draft-ietf-avtcore-srtp-vbr-audio-02.txt   draft-ietf-avtcore-srtp-vbr-audio-03.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: October 30, 2011 Octasic Inc. Expires: January 7, 2012 Octasic Inc.
April 28, 2011 July 6, 2011
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-ietf-avtcore-srtp-vbr-audio-02.txt draft-ietf-avtcore-srtp-vbr-audio-03.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 October 30, 2011. This Internet-Draft will expire on January 7, 2012.
Copyright Notice Copyright Notice
Copyright (c) 2011 IETF Trust and the persons identified as the Copyright (c) 2011 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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In practical SRTP scenarios, it must also be considered how In practical SRTP scenarios, it must also be considered how
significant the information leak is when compared to other SRTP- significant the information leak is when compared to other SRTP-
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 unstructured 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.
applications would be particularly vulnerable since an evesdropper Interactive voice response (IVR) applications would be particularly
could easily use the rate information to easily recognize the prompts vulnerable since an evesdropper could easily use the rate information
being played out. to easily recognize the prompts 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
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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.
If VAD is used on an encrypted speech signal, then some information If VAD is used on an encrypted speech signal, then some information
about the characteristics of that speech signal can be determined by about the characteristics of that speech signal can be determined by
watching the patterns of voice activity. This information leakage is watching the patterns of voice activity. This information leakage is
less than with VBR coding since there are only two rates possible. less than with VBR coding since there are only two rates possible.
The information leakage due to VAD in SRTP audio sessions can be much The information leakage due to VAD in SRTP audio sessions can be much
reduced if the sender adds an unpredictable "overhang" period to the reduced if the sender adds an unpredictable "overhang" period to the
end of active speech intervals, so obscuring their actual length. an end of active speech intervals, so obscuring their actual length. An
RTP sender using VAD with encrypted SRTP audio SHOULD insert such an RTP sender using VAD with encrypted SRTP audio SHOULD insert such an
overhang period at the end of each talkspurt, delaying the start of overhang period at the end of each talkspurt, delaying the start of
the silence/comfort noise by a random interval. The length of the the silence/comfort noise by a random interval. The length of the
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. This may be more important
overhang period must be packetised and transmitted in RTP packets in for short talk spurts, since is seems easier to distinguish between
a manner that is indistinguishable from the other data in the different single word reponses based on the exact word length, than
talkspurt. to glean meaning from the duration of a longer phrase. The audio
data comprising the overhang period must be packetised and
transmitted in RTP packets in a manner that is indistinguishable from
the other data in the 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 a few hundred milliseconds (this should be sufficient "half life" of a few hundred milliseconds (this should be sufficient
to obscure the presence of inter-word pauses and the lengths of to obscure the presence of inter-word pauses and the lengths of
single words spoken in isolation, for example the digits of a credit single words spoken in isolation, for example the digits of a credit
card number clearly enunciated for an automated system, but not so card number clearly enunciated for an automated system, but not so
long as to significantly reduce the effectiveness of VAD for long as to significantly reduce the effectiveness of VAD for
detecting listening pauses). Despite the overhang (and no matter detecting listening pauses). Despite the overhang (and no matter
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