Security Practices and Tips
Bob spends the next half hour or so sipping coffee and jotting notes on what he’s learned so far, and, like Sam, generally doing business-related busy work.
“Sam, I have just received a message from my wife,” Bob announces, “You will be interested to hear what she says”:
Dear Bobby, how glad I am to hear from you, and that you have successfully boarded your plane--you must be enjoying the first class treatment! You know how worried I get when I do not hear from you when I expect to--but I am always keeping my eyes on you. And of course, Walter and I will meet your flight. We are both looking forward to meeting this new friend of yours…
Bob trails off, and Sam presses him to continue. “Oh, that’s about all she wrote,” stammers Bob, “Just, you know, ‘love and kisses, Eve’, more or less,” but Bob reads on, realizing that Sam has control over his network connection and therefore can see the entire message himself. “She wrote”:
…this new friend of yours.
I missed my snookum-wookums so much. Love and kisses.
Eve
“Yes, that’s delightful,” says Sam as he cringes inwardly, “I, too, look forward to meeting your lovely wife and cat. You had more questions about encryption?”
“Actually, yes I do,” says Bob, “a few. First, please explain how I make sure a software file I download is properly verified?”
8.1 Verifying Software Downloads
“One common attack is to trick you into installing malicious software that gives the attacker control over your system. One way to do that is tricking you into thinking it is software that you really want,” Sam begins.
“Oh, like GnuPG? I downloaded it while you were monitoring my network activities in the United States. Have I downloaded a malicious version that you created just for me? One that keeps track of everything I do on my computer?” asks Bob.
“Well, uh, no, of course we would never do something like that to you, Bob, but maybe the SSS has a version that monitors all your secrets,” says Sam. “So, to be sure you have the right version of GnuPG, you must verify the signature file for the download.”
Bob asks, “OK, I downloaded GnuPG and installed it--but how can I be sure that version of GnuPG hasn’t been hacked and delivered to me by an attacker instead of the real program? I can’t very well verify the signature of the hacked software with the hacked software--that can’t be right.” Bob looks directly at Sam.
“That’s right, I mean, not right. You should never use software you download to verify itself.” Sam seems to be getting a bit uncomfortable as Bob continues: “I think that one way to verify this software is to try downloading it from other sites and compare all the downloads; if they are all the same, then maybe it’s OK.”
Sam brightens visibly and says, “Yes, exactly! Just download the software, with the signature file and signing keys from a couple of different mirrors and compare with what you’ve got--that should work!”
Bob’s not buying it though, “That’s what I thought, but if my attacker--I will call him ‘Mallory’, like GnuPG says--Mallory sees my network activity, and if she can replace a download once, then I think she can also replace the download two or three or fifty times, and every time it will look the same.”
Sam says nothing as Bob continues: “So I asked Alice--a lovely young woman, and very bright. Also my wife’s niece, did you know that?” Bob goes on, “I asked Alice if she would get me a copy of GnuPG, she gave me the Liveboot version of Ubuntu Linux, which I ran from my DVD drive, and I discovered that the version of GnuPG I downloaded was different from the one she gave me--which she verified. I decided to trust Alice, so I’ve been using her version.”
Sam continues to say nothing as Bob continues: “I checked the signing keys on public keyservers and verified that the signature on the code Alice gave me was correct, and that the signatures on the files I downloaded from Mallory are not correct.” Bob pauses, then says, “I think GnuPG worked quite well for me.”
8.1.1 Verifying a Download With a Hash Value
Sam says, “Um. That’s great, Bob. You know if you don’t have a copy of GnuPG that you trust, you can do a hash of the download and compare that to the good hash value published online?”
“Yes, I discovered I can get the SHA256 checksum value from the download site, and can run a program on my system to calculate the SHA256 checksum of the file I downloaded. But I haven’t been able to figure out how to do that part, can you explain how that works, Sam?”
Sam, glad to change the topic, answers, “SHA256 is a 256-bit secure hash algorithm. That means it’s considered a cryptographically secure way to generate a unique 256-bit value for any given file. When a software developer publishes a download file, they calculate the hash for the file and publish it as well. When you download the file, you calculate the hash on your download and compare it to the published hash. If they match, you can be confident the two files are identical.”
“The hash itself doesn’t give any information about the owner or publisher of the file (like a digital signature does) but it will give an indication of whether the file you download is the same as the file originally published online.” Sam finishes.
