Encryption
Encryption has long been used by militaries and governments to facilitate secret communication. It is now commonly used in protecting information within many kinds of civilian systems. For example, the Computer Security Institute reported that in 2007, 71% of companies surveyed utilized encryption for some of their data in transit, and 53% utilized encryption for some of their data in storage. Encryption can be used to protect data "at rest", such as information stored on computers and storage devices (e.g. USB flash drives). In recent years, there have been numerous reports of confidential data, such as customers' personal records, being exposed through loss or theft of laptops or backup drives; encrypting such files at rest helps protect them if physical security measures fail. Digital rights management systems, which prevent unauthorized use or reproduction of copyrighted material and protect software against reverse engineering (see also copy protection), is another somewhat different example of using encryption on data at rest.
Encryption layer in storage stack
There is a plurality of terms that are used to describe implementations of disk encryption: on-the-fly encryption (OTFE); full disk encryption (FDE), whole disk encryption; filesystem-level encryption, encrypted filesystem, cryptographic filesystem, etc. All of them refer to an encryption layer in the storage stack. This article describes these techniques and explains their pros and cons. This article does not reference any software or hardware implementing these techniques (see disk encryption software and hardware), and it does not describe cryptographic theory behind it (see disk encryption theory).