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Scientists Extract RSA Key From GnuPG Using Sound of CPU
kthreadd writes "In their research paper titled RSA Key Extraction via Low-Bandwidth Acoustic Cryptanalysis, Daniel Genkin, Adi Shamir and Eran Tromer et al. present a method for extracting decryption keys from the GnuPG security suite using an interesting side-channel attack. By analysing the acoustic sound made by the CPU they were able to extract a 4096-bit RSA key in about an hour (PDF). A modern mobile phone placed next to the computer is sufficient to carry out the attack, but up to four meters have been successfully tested using specially designed microphones."
What sounds does a cpu make?
They describe that in the paper's summary.
Or better yet how does a CPU make sound?
They describe that in the first line of the paper's summary, and also in question 2 of the Q&A.
The clock speeds are in the GHZ range so it is so far outside of the sound range of any microphone it just is not funny.
They address that at the end of the first paragraph of the paper's summary, and also in question 8.
Throw in that all cpus today have more than one core you will have a more than one code stream executing at one time.
They address that in question 12.
Throw in the sound of the fans running to make picking up the sound just seem very unlikely.
Also in question 12.
Until it is duplicated I would really doubt it.
OK, thanks for the valuable feedback.
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Ok, I'm impressed.
For those that didn't want to RTFA, this works by letting the target computer spin on a carefully chosen piece of text. That text is chosen such that the CPU will do some predictable math (such as big equations that == 0). Then, those places where the CPU hits 0 can be heard through a sensitive microphone.
The neat part is that you're not looking for a 4096-bit key. Computers don't actually handle things with that large of a size, they have to break it down into 32-bit/64-bit chunks to be able to do the math. That's the real vulnerability - despite the key length itself being massive, because the number gets broken down into small chunks, you can start to handle it. The paper goes through a very complicated way of sensing each section of a large key, and then piecing it all together. This is not a case of hearing a specific noise, and looking it up in a table. It's not even a case of looking up 32-bit chunks in a table.
So, it is a real attack, that is mostly dependent on the breakdown of the 4096bit key into bitesize chunks, that go through known math routines. If you can get the target to nicely decrypt several well-crafted messages for you, and you can get a good microphone near their CPU while they do it, and you can let this process go on long enough (so the attack program can listen to the CPU for a while to build up a profile, etc.), it really can work.
I'll say that it needs kind of an ideal scenario to get all those things lined up, but it's not impossible.
Preventing it fully is really only possible with two ways. Either switch your CPU to not use those bite-size chunks, and have the decryption take place all in one massive math operation (not realistic), or change the math that occurs on the bite-size chunks to be irregular in terms of any recognizable patterns (very realistic).
>The "audio" in question is most likely all below 24 kHz, that being the Nyquist limit for the 48 kHz sampling hardware, unless it happens that some phones can actually sample faster, and have microphones that can respond to higher frequencies. The instruction rate of the CPUs in question is many times that frequency. It doesn't sound likely.
:)
Your objection was directly addressed in the article:
"Cryptanalytic side-channel attacks typically require measurements with temporal resolution similar to the time scale of the target operation, but here the target cryptographic computation is many orders of magnitude faster....the key extraction attack relies on crafting chosen ciphertexts that cause numerical cancellations deep inside GnuPG's modular exponentiation algorithm. This causes the special value zero to appear frequently in the innermost loop of the algorithm, where it affects control flow. A single iteration of that loop is much too fast for direct acoustic observation, but the effect is repeated and amplified over many thousands of iterations, resulting in a gross leakage effect that is discernible in the acoustic spectrum over hundreds of milliseconds
I dare suggest that sometimes even the experts need to RTFA.
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