8.1.2 Calculating a Hash Value on Different OSes
“It’s easiest to calculate the hash value on a file with Linux,” says Sam. “Here’s the command”:
“sha256sum is a command that comes with Linux; there are options you can use, but for checking a hash, this will work”:
“Compare that result to the value published with the download.” Sam says. “It’s about the same on Windows, except the command is named sha256sum.exe. On OS X, the command is shasum”:
“Verifying the hash value is good, but not always good enough, especially if you are comparing to a hash value supplied directly with the download--you need to be confident that the hash value you’re comparing is trustworthy.” Sam continues, “It’s best to use a signature and signing keys.”
“Yes,” says Bob, “that’s what I thought. By the way, Sam, how much do you know about my country?”
“Well, ah, that’s an interesting question,” begins Sam, but Bob cuts him off: “Yes, before you answer, I know you claim to be a spy with dealings in my country, but I think you know very little about Sylvania. You did not recognize the name of our leader. I think you may have other gaps in your knowledge. You are aware of our leader’s policy against publishing dissent or criticism of regime?”
“Yes, of course, it’s horrible--you must be horribly repressed in your country…” begins Sam, but Bob cuts him off:
“Actually, it is not so horrible in my homeland. In many ways, yes, Sylvania is backward, yet we have great regard for traditions of great democracies like your United States. But Chairman Bob is old-fashioned, and when he came to power in 1936, he took on the trappings of a dictator. It was the fashion, as you know.”
Sam, boggling at the revelation that the Sylvanian Leader has been ruling for over 75 years, opens his mouth to ask and is cut off again by Bob, “Yes, Leader is 124 years old this year, but this is not unusual in our nation: something to do with our yoghurt, I am told. However,” he continues, “while it is treason to print criticism, dissent is encouraged as long as it is not in print. Anyone may say anything they like, and artistic representations are encouraged: music, paintings, even theater.”
“You may well ask why this is so: our Chairman is popular because he rules in accordance with wishes of citizens--citizens present their cases and achieve consensus in the marketplace of ideas, and then Chairman Bob announces the laws and rulings that best meet the needs of Sylvania’s citizens.”
Sam, amazed, asks, “But your Internet access is filtered--what about freedom of surfing? Don’t you want to be able to go to any web site you like?”
Bob says, “In Sylvania, Internet access is supported for furthering the arts, science, and commerce in Sylvania. Bandwidth in my country, like petroleum, is a limited resource, so we try not to waste it on memes and pictures of cats.”
“Sam, life in Sylvania is not so bad: we have our freedoms, even if it doesn’t seem that way at first glance. Our culture is different from yours, but we believe as you do in the rights of all. You will see when we arrive.”
Bob, pausing to sip some coffee, adds, “Also, you should know that citizens who criticize Sylvanian system or Sylvanian leaders are not actually executed: the usual punishment for a first offense is a 10-year term as support staff in the Sylvanian Legislature. It is punitive--Sylvanian legislators are very bad bosses--but not excessively so. Let’s get back to talking about keeping secrets, please.”
“Sam, you’ve mentioned several times to use strong passphrases, can you explain what you mean by that?”
8.2 Passphrases: Doing Them Right
Sam says, “Conventional wisdom insists that ‘strong’ passphrases have at least eight characters and include upper- and lowercase letters, numerals, and special characters. The idea is to defend against password cracking, and particularly against brute-force attacks: if a password is a name followed by a number, it can be discovered fairly quickly with a brute-force attack; short passwords with just six characters, even if they include numerals and symbols, are also easily cracked.”
“The math is easy: with 95 characters available for passphrases1 we have exactly 95 one-character passphrases (‘E’ or ‘?’, for example), so you could brute force a one-character passphrase by hand, by trying every letter, number and character. If you are very lucky, that attack could succeed in one try; if you are very unlucky, it would take the maximum of 95 tries; on average you’d succeed after trying about half of all possible combinations. If it takes 1 second to try each single-character passphrase, you’d need between 1 and 95 seconds to crack it, but on average you can expect to spend about 43 seconds.”
Sam continues: “For a 2-character passphrase, there are 95 options for the first character, and each of those options can be coupled with 95 options for the second character. Total number of combinations is 95×95=9025. Now, a brute-force attack will succeed in about an hour and a quarter instead of under a minute.”
