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Full Disk Encryption – Two Out of Three Aren’t Bad

Security is a core interest of mine.  I have written and taught about security for many years; consistently keeping our team focused on secure solutions, and am in pursuit of earning the CISSP certification.  Some aspects of security are hard to make work effectively and other aspects are fairly simple, having more to do with common sense than technical expertise.

In this latter category I would put full disk encryption.  Clearly there are still many companies and individuals who have not embraced this technique.  The barrage of news articles describing lost and stolen computers containing sensitive information on unencrypted hard drives makes this point every day.

This leads me to the question of why people don’t use this technology.  Is it a lack of information, limitations in the available products or something else?  For my part I’ll focus this posting on providing information regarding full disk encryption, based on experience. A future post will describe Blue Slate’s deployment of full disk encryption.

Security focuses on three major concepts, Confidentiality, Integrity and Availability (CIA).  These terms apply across the spectrum of potential security-related issues.  Whether considering the physical environment, hardware, applications or data, there are techniques to protect the CIA within that domain.

For instance, we can apply these concepts to our physical environment.  We use locks to protect our physical assets.  This maintains the confidentiality of physical records.   We place a piece of tape containing our name under our favorite desk chair.  This assures the integrity of the seat, or at least lets us know if someone swaps it with another one. We use fire suppression systems to protect our buildings, which maintain the availability of office space.

Encryption supports both confidentiality and integrity (2 out of 3 as the title says).  The first, confidentiality, is what most of us immediately consider when thinking about encryption.

We use encryption when we want to protect the secrecy of information in some way.  Keeping the information confidential, between the appropriate parties, is a common requirement in our personal and business environments.  When we use Hypertext Transfer Protocol (HTTP) over Secure Sockets Layer (SSL) (e.g. HTTPS) to protect the information traveling between our web browser and server we are doing so in order to keep the information confidential.

Being able to trust that information is accurate is also vital.  In this case we need to ensure the integrity of data.  When we use an MD5 hash to verify the authenticity of downloaded software, we are using encryption to guarantee the integrity of the program.

Using encryption in these ways is fairly automatic for many of us in the IT realm.  We know that information flowing across the network needs to be protected and verified.  However, information at rest (stored on a device) requires the same protections.  Just as packets flowing across a network can be stolen (sniffers, man-on-the-middle) so too a computer, hard drive, USB drive, and CD out of our physical control can be stolen.

Encrypting information at rest is a well understood problem.  Multiple products, both commercial and open source, exist to address this need.  Their use should be as prevalent for physical devices as SSL is for network-based communications.

A key point regarding full disk encryption is that it only protects data when it is off-line. In other words, the technique protects computers and removable storage devices when they are dormant (disconnected removable drives, computers that are shutdown or hibernating).  What do you gain from employing such a technique?

Principally, the use of full disk encryption maintains the confidentiality of all the information on the device when it is out of your control. The approach removes any concern that the user will forget to encrypt a particular file – it is all encrypted automatically.  Further, system data such as hibernation and swap files, which can contain copies of sensitive information, are also encrypted.

Full disk encryption also provides system integrity protection. This is not always perceived as an advantage of the approach.  If a user has a device that is out of his or her control for some period of time, how does he or she know that someone didn’t gain physical access to the device and replace or introduce a new file onto it?  If it is encrypted then such an attack is thwarted since someone can’t easily copy a new file onto the encrypted hard disk, USB stick or CD.

As with any security approach, there are limits to the protection offered by full disk encryption. Certainly it does nothing for the availability of the system or data.  If the disk or device is physically damaged or a magnetic storage device is exposed to strong magnetic fields, the information will be damaged and the machine won’t boot or the data won’t be readable.  The technique will not protect a running computer from being infected with a virus or other malware.  It will not protect a machine with an unlocked user session from having someone walk up and replace or add files.

The advantages of full disk encryption, which deals with protection of the information when physical control has been lost (the same reason for network-based encryption), are enormous. Once the physical controls are not able to protect the information (e.g. it is outside of the office confines) we can leverage encryption to block an attacker’s access to the information on the device.  This is a clear example of applying security in depth.  We still need virus and malware protections, screen locking policies and so forth.  Those techniques do us no good, however, when someone else controls the media.

When companies consider this technique there are some common concerns. Traditionally, concerns are related to control of keys, loss of data, and system performance. The process for controlling encryption keys is impacted by the size of the organization and the number of devices to be encrypted.  The other two can be discussed in a more general sense.

When dealing with encryption keys we want to assure, under normal conditions, that the user of the computer is the only person with the ability to unlock the machine. Where this leads to concern is when dealing with the company’s desire to access the information on the system after an individual is no longer associated with the company (or is accused of a crime involving the system).  In this case we need a way to hold a key that can be used in extenuating circumstances to decrypt the device.  That process must be defined by each company.

Concerns regarding data loss are tied to the fact that encrypted devices are very sensitive to corruption of the stored information. If the encrypted data cannot be decrypted due to a physical failure or erroneously written bit, it may be close to impossible to access any of the information on the device using common tools.

When a non-encrypted device suffers such a failure we can directly read sectors and extract subsets of information.  Often we can recover most of the device’s content in this manner.  Tools that simplify such recovery won’t work with encrypted drives.

One way to deal with this limitation is to use effective and secure backup techniques and processes.  Clearly the backups must be encrypted.  Once a backup process is in place a device failure will not prevent recovery of the information.

The concern around performance is understandable. The computer is doing more work whenever data is read from or written to the encrypted media.  This processing must occur in addition to that of the software with which the user is interacting.

For instance, if I am editing a document using my word processing program and I save the file, all the same processing has to occur with an additional encryption step.  Luckily Moore’s law and very efficient encryption software help mitigate this concern.

Lest you consider this entry to be mere theory, I will use our company as a case study for full disk encryption.  Look for that in an upcoming post.  (Update: the follow-up post discussing our company’s full disk encryption rollout can be found at: http://monead.com/blog/?p=325)

In the meantime I always enjoy talking about security. I am interested in hearing from others regarding their use of full disk encryption or concerns with the approach.  How are companies protecting data on portable devices?  Do you think that the use of full disk encryption even for non-mobile devices, like servers, has value?

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