Entries Tagged "side-channel attacks"

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Side-Channel Attacks on Encrypted Web Traffic

Nice paper: “Side-Channel Leaks in Web Applications: a Reality Today, a Challenge Tomorrow,” by Shuo Chen, Rui Wang, XiaoFeng Wang, and Kehuan Zhang.

Abstract. With software-as-a-service becoming mainstream, more and more applications are delivered to the client through the Web. Unlike a desktop application, a web application is split into browser-side and server-side components. A subset of the application’s internal information flows are inevitably exposed on the network. We show that despite encryption, such a side-channel information leak is a realistic and serious threat to user privacy. Specifically, we found that surprisingly detailed sensitive information is being leaked out from a number of high-profile, top-of-the-line web applications in healthcare, taxation, investment and web search: an eavesdropper can infer the illnesses/medications/surgeries of the user, her family income and investment secrets, despite HTTPS protection; a stranger on the street can glean enterprise employees’ web search queries, despite WPA/WPA2 Wi-Fi encryption. More importantly, the root causes of the problem are some fundamental characteristics of web applications: stateful communication, low entropy input for better interaction, and significant traffic distinctions. As a result, the scope of the problem seems industry-wide. We further present a concrete analysis to demonstrate the challenges of mitigating such a threat, which points to the necessity of a disciplined engineering practice for side-channel mitigations in future web application developments.

We already know that eavesdropping on an SSL-encrypted web session can leak a lot of information about the person’s browsing habits. Since the size of both the page requests and the page downloads are different, an eavesdropper can sometimes infer which links the person clicked on and what pages he’s viewing.

This paper extends that work. Ed Felten explains:

The new paper shows that this inference-from-size problem gets much, much worse when pages are using the now-standard AJAX programming methods, in which a web “page” is really a computer program that makes frequent requests to the server for information. With more requests to the server, there are many more opportunities for an eavesdropper to make inferences about what you’re doing—to the point that common applications leak a great deal of private information.

Consider a search engine that autocompletes search queries: when you start to type a query, the search engine gives you a list of suggested queries that start with whatever characters you have typed so far. When you type the first letter of your search query, the search engine page will send that character to the server, and the server will send back a list of suggested completions. Unfortunately, the size of that suggested completion list will depend on which character you typed, so an eavesdropper can use the size of the encrypted response to deduce which letter you typed. When you type the second letter of your query, another request will go to the server, and another encrypted reply will come back, which will again have a distinctive size, allowing the eavesdropper (who already knows the first character you typed) to deduce the second character; and so on. In the end the eavesdropper will know exactly which search query you typed. This attack worked against the Google, Yahoo, and Microsoft Bing search engines.

Many web apps that handle sensitive information seem to be susceptible to similar attacks. The researchers studied a major online tax preparation site (which they don’t name) and found that it leaks a fairly accurate estimate of your Adjusted Gross Income (AGI). This happens because the exact set of questions you have to answer, and the exact data tables used in tax preparation, will vary based on your AGI. To give one example, there is a particular interaction relating to a possible student loan interest calculation, that only happens if your AGI is between $115,000 and $145,000—so that the presence or absence of the distinctively-sized message exchange relating to that calculation tells an eavesdropper whether your AGI is between $115,000 and $145,000. By assembling a set of clues like this, an eavesdropper can get a good fix on your AGI, plus information about your family status, and so on.

For similar reasons, a major online health site leaks information about which medications you are taking, and a major investment site leaks information about your investments.

The paper goes on to talk about mitigation—padding page requests and downloads to a constant size is the obvious one—but they’re difficult and potentially expensive.

More articles.

Posted on March 26, 2010 at 6:04 AM • View Comments

Data Leakage Through Power Lines

The NSA has known about this for decades:

Security researchers found that poor shielding on some keyboard cables means useful data can be leaked about each character typed.

By analysing the information leaking onto power circuits, the researchers could see what a target was typing.

The attack has been demonstrated to work at a distance of up to 15m, but refinement may mean it could work over much longer distances.

These days, there’s lots of open research on side channels.

Posted on July 15, 2009 at 6:17 AM • View Comments

Eavesdropping on Dot-Matrix Printers by Listening to Them

Interesting research.

