How To: Encrypt your data

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How To: Encrypt your data

Don't expose your data to hackers and thieves. This tutorial explains encryption and shows you how to protect your own files.

The idea of encryption conjures up images of espionage and military operations. But as more and more information is stored and transferred electronically, it’s becoming a part of daily life.

Happily, it’s also becoming easier. For much of the 20th century, encryption required special equipment, such as the famous Enigma machine used by German forces in World War II. Today, encrypting confidential files is a simpler process – and a more reliable one. Where the allies were able to decode many Enigma messages during the war, modern encryption techniques are so impenetrable that cracking a single encrypted email would take many lifetimes of guesswork.

Of course, it’s unlikely that the files on your PC will relate to matters of life and death, like the Enigma messages. But encryption isn’t only useful for sending secret information, it’s also a protection against data loss.

For example, if you accidentally leave a USB flash drive in a taxi, or have your laptop stolen, all the unencrypted information is compromised. That could mean the loss of your bank details, or of commercially sensitive data. It could even put you in breach of your data protection obligations: UK employment services company A4e was fined $92,000 by the Information Commissioner’s Office last year after losing a laptop containing unencrypted client information.

Encryption can be valuable for email, too: if you send your messages and attachments in plain text, it’s possible for an eavesdropping hacker to snoop on your sensitive information.On these pages, we’ll reveal how to protect your data, and yourself.


Computer encryption works by applying some sort of systematic transformation to the binary data within your files. For example, one simple algorithm for encrypting messages, known as ROT13, works by moving all the letters of the alphabet forward (or backward) by 13 places, so that ABCXYZ becomes NOPKLM.

ROT13 was popular in the early days of the internet for concealing information that someone might not want to see by accident – such as spoilers for a film plot, or hints for an adventure game. Clearly, though, it’s far too rudimentary to protect sensitive data, not least because no password is used, so anybody can decode it by simply applying the ROT13 process again.

Serious encryption employs much more complex mathematical algorithms, involving passwords and lengthy encryption “keys”, to turn structured data files into what looks like a completely disordered string of bytes. Without the password, and without knowing exactly what process was used to create the file, it’s all but impossible to recreate the original file.

One of the most popular encryption methods is the Advanced Encryption Standard, or AES. The standard was ratified by the US Government in 2001 – but although it’s used for confidential governmental business, it’s an open standard that you’re free to use for your data.

The mathematics of AES is somewhat abstruse – if you’re feeling brave, you can download the standard in PDF format from Its benefits, however, are easy to understand. In short, computer processors can apply AES encryption to files very quickly, and decrypt them just as swiftly – given the correct password.

If you don’t have the password, however, the only way to find it is by guesswork – and this isn’t feasible because the “keys” AES uses to encrypt your data are very long indeed. Even the simplest form of AES encryption uses a 128-bit encryption key – which means there are 2128 = 340,282,366,920,938,000,000,000,000,000,000,000,000 possible combinations. The maximum security CyberGhost is a free public VPN service version of AES uses a 256-bit key, giving 2128 times as many possible combinations.

This doesn’t mean everything encrypted with AES is necessarily impenetrable. Encryption tools typically work by asking you to come up with a password, which is then used to mathematically derive the key. If you pick a very simple password (such as “123”) then it’s still possible for an interloper to guess it, feed it into the program and gain access to your key.

However, if you choose a strong password you have little to worry about. It’s recommended that you use a long mix of capital and lower-case letters, numbers and punctuation marks – and steer clear of dictionary words and easy transformations thereof. This makes a cracker’s job almost impossible. For example, to find the password “Dari^en’sComp^uter” by brute force would take millions of years with current technology.


If you want to protect the files on your PC using AES or a similar algorithm, you have a choice of encryption software. Windows 7 Professional and Ultimate come with Microsoft’s own Encrypting File System (EFS), which lets you automatically encrypt and decrypt files as they’re written to and read back from NTFS drives. The Ultimate edition also offers BitLocker Drive Encryption, which lets you encrypt entire local hard disks and external drives.

If you’re using a home edition of Windows, these options aren’t available to you, but there are plenty of alternatives. One of the most popular is a tool called TrueCrypt. It’s free – in fact, it’s open source, so anyone can audit the code and confirm that the encrypted files it produces don’t have any weaknesses or “back doors” that might expose your data.

