hidden within the free space of the more obvious "outer" volumes. Such strategies provide plausible deniability. Well-known examples of disk encryption software include: BitLocker for Windows; FileVault for Apple OS/X; LUKS a standard free software mainly for Linux and TrueCrypt, a non-commercial freeware application, for Windows, OS/X and Linux. A 2008 study found data remanence in dynamic random access memory (DRAM), with data retention of seconds to minutes at room temperature and much longer times when memory chips were cooled to low temperature. The study authors were able to demonstrate a cold boot attack to recover cryptographic keys for several popular disk encryption systems despite some memory degradation, by taking advantage of redundancy in the way keys are stored after they have been expanded for efficient use. The authors recommend that computers be powered down, rather than be left in a "sleep" state, when not under physical control by the computer's legitimate owner. This method of key recovery, however, is suited for controlled laboratory settings and is extremely impractical for "field" use due to the equipment and cooling systems required. Other Features Plausible deniability Some disk encryption systems, such as VeraCrypt, CipherShed (active open source forks of the discontinued TrueCrypt project), BestCrypt (proprietary trialware), offers levels of plausible deniability, which might be useful if a user is compelled to reveal the password of an encrypted volume. Hidden volumes Hidden volumes are a steganographic feature that allows a second, "hidden", volume to reside within the apparent free space of a visible "container" volume (sometimes known as "outer" volume). The hidden volume has its own separate file system, password, and encryption key distinct from the container volume. The content of the hidden volume is encrypted and resides in the free space of the file system of the outer volume—space which would otherwise be filled with random values if the hidden volume did not exist. When the outer container is brought online thorough the disk encryption software, whether the inner or outer volume is mounted depends on the password provided. If the "normal" password/key of the outer volume proves valid, the outer volume is mounted; if the password/key of the hidden volume proves valid, then (and only then) can the existence of hidden volume even be detected, and it is mounted; otherwise if the password/key does not successfully decrypt either the inner or outer volume descriptors, then neither is mounted. Once a hidden