draft-perkins-avt-srtp-vbr-audio-04.txt   draft-perkins-avt-srtp-vbr-audio-05.txt 
Network Working Group C. Perkins Network Working Group C. Perkins
Internet-Draft University of Glasgow Internet-Draft University of Glasgow
Intended status: BCP JM. Valin Intended status: BCP JM. Valin
Expires: January 10, 2011 Octasic Inc. Expires: June 15, 2011 Octasic Inc.
July 9, 2010 December 12, 2010
Guidelines for the use of Variable Bit Rate Audio with Secure RTP Guidelines for the use of Variable Bit Rate Audio with Secure RTP
draft-perkins-avt-srtp-vbr-audio-04.txt draft-perkins-avt-srtp-vbr-audio-05.txt
Abstract Abstract
This memo discusses potential security issues that arise when using This memo discusses potential security issues that arise when using
variable bit rate audio with the secure RTP profile. Guidelines to variable bit rate audio with the secure RTP profile. Guidelines to
mitigate these issues are suggested. mitigate these issues are suggested.
Status of this Memo Status of this Memo
This Internet-Draft is submitted in full conformance with the This Internet-Draft is submitted in full conformance with the
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Internet-Drafts are working documents of the Internet Engineering Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet- working documents as Internet-Drafts. The list of current Internet-
Drafts is at http://datatracker.ietf.org/drafts/current/. Drafts is at http://datatracker.ietf.org/drafts/current/.
Internet-Drafts are draft documents valid for a maximum of six months Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress." material or to cite them other than as "work in progress."
This Internet-Draft will expire on January 10, 2011. This Internet-Draft will expire on June 15, 2011.
Copyright Notice Copyright Notice
Copyright (c) 2010 IETF Trust and the persons identified as the Copyright (c) 2010 IETF Trust and the persons identified as the
document authors. All rights reserved. document authors. All rights reserved.
This document is subject to BCP 78 and the IETF Trust's Legal This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of (http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents publication of this document. Please review these documents
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described in the Simplified BSD License. described in the Simplified BSD License.
Table of Contents Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3 1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Scenario-Dependent Risk . . . . . . . . . . . . . . . . . . . . 3 2. Scenario-Dependent Risk . . . . . . . . . . . . . . . . . . . . 3
3. Guidelines for use of VBR Audio with SRTP . . . . . . . . . . . 4 3. Guidelines for use of VBR Audio with SRTP . . . . . . . . . . . 4
4. Guidelines for use of Voice Activity Detection with SRTP . . . 4 4. Guidelines for use of Voice Activity Detection with SRTP . . . 4
5. Padding the output of VBR codecs . . . . . . . . . . . . . . . 5 5. Padding the output of VBR codecs . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5 6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5 7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6 8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6 9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . . 6 9.1. Normative References . . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . . 6 9.2. Informative References . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6 Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
1. Introduction 1. Introduction
The secure RTP framework (SRTP) [RFC3711] is a widely used framework The secure RTP framework (SRTP) [RFC3711] is a widely used framework
for securing RTP sessions. SRTP provides the ability to encrypt the for securing RTP sessions. SRTP provides the ability to encrypt the
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related information, such as the fact that the source and destination related information, such as the fact that the source and destination
IP addresses are available. IP addresses are available.
3. Guidelines for use of VBR Audio with SRTP 3. Guidelines for use of VBR Audio with SRTP
It is the responsibility of the application designer to determine the It is the responsibility of the application designer to determine the
appropriate trade-off between security and bandwidth overhead. As a appropriate trade-off between security and bandwidth overhead. As a
general rule, VBR codecs should be considered safe in the context of general rule, VBR codecs should be considered safe in the context of
encrypted one-to-one calls. However, applications that make use of encrypted one-to-one calls. However, applications that make use of
pre-recorded messages where the contents of such pre-recorded pre-recorded messages where the contents of such pre-recorded
messages may be of any value to an evesdropper (i.e. messages beyond messages may be of any value to an evesdropper (i.e., messages beyond
standard greeting messages) SHOULD NOT use codecs in VBR mode. IVR standard greeting messages) SHOULD NOT use codecs in VBR mode. IVR
applications would be particularly vulnerable since an evesdropper applications would be particularly vulnerable since an evesdropper
could easily use the rate information to easily recognize the prompts could easily use the rate information to easily recognize the prompts
being played out. being played out.
It is safe to use variable rate coding to adapt the output of a voice It is safe to use variable rate coding to adapt the output of a voice
codec to match characteristics of a network channel, for example for codec to match characteristics of a network channel, for example for
congestion control purposes, provided this adaptation done in a way congestion control purposes, provided this adaptation done in a way
that does not expose any information on the speech signal. That is, that does not expose any information on the speech signal. That is,
if the variation is driven by the available network bandwidth, not by if the variation is driven by the available network bandwidth, not by
the input speech (i.e. if the packet sizes and spacing are constant the input speech (i.e., if the packet sizes and spacing are constant
unless the network conditions change). VBR speech codecs can safely unless the network conditions change). VBR speech codecs can safely
be used in this fashion with SRTP while avoiding leaking information be used in this fashion with SRTP while avoiding leaking information
on the contents of the speech signal that might be useful for traffic on the contents of the speech signal that might be useful for traffic
analysis. analysis.
4. Guidelines for use of Voice Activity Detection with SRTP 4. Guidelines for use of Voice Activity Detection with SRTP
Many speech codecs employ some form of voice activity detection (VAD) Many speech codecs employ some form of voice activity detection (VAD)
to either suppress output frames, or generate some form of lower-rate to either suppress output frames, or generate some form of lower-rate
comfort noise frames, during periods when the speaker is not active. comfort noise frames, during periods when the speaker is not active.
