Network Working Group                                         C. Perkins
Internet-Draft                                     University of Glasgow
Intended status: BCP                                           JM. Valin
Expires: June 15, 2011                                      Octasic Inc.
                                                       December 12, 2010

   Guidelines for the use of Variable Bit Rate Audio with Secure RTP


   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 June 15, 2011.

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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 . . . . . . . . . . . . . . . . . . . . . . 6
   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

   The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
   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.  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

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
   estimate the length of that talkspurt.  The audio data comprising the
   overhang period must be packetised and transmitted in RTP packets in

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   a manner that is indistinguishable from the other data in the

   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

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.

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7.  IANA Considerations

   No IANA actions are required.

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, Koen Vos, and Ingemar Johansson 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

              Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media
              Path Key Agreement for Secure RTP",
              draft-zimmermann-avt-zrtp-22 (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.

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Authors' Addresses

   Colin Perkins
   University of Glasgow
   School of Computing Science
   Glasgow  G12 8QQ


   Jean-Marc Valin
   Octasic Inc.
   4101 Molson Street, Suite 300
   Montreal, Quebec  H1Y 3L1


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