You can easily get the latest version of TrueCrypt by downloading it free from It works on OS X and Linux, too, so it’s convenient for multi-platform use – an advantage over Microsoft’s proprietary encryption systems.

TrueCrypt uses a “container” model of encryption: rather than focusing on individual files, the software presents a virtual encrypted hard disk that you can use to store sensitive data. The disk is, in reality, a file on your hard disk – called a container – and until you provide the host software with the correct password, it can’t be accessed within Windows. Once the password is entered, you can load, save and run files from the secure drive as if it were a real disk or an external hard disk. This way, you need only enter your password once per session. Indeed, if your secure volume is on an external device such as a USB flash drive, you can cache the password so you don’t need to enter it at all – but it will still be required to access your files from any other computer.

The first thing to do after you install TrueCrypt, therefore, is to create your container – see the walkthrough on p89 for a guide. Alternatively, you can choose to encrypt an entire partition or disk, but it’s more convenient to use a container. This stores all your secure data in a single file, so you can back it up, without having to decrypt your files and leave them potentially vulnerable. It’s also more efficient: although your virtual volume may appear to Windows as several gigabytes in size, the container file will grow to accommodate only the files you’ve written to it.

TrueCrypt offers many advanced features that could be useful to those who work with particularly sensitive data. It defaults to 256-bit AES encryption, but there are two other algorithms you can use instead – the exotically named Serpent and Twofish systems – to make life harder for would-be crackers. It’s possible to use encryption “cascades”, encrypting all your data with first one algorithm, then another, so that even if someone were somehow to successfully crack one level of encryption, they would still be left with only an incomprehensible string of binary data.

The software can even encrypt your entire Windows installation, so that the password must be entered before Windows will boot. This could be a good idea if you regularly access sensitive data over the net, for example, as it ensures that not only your saved files but also temporary files, browser histories and web caches are inaccessible. If the password isn’t entered correctly, a fake “missing operating system” message is shown, giving the impression that the disk is corrupted. If you’re really paranoid, you can even install a second, decoy operating system – so someone can watch you boot up the PC, and never see your hidden files.

You’ll find extensive documentation on these features, plus technical explanations of the processes used, at the TrueCrypt website; but if you only want to ensure your files can’t be read by outsiders, TrueCrypt’s basic features provide peace of mind in a few clicks.


When you’re using encryption, every byte you read or write has to go through a complex mathematical process. This takes time, so encrypted files ordinarily open and save more slowly than regular ones.

This isn’t always the case. Recent Intel chips – including most Core i5 and i7 models launched since the start of 2010 – feature new extensions (known as AES-NI) that can perform encryption and decryption at very high speeds. Running TrueCrypt on a Core i7-2600 processor, we’ve found using AES encryption is no slower than reading and writing unencrypted data.

Encryption can also be built into external hard disks and flash drives. In such cases, you’ll probably need to use the manufacturer’s own software to offload the encryption process to the drive controller. Again, this gives transfer speeds comparable to what you’d see with unencrypted data.

If you don’t have dedicated hardware support, though, encryption can have a tangible effect on the speed of disk accesses, especially with a low-powered CPU. To illustrate this, we used an Asus Eee PC, with an Atom N270 processor, to run our large-file copy tests on a TrueCrypt encrypted container file hosted on the local hard disk.

As the graph shows, applying AES encryption to the files as they were read or written slashed performance by two-thirds or more – and using a cascade of three encryption algorithms halved that again. That’s something to bear in mind if you’re considering encrypting your entire system: if you want the best performance, encrypt only the minimum of data that needs protecting.


The encryption algorithms used by TrueCrypt are what’s known as symmetric-key algorithms, which means they use the same key to encrypt and decrypt your data. This is convenient for personal storage, but it isn’t ideal for ad hoc transactions such as online shopping. In order to communicate with an online merchant using AES, you’d need to find a secure way of agreeing on a password in advance.

The answer is a different approach – one that uses two keys, known as the private key and the public key (hence the name “public-key encryption”). These keys are both long strings of random-looking data, which can be generated by your encryption software or by a third-party certification authority. They’re connected by an ingenious mathematical relationship, which means that anybody can use your public key to encrypt a message, but it can only be decrypted with your private key. This makes it easy for anyone to share information with you securely without pre-arranging a password.

