server through various means, including,
but not limited to, the transmission of bogus media packets or
attempt to gain access to the plaintext of the media.
Switching conference server - A conference server that does not
decrypt RTP media flows or perform processing on the media payload,
but instead simply forwards the received media from a sender to the
other participants in a multimedia conference. A switching
conference server may modify some RTP headers.
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4. Background
Traditional multimedia conferencing servers would mix, transcode,
transrate, and/or recompose media flows from one or more conference
participants, sending out a different audio and video flow to each
participant. For audio, this might entail mixing some number of
input flows that appear to contain audio intended to be heard by the
other participants, with each participant receiving a flow that does
not contain that participant's own audio. For video, the conference
server may elect to send only video showing the current active
speaker, a tiled composition of all participants or the most recent
active speakers, a video flow with the active speaker presented
prominently with other participants presented as thumbnail images, or
some other composite arrangement. It is also common for audio or
video to be transcoded. A typical traditional conferencing server is
depicted in Figure 1.
+-------------------+
+---+ --{A}--> | | <--{C}-- +---+
| A | | Media Composition | | C |
+---+ <-{BCD}- | | -{ABD}-> +---+
| Transcoders |
+---+ --{B}--> | Transraters | <--{D}-- +---+
| B | | | | D |
+---+ <-{ACD}- | Decrypt/Encrypt | -{ABC}-> +---+
+-------------------+
Figure 1 - Traditional Conferencing Server
Traditional conference servers require a significant amount of
processing power, which in turn translates into a high cost for
conferencing hardware manufacturers. Significantly, too, it is very
difficult to deploy these servers in a cloud environment due to the
high processing demands, as the specialized hardware found in the
traditional voice and video conferencing server does not exist in a
cloud environment.
To enable the traditional conferencing server to perform its job, the
server establishes an SRTP session with each of the conference
participants so that it can get the keys required to decrypt and
encrypt media flows from and to each participant. This means that
the conference server is necessarily a fully trusted entity in the
communication path. Anytime these servers are deployed in a network
that is not tightly controlled, it increases the risk that an
attacker might gain access to cryptographic key material, thus
allowing the attacker to be able to see and listen to ongoing
conferences. In some instances, depending on how the hardware is
designed and how keys and certificates are managed, it might be
possible for an attacker to see and listen to previously recorded
conferences or future conferences.
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The Secure Real-time Transport Protocol (SRTP) [RFC3711] is a profile
of RTP, which can provide confidentiality, message authentication,
and replay protection to the RTP traffic and to the RTP Control
Protocol (RTCP). Encryption of header extension in SRTP [RFC6904]
provides a mechanism extending the mechanisms of [RFC3711], to
selectively encrypt RTP header extensions in SRTP. [RFC3711] and
[RFC6904] solves end-to-end use cases between two endpoints, and does
not consider use cases where a sender delivers media to a receiver
via a cloud-based conferencing service.
5. Motivation for Private Media in Switched Conferencing
5.1. Switched Conferencing in Cloud Services
There is a trend in the industry for enterprises to use cloud
services to host multi-party conferences and meet-me services, either
exclusively or to meet peak loads on-demand. At the same time, there
is huge shift toward using light-weight, cost-effective switching
conference servers in cloud services that do not necessarily need to
mix audio or composite/transcode video. Also fueling the use of such
light-weight conference servers is the desire to fully exploit
virtualized computing resources and dynamic scalability potential
available in cloud computing environments.
The increased use of cloud services has exposed a problem. There are
two different trust domains from a media perspective: endpoints and
other devices in a trusted domain, and conference servers controlled
by the cloud service in an untrusted domain. Other examples of
conference devices spread across trusted and untrusted domains are
likely, but the cloud service trend is triggering the urgency to
address the need to allow for lightweight media conference while
enabling media privacy at the same time.
With a switching conference server, each participant transmits media
to the server as it would with a traditional conferencing server.
However, the switching conference server merely forwards media to the
other participants in the conference (where the other participant may
be associated with a cascaded conference server or an endpoint on the
same server), leaving composition to the receiving endpoint. Since
some endpoints may have a limited amount of bandwidth, each endpoint
might negotiate with the switching conference server to receive only
a subset of the available media flows. Each transmitting endpoint
might also send multiple media flows of varying frame sizes and/or
frame rates (e.g., simulcast or scalability layers), so that the
server can select the streams most appropriate for each receiver's
bandwidth and capabilities. This allows, for example, an endpoint to
receive and display higher quality video for the active speaker and
thumbnails for other participants. It is also worth noting that, for
switched media to work successfully, each endpoint in the conference
must support the media formats transmitted by all other entities in
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the conference. More modern endpoints support multiple codecs and
formats, making this commercially practical.