“Each character you add to the passphrase multiplies the total number of possible passphrases by 95, so a 10-character passphrase has 95 raised to the tenth power. That’s 95×95×95×95×95×95×95×95×95×95, about 60,000,000,000,000,000,000 different combinations. That’s 60 billion billion.”
“If you used a lower-case-only passphrase, you’d have far fewer possible combinations: it would be 26 (number of lowercase letters) raised to the tenth power: 2610. That comes to about 147,000,000,000,000, or about 147,000 billion, a tiny fraction (about 1/400,000th) of the possibilities when you use upper- and lowercase letters, numerals and symbols.”
“So, 10-character passphrases should be safe, then, no?” Bob asks.
“Oh, goodness no,” replies Sam. “Well, not necessarily, anyway. It depends on who wants to crack your password, and how many computers they have at their disposal, as well as whether your password is ‘easy’ to guess (by that, I mean, using ‘123456password’, or any passphrase that might be on a list of easy-to-guess passphrases). Let’s say your passphrase is reasonably random-seeming. With a 10-character lower-case-only passphrase, it takes (on average) about 70,000 billion trials to discover the passphrase. If one computer can try 1,000 passphrases per second (a reasonable supposition), it would take that computer about 70 billion seconds, or a couple of thousand years.”
“Is that good for my passphrase?” asks Bob.
“Not really. If it takes one computer two or three thousand years, you can crack the passphrase in two or three years with 1,000 computers. That drops to a week or so with 10,000 computers--a couple of hours with a million computers. If one computer costs $100--that cheap, because you’re buying in bulk, plus you don’t need individual disk drives, video display cards, and so on--that means you can crack almost any 10-character (lower-case-only) passphrase in an hour or two, for just $100 million,” says Sam.
“That seems like a lot of money, so I shouldn’t worry too much, right?” asks Bob, but Sam says, “We’re talking about multinational corporations and government agencies--with those guys, $100 million is a rounding error, it’s petty cash. The Pentagon spends about that much on one F-35 fighter jet.”
“It could still take years to brute force a strong 10-character passphrase (with upper- and lowercase letters, numerals, and symbols), but the people who write password cracking software rely on users picking passphrases with some pattern in them, like names followed by numbers, so they focus attacks on likely combinations rather than simply trying every passphrase from ‘A’ to ‘zzzzzzzzzz’.”
“If you use a 12-character passphrase without patterns, you should be safe--from brute-force passphrase cracking. You still have to defend against keylogging and network monitoring and spoofing and social engineering2 and rubber-hose cryptanalysis3 and all the other strategies for defeating your passphrase.” Sam pauses, but starts quickly before Bob can ask his next question:
“That’s not all, Bob. Remembering 12 random-seeming characters is difficult, and with GnuPG, if you forget your public key passphrase, you’ve lost the ability to use that passphrase completely. No passphrase recovery (other than trying to use passphrase cracking software yourself). So, most users wind up writing their passphrases down, or even using passphrase keeper software4. In many cases, cracking passphrases is as easy as looking for yellow-stickies on, around, or under the computer itself.”
“Is there nothing to be done?” Bob asks. “Why use passphrases at all then?”
“You can use a passphrase ‘safe’, but you’ve got to have a very strong passphrase to access the safe, and even then, just having it could be an invitation to enhanced interrogation.” Sam continues: “A more secure option is to use a sentence or verse or phrase that you can easily remember (but hard to guess), and build a passphrase from the first letter (or two or three) of each word, using punctuation and numerals where appropriate5. For example: ‘Mary had a little lamb, its fleece was white as snow. And everywhere that Mary went, that lamb was sure to go.’ can be turned into a passphrase like this”:
“Notice how I used punctuation and the numeral 2 (instead of the ‘t’ from the word ‘to’); both make the passphrase harder to guess. That’s a 26-character passphrase, but easy to remember. I wouldn’t use that one because it’s obvious, but you could use some other phrase or verse that you’re likely to know and remember but that an attacker would not know about. According to some experts, passphrases have outlived their usefulness, and should be supplemented with a second form of authentication6. For now, a good passphrase is fine, just keep it safe.”
Bob ponders a bit, and then asks, “You also mentioned about RAMs and caches, how do they expose my passphrases or plaintexts? Explain please.”
