Guidelines for the Use of Variable Bit Rate Audio with Secure RTP
draft-ietf-avtcore-srtp-vbr-audio-04

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
 draft-ietf-avtcore-srtp-vbr-audio-04.txt
Abstract
 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 http://datatracker.ietf.org/drafts/current/.
 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
 (http://trustee.ietf.org/license-info) in effect on the date of
 publication of this document. Please review these documents
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 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",
 "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 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
 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, 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
 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
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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
 UK
 Email: csp@csperkins.org
 Jean-Marc Valin
 Octasic Inc.
 4101 Molson Street, Suite 300
 Montreal, Quebec H1Y 3L1
 Canada
 Email: Jean-Marc.Valin@octasic.com
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