Figure 2 depicts an example of a switching conference server wherein
each participant is receiving the media flows transmitted by each of
the other participants in the conference.
+--------------------+
+---+ --{A}--> | | <-{C}--- +---+
| A | <-{B}--- |Switching Conference| --{A}--> | C |
| | <-{C}--- | Server | --{B}--> | |
+---+ <-{D}--- | | --{D}--> +---+
| Packet |
+---+ --{B}--> | Authentication | <-{D}--- +---+
| B | <-{A}--- | | --{A}--> | D |
| | <-{C}--- | | --{B}--> | |
+---+ <-{D}--- | Media Privacy | --{C}--> +---+
+--------------------+
Figure 2 - Switching Conference Server
Note - The use of multiple arrows directed toward each endpoint is
not intended to suggest the use of separate RTP sessions.
By using methods such as those described in [RFC6464], it is possible
for the switching conference server to transmit the appropriate audio
and video flows to conference participants without having knowledge
of the contents of the encrypted media. The examples that follow
help to illustrate this point.
In the Figure 3 below, endpoints A, B and D receive the video streams
from endpoint C, the currently active speaker, which is receiving
video from endpoint A, the previous active speaker. Later when
endpoint B becomes the active speaker (Figure 4), endpoints A, C and
D will start to receive video from B, while endpoint B continues to
receive video from endpoint C. Finally in Figure 5, endpoint A
becomes the active speaker.
+--------------------+
+---+ --{A}--> | | <--{C}-- +---+
| A | |Switching Conference| | C |*
+---+ <-{C}--- | Server | ---{A}-> +---+
| |
+---+ --{B}--> | | <--{D}-- +---+
| B | | | | D |
+---+ <-{C}--- | | ---{C}-> +---+
+--------------------+
Figure 3 - Endpoint "C" is the Active Speaker
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+--------------------+
+---+ --{A}--> | | <--{C}-- +---+
| A | |Switching Conference| | C |
+---+ <-{B}--- | Server | ---{B}-> +---+
| |
+---+ --{B}--> | | <--{D}-- +---+
*| B | | | | D |
+---+ <-{C}--- | | ---{B}-> +---+
+--------------------+
Figure 4 - Endpoint "B" is the Active Speaker
+--------------------+
+---+ --{A}--> | | <--{C}-- +---+
*| A | |Switching Conference| | C |
+---+ <-{B}--- | Server | ---{A}-> +---+
| |
+---+ --{B}--> | | <--{D}-- +---+
| B | | | | D |
+---+ <-{A}--- | | ---{A}-> +---+
+--------------------+
Figure 5 - Endpoint "A" is the Active Speaker
Switched conferencing can also enable conferences to scale to include
many more simultaneous participants than would be possible with a
traditional conferencing server. Like traditional conferencing
servers, switching conference servers can also be cascaded or
interconnected in a meshed topology to increase the size of the
conference without putting undue burden on any particular server.
5.2. Private Media Security through Switching
A traditional conferencing server, or MCU, establishes an SRTP
session with each participating endpoint separately, and needs
to decrypt packets containing media presented to other endpoints. By
using a switching conference server, it is possible to keep the media
encryption keys private to the endpoints such that the conference
server does not have access to the keys used for media encryption.
The switching conference server just forwards media received to each
of the other participants in the conference.
This provides for a significantly improved security model, as one
can, for example, utilize conferencing resources in the cloud that do
not necessarily have to be trusted. That said, there may be
situations where the switching conference server needs to modify the
RTP packet received from an endpoint, such as by adding or removing
an RTP header extension, modifying the payload type value, etc. It
would be the responsibility of the switching conference server to
ensure that media of the expected type and containing the correct
information is received by a recipient.
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Thus, there is a need to utilize an end-to-end encryption and
authentication key (or pair of keys) and a hop-by-hop encryption and
authentication key (or pair of keys). The purpose for the hop-by-hop
encryption key is to optionally encrypt RTP header extensions. The
current SRTP specification and related specifications do not define
use of a dual-key approach presently. However, such an approach is
possible and would result in ensuring the privacy of media while also
enabling the more scalable switched conferencing model.
The assumptions with this model are that the endpoints are trusted
entities, as they clearly have access to the media keys for
encryption. Some call processing functions for the administrative
domain, such as SIP [RFC3261] proxy servers or B2BUAs, are trusted in
exactly the same way they are with the traditional conferencing
model, meaning they must be trusted to keep signaling secure as
certificate information (e.g., fingerprints) might be conveyed via
signaling. The switching conference server is not fully trusted and
is not given visibility into the actual contents of the SRTP payload.