First, we develop a novel feature design that borrows from commonly used techniques for feature extraction in speech recognition and music processing. These techniques are geared towards the human ear, which is limited to approx. 20 kHz and whose sensitivity is logarithmic in the frequency; for printers, our experiments show that most interesting features occur above 20 kHz, and a logarithmic scale cannot be assumed. Our feature design reflects these observations by employing a sub-band decomposition that places emphasis on the high frequencies, and spreading filter frequencies linearly over the frequency range. We further add suitable smoothing to make the recognition robust against measurement variations and environmental noise.

Second, we deal with the decay time and the induced blurring by resorting to a word-based approach instead of decoding individual letters. A word-based approach requires additional upfront effort such as an extended training phase as the dictionary grows larger, and it does not permit us to increase recognition rates by using, e.g., spell-checking. Recognition of words based on training the sound of individual letters (or pairs/triples of letters), however, is infeasible because the sound emitted by printers blurs so strongly over adjacent letters.

Third, we employ speech recognition techniques to increase the recognition rate: we use Hidden Markov Models (HMMs) that rely on the statistical frequency of sequences of words in text in order to rule out incorrect word combinations. The presence of strong blurring, however, requires to use at least 3-grams on the words of the dictionary to be effective, causing existing implementations for this task to fail because of memory exhaustion. To tame memory consumption, we implemented a delayed computation of the transition matrix that underlies HMMs, and in each step of the search procedure, we adaptively removed the words with only weakly matching features from the search space.

We built a prototypical implementation that can bootstrap the recognition routine from a database of featured words that have been trained using supervised learning. Afterwards, the prototype automatically recognizes text with recognition rates of up to 72 %.

Researchers have done lots of work on eavesdropping on remote devices. (One example.) And we know the various intelligence organizations of the world have been doing this sort of thing for decades.

Posted on June 23, 2009 at 6:16 AM • View Comments

Reading a Letter from the Envelope it Was In

Fascinating:

Paul Kelly and colleagues at Loughborough University found that a disulfur dinitride (S2N2) polymer turned exposed fingerprints brown, as the polymer reaction was initiated from the near-undetectable remaining residues.

Traces of inkjet printer ink can also initiate the polymer. The detection limit is so low that details of a printed letter previously in an envelope could be read off the inside of the envelope after being exposed to S2N2.

“A one-covers-all versatile system like this has obvious potential,” says Kelly.

“This work has demonstrated that it is possible to obtain fingerprints from surfaces that hitherto have been considered extremely difficult, if not impossible, to obtain,” says Colin Lewis, scientific advisor at the UK Ministry of Defence. “The method proposed has shown that this system could well provide capabilities which could significantly enhance the tools available to forensic scientists in the future.”

Posted on November 11, 2008 at 7:55 AM • View Comments

Remotely Eavesdropping on Keyboards

Clever work:

The researchers from the Security and Cryptography Laboratory at Ecole Polytechnique Federale de Lausanne are able to capture keystrokes by monitoring the electromagnetic radiation of PS/2, universal serial bus, or laptop keyboards. They’ve outline four separate attack methods, some that work at a distance of as much as 65 feet from the target.

In one video demonstration, researchers Martin Vuagnoux and Sylvain Pasini sniff out the the keystrokes typed into a standard keyboard using a large antenna that’s about 20 to 30 feet away in an adjacent room.

Website here.

Posted on October 23, 2008 at 12:48 PM • View Comments

Eavesdropping on Encrypted Compressed Voice

Traffic analysis works even through the encryption:

The new compression technique, called variable bitrate compression produces different size packets of data for different sounds.

That happens because the sampling rate is kept high for long complex sounds like “ow”, but cut down for simple consonants like “c”. This variable method saves on bandwidth, while maintaining sound quality.

VoIP streams are encrypted to prevent eavesdropping. However, a team from John Hopkins University in Baltimore, Maryland, US, has shown that simply measuring the size of packets without decoding them can identify whole words and phrases with a high rate of accuracy.

The technique isn’t good enough to decode entire conversations, but it’s pretty impressive.