8.3 Dangers of RAM Cache and Other System Artifacts
“Your computer changes its ‘state’--contents of its working memory, or RAM, as well as contents of its hard drives--whenever you do anything with it. A program may create a temporary file to keep track of what files you have open, or maintain a log of every song and video you play. Web browsers notoriously store all kinds of history files and logs on everything you do online. Your browser may expose all kinds of information, including all personal information, credit card numbers, and web site passphrases.” says Sam.
“So maybe I should encrypt my entire computer?” asks Bob.
“Yes, actually, you should; that’s my next topic. You should encrypt your hard drives, but there’s more to it than that. Even programs that don’t write files with sensitive information may still store sensitive data (like your passphrases) in the computer’s working memory--the RAM. When you power down your computer, whatever is in RAM eventually dissolves away7. And powering off means powering everything off, no hibernation or sleep mode: those store system state--the contents of RAM--making it easier for an attacker to subvert your system.”
“Remember, even though passphrases and plaintexts may not be written to files, they can still be vulnerable while your computer is turned on because that data is in RAM.” Sam adds, “You should also be careful with your command history when you’re using GnuPG at the command line.”
“The command line history can trip you up, since it documents your computer activities. In the Windows command prompt, pressing F7 displays command history, but only for the current session. When you close the command prompt window in Windows, that history disappears, but things are different in Linux and OS X, in both good and bad ways. Good because your history is saved (so you can more easily browse through old commands) and the default number of commands is pretty high8, but that can be bad, since saving your history means saving evidence of your activities.”
“The thing to do, if you’re worried about leaving evidence behind, is to remove it: On Windows, that’s as easy as closing the command prompt window, but on OS X and Linux, you can clear the history with the command history -c, but also securely delete the history file itself (.bash_history, found in the GnuPG home directory9; note the filename starts with a period) containing the history10.”
Sam continues: “Another thing that happens is that GnuPG, through the Pinentry program, takes your passphrase and holds it in memory (the RAM) to unlock access to your key. By default, GnuPG waits for 10 minutes before erasing your passphrase from RAM. That way you can decrypt or sign more than once without having to re-enter the passphrase.”
“If you’re really nervous, you can change the configuration to reduce the time to live for the cached passphrase, or even turn caching off entirely by setting the time-to-live value to zero.” Sam thinks, and says, “If you’re using OS X, one thing you really shouldn’t do is to click on the ‘Save in Keychain’ box when Pinentry opens up and you enter your passphrase. If you do, you can lose control of your public key entirely--because your passphrase is now only protected by the strength of your system passphrase. And you might not even realize you’re doing it. I’ve done it. It’s bad enough that the system doesn’t forget your passphrase, but when you actively ask the system to save it for you then you’re in trouble.”
Sam adds, “The system holds onto your cached passphrase for 10 minutes (the default) even if you close the terminal window. So be careful: it’s one of those things that make me believe Linux is a better choice than OS X or Windows for doing anything related to security.”
“Thanks Sam,” says Bob, “that’s helpful. But what about encrypting my hard drive? Is your hard drive encrypted?”
“Oh yes,” answers Sam, “mine is definitely encrypted. It’s pretty easy, too; you can get the step-by-step details online, it’s different on Windows, OS X, and Linux, but not too hard. It is definitely worth doing, though.”
8.4 Full Disk Encryption
“Broad strokes here: doing full disk encryption (FDE) means that your entire hard drive is encrypted. If you take the hard drive out and put it on another system, or if you boot from a rescue disc, all you’ll see is ciphertext.” Sam pauses expectantly, so Bob asks, “If it’s all encrypted, how can I use any of the data on it? Do I have to decrypt it all every time I log in?”
“Great question!” Sam replies. “When you log back in to a system with FDE, there’s a little program that encrypts any data being written to the disk and that decrypts any data that is being read from the disk. It’s very efficient, so it doesn’t really affect system performance. If you log in with the right passphrase, that program will work; if you don’t have the passphrase, you won’t be able to read any data from that disk.”
“But Sam, in that case, when I’m using the computer, it’s as if the disk isn’t encrypted and all of my system is an open book,” asks Bob, uncertainly adding, “Isn’t it?”
