Compressed VoIP Calls Vulnerable To Bugging

holy_calamity writes "Security researchers at Johns Hopkins report that a variable bit-rate compression scheme being rolled out on VoIP systems leaves encrypted calls vulnerable to bugging. Simpler syllables are squeezed into smaller data packets, with more complex ones taking up more space; the researchers built software that uses this to spot phrases of interest in encrypted calls simply by measuring packet size."

Easy Solution:

Easy Solution. Music in the background.

Re:Easy Solution:

Better solution: Fix the stupid, broken protocol.

For instance, the concept of RSA blinding had to be invented because people discovered that certain bits of the SSL private key can be determined simply by measuring the time it takes to encode messages. This was due to some implementation details inside SSLeay where it switched from one multiplication algorithm to a different one depending on the size of certain numbers in the algorithm.

OAEP had to be invented for similar reasons

"Music in the background" is not a security solution. In fact, that's a freaking joke.

Re:Easy Solution:

""Music in the background" is not a security solution. In fact, that's a freaking joke."

Yes, but a joke you can dance on.

Protocol isn't broken - it's badly mixed

This isn't a simple case of a broken protocol - it's an effect of mixing different protocols in ways that don't work together.

Voice codecs are designed to support a given level of audio quality subject to bit rate and computational complexity limitations. Most codecs are fixed-rate, or fixed-rate with silence suppression. Encryption isn't part of their design; it's somebody else's problem, and many VOIP systems aren't encrypted anyway (for instance, connections between an office phone and a PBX usually aren't.) Variable bit rate codecs are sometimes a good choice, depending on the kind of sounds you're trying to compress and the networks you're transmitting them on, and they're at least an alternative to the usual fixed-rate codecs.

Encryption systems usually aren't designed to deal with real-time message streams or timing attacks. Typically VOIP encryption protocols are designed for constant bit rate codec output, which is what most codecs provide, and the codecs usually package up 10, 20, or 30ms audio samples into a data packet for transmission over IP.

The problem occurs when you're choosing your codec and encryption separately, and you take a crypto system designed for fixed-rate codecs and use a variable-bit-rate codec instead. It's difficult to keep people from doing that sort of thing, especially if they're using huge-overhead approaches like VOIP inside IPSEC as opposed to VOIP systems with the crypto built in. It's also difficult to prevent people from making bad choices like that when they're using open-source software applications, as opposed to proprietary phones that only have the small set of codecs the manufacturer built in (typically uncompressed G.711, or G.729 or a GSM codec, all of which are fixed-rate except for silence suppression.)

Not really...

First, the paper was testing the Speex codec [speex.org], and in based in principle on looking at codecs which use variable bit-rate CELP, a compression scheme which is tailored to speech, not music (music sounds terrible through one of these codecs, because their dictionaries are filled with speech sounds). Having music in the background is only likely to confuse the codec, making the speech sound terrible too, possibly to the point of unintelligibility.

The conclusions do not apply to more standardized codecs like G.711 and G.729a, which use fixed size packets.

The paper itself can be downloaded from here [jhu.edu]. Get it quick, before the IEEE figures this out and make the author remove it so they can extort their fee.

Do what my grandparents do

Anyone wanting to avoid detection could just follow what my German-speaking grandparents do when they don't want us kids listening into the conversation: randomly switch languages on different topics (though I think that this is sometimes also because some concepts are also easier to portray in a given language).

Random switches between languages would probably confuse the heck out of filters guessing compressed data. That or you could just learn Russian... I don't think they *have* any simple-syllable words in Russian :-)

Re:Do what my grandparents do

That or you could just learn Russian... I don't think they *have* any simple-syllable words in Russian :-)

In Soviet Russia, VoIP bugs you!

Re:Do what my grandparents do

>That or you could just learn Russian... I don't think they *have* any simple-syllable words in Russian :-)

Da!

Re:Do what my grandparents do

Just speak arabic!! We already know the FBI and CIA don't have enough translators.

Re:Do what my grandparents do

Depends upon how you define "translators." One of my best friends just got out of the Army, he is a really good linguist and knows several langauges, but he flunked out of the Arabic program because its not just hard to learn, you have to learn hundreds of dialects even for Iraq. He could understand it well enough but to be able to go out on the street and translate you have to be certain you won't accidentally offend with a mistranslation. Apparently virtual no non native arabic speakers ever make it through this program. Anyways he go reassigned to listen to and interpret radio broadcast and other incoming information. Not officially a translator. The point to this story?? I don't know..

Re:

Going from one language to two would only make the process of breaking the message a bit more complex, and by that I mean precisely one bit more complex, because there would be about twice as many phrases to look for. This is not a solution. The solution

Re:

That or you could just learn Russian...

Which would give you an advantage, if you ever have to pilot a bleeding-edge mind-controlled Russian jet fighter.

Evasive, ummm, technology

FTFA

In tests on example conversations, the software correctly identified phrases with an average accuracy of about 50%. But that jumped to 90% for longer, more complicated words. Wright thinks these phrases may be the most important. "I think the attack is much more of a threat to calls with some sort of professional jargon where you have lots of big words that string together to make long, relatively predictable phrases," he says. "Informal conversational speech would be tougher because it's so much more random."

So, ummm, what we should do to, umm, well, protect ourselves from, ummm, yaknow, eavesdroppers, heh-heh, is well, make sure there's enough, ummmmmmm, yaknow, like extra noise, like, mixed in, dude.

Re:Evasive, ummm, technology

So, ummm, what we should do to, umm, well, protect ourselves from, ummm, yaknow, eavesdroppers, heh-heh, is well, make sure there's enough, ummmmmmm, yaknow, like extra noise, like, mixed in, dude.

