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
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RFC 6562 VBR Audio with SRTP March 2012
Table of Contents
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.
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.
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RFC 6562 VBR Audio with SRTP March 2012
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.
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.
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RFC 6562 VBR Audio with SRTP March 2012
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.
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.
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RFC 6562 VBR Audio with SRTP March 2012
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.
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
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RFC 6562 VBR Audio with SRTP March 2012 8.2. Informative References[RFC6189] Zimmermann, P., Johnston, A., and J. Callas, "ZRTP: Media
