How to protect the sensitive files on your computer

الملف الشخصي · 06-04-2011 11:25 PM

How to protect the sensitive files on your computer

How to protect the sensitive files on your computer

Unauthorised access to the information on your computer or portable storage devices can be carried out remotely, if the 'intruder' is able to read or modify your data over the Internet; or physically, if he manages to get hold of your hardware. You can protect yourself against either type of threat by improving the physical and network security of your data, as discussed in Chapter 1: How to protect your computer from malware and hackers and Chapter 2: How to protect your information from physical threats. It is always best to have several layers of defence, however, which is why you should also protect the files themselves. That way, your sensitive information is likely to remain safe even if your other security efforts prove inadequate.

There are two general approaches to the challenge of securing your data in this way. You can encrypt your files, making them unreadable to anyone but you, or you can hide them in the hope that an intruder will be unable to find your sensitive information. There are tools to help you with either approach, including a FOSS application called TrueCrypt, which can both encrypt and hide your file.
Background scenario

Claudia and Pablo work with a human rights NGO in a South American country. They have spent several months collecting testimonies from witnesses to the human rights violations that have been committed by the military in their region. If the details of who provided these testimonies were to become known, it would endanger both the courageous people who testified and members of the organisation in that region. This information is currently stored in a spreadsheet on the NGO's Windows XP computer, which is connected to the Internet. Being security conscious, Claudia has made sure to store a backup of the data on a CD, which she keeps outside the office.
What you can learn from this chapter

How to encrypt information on your computer
What risks you might face by keeping your data encrypted
How to protect data on USB memory sticks, in case they are lost or stolen
What steps you can take to hide information from physical or remote intruders

Encrypting your information

Pablo: But my computer is already protected by the Windows login password! Isn't that good enough?

Claudia: Actually, Windows login passwords are usually quite easy to break. Plus, anybody who gets his hands on your computer for long enough to restart it with a LiveCD in the drive can copy your data without even having to worry about the password. If they manage to take it away for a while, then you're in even worse trouble. It's not just Windows passwords you need to worry about, either. You shouldn't trust Microsoft Word or Adobe Acrobat passwords either.

Encrypting your information is a bit like keeping it in a locked safe. Only those who have a key or know the lock's combination (an encryption key or password, in this case) can access it. The analogy is particularly appropriate for TrueCrypt and tools like it, which create secure containers called 'encrypted volumes' rather than simply protecting one file at a time. You can put a large number of files into an encrypted volume, but these tools will not protect anything that is stored elsewhere on your computer or USB memory stick.

Hands-on: Get started with the TrueCrypt Guide

While other software can provide encryption that is equally strong, TrueCrypt was designed specifically to make this kind of secure file storage as simple as possible. Furthermore, its support for carrying encrypted volumes on portable storage devices, the fact that it is a [FOSS](/en/glossary#FOSS" title=) tool, and the 'deniability' features described in the Hiding your sensitive information section below, give TrueCrypt a distinct advantage over many built-in proprietary encryption tools, such as Windows XP's 'bitlocker'.

Pablo: Alright, now you have me worried. What about other users on the same computer? Does this mean they can read files in the 'My Documents' folder?

Claudia: I like the way you're thinking! If your Windows password doesn't protect you from intruders, how can it protect you from other people with accounts on the same computer? In fact, your My Documents folder is normally visible to anybody, so other users wouldn't even have to do anything clever to read your unencrypted files. You're right, though, even if the folder is made 'private,' you're still not safe unless you use some kind of encryption.
Tips on using file encryption safely

Storing confidential data can be a risk for you and for the people you work with. Encryption reduces this risk but does not eliminate it. The first step to protecting sensitive information is to reduce how much of it you keep around. Unless you have a good reason to store a particular file, or a particular category of information within a file, you should simply delete it (see Chapter 6: How to destroy sensitive information for more information about how to do this securely). The second step is to use a good file encryption tool, such as TrueCrypt.

Claudia: Well, maybe we don't actually need to store information that could identify the people who gave us these testimonies. What do you think?

Pablo: Agreed. We should probably write down as little of that as possible. Plus, we should think up a simple code we can use to protect names and locations that we absolutely have to record.

Returning to the analogy of a locked safe, there are a few things you should bear in mind when using TrueCrypt and tools like it. No matter how sturdy your safe is, it won't do you a whole lot of good if you leave the door open. When your TrueCrypt volume is 'mounted' (whenever you can access the contents yourself), your data may be vulnerable, so you should keep it closed except when you are actually reading or modifying the files inside it.

There are a few situations when it is especially important that you remember not to leave your encrypted volumes mounted:

Disconnect them when you walk away from your computer for any length of time. Even if you typically leave your computer running overnight, you need to ensure that you do not leave your sensitive files accessible to physical or remote intruders while you are gone.
Disconnect them before putting your computer to sleep. This applies to both 'suspend' and 'hibernation' features, which are typically used with laptops but may be present on desktop computers as well.
Disconnect them before allowing someone else to handle your computer. When taking a laptop through a security checkpoint or border crossing, it is important that you disconnect all encrypted volumes and shut your computer down completely.
Disconnect them before inserting an untrusted USB memory stick or other external storage device, including those belonging to friends and colleagues.
If you keep an encrypted volume on a USB memory stick, remember that just removing the device may not immediately disconnect the volume. Even if you need to secure your files in a hurry, you have to dismount the volume properly, then disconnect the external drive or memory stick, then remove the device. You might want to practice until you find the quickest way to do all of these things.