Public-key cryptography can also be used to prove that a message or file comes from a trusted source. For this application the public and private keys swap roles: the encryption key, which would ordinarily be the public one, must be kept private, and the decryption key is made public. Anyone who can decrypt your messages using your decryption key can be certain that the message or file was sent from someone in possession of the encryption key.


Microsoft Outlook 2007 and 2010 have built-in support for public-key encryption, so if you’re already using Office it’s simple to exchange secure email with other Outlook users. Configuring the feature takes a few steps, but after the initial setup the process is effortless.

First, you’ll need what Microsoft calls a “Digital ID” – a pair of encryption keys, combined with a certificate from a trusted certifying authority, such as VeriSign or IntelliSafe. This certificate confirms that an encrypted message came from a particular person, adding extra security.

Most certifying authorities will charge you for a Digital ID, but you can get a free certificate for personal use from security vendor Comodo. Sign up at – we suggest you visit in Internet Explorer; rather annoyingly, we’ve found other browsers can’t always handle the process.

Once you’re at the site, click the Free Download button, then fill in your name and email address, along with a password for revoking your certificate should it become compromised (such as if your PC is stolen). Your certificate will be tied to the email address you provide here, so make sure you enter the right one. A minute or two later, you’ll receive an email from Comodo containing a link that you can follow (again, in Internet Explorer) to install your new certificate in Windows.

Once your certificate is installed, it may look like nothing’s changed. The next time you create an email in Outlook, however, you should see two new buttons under the Options tab: Encrypt and Sign. If you click on Encrypt, Outlook will send your email in an encrypted format that can only be read by the intended recipient.

At least, it will try: the first time you try sending an encrypted email, you may see a warning that your message can’t be encrypted. Because of the way public-key encryption works, you can send only encrypted email to recipients whose public keys are known to Outlook – and, likewise, your colleagues can send you encrypted mail only once they have your key. You can give it to them by simply selecting Sign on an outgoing message: their copy of Outlook will automatically store your key. Similarly, once someone else has sent you a signed message, you’ll be able to encrypt your emails to them. Encrypted messages are shown with a blue padlock, while signed messages have a small red rosette.

Although encrypted messages are illegible to anyone spying on your mail server or your network traffic, you should note that they’re automatically decrypted as they arrive in your inbox, so they’re readable by anyone with access to your computer. If that’s a concern, you may wish to consider additional security measures, such as encrypting the system disk (see above).

If you want Outlook to encrypt and sign your messages by default, you’ll find the option hidden away in the Office Trust Center. Click on the File tab (or the Orb in Outlook 2007), then Options | Trust Center | Trust Center Settings | E-mail Security. In the window that appears, tick the two top tickboxes to turn on encrypt and digital signing by default. If you have more advanced needs, you can configure additional settings, and save multiple encryption profiles, by clicking Settings.

If you’re using a different mail client, you can usually add public-key encryption capabilities with a plugin. The open-source GPG tool – short for GNU Privacy Guard – can be added to clients including Thunderbird and Eudora. Alternatively, switch to a client that has public-key encryption built in, such as Claws Mail or Evolution. You’ll find more information about GPG at

Step 1:

When you load up TrueCrypt and click “Create Volume”, you’ll be asked whether you want to create a container or encrypt a disk. Typically, it’s convenient to use a container file, which can be easily backed up and encrypts only your most sensitive files. Click Next and choose whether you want your volume to be hidden: if you do, a thief or intruder won’t even see the container file.

Step 2:

Next, choose a location for your virtual volume. Click “Select File”, browse to the disk location you want and type in a name for the container file. You can always move it later. You’ll be asked how large you want this container to be: the size you choose is a maximum, so even if you specify a 2GB container, it won’t take up that much space on disk until you’ve saved that much data

Step 3:

After you’ve chosen a location for your container, you must choose an encryption algorithm. AES, Serpent and Twofish are all very well-respected systems. You can also use a cascade of two or three algorithms, but remember that this will have an impact on read and write performance (see previous page). You can choose a hash algorithm here too: this is used to add a degree of randomness to the encryption, making your data more mathematically obfuscated.

Step 4:

Next, enter the desired size of your container (see step 2), then proceed to enter a password. The last step, shown above, is to format your new container. TrueCrypt uses random numbers here, too: wiggle your mouse as instructed to generate an unguessable number before clicking Format. Once that’s done your container is ready. To use it, click Select File in the main TrueCrypt interface, then click on a drive letter, click Mount and enter your password.

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