However, the switching conference server in the untrusted domain is
at least trusted to perform its core duties of forwarding media and
processing signaling; it simply isn't trusted with the media
encryption keys.
The assumption is that no changes are made to SRTCP, i.e. SRTCP is
protected hop-by-hop with a single security context.
This dual-key model does necessitate a change in the way that keys
are managed. However, the topic of key management is outside the
scope of this requirements document. However, high-level assumptions
like if the end-to-end contexts use a group key as SRTP master key or
if individual SRTP master keys (that may be derived/negotiated from
another group key) is likely to influence the solution derived from
this document.
6. Goals and Non-Goals
6.1. Goals
6.1.1. Ensure End-To-End Confidentiality
The content of the communication and all media needs to be
confidential within the group of entities explicitly invited into the
conference. An external monitoring adversary should not be able to
deduce the human-to-human communication that actually occurred from
capturing the media packets.
At the same time, it is necessary to allow switching media servers to
manipulate certain RTP header fields like the payload type value.
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6.1.2. Ensure End-To-End Source Authentication of Media
In a conference system with multiple participants it is vital that
the multimedia content presented to any of the human participants is
from the stated participant, and not an adversary that attempts to
inject misleading content. Nor should an adversary be able to fool
the system into becoming a trusted party in the conference. Only
explicitly invited parties shall be able to contribute content.
6.1.3. Provide a More Efficient Service than "Full-Mesh"
A multi-party conference that has the goals of confidentiality and
source authentication can be established as a "full mesh" (i.e., each
participating endpoint directly addresses each of the other
participants). However, this has a significant issue with the amount
of consumed resources in both the uplink and the downlink from each
participant.
A switched conferencing model would yield the efficiencies desired.
6.1.4. Support Cloud-Based Conferencing
To achieve cost-effective and scalable conferencing, it must be
possible to run the conference node instances in a cloud-based
virtualized environment.
From a security standpoint, this is a significant issue since the
virtualized server instance and the underlying hardware and software
upon which it runs might not be secure from an adversary.
6.1.5. Limiting a User's Access to Content
Since an invited user will be provided with the content protection
keys, the user can decrypt content from time periods before and after
the user joined the conference. However, this is not always
desirable. It should be possible to re-key the content protection
keys every time a user joins or leaves the conference so each
particular set of conference participants uses a unique key.
This also changes the trust level required on the conference roster
handling at any point and how to keep that accurate and secured.
It should be noted that timely completion of the re-keying operations
become an obstacle in system design and operation. Thus, it is a
goal to allow for this possibility when it is deemed essential, but
it should not be a requirement on a system to re-key each time the
participant list changes.
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6.1.6. Compatibility with the WebRTC Security Architecture
It is a goal of this work to ensure compatibility with the WebRTC
security architecture as described in [I.D-rtcweb-security-arch]. As
an example, local resources that are considered a part of the trusted
computing base (TCB), such as keying material derived using DTLS-
SRTP, will remain within the TCB and not exposed to untrusted
entities.
The browser is reliant on an external calling service to convey
signaling information that may open the door for a man-in-the-middle
attack, such as the conveyance of certificate fingerprints over the
interface between the browser and the calling service. However, as
described in [I.D-rtcweb-security-arch], the browser may utilize
additional services, such as a trusted identify provider, to mitigate
such risks.
6.2. Non-Goals
6.2.1. Securing the Endpoints
The security of a communication session requires that the endpoints
are not compromised and that the users are trustworthy. If not,
credentials and decrypted content may be shared with third parties.
However, this is hard to prevent through system design. Thus, it
should be assumed that the endpoint is secure and the user is
trustworthy; how to achieve this is out of scope this document.
6.2.2. Concealing that Communication Occurs
A non-goal is to attempt to prevent a pervasive monitoring adversary
from knowing that the communication session has occurred. The reason
for excluding this as a goal is that it is extremely difficult to
achieve, as a pervasive monitoring adversary can be expected to be
able to have knowledge of all IP flows that enter or exit local ISPs,
across links that straddle nation borders or internet exchange
points. To hide the fact communication occurred, the flows required
to achieve the communication session need to be highly difficult to
correlate between different legs of the communication.
At this stage this is deemed too difficult to attempt and will need
to be a subject for further study. Existing attempts include The
Onion Router (TOR), against which it has been claimed to be possible
to monitor, at least partially, by an adversary with sufficient
reach.
Also of consideration is that trying to conceal the fact that
communication occurred actually makes it more difficult for network
administrators to effectively manage and troubleshoot issues with
conference calls.