Oh my god, thats like, totally, like, a great idea, yaknow. I mean, like, they'll never figure out what we're, like, saying, yaknow?

Oryoucouldspeakreallyfastwithoutpausesbetweenwords. Thatwaythey'llneverknowwhatyousaid =)

Or. We. Could. All. Speak. Like. Shatner. Random. Long. Pauses. Genius.

Cheers

It's easy to encrypt your conversations

Just st-st-stuh-stutter when you talk. And use a lot of, uh, you know, um, non-word sounds between, uh, like, your phrases. And don't use any complexificated words without Bushifying them first. Better yet, only speak in Klingon.

Or maybe you shouldn't say anything on VoIP that you don't want anyone else to hear.

Re:

Or maybe you shouldn't say anything on VoIP that you don't want anyone else to hear.

A couple honest questions...

1) Why do I see so much about wiretapping/bugging VoIP lately? I guess I've always assumed that VoIP was just as vulnerable to bugging as POTS

Bad science

First, the article mixes things :
vowels actually are simpler than consonant to compress (because of spectral complexity - consonant use much more different frequencies. They are mostly noises and have a more "random"-like wave form making them harder to compress). They got it completely in reverse.

Then TFA doens't show a method to magically guess was is being said over a crypted channel only by looking at the bitrates, it only says that it finds some predetermined pattern in a given set of samples to test against. The whole thing would only be able to answer to some very simple questions like "did the words XYZ appear in the conversation ? or did ABC appear in the conversation ?" - with a rather bad success rate if those words are long and complex enough - which hardly makes it enough to obtain personal information or otherwise efficiently spy on someone.

Then the whole system has a lot of short comings :
- As said before it assumes that the spy know exactly that some phrase has to be said - if the spy doesn't guess exactly what words he must search for the attack fails (the users may be speaking in a foreign language to begin with).
- It assumes that the speech-generator-made needle they are looking for in the hay sack will be close to what they are looking for. The users may have an accent and pronounce words differently (cf alumnium vs. aluminium, etc...)
- And worse of all, it assume that the granularity of the packed will be small enough so that the phonemes will have an influence on the bit rate. Whereas in reality, short packets have a big overhead of bandwidth, longer packets increases the latency. But lots of VoIP users are happy with a 500ms latency because it really diminishes the overhead. At 500ms you can have a couple of words in a single packet. The whole packet will tend to have a corresponding bandwidth close to the average (there will be small difference between phonemes, but these will all be packed into the same packet and will average).
- It fails to take into account an interleaved video stream. Video conferencing is really popular, and its own bandwith will completely dwarf the bandwidth used by audio. So unless the VoIP uses 2 separate stream (some VoIP systems do), and only encrypt at the stream level, and the transmission is happening over a non crypted channel (no sane person should do that), this method will fail epically.

ode-cay

Ust-jay eak-spay in ode-cay.

Re:Randomize the packets slightly

Time/space attacks are well known. Somebody who actually, hmm, UNDERSTOOD cryptographic security would never have designed the protocol this way in the first place.

The people suggesting that we should just inject noise or background patterns are being ridiculous. Why sacrifice communication quality when there are BETTER ways to fix it? DO IT RIGHT.

Re:

Hahaha! Compressing encrypted data?! My sides are splitting!

In case you can't figure it out: good encryption makes data look completely random. Do you know of any algorithms which compress PURELY RANDOM data? I sure as hell don't.

Re:Here's a thought

There's a reason for that. With a good encryption mechanism, the ciphertext will have maximum entropy (one bit of entropy per bit of ciphertext). Random data also has maximum entropy.

The point of compression is to take data that's expressed in a way that doesn't maximize entropy and reexpress it in a way that is higher-entropy (more information per bit). As such, maximum-entropy data is, by its nature, incompressible.

Re:Here's a thought

Except that might not help here.
The issue is that VOIP is an application that needs low latency. You have to send the data you have within (.1 seconds? something small) a specific amount of time, and can't wait for the buffer to fill before sending it, compressed, encrypted or not. Thus you get packets that are different sizes.

This isn't sending the whole conversation at once, this is a constant stream of data with specific requirements on latency.

A solution would be to make each packet the same size by padding it with random data that the other side will discard. But that eliminates some of the benefit of compression.

Maybe just use a fixed bit rate, as opposed to a VBR encoding?

Re:

What idiot modded this up? Encrypted data is (pretty much by definition) uncompressable. Encryption works by hiding information and removing redundancy. Compression works by identifying and removing redundancy. The two concepts simply CANNOT BE APPLIED IN

Re:Here's a thought

Voice data just CAN'T be securely encrypted.

Really? I have a Top Secret phone on my desk, and I can assure you it's pretty secure. (And no, it's not a shoe.)

Re:Here's a thought

Voice data just CAN'T be securely encrypted. That's because the spacetime information HAS to be there because we inherently interpret voice data according to these characteristics. Either you reveal this information in the stream, or you must increase the latency to the point that communication is impossible. If you want security, don't speak, WRITE, and use a cryptosystem that isn't a piece of shit.

I disagree. The problem pointed at in this article can be easily solved on many SIP endpoints. I spend all day working on VoIP phones from vendors such as Linksys, Polycom, Aastra, Cisco, and if I really have to snom. Most of these have an option where it'll just send blank full bitrate audio rather than the usual "put silence here" instructions on G.711 calls. In fact that is the default behavior on some, since it makes the latency a bit more predictable to have a constant-rate data stream. If you want to use a VBR codec, of course this is a problem, but don't act like it's impossible or even hard to solve. If you are concerned enough to encrypt your conversations, use a CBR codec. 64 kbit/sec is not hard to free up.