Subject: [AVT] VBR with SRTP information leak

From: Koen Vos <[email-masked]>
Date: Wed, 09 Sep 2009 22:16:08 -0700

Hi,

I saw this draft suggesting that variable bitrate (VBR) speech codecs  
not be used in combination with SRTP:
http://www.ietf.org/id/draft-perkins-avt-srtp-vbr-audio-01.txt
I didn't find much discussion about this in the archive, and wonder  
how serious the proposal is.

In particular, I would like to know how someone in the future should  
use a VBR codec in combination with encryption?  Or will the IETF  
decide that what is prevalent today because of its pragmatic trade-off  
between security and efficiency is simply no longer acceptable tomorrow?

thanks,
koen.

---

From: Koen Vos <[email-masked]>
Date: Mon, 14 Sep 2009 19:52:20 -0700

Dear Mr. Perkins,

I'd like to follow up on your draft "Guidelines for the use of  
Variable Bit Rate Audio with Secure RTP"  
(http://www.ietf.org/id/draft-perkins-avt-srtp-vbr-audio-01.txt),  
which proposes that "VBR audio codecs that alter the size or spacing  
of their output according to the characteristics of the input speech  
signal SHOULD NOT be used with encrypted SRTP sessions." As already  
expressed in my previous email, I do not agree with such a proposal.  
Please let me explain why:

1. The decision what codec to use does not belong in SRTP or with the  
IETF. Just like SRTP should not prescribe that only hardware  
encryption be used, or that all calls be routed through secure  
proxies. While these kind of requirements would boost security, they  
also come at a cost, and it is not the role of SRTP or the IETF to  
decide the best trade-off between security and cost.

2. The cost of constant bitrate (CBR) coding is not insignificant: the  
average bitrate can easily double compared to variable bitrate (VBR).  
Many situations exist where Internet access is expensive or limited.

3. Many users place limited value on security, as witnessed by the  
wide use of PSTN and GSM. Such users may be happy with "cheap"  
security such as encryption, but would rather avoid the inconvenience,  
higher price or lower quality that comes with more drastic security  
measures.

4. Combining (2) and (3) suggests that encrypted VBR could be the  
"sweet spot" for some or even most users. As it happens, most VoIP  
calls today are made using software-encrypted VBR.

5. The [spot-me] reference in the draft shows that sentences spoken in  
a controlled environment can be recognized with some degree of  
accuracy. Given that there were 122 sentences in their "dictionary",  
and assuming at least 7 words per sentence, that suggests an extracted  
information rate of less than 1 bit per word, on average. Without  
major improvements, such a system could thus not be used to recognize  
natural speech which has a higher entropy.

6. You may disagree with (5) based on the view that it is very  
dangerous to downplay the threat from any information leak. But then  
your proposed method of adding overhang periods to active speech  
intervals is equally dangerous, as all it does is reduce the size of  
the leak. My personal view is that it is dangerous to jump to  
conclusions without further analysis or common sense.

I applaud any effort to raise awareness about security limitations and  
risks. But the goal should be to inform developers and users who want  
to balance security risk and cost as they see fit.

best,
koen.

Quoting Koen Vos <[email-masked]>:

> Hi,
>
> I saw this draft suggesting that variable bitrate (VBR) speech  
> codecs not be used in combination with SRTP:
> decide that what is prevalent today because of its pragmatic  
> trade-off between security and efficiency is simply no longer  
> acceptable tomorrow?
>
> thanks,
> koen.
> _______________________________________________
> Audio/Video Transport Working Group
> [email-masked]
> https://www.ietf.org/mailman/listinfo/avt
>