volume has been created inside the visible container volume, the user will store important-looking information (but which the user does not actually mind revealing) on the outer volume, whereas more sensitive information is stored within the hidden volume. If the user is forced to reveal a password, the user can reveal the password to the outer volume, without disclosing the existence of the hidden volume. The hidden volume will not be compromised, if the user takes certain precautions in overwriting the free areas of the "host" disk. No identifying features Volumes, be they stored in a file or a device/partition, may intentionally not contain any discernible "signatures" or unencrypted headers. As cipher algorithms are designed to be indistinguishable from a pseudorandom permutation without knowing the key, the presence of data on the encrypted volume is also undetectable unless there are known weaknesses in the cipher. This means that it is impossible to prove that any file or partition is an encrypted volume (rather than random data) without having the password to mount it. This characteristic also makes it impossible to determine if a volume contains another hidden volume. A file hosted volume (as opposed to partitions) may look out of place in some cases since it will be entirely random data placed in a file intentionally. However, a partition or device hosted volume will look no different from a partition or device that has been wiped with a common disk wiping tool such as Darik's Boot and Nuke. One can plausibly claim that such a device or partition has been wiped to clear personal data. Portable or "traveller mode" means the encryption software can be run without installation to the system hard drive. In this mode, the software typically installs a temporary driver from the portable media. Since it is installing a driver (albeit temporarily), administrative privileges are still required. Resizable volumes Some disk encryption software allows encrypted volumes to be resized. Not many systems implement this fully, and resort to using "sparse files" to achieve this. Backups Encrypted volumes contain "header" (or "CDB") data, which may be backed up. Overwriting these data will destroy the volume, so the ability to back them up is useful. Restoring the backup copy of these data may reset the volume's password to what it was when the backup was taken. See also Disk encryption theory Disk encryption hardware Comparison of disk encryption software