“Exactly,” answers Sam. “That’s why you should never leave your computer turned on when you’re not using it, or when it’s out of your control. And also why you should keep your computer off any networks, and also don’t let anyone plug in to your USB ports. Because there are forensic software programs, sometimes used by law enforcement agencies, that can copy the contents of your RAM, or your entire hard drive.”
“Well, then why bother encrypting my hard drive at all, Sam?” Bob asks.
8.4.1 How Good Is FDE?
“FDE is an excellent security practice, as long as you are aware of the weaknesses,” Sam says. “Because as long as you are vigilant, and keep the system powered down when you don’t have control over it, you’ll be pretty safe.”
“Thank you, Sam, that explains what Walter was doing earlier, while you were in the washroom.” Bob turns to face a burly gentleman standing at the entry to the first class compartment and says, “Walter, allow me to introduce you to my new friend, Mallory.” Turning back to Sam, Bob says, “Mallory, this is Walter, one very cool cat who works for my wife. I should have known that he would be on this flight; he watches over us, particularly when we may need some protection.”
Sam begins to panic, glancing back and forth, at Bob and then at Walter. “When you went to the washroom, I discovered Walter in coach and explained that you were trying to convince me to betray my nation. The first thing he did was to plug into your notebook and did some fooling around, I don’t know what, exactly, but I think now that you have been p0wned--is that the right word, Walter?”
As Walter nods somberly, Bob continues: “So, Sam, rather than I working for you, it is you who will be working for my wife. Chin up, though, it is not so bad to live in Sylvania: you will be given an entry-level job, something honorable yet not too pleasant, perhaps school lunch server or maybe plumber’s helper. And in the evenings you will be called upon to help train members of our security service.”
“In any case,” continues Bob, “Please explain how effective FDE is; you can be sure I will not leave my system turned on when it is unattended. But take your time, and have a beverage if you need a moment to collect your wits, by all means.”
Walter removes Sam’s computer from his tray table, replacing it with a tumbler of scotch, and as Bob plays Scramble with Friends11 Sam attempts to regain enough composure to answer Bob’s question.
Sam begins: “When used correctly, FDE can be extremely resistant even to efforts by the government.12 I mean, the US government, but I guess any other government will have the same problem. The two big things to remember are keep the computer turned off when unattended and use a strong passphrase.”
Bob interrupts: “Yes, that’s funny, as Walter told me your FDE passphrase was ‘password123’. I think that is not a strong passphrase, do you agree?”
“Well, no, unh, I guess it’s not too strong, Bob,” mutters Sam.
Bob touches Sam’s wrist and says, “Well, never mind. Everything will be fine for you, don’t worry, you’ll see. But before you return to Walter’s seat in coach, tell me how to do FDE.”
“Aw, heck, Bob, here’s a FAQ I wrote,” Sam says, handing a sheet of paper to Bob. “I’m coming Walter, where were you sitting?”
“Oh, a very nice seat, just across from the toilet, all the way back. There are only two babies in the seats behind, and you have the honor of sitting between two of Sylvania’s most popular wrestlers.”
Bob says, “See you later, Mallory,” as he turns to peruse Sam’s FAQ:
8.5 Encrypting Your System Hard Drive FAQ
Encrypting the disk can be done with encryption programs included with Windows and OS X, or with the TrueCrypt program on Windows, OS X, or Linux. Not all Windows editions include the Microsoft BitLocker program; if your system does not, you can still encrypt the disk with TrueCrypt.
8.5.1 How it Works
The process begins by starting the FDE encryption program of choice (FileVault, BitLocker, or TrueCrypt) to encrypt the fixed drive on your computer. It may take hours to actually finish encrypting, though you can continue to work with the system while it is encrypting. When done, everything on your drive will be encrypted and accessible only by the authorized user, when logged in.
The encrypted disk is most secure when turned off. When you must turn it on, you should not permit any connection, either by wi-fi, network cable, or any other hardware connection.
If you’re using a cloud service to sync files, all your files on that service are stored in plaintext. If the provider encrypts stored data, that data will most likely be encrypted to keys held by the service provider, which cedes control over your sensitive data to the cloud provider. The same goes for running a backup service, like Apple’s Time Machine, on a removable disk. All the data on the backup device will be plaintext.
