May 22, 2022

Archives for February 2008

pesky details with getting a voting system correct

Today was the last day of early voting in Texas’s primary election. Historically, I have never voted in a primary election. I’ve never felt I identified enough with a particular political party to want to have a say in selecting their candidates. Once I started working on voting security, I discovered that this also allowed me to make a legitimate claim to being “non-partisan.” (While some election officials, political scientists, and others who you might perhaps prefer to be non-partisan do have explicit partisan views, many more make a point of similarly obscuring their partisan preferences like I do.)

In Texas, you are not required to register with a party in order to vote in their primary. Instead, you just show up and ask for their primary ballot. In the big city of Houston, any registered voter can go to any of 35 early voting locations over the two weeks of early voting. Alternately, they may vote in their home, local precinct (there are almost a thousand of these) on the day of the election. There have been stories of long lines over the past two weeks. My wife wanted to vote, but procrastinating, we went on the final night to a gigantic supermarket near campus. Arriving at 5:50pm or so, she didn’t reach the head of the queue until 8pm. Meanwhile, I took care of our daughter and tried to figure out the causes of the queue.

There were maybe twenty electronic voting machines, consistently operating at between 50-70% utilization (i.e., as many as half of the voting machines were unused at any given time). Yet the queue was huge. How could this be? Turns out there were four people at the desk in front dealing with the sign-in procedure. In a traditional, local precinct, this is nothing fancier than flipping open a paper printout to the page with your name. You sign next to it, and then you go vote. Simple as can be. Early voting is a different can of worms. They can’t feasibly keep a printout with over a million names of it in each of 35 early voting centers. That means they need computers. Our county’s computers had some kind of web interface that they could use to verify the voter’s registration. They then print a sticker with your name on it, you sign it, and it goes into a book. If a voter happens to present their voter registration card (my wife happened to have hers with her), the process is over in a hurry. Otherwise, things slow down, particularly if, say, your driver’s license doesn’t match up with the computer. “What was your previous address?” Unsurprisingly, the voter registration / sign-in table was the bottleneck. I’ve seen similar effects beforehand when voting early.

How could you solve this problem? You could have an explicit “fast path” for voters who match quickly versus a “slow path” with a secondary queue for more complicated voters. You can have more registration terminals. You could have roving helpers with PDAs and battery-powered printers that try to get further back into the queue and help voters reconcile themselves with their “true” identity. There’s no lack of creativity that’s been applied to solving this class of problems outside of the domain of election management.

Now, these voter registration systems are not subject to any of the verification and testing procedures that apply to the electronic voting machines themselves. Any vendor can sell pretty much anything and the state government doesn’t have much to say about it. That’s both good and bad. It’s clearly bad because any vetting process might have tried to consider these queueing issues and would have issued requirements on how to address the problem. On the flip side, one of the benefits of the lack of regulation is that the vendor(s) could ostensibly fix their software. Quickly.

To the extent there’s a moral to this story, it’s that the whole system matters. For the most part, we computer security folks have largely ignored voter registration as being somebody else’s problem. Maybe there’s a market for some crack programmer to crank out a superior solution in the time it took to read this blog post and get us out of the queue and into the voting booth.

(Sidebar: Turns out, the Texas Democratic Party has both a primary election and a caucus. Any voter who casts a vote in the primary is elgible to caucus with the party. The caucus locations are the same as the local polling places, with caucusing starting 15 minutes after the close of the polls. Expect stories about crowding, confusion, and chaos, particularly given the crowded, small precinct rooms and relatively few people with experience in the caucusing process. Wikipedia has some details about the complex process by which the state’s delegates are ultimately selected. There may or may not be lawsuits over the process as well.)

Cold Boot Attacks: Vulnerable While Sleeping

Our research on cold boot attacks on disk encryption has generated lots of interesting discussion. A few misconceptions seem to be floating around, though. I want to address one of them today.

As we explain in our paper, laptops are vulnerable when they are “sleeping” or (usually) “hibernating”. Frequently used laptops are almost always in these states when they’re not in active use – when you just close the lid on your laptop and it quiets down, it’s probably sleeping.

When a laptop goes to sleep, all of the data that was in memory stays there, but the rest of the system is shut down. When you re-open the lid of the laptop, the rest of the system is activated, and the system goes on running, using the same memory contents as before. (Hibernating is similar, but the contents of memory are copied off to the hard drive instead, then brought back from the hard drive when you re-awaken the machine.) People put their laptops to sleep, rather than shutting them down entirely, because a sleeping machine can wake up in seconds with all of the programs still running, while a fully shut-down machine will take minutes to reboot.

Now suppose an attacker gets hold of your laptop while it is sleeping, and suppose the laptop is using disk encryption. The attacker can take the laptop back to his lair, and then open the lid. The machine will reawaken, with the same information in memory that was there when you put the machine to sleep – and that information includes the secret key that is used to encrypt the files on your hard disk. The machine may be screen-locked – that is, it may require entry of your password before you can interact with the desktop – but the attacker won’t care. All he cares about is that the encryption key is in memory.

The attacker will then insert a special thumb drive into the laptop, yank out the laptop’s battery, quickly replace the battery, and push the power button to reboot the laptop. The encryption key will still be in memory – the memory will not have lost its contents because the laptop was without power only momentarily while the battery was out. It doesn’t matter how long the laptop takes to reboot, because the memory contents are fading only momentarily while the battery is out. When the laptop boots, software from the thumb drive will read the contents of memory, find the secret encryption key, and proceed to unlock the encrypted files on your hard drive.

In short, the adversary doesn’t need to capture your laptop while the laptop is open and in active use. All he needs is to get your laptop while it is sleeping – which it is probably doing most of the time.

New Research Result: Cold Boot Attacks on Disk Encryption

Today eight colleagues and I are releasing a significant new research result. 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 research team includes J. Alex Halderman, Seth D. Schoen, Nadia Heninger, William Clarkson, William Paul, Joseph A. Calandrino, Ariel J. Feldman, Jacob Appelbaum, and Edward W. Felten.

Our site has links to the paper, an explanatory video, and other materials.

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.

For more details, see the paper site.