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rfc:rfc6562

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.
 This document is subject to BCP 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 2012

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

Perkins & 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 information

Perkins & 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 information

Perkins & 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. 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.

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

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

Perkins & Valin Standards Track [Page 6]

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