Internet DRAFT - draft-ietf-avtcore-srtp-vbr-audio
draft-ietf-avtcore-srtp-vbr-audio
Network Working Group C. Perkins
Internet-Draft University of Glasgow
Intended status: BCP JM. Valin
Expires: July 2, 2012 Octasic Inc.
December 30, 2011
Guidelines for the use of Variable Bit Rate Audio with Secure RTP
draft-ietf-avtcore-srtp-vbr-audio-04.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 in full conformance with the
provisions of BCP 78 and BCP 79.
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This Internet-Draft will expire on July 2, 2012.
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 . . . . . . . . . . . . . . . . . . . . 6
7. IANA Considerations . . . . . . . . . . . . . . . . . . . . . . 6
8. Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . 6
9. References . . . . . . . . . . . . . . . . . . . . . . . . . . 6
9.1. Normative References . . . . . . . . . . . . . . . . . . . 6
9.2. Informative References . . . . . . . . . . . . . . . . . . 7
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . . 7
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1. Introduction
The secure RTP framework (SRTP) [RFC3711] is a widely used framework
for securing RTP [RFC3550] 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. This is potentially a security risk
for some applications. 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, and [fon-iks]
shows that approximate transcripts of encrypted VBR calls can be
derived for some codecs without breaking the encryption. How
significant these results are, and how they generalise to other
codecs, is still an open question. This memo discusses ways in which
such traffic analysis risks 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 leaks and attacks discussed in [spot-me],
[fon-iks], and similar works are significant is highly dependent on
the application and use scenario. 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. Recognizing unconstrained conversational speech
from the rate information alone is unreliable and computationally
expensive at present, but does appear possible in some circumstances.
These attacks are only likely to improve over time.
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
low-value encrypted unstructured 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. Interactive voice response (IVR) applications would be
particularly vulnerable since an evesdropper could easily use the
rate information to easily recognize the prompts being played out.
Applications conveying highly sensitive unstructured information
SHOULD NOT use codecs in VBR mode.
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 reliably
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estimate the length of that talkspurt. This may be more important
for short talk spurts, since is seems easier to distinguish between
different single word reponses based on the exact word length, than
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
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 a few hundred milliseconds (this should be sufficient
to obscure the presence of inter-word pauses and the lengths of
single words spoken in isolation, for example the digits of a credit
card number clearly enunciated for an automated system, but not so
long as to significantly reduce the effectiveness of VAD for
detecting listening pauses). Despite the overhang (and no matter
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
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, a constant bit rate
(CBR) codec SHOULD be negotiated and used instead, or the VBR codec
SHOULD be operated in a 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 to a small range of sizes ([spot-me] achieves good results by
padding to the next multiple of 16 octets, but the amount of padding
needed to hide the variation in packet size will depend on the codec
and the sophistication of the attacker), or may be padded to a size
that varies with time. The most secure, and RECOMMENDED, option is
to pad all packets throughout the call to the same size.
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
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"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
This entire memo is about security. The security considerations of
[RFC3711] also apply.
7. IANA Considerations
No IANA actions are required.
8. Acknowledgements
ZRTP [RFC6189] contains similar recommendations; 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,
Koen Vos, Ingemar Johansson, and Stephen Farrell 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.
[RFC3550] Schulzrinne, H., Casner, S., Frederick, R., and V.
Jacobson, "RTP: A Transport Protocol for Real-Time
Applications", STD 64, RFC 3550, July 2003.
[RFC3711] Baugher, M., McGrew, D., Naslund, M., Carrara, E., and K.
Norrman, "The Secure Real-time Transport Protocol (SRTP)",
RFC 3711, March 2004.
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9.2. Informative References
[RFC6189] Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media
Path Key Agreement for Unicast Secure RTP", RFC 6189,
April 2011.
[fon-iks] White, A., Matthews, A., Snow, K., and F. Monrose,
"Phonotactic Reconstruction of Encrypted VoIP
Conversations: Hookt on fon-iks", Proceedings of the IEEE
Symposium on Security and Privacy 2011, May 2011.
[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
School of Computing Science
Glasgow G12 8QQ
UK
Email: csp@csperkins.org
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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