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Network Working Group C. Perkins
Internet-Draft University of Glasgow
Intended status: BCP JM. Valin
Expires: September 9, 2010 Octasic Inc.
March 8, 2010
Guidelines for the use of Variable Bit Rate Audio with Secure RTP
draft-perkins-avt-srtp-vbr-audio-03.txt
Abstract
This memo discusses potential security issues that arise when using
variable bit rate audio with the secure RTP profile. Guidelines to
mitigate these issues are suggested.
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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Copyright Notice
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Scenario-Dependent Risk . . . . . . . . . . . . . . . . . . . . 3
3. Guidelines for use of VBR Audio with SRTP . . . . . . . . . . . 4
4. Guidelines for use of Voice Activity Detection with SRTP . . . 4
5. Padding the output of VBR codecs . . . . . . . . . . . . . . . 5
6. Security Considerations . . . . . . . . . . . . . . . . . . . . 5
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 5
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . . 6
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 6
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1. Introduction
The secure RTP framework (SRTP) [RFC3711] is a widely used framework
for securing RTP sessions. SRTP provides the ability to encrypt the
payload of an RTP packet, and optionally add an authentication tag,
while leaving the RTP header and any header extension in the clear.
A range of encryption transforms can be used with SRTP, but none of
the pre-defined encryption transforms use any padding; the RTP and
SRTP payload sizes match exactly.
When using SRTP with voice streams compressed using variable bit rate
(VBR) codecs, the length of the compressed packets will therefore
depend on the characteristics of the speech signal. This variation
in packet size will leak a small amount of information about the
contents of the speech signal. For example [spot-me] shows that
known phrases in an encrypted call using the Speex codec in VBR mode
can be recognised with high accuracy in certain circumstances,
without breaking the encryption. Other work, referenced from
[spot-me], has shown that the language spoken in encrypted
conversations can also be recognised. This is potentially a security
risk for some applications. How significant these results are and
how they generalise to other codecs is still an open question. This
memo discusses ways in which this traffic analysis risk may be
mitigated.
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].
2. Scenario-Dependent Risk
Whether the information leak analysed in [spot-me] is significant
highly depends on the application. In the worst case, using the rate
information to recognize a pre-recorded message knowing the set of
all possible messages would lead to near-perfect accuracy. Even when
the audio is not pre-recorded, there is a real possibility of being
able to recognize contents from encypted audio when the dialog is
highly structured (e.g. when the evesdropper knows that only a
handful of possible sentences are possible) and thus contain only
little information. On the other end, recognizing unconstrained
conversational speech from the rate information alone appears to be
highly unlikely at best. In fact, such a task is already considered
a hard problem even when one has access to the unencrypted audio.
In practical SRTP scenarios, it must also be considered how
significant the information leak is when compared to other SRTP-
related information, such as the fact that the source and destination
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IP addresses are available.
3. Guidelines for use of VBR Audio with SRTP
It is the responsibility of the application designer to determine the
appropriate trade-off between security and bandwidth overhead. As a
general rule, VBR codecs should be considered safe in the context of
encrypted one-to-one calls. However, applications that make use of
pre-recorded messages where the contents of such pre-recorded
messages may be of any value to an evesdropper (i.e. messages beyond
standard greeting messages) SHOULD NOT use codecs in VBR mode. In
particular, IVR applications would be particularly vulnerable since
an evesdropper could easily use the rate information to easily
recognize the prompts being played out.
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
congestion control purposes, provided this adaptation done in a way
that does not expose any information on the speech signal. That is,
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
unless the network conditions change). VBR speech codecs can safely
be used in this fashion with SRTP while avoiding leaking information
on the contents of the speech signal that might be useful for traffic
analysis.
4. Guidelines for use of Voice Activity Detection with SRTP
Many speech codecs employ some form of voice activity detection (VAD)
to either suppress output frames, or generate some form of lower-rate
comfort noise frames, during periods when the speaker is not active.
If VAD is used on an encrypted speech signal, then some information
about the characteristics of that speech signal can be determined by
watching the patterns of voice activity. This information leakage is
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
reduced if the sender adds an unpredictable "overhang" period to the
end of active speech intervals, so obscuring their actual length. 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
the silence/comfort noise by a random interval. The length of the
overhang applied to each talkspurt must be randomly chosen in such a
way that it is computationally infeasible for an attacker to predict
the length of that talkspurt. The audio data comprising the overhang
period must be packetised and transmitted in RTP packets in a manner
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that is indistinguishable from the other data in the talkspurt.
The overhang period SHOULD have an exponentially-decreasing
probability distribution function. This ensures a long tail, while
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
matter what the duration is), there is still a small amount of
information leak 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.
The application of a random overhang period to each talkspurt will
reduce the effectiveness of VAD in SRTP sessions when compared to
non-SRTP sessions. It is, however, still expected that the use of
VAD will provide a significant bandwidth saving for many encrypted
sessions.
5. Padding the output of VBR codecs
For scenarios where VBR is considered unsafe, the codec SHOULD be
operated in CBR mode. However, if the codec does not support CBR,
RTP padding SHOULD be used to reduce the information leak to an
insignificant level. Packets may be padded to a constant size, or
may be padded to a size that varies with time. In the case where the
size of the padded packets varies in time, the same concerns as for
VAD apply. That is, the padding SHOULD NOT be reduced without
waiting for a certain (random) time. The RECOMMENDED "hold time" is
the same as the one for VAD.
Note that SRTP encrypts the count of the number of octets of padding
added to a packet, but not the bit in the RTP header that indicates
that the packet has been padded. For this reason, it is RECOMMENDED
to add at least one octet of padding to all packets in a media
stream, so an attacker cannot tell which packets needed padding.
6. Security Considerations
The security considerations of [RFC3711] apply.
7. IANA Considerations
No IANA actions are required.
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8. Acknowledgements
This memo is based on the discussion in [spot-me]. Recent versions
of ZRTP [I-D.zimmermann-avt-zrtp] contain a similar recommendation;
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
Phil Zimmermann, Stefan Doehla, Mats Naslund, Gregory Maxwell, David
McGrew, Mark Baugher, and Koen Vos for their comments and feedback on
this memo.
9. References
9.1. 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.
9.2. Informative References
[I-D.zimmermann-avt-zrtp]
Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media
Path Key Agreement for Secure RTP",
draft-zimmermann-avt-zrtp-17 (work in progress),
January 2010.
[spot-me] Wright, C., Ballard, L., Coull, S., Monrose, F., and G.
Masson, "Spot me if you can: Uncovering spoken phrases in
encrypted VoIP conversation", Proceedings of the IEEE
Symposium on Security and Privacy 2008, May 2008.
Authors' Addresses
Colin Perkins
University of Glasgow
Department of Computing Science
Glasgow G12 8QQ
UK
Email: csp@csperkins.org
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Jean-Marc Valin
Octasic Inc.
4101 Molson Street, Suite 300
Montreal, Quebec H1Y 3L1
Canada
Email: Jean-Marc.Valin@octasic.com
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