8.5.2 Enabling FDE
Most people do FDE once: they set it up and let it run. Here are quick summaries of how to do it:
TrueCrypt (all OSes): Download the appropriate program file from www.truecrypt.org (don’t forget to download and authenticate the signature). For Windows and OS X, the download is an executable program, run it to begin. Linux users may need to do a bit more research before installation.
BitLocker (Windows): Setup may vary from one version/edition to another, but for example, on Windows 7 Ultimate or Enterprise editions, BitLocker can be turned on by entering the Control Panel application, choosing Security, clicking on BitLocker Drive Encryption, choose a drive to encrypt and follow instructions to enable drive encryption.
FileVault (OS X): Open the System Preferences application; choose Security & Privacy (from the top row). Choose the FileVault panel and follow instructions to activate FDE.
Things to be aware of:
Passphrase: use a strong one, as it is the only thing standing between your private data and an attacker.
Recovery key: if you are offered the option of a recovery key, or any other aid to recovering the encrypted disk, be sure to record it and store the record securely, preferably where only you can access it, and preferably physically removed from where the encrypted system is used.
8.5.3 About Microsoft BitLocker
BitLocker is proprietary software produced by Microsoft, for Microsoft Windows. In other words, it should be the “best solution” for FDE on Windows systems, because it is designed to work only with Windows, and by the same organization that created Windows.
However, Microsoft includes BitLocker in the Enterprise and Ultimate editions of Windows Vista and Windows 7, and in the Pro and Enterprise editions of Windows 8--which means that you may need to upgrade Windows to be able to use BitLocker.
Among the benefits of using BitLocker, particularly for larger organizations, is that encryption recovery can be administered centrally. This is particularly important to maintaining security of data on organizational computers while at the same time retaining access to organizational data assets for authorized users in the organization.
Individuals using consumer editions of Windows will need to upgrade their version of Windows to get access to BitLocker FDE, and may prefer to choose another option to avoid the expense of an upgrade.
8.5.4 About Apple FileVault
As with BitLocker for Windows, users of Apple’s OS X will find FileVault to be easy to use and in fact completely transparent to the end user, while at the same time offering organizations mechanisms for authorized users to recover encrypted data.
FileVault is incorporated into the current version of OS X, and can be used on any computer running that OS.
8.5.5 About TrueCrypt
The open source community does not accept the TrueCrypt license as “open.” Although it can be downloaded and used for free, and the source code can be reviewed and modified, there are some subtle aspects of the license that make it unacceptable to many free/open source advocates. What this means is that TrueCrypt is not included in any major Linux distributions, but it is still the leading noncommercial solution for FDE on any OS.
Because it is free, popular, and source code available, TrueCrypt should be an adequate solution for users on any platform.
User should be aware that TrueCrypt documentation can be spotty; answers to many questions about installation and configuration may best be found through a well-crafted search query.
1Includes 26 uppercase and 26 lowercase letters, 10 numerals, and 33 characters.
2Social engineering: use of interpersonal interactions to convince a person to reveal a passphrase or take some action against their own interest.
3Rubber-hose cryptanalysis: use of torture or coercion to recover a passphrase.
4Bruce Schneier’s Password Safe (http://pwsafe.org/) works on Windows, numerous “related projects” support other platforms (http://pwsafe.org/relatedprojects.shtml).
5For example, see “Strong passphrases and password cracking” (http://crypto.loshin.com/2013/02/01/strong-passphrases-and-password-cracking/), also “Password Tips” from Purdue University’s SecurePurdue web site (http://www.purdue.edu/securePurdue/bestpractices/passtips.cfm).
6“Two-factor authentication” (http://crypto.loshin.com/2013/02/01/two-factor-authentication/).
7On some systems, data in RAM can persist for minutes, even longer when the system is cooled before powering off. See “Lest We Remember: Cold Boot Attacks on Encryption Keys” (https://citp.princeton.edu/research/memory/).
8The default for OS X is 500 commands and for Linux, 1,000.
9Use the command gpg --help to find the GnuPG home directory.
10For more, see “Managing shell command history in OS X/Linux” (http://crypto.loshin.com/2013/02/13/managing-shell-command-history-in-os-xlinux/).
11Scramble with Friends is a popular time killer often played on a smartphone while sitting on an airplane. It has nothing to do with encryption.
12See “Efficacy of full disk encryption” http://crypto.loshin.com/2012/11/19/efficacy-of-full-disk-encryption/.