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overhang applied to each talkspurt must be randomly chosen in such a overhang applied to each talkspurt must be randomly chosen in such a
way that it is computationally infeasible for an attacker to reliably way that it is computationally infeasible for an attacker to reliably
estimate the length of that talkspurt. The audio data comprising the estimate the length of that talkspurt. The audio data comprising the
overhang period must be packetised and transmitted in RTP packets in overhang period must be packetised and transmitted in RTP packets in
a manner that is indistinguishable from the other data in the a manner that is indistinguishable from the other data in the
talkspurt. talkspurt.
The overhang period SHOULD have an exponentially-decreasing The overhang period SHOULD have an exponentially-decreasing
probability distribution function. This ensures a long tail, while probability distribution function. This ensures a long tail, while
being easy to compute. It is RECOMMENDED to use an overhang with a being easy to compute. It is RECOMMENDED to use an overhang with a
"half life" of at least 1 second. Despite the overhang (and no "half life" of a few hundred milliseconds (this should be sufficient
matter what the duration is), there is still a small amount of to obscure the presence of inter-word pauses and the lengths of
information leaked about the start time of the talkspurt due to the single words spoken in isolation, for example the digits of a credit
fact that we cannot apply an overhang to the start of a talkspurt card number clearly enunciated for an automated system, but not so
without unacceptably affecting intelligibility. For that reason, VAD long as to significantly reduce the effectiveness of VAD for
SHOULD NOT be used in encrypted IVR applications where the content of detecting listening pauses). Despite the overhang (and no matter
pre-recorded messages may be of any value to an eavesdropper. what the duration is), there is still a small amount of information
leaked about the start time of the talkspurt due to the fact that we
cannot apply an overhang to the start of a talkspurt without
unacceptably affecting intelligibility. For that reason, VAD SHOULD
NOT be used in encrypted IVR applications where the content of pre-
recorded messages may be of any value to an eavesdropper.
The application of a random overhang period to each talkspurt will The application of a random overhang period to each talkspurt will
reduce the effectiveness of VAD in SRTP sessions when compared to reduce the effectiveness of VAD in SRTP sessions when compared to
non-SRTP sessions. It is, however, still expected that the use of non-SRTP sessions. It is, however, still expected that the use of
VAD will provide a significant bandwidth saving for many encrypted VAD will provide a significant bandwidth saving for many encrypted
sessions. sessions.
5. Padding the output of VBR codecs 5. Padding the output of VBR codecs
For scenarios where VBR is considered unsafe, the codec SHOULD be For scenarios where VBR is considered unsafe, the codec SHOULD be
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No IANA actions are required. No IANA actions are required.
8. Acknowledgements 8. Acknowledgements
This memo is based on the discussion in [spot-me]. Recent versions This memo is based on the discussion in [spot-me]. Recent versions
of ZRTP [I-D.zimmermann-avt-zrtp] contain a similar recommendation; of ZRTP [I-D.zimmermann-avt-zrtp] contain a similar recommendation;
the purpose of this memo is to highlight these issues to a wider the purpose of this memo is to highlight these issues to a wider
audience, since they are not specific to ZRTP. Thanks are due to audience, since they are not specific to ZRTP. Thanks are due to
Phil Zimmermann, Stefan Doehla, Mats Naslund, Gregory Maxwell, David Phil Zimmermann, Stefan Doehla, Mats Naslund, Gregory Maxwell, David
McGrew, Mark Baugher, and Koen Vos for their comments and feedback on McGrew, Mark Baugher, Koen Vos, and Ingemar Johansson for their
this memo. comments and feedback on this memo.
9. References 9. References
9.1. Normative References 9.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate [RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119, March 1997. Requirement Levels", BCP 14, RFC 2119, March 1997.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K. [RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)", Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004. RFC 3711, March 2004.
9.2. Informative References 9.2. Informative References
[I-D.zimmermann-avt-zrtp] [I-D.zimmermann-avt-zrtp]
Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media
Path Key Agreement for Secure RTP", Path Key Agreement for Secure RTP",
draft-zimmermann-avt-zrtp-17 (work in progress), draft-zimmermann-avt-zrtp-22 (work in progress),
January 2010. January 2010.
[spot-me] Wright, C., Ballard, L., Coull, S., Monrose, F., and G. [spot-me] Wright, C., Ballard, L., Coull, S., Monrose, F., and G.
Masson, "Spot me if you can: Uncovering spoken phrases in Masson, "Spot me if you can: Uncovering spoken phrases in
encrypted VoIP conversation", Proceedings of the IEEE encrypted VoIP conversation", Proceedings of the IEEE
Symposium on Security and Privacy 2008, May 2008. Symposium on Security and Privacy 2008, May 2008.
Authors' Addresses Authors' Addresses
Colin Perkins Colin Perkins
University of Glasgow University of Glasgow
Department of Computing Science School of Computing Science
Glasgow G12 8QQ Glasgow G12 8QQ
UK UK
Email: csp@csperkins.org Email: csp@csperkins.org
Jean-Marc Valin Jean-Marc Valin
Octasic Inc. Octasic Inc.
4101 Molson Street, Suite 300 4101 Molson Street, Suite 300
Montreal, Quebec H1Y 3L1 Montreal, Quebec H1Y 3L1
Canada Canada
Email: Jean-Marc.Valin@octasic.com Email: Jean-Marc.Valin@octasic.com
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