Posted on June 19, 2008 at 6:27 AM • View Comments

Cold Boot Attacks Against Disk Encryption

Nice piece of research:

We show that disk encryption, the standard approach to protecting sensitive data on laptops, can be defeated by relatively simple methods. We demonstrate our methods by using them to defeat three popular disk encryption products: BitLocker, which comes with Windows Vista; FileVault, which comes with MacOS X; and dm-crypt, which is used with Linux.

[…]

The root of the problem lies in an unexpected property of today’s DRAM memories. DRAMs are the main memory chips used to store data while the system is running. Virtually everybody, including experts, will tell you that DRAM contents are lost when you turn off the power. But this isn’t so. Our research shows that data in DRAM actually fades out gradually over a period of seconds to minutes, enabling an attacker to read the full contents of memory by cutting power and then rebooting into a malicious operating system.

Interestingly, if you cool the DRAM chips, for example by spraying inverted cans of “canned air” dusting spray on them, the chips will retain their contents for much longer. At these temperatures (around -50 °C) you can remove the chips from the computer and let them sit on the table for ten minutes or more, without appreciable loss of data. Cool the chips in liquid nitrogen (-196 °C) and they hold their state for hours at least, without any power. Just put the chips back into a machine and you can read out their contents.

This is deadly for disk encryption products because they rely on keeping master decryption keys in DRAM. This was thought to be safe because the operating system would keep any malicious programs from accessing the keys in memory, and there was no way to get rid of the operating system without cutting power to the machine, which “everybody knew” would cause the keys to be erased.

Our results show that an attacker can cut power to the computer, then power it back up and boot a malicious operating system (from, say, a thumb drive) that copies the contents of memory. Having done that, the attacker can search through the captured memory contents, find any crypto keys that might be there, and use them to start decrypting hard disk contents. We show very effective methods for finding and extracting keys from memory, even if the contents of memory have faded somewhat (i.e., even if some bits of memory were flipped during the power-off interval). If the attacker is worried that memory will fade too quickly, he can chill the DRAM chips before cutting power.

There seems to be no easy fix for these problems. Fundamentally, disk encryption programs now have nowhere safe to store their keys. Today’s Trusted Computing hardware does not seem to help; for example, we can defeat BitLocker despite its use of a Trusted Platform Module.

The paper is here; more info is here. Articles here.

There is a general security problem illustrated here: it is very difficult to secure data when the attacker has physical control of the machine the data is stored on. I talk about the general problem here, and it’s a hard problem.

EDITED TO ADD (2/26): How-to, with pictures.

Posted on February 21, 2008 at 1:29 PM • View Comments

Information Leakage in the Slingbox

Interesting:

…despite the use of encryption, a passive eavesdropper can still learn private information about what someone is watching via their Slingbox Pro.

[…]

First, in order to conserve bandwidth, the Slingbox Pro uses something called variable bitrate (VBR) encoding. VBR is a standard approach for compressing streaming multimedia. At a very abstract level, the idea is to only transmit the differences between frames. This means that if a scene changes rapidly, the Slingbox Pro must still transmit a lot of data. But if the scene changes slowly, the Slingbox Pro will only have to transmit a small amount of data—a great bandwidth saver.

Now notice that different movies have different visual effects (e.g., some movies have frequent and rapid scene changes, others don’t). The use of VBR encodings therefore means that the amount data transmitted over time can serve as a fingerprint for a movie. And, since encryption alone won’t fully conceal the number of bytes transmitted, this fingerprint can survive encryption!

We experimented with fingerprinting encrypted Slingbox Pro movie transmissions in our lab. We took 26 of our favorite movies (we tried to pick movies from the same director, or multiple movies in a series), and we played them over our Slingbox Pro. Sometimes we streamed them to a laptop attached to a wired network, and sometimes we streamed them to a laptop connected to an 802.11 wireless network. In all cases the laptop was one hop away.

We trained our system on some of those traces. We then took new query traces for these movies and tried to match them to our database. For over half of the movies, we were able to correctly identify the movie over 98% of the time. This is well above the less than 4% accuracy that one would get by random chance.

More details in the paper.

Posted on June 4, 2007 at 1:24 PM • View Comments

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Sidebar photo of Bruce Schneier by Joe MacInnis.