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

The information below is for an old version of the document that is already published as an RFC.
Document Type
This is an older version of an Internet-Draft that was ultimately published as RFC 6562.
Authors Colin Perkins , Jean-Marc Valin
Last updated 2015-10-14 (Latest revision 2011-12-30)
Replaces draft-perkins-avt-srtp-vbr-audio
RFC stream Internet Engineering Task Force (IETF)
Intended RFC status Proposed Standard
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Other - see Comment Log
Document shepherd Magnus Westerlund
Shepherd write-up Show Last changed 2011-09-30
IESG IESG state Became RFC 6562 (Proposed Standard)
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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


   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.

   Internet-Drafts are working documents of the Internet Engineering
   Task Force (IETF).  Note that other groups may also distribute
   working documents as Internet-Drafts.  The list of current Internet-
   Drafts is at

   Internet-Drafts are draft documents valid for a maximum of six months
   and may be updated, replaced, or obsoleted by other documents at any
   time.  It is inappropriate to use Internet-Drafts as reference
   material or to cite them other than as "work in progress."

   This Internet-Draft will expire on July 2, 2012.

Copyright Notice

   Copyright (c) 2011 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

   This document is subject to BCP 78 and the IETF Trust's Legal
   Provisions Relating to IETF Documents
   ( in effect on the date of
   publication of this document.  Please review these documents
   carefully, as they describe your rights and restrictions with respect
   to this document.  Code Components extracted from this document must
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   the Trust Legal Provisions and are provided without warranty as
   described in the Simplified BSD License.

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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",
   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

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

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


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


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