If you decide to keep your TrueCrypt volume on a USB memory stick, you can also keep a copy of the TrueCrypt program with it. This will allow you to access your data on other people's computers. The usual rules still apply, however: if you don't trust the machine to be free of malware, you probably shouldn't be typing in your passwords or accessing your sensitive data.
Hiding your sensitive information

One issue with keeping a safe in your home or office, to say nothing of carrying one in your pocket, is that it tends to be quite obvious. Many people have reasonable concerns about incriminating themselves by using encryption. Just because the legitimate reasons to encrypt data outnumber the illegitimate ones does not make this threat any less real. Essentially, there are two reasons why you might shy away from using a tool like TrueCrypt: the risk of self-incrimination and the risk of clearly identifying the location of your most sensitive information.
Considering the risk of self-incrimination

Encryption is illegal in some countries, which means that downloading, installing or using software of this sort might be a crime in its own right. And, if the police, military or intelligence services are among those groups from whom you are seeking to protect your information, then violating these laws can provide a pretext under which your activities might be investigated or your organisation might be persecuted. In fact, however, threats like this may have nothing to do with the legality of the tools in question. Any time that merely being associated with encryption software would be enough to expose you to accusations of criminal activity or espionage (regardless of what is actually inside your encrypted volumes), then you will have to think carefully about whether or not such tools are appropriate for your situation.

If that is the case, you have a few options:

You can avoid using data security software entirely, which would require that you store only non-confidential information or invent a system of code words to protect key elements of your sensitive files.
You can rely on a technique called steganography to hide your sensitive information, rather than encrypting it. There are tools that can help with this, but using them properly requires very careful preparation, and you still risk incriminating yourself in the eyes of anyone who learns what tool you have used.
You can try to store all of your sensitive information in a secure webmail account, but this demands a reliable network connection and a relatively sophisticated understanding of computers and Internet services. This technique also assumes that network encryption is less incriminating than file encryption and that you can avoid accidentally copying sensitive data onto your hard drive and leaving it there.
You can keep sensitive information off of your computer by storing it on a USB memory stick or portable hard drive. However, such devices are typically even more vulnerable than computers to loss and confiscation, so carrying around sensitive, unencrypted information on them is usually a very bad idea.

If necessary, you can employ a range of such tactics. However, even in circumstances where you are concerned about self-incrimination, it may be safest to use TrueCrypt anyway, while attempting to disguise your encrypted volume as best you can.

If want to make your encrypted volume less conspicuous, you can rename it to look like a different type of file. Using the '.iso' file extension, to disguise it as a CD image, is one option that works well for large volumes of around 700 MB. Other extensions would be more realistic for smaller volumes. This is a bit like hiding your safe behind a painting on the wall of your office. It might not hold up under close inspection, but it will offer some protection. You can also rename the TrueCrypt program itself, assuming you have stored it as you would a regular file on your hard drive or USB memory stick, rather than installing it as a program. The TrueCrypt Guide explains how to do this.
Considering the risk of identifying your sensitive information

Often, you may be less concerned about the consequences of 'getting caught' with encryption software on your computer or USB memory stick and more concerned that your encrypted volume will indicate precisely where you store the information that you most wish to protect. While it may be true that no one else can read it, an intruder will know that it is there, and that you have taken steps to protect it. This exposes you to various non-technical methods through which that intruder might attempt to gain access, such as intimidation, blackmail, interrogation and torture. It is in this context that TrueCrypt's deniability feature, which is discussed in more detail below, comes into play.

TrueCrypt's deniability feature is one of the ways in which it goes beyond what is typically offered by file encryption tools. This feature can be thought of as a peculiar form of steganography that disguises your most sensitive information as other, less sensitive, hidden data. It is analogous to installing a subtle 'false bottom' inside that not-so-subtle office safe. If an intruder steals your key, or intimidates you into giving her the safe's combination, she will find some convincing 'decoy' material, but not the information that you truly care about protecting.

Only you know that your safe contains a hidden compartment in the back. This allows you to 'deny' that you are keeping any secrets beyond what you have already given to the intruder, and might help protect you in situations where you must reveal a password for some reason. Such reasons might include legal or physical threats to your own safety, or that of your colleagues, associates, friends and family members. The purpose of deniability is to give you a chance of escaping from a potentially dangerous situation even if you choose to continue protecting your data. As discussed in the Considering the risk of self-incrimination section, however, this feature is much less useful if merely being caught with a safe in your office is enough to bring about unacceptable consequences.

TrueCrypt's deniability feature works by storing a 'hidden volume' inside your regular encrypted volume. You open this hidden volume by providing an alternate password that is different from the one you would normally use. Even if a technically sophisticated intruder gains access to the standard volume, he will be unable to prove that a hidden one exists. Of course, he may very well know that TrueCrypt is capable of hiding information in this way, so there is no guarantee that the threat will disappear as soon as you reveal your decoy password. Plenty of people use TrueCrypt without enabling its deniability feature, however, and it is generally considered impossible to determine, through analysis, whether or not a given encrypted volume contains this kind of 'false bottom'. That said, it is your job to make sure that you do not reveal your hidden volume through less technical means, such as leaving it open or allowing other applications to create shortcuts to the files that it contains. The Further reading section, below, can point you to more information about this.

Claudia: Alright, so let's toss some junk into the standard volume, and then we can move all our testimonies into the hidden one. Do you have some old PDFs or something we can use?

Pablo: Well, I was thinking about that. I mean, the idea is for us to give up the decoy password if we have no other choice, right? But, for that to be convincing, we need to make sure those files look kind of important, don't you think? Otherwise, why would we bother to encrypt them? Maybe we should use some unrelated financial documents or a list of website passwords or something.

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