Internet Engineering Task Force (IETF)                        C. Perkins
Request for Comments: 6562                         University of Glasgow
Category: Standards Track                                      JM. Valin
ISSN: 2070-1721                                      Mozilla Corporation
                                                              March 2012

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

Abstract

   This memo discusses potential security issues that arise when using
   variable bit rate (VBR) audio with the secure RTP profile.
   Guidelines to mitigate these issues are suggested.

Status of This Memo

   This is an Internet Standards Track document.

   This document is a product of the Internet Engineering Task Force
   (IETF).  It represents the consensus of the IETF community.  It has
   received public review and has been approved for publication by the
   Internet Engineering Steering Group (IESG).  Further information on
   Internet Standards is available in Section 2 of RFC 5741.

   Information about the current status of this document, any errata,
   and how to provide feedback on it may be obtained at
   http://www.rfc-editor.org/info/rfc6562.

Copyright Notice

   Copyright (c) 2012 IETF Trust and the persons identified as the
   document authors.  All rights reserved.

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Table of Contents

   1.  Introduction ...................................................2
   2.  Scenario-Dependent Risk ........................................2
   3.  Guidelines for Use of VBR Audio with SRTP ......................3
   4.  Guidelines for Use of Voice Activity Detection with SRTP .......3
   5.  Padding the Output of VBR Codecs ...............................4
   6.  Security Considerations ........................................5
   7.  Acknowledgements ...............................................5
   8.  References .....................................................5
       8.1. Normative References ......................................5
       8.2. Informative References ....................................6

1.  Introduction

   The Secure RTP (SRTP) framework [RFC3711] is a widely used framework
   for securing RTP sessions [RFC3550].  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 predefined 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 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 recognized
   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 generalize 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 prerecorded message knowing the set of all
   possible messages would lead to near-perfect accuracy.  Even when the

   audio is not prerecorded, there is a real possibility of being able
   to recognize contents from encrypted audio when the dialog is highly
   structured (e.g., when the eavesdropper 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, how significant the information leak is
   when compared to other SRTP-related information must be considered,
   such as the fact that the source and destination 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 prerecorded messages where the contents of such
   prerecorded messages may be of any value to an eavesdropper (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 eavesdropper could easily use the
   rate information to 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, provided this
   adaptation is done in a way that does not expose any information on
   the speech signal.  For example, VBR audio can be used for congestion
   control purposes, where the variation is driven by the available
   network bandwidth, not by the input speech (i.e., 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, 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.  This may be more important
   for short talkspurts, since it seems easier to distinguish between
   different single word responses 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 packetized 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 interword 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 prerecorded
   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.  However, it is still expected that the use of VAD
   will provide significant bandwidth savings 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
   "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.  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.

8.  References

8.1.  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.

8.2.  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
   Mozilla Corporation
   650 Castro Street
   Mountain View, CA  94041
   USA

   Phone: +1 650 903-0800
   EMail: jmvalin@jmvalin.ca