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Internet Engineering Task Force (IETF)                        C. PerkinsRequest for Comments: 6562                         University of GlasgowCategory: Standards Track                                      JM. ValinISSN: 2070-1721                                      Mozilla Corporation                                                              March 2012Guidelines for the Use ofVariable Bit Rate Audio with Secure RTPAbstract   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 inSection 2 of RFC 5741.   Information about the current status of this document, any errata,   and how to provide feedback on it may be obtained athttp://www.rfc-editor.org/info/rfc6562.Copyright Notice   Copyright (c) 2012 IETF Trust and the persons identified as the   document authors.  All rights reserved.   This document is subject toBCP 78 and the IETF Trust's Legal   Provisions Relating to IETF Documents   (http://trustee.ietf.org/license-info) 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   include Simplified BSD License text as described in Section 4.e of   the Trust Legal Provisions and are provided without warranty as   described in the Simplified BSD License.Perkins & Valin              Standards Track                    [Page 1]

RFC 6562                   VBR Audio with SRTP                March 2012Table of Contents1.  Introduction ...................................................22.  Scenario-Dependent Risk ........................................23.  Guidelines for Use of VBR Audio with SRTP ......................34.  Guidelines for Use of Voice Activity Detection with SRTP .......35.  Padding the Output of VBR Codecs ...............................46.  Security Considerations ........................................57.  Acknowledgements ...............................................58.  References .....................................................58.1. Normative References ......................................58.2. Informative References ....................................61.  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 inRFC 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 thePerkins & Valin              Standards Track                    [Page 2]

RFC 6562                   VBR Audio with SRTP                March 2012   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 informationPerkins & Valin              Standards Track                    [Page 3]

RFC 6562                   VBR Audio with SRTP                March 2012   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 informationPerkins & Valin              Standards Track                    [Page 4]

RFC 6562                   VBR Audio with SRTP                March 2012   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.  References8.1.  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.Perkins & Valin              Standards Track                    [Page 5]

RFC 6562                   VBR Audio with SRTP                March 20128.2.  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.caPerkins & Valin              Standards Track                    [Page 6]