Full disk encryption - also known as whole encryption - is the most effective way to prevent confidential data being taken from a laptop that has been lost, stolen or left unattended in a hotel room.

How Full Disk Encryption Works

FDE works by encrypting a system's entire hard drive - all the confidential data stored on it, but also the operating system and all applications. When the system is started, the user is prompted for the encryption key, which enables the system to decrypt enough to boot and run normally.

Most full disk encryption products allow users to provide the system's encryption key at the pre-boot stage in several ways:

  • in the form of a password or passphrase;
  • by inserting a USB drive containing the encryption key;
  • using a one-time password-generating device such as an RSA token;
  • or using a biometric device such as a fingerprint reader (usually connected to a Trusted Platform Module which holds the actual encryption key)

A combination of two or more of these methods can be used to create multifactor authentication, for greater encryption strength and added security.

As information is read from the disk that is protected by full disk encryption, it is decrypted on the fly and stored in memory - and any information written to the disk is also encrypted on the fly. Without the encryption key, the data stored on the disk remains inaccessible to thieves and hackers.

Full Disk Encryption vs. File Level Encryption

Full disk encryption differs from file level encryption (FLE) in that it secures all data stored on your hard drives automatically and transparently - including swap files and hidden files that may contain confidential data - without any user intervention. In contrast, FLE only protects specific files that are manually encrypted. And FLE generally depends on the user to perform some action to ensure that files are encrypted before storage.

A drawback of whole encryption, however, is that it does nothing to protect files "in motion." Once a file is sent via email or copied to a memory stick, it is no longer encrypted. For that reason you may want to consider deploying FLE in conjunction with full disk encryption, so that users have the option to manually encrypt files that need to be shared with others.

Full Disk Encryption Security Shortfalls

No security system is 100 percent secure, and whole disk encryption can be vulnerable to various attacks including:

  • Accessing the encryption key. When users store a USB drive containing the encryption key along with a computer, accessing the encryption key becomes trivial for a thief. Users can also be fooled into revealing their password through social engineering.
  • Theft of the laptop while it is running. Full disk encryption only protects data when the computer is turned off. So if a laptop is stolen while it is running but unattended (or while the user is distracted), the data will be fully accessible to the thief.
  • Advanced in-memory techniques. FDE systems require encryption keys to be held in memory while the system is running. Since the contents of DRAM chips persists for a period of seconds to minutes after a system is shut down - and this time period can be extended by chilling the DRAM with canned air - it is possible to cut the power to a laptop that has been left unattended and boot it from a memory stick or CD into another operating system, then read (and save) the contents of the DRAM. The key can then be extracted from this data and used in a subsequent attack.

It's also worth noting that some software applications place information on the main drive's boot sector, and this can get overwritten by full disk encryption systems, causing them to stop working.

How to Evaluate Full Disk Encryption Software

All full disk encryption systems encrypt a system's entire disk, but they are certainly not all identical. Here are four key capabilities to look for when choosing full disk encryption software.

Centralized Deployment and Management

Some full disk encryption software - notably most open source whole encryption products - is designed for personal use and must be installed and configured on the device itself.

In an enterprise setting, though, it is not practical to visit every laptop to install whole encryption software. FDE software should be configured centrally to ensure uniformity and to make it easy to send any configuration changes to every laptop whenever necessary. It is also generally desirable for admins to be able to lock down the configuration of a full disk encryption system, so it can't be changed by end users.

Thus a key consideration is a centralized management system. (Some full disk encryption systems such as Microsoft's BitLocker can be used as standalone software, but a management component is also available for use in an enterprise context.)

It may also be convenient to look for a management system that integrates or is part of a broader system that can manage all aspects of endpoint security, such as anti-virus software, as well as full disk encryption.

Other key functions to look for in a management system are remote patching and updating, and the ability to update the underlying cryptographic system.

Authentication

A whole encryption system is only as good as the authentication system that allows users to access their computers, so ensure any system you consider offers a range of two factor authentication methods such as the use of a card-based authenticator or a USB key. Some products also allow biometrics to be used as a second factor.

For ease of management, it may be most convenient to use a system that can tie in with your existing corporate authentication system and directory service such as Active Directory.

Key Management and Recovery

One of the most common problems with full disk encryption is that users can get locked out of their computers and unable to work because they have forgotten their password or lost their second factor authentication credential. Laptops and the data they contain can also become inaccessible if a staff member leaves the organization and no knowledge transfer occurs.

That means it is important to verify that any FDE solution you consider has an adequate key management and recovery system that meets the security policies of your organization.

For example, some management systems offer self-service key recovery - allowing users to get back into their systems quickly after supplying information such as their date of birth or Social Security number. But if that provides insufficient security for some or all users, you should look for a system that offers key recovery only via an administrator.

Operating System Support

Since it is only practical to deploy and manage full disk encryption centrally, it follows that it is important to ensure that any FDE product you consider supports the full range of operating systems in use by employees. In particular, if your organization has a BYOD culture then you should investigate whether OS X (and even Linux) are supported and, if so, whether all features are supported on those OSes.

Other Full Disk Encryption Questions to Consider

In addition to researching the above features, enterprises will want to seek answers to these four questions:

  1. What cryptographic system is used, and has it been implemented securely? Any system you consider should use strong, standard, certified encryption algorithms such as the Advanced Encryption Standard (AES) with 256 bit keys. To ensure that the cryptography subsystem has been implemented securely, look for FIPS 140-2 certification.
  2. Is the system compatible with any uncommon software you use?
  3. Full disk encryption products may overwrite parts of the disk (such as the boot sector) that other software already uses. If that's the case, whole encryption may make this software unusable. In most cases, the only way to be sure is to conduct thorough testing.
  4. How does the full disk encryption software handle brute force attacks?
  5. The only practical way to decrypt encrypted drives without access to the key is to make repeated attempts to guess the password. This can be prevented by ensuring that the full disk encryption product you choose has a password lockout that disables logins either permanently or for a fixed period (perhaps two hours) after a certain number of failed login attempts.
  6. Does the product support Intel's AES-NI instructions for hardware accelerated encryption and decryption?
  7. Intel introduced a set of seven new instructions after 2008 to improve the speed of applications performing encryption and decryption using AES. Full disk encryption systems involve some processor (and therefore power) overhead to carry out the on-the-fly encryption and decryption; the impact of this depends on the amount of disk I/O that individual applications demand. For users carrying out typical email and office productivity activities, the performance impact is unlikely to be noticeable - but it can be significant for very data-intensive activities such as video processing, unless the computer's main processor and the full disk encryption product both support Intel's AES-NI instructions.

Paul Rubens is an award-winning technology journalist who has been covering IT security for over 20 years. He has written for leading international publications including The Economist, The Times, The Financial Times, The Guardian, the BBC, and Computing.