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What is a side channel attack? How these end-runs around encryption put everyone at risk

Apr 17, 20195 mins

Oh, look! A van across the street.

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Credit: CSA Images / Getty Images

Side channel attack definition

So, you want to break cryptography.

Brute force attacks on cryptography could take billions of years, which no one has to spare. Maybe you live in a country where rubber hose cryptography is, shall we say, frowned upon. Hacking a target’s endpoint is an option, but what if you get caught? Better to use an attack that leaves no forensic traces behind.

Enter side channel attacks. A side channel attack breaks cryptography by using information leaked by cryptography, such as monitoring the electromagnetic field (EMF) radiation emitted by a computer screen to view information before it’s encrypted in a van Eck phreaking attack, aka Transient Electromagnetic Pulse Emanation STandard (TEMPEST). Other well-known side channel attacks include spying on the power consumption of an electronic device to steal an encryption key, or acoustic attacks that record the sound of a user’s key strokes to steal their passphrase.

These side channel attacks are not theoretical and have been known about for decades. The oldest, and most common, and used by intelligence agencies and police around the world, is the park your van across the street from the victim and spy on their computer screens using a TEMPEST attack.

What is a TEMPEST side channel attack?

Is that van across the street really from the cable company? If you’re a politician, journalist, diplomat, tech executive or anyone else with a nation-state in their threat model, you might take a closer look. TEMPEST attacks have been around for decades. The U.S. Department of Defense (DoD) requires military electronics to meet TEMPEST-resistant standards, including secure casings for laptops to minimize EMF radiation emanated by the device.

1983 grid tempest tested compass photo GRiD Computer

An early TEMPEST-tested laptop PC from around 1983

TEMPEST attacks work because screens emit EMF radiation that can be sniffed from as far as hundreds of meters away (and certainly from across the street) and used to view in real-time the victim’s screen. This BBC television special demonstrated the attack decades ago. While the move away from old-fashioned ginormous CRT monitors to flat-screen LCD displays changed the EMF frequencies, laptop screens and flat-screen monitors have been shown to be vulnerable to such attacks as well.

It’s widely believed that intelligence agencies around the world use TEMPEST attacks on a regular basis, and it’s almost certain that the FBI and other domestic police forces use this kind of attack in their investigations when spying on both criminals and journalists.

The practical defense against TEMPEST attacks has also been known for decades. A Faraday cage blocks such EMF radiation and can be built with chicken wire in your home office (the wave length of the radiation is larger than the holes in chicken wire and so can’t escape) or if you’re the government, you build a  sensitive compartmented information facility (SCIF). How Stuff Works has a good deep dive on Faraday cages for those wanting to go down that particular rabbit hole.

SANS has also a great deep dive on TEMPEST for those wanting to learn more about just how vulnerable consumer devices are to this kind of side channel attack.

What is a power consumption side channel attack?

Cryptography is energy intensive, and like any computationally intensive activity, that energy can be measured and analyzed. Attacks on power outlets and power cables have proven successful at recovering RSA private keys in the lab, for example, and as the old saying goes, today’s academic attacks are yesterday’s nation-state attacks are tomorrow’s script kiddie attacks. Simple power analysis can “be used to break RSA implementations by revealing differences between multiplication and squaring operations,” this 1998 paper entitled “Introduction to Differential Power Analysis and Related Attacks” concludes.

What is a timing analysis side channel attack?

Cryptography takes time to run, and that time can be measured, analyzed and used to statistically break cryptography. Both Meltdown and Spectre, the much-touted hardware security flaws, are timing attacks on Intel hardware. As Paul C. Kocher makes clear in this early paper on timing attacks, “By carefully measuring the amount of time required to perform private key operations, attackers may be able to find fixed Diffie-Hellman exponents, factor RSA keys, and break other cryptosystems.”

In response to these discoveries, countermeasures that add timing “noise” to encryption processes have been deployed in many cryptographic libraries to mitigate the risk of a timing side channel attack.

What is an acoustic side channel attack?

Listen to the sound of your victim typing the passphrase into their laptop and steal their private key. Sounds like science fiction but it’s been around for decades, and the advent of ubiquitous smart phones means there are microphones listening everywhere.

Unlike TEMPEST attacks, which are cheap enough that pretty much any moderately skilled geek with a couple hundred bucks can mount such an attack, an acoustic attack requires a fairly sophisticated machine learning model, including enough training data to distinguish one key press from another. This makes acoustic attacks, at least for the moment, the bailiwick of nation-state intelligence agencies and police forces like the FBI.

What is the future of side channel attacks?

Side channel attacks have been around for ages but have not gotten the same level of attention as remote attacks have. You can hack someone on the other side of the planet, but the physical proximity required to carry out most side channel attacks has limited adversaries to spies and cops.

However, the advent of smartphones and drones means there are now microphones and sensors everywhere that can be used to launch side channel attacks on victims. Such attacks are only going to get easier and cheaper over time. 

Senior Writer

J.M. Porup got his start in security working as a Linux sysadmin in 2002. Since then he's covered national security and information security for a variety of publications, and now calls CSO Online home. He previously reported from Colombia for four years, where he wrote travel guidebooks to Latin America, and speaks Spanish fluently with a hilarious gringo-Colombian accent. He holds a Masters degree in Information and Cybersecurity (MICS) from UC Berkeley.

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