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6.2.3. Individual Media Source Authentication
Although the participants in the conference are authenticated, it is
not a goal to provide source authentication of the media at the
individual user level, instead being satisfied with being able to
authenticate media as coming from an invited conference participant
or not.
There exist solutions that can provide individual media source
authentication (e.g., TESLA). However, they impact the performance
or security properties they provide. Thus, further study is required
to determine impact and resulting security properties if desired to
have individual source authentication.
6.2.4. Support for Multicast in Switched Conferencing
Multicast traffic is, by design, transmitted to every participant in
a conference. The focus of this document is only on centralized
unicast conferencing that utilizes a switched conferencing
architecture.
7. Requirements
The following are the security solution requirements for switched
conferencing that enable end-to-end media privacy between all
conference participants.
Note that while some switching media servers might be fully trusted
entities, the intent of this solution and purpose for these private
media (PM) requirements is to address those servers that are not
fully trusted.
PM-01: Switching conference server MUST be able to switch the media
between participants in a conference without having access to
the media encryption keys.
PM-02: Solution MUST maintain all current SRTP security goals,
namely the ability to provide for confidentiality, provide
replay protection, and ensure message integrity.
PM-03: Solution MUST extend replay attacks protection to cover each
hop in the media path. It MUST be possible to detect if a
packet received by either an endpoint or a switching
conference server was previously received by that entity or
if the packet is not intended for that entity.
PM-04: Keys used for end-to-end encryption and authentication of RTP
payloads and other information deemed unsuitable for access
by the switching conference server MUST NOT be generated by
or accessible to any component that is not in the fully
trusted domain.
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PM-05: The switching conference server MUST be capable of making
changes to the RTP header and, optionally, the RTP header
extensions.
PM-06: The switching conference server, or any entity that is not
fully trusted, MUST NOT be involved in the authentication of
identities for the purpose of media key distribution.
PM-07: The switching conference server MUST be able to switch an
already active SRTP stream to a new receiver, while
guaranteeing the timely synchronization between the SRTP
context of the transmitter and its current and new receivers.
PM-08: It MUST be possible for the switching conference server to
determine if a received media packet was transmitted by a
valid conference participant.
PM-09: It MUST be possible for a conference to be optionally re-
keyed as desired, such as each time a participant joins or
leaves the conference.
PM-10: To decrypt packets, the receiving endpoint needs to be able
to know the SSRC and RTP sequence number used by the sending
endpoint. These values need to be integrity protected end-
to-end, either explicitly by inclusion in an end-to-end MAC
or implicitly like the MKI field in [RFC3711].
8. IANA Considerations
There are no IANA considerations for this document.
9. Security Considerations
[TBD]
10. References
10.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol
(SRTP)", RFC 3711, March 2004.
[RFC6464] Lennox, J., Ivov, E., and E. Marocco, "A Real-time
Transport Protocol (RTP) Header Extension for Client-to-
Mixer Audio Level Indication", RFC 6464, December 2011.
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[I.D-rtcweb-security-arch]
E. Rescorla, "WebRTC Security Architecture", Work in
Progress, July 2014.
[RFC6904] J. Lennox, "Encryption of Header Extensions in the Secure
Real-time Transport Protocol (SRTP)", RFC 6904, December
2013.
10.2. Informative References
[RFC3261] 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.
11. Acknowledgments
The authors would like to thank Marcello Caramma, Matthew Miller,
Christian Oien, Magnus Westerlund, Cullen Jennings, Christer
Holmberg, and Bo Burman for their invaluable input.
12. Contributors
[TBD]
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Authors' Addresses
Paul E. Jones
Cisco Systems, Inc.
7025 Kit Creek Rd.
Research Triangle Park, NC 27709
USA
Phone: +1 919 476 2048
Email: paulej@packetizer.com
Nermeen Ismail
Cisco Systems, Inc.
170 W Tasman Dr.
San Jose
USA
Email: nermeen@cisco.com
David Benham
Cisco Systems, Inc.
170 W Tasman Dr.
San Jose
USA
Email: dbenham@cisco.com
Nathan Buckles
Cisco Systems, Inc.
170 W Tasman Dr.
San Jose
USA
Email: nbuckles@cisco.com
John Mattsson
Ericsson AB
SE-164 80 Stockholm
Sweden
Phone: +46 10 71 43 501
Email: john.mattsson@ericsson.com
Yi Cheng
Ericsson
SE-164 80 Stockholm
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Sweden
Phone: +46 10 71 17 589
Email: yi.cheng@ericsson.com
Richard Barnes
Mozilla
331 E Evelyn Ave.
Mountain View
USA
Email: rlb@ipv.sx
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