Some storage devices (e.g., some solid-state drives, including USB flash drives) and some file systems utilize so-called wear-leveling mechanisms to extend the lifetime of the storage device or medium. These mechanisms ensure that even if an application repeatedly writes data to the same logical sector, the data is distributed evenly across the medium (logical sectors are remapped to different physical sectors). Therefore, multiple "versions" of a single sector may be available to an attacker. This may have various security implications. For instance, when you change a volume password/keyfile(s), the volume header is, under normal conditions, overwritten with a re-encrypted version of the header. However, when the volume resides on a device that utilizes a wear-leveling mechanism, TrueCrypt cannot ensure that the older header is really overwritten. If an adversary found the old volume header (which was to be overwritten) on the device, he could use it to mount the volume
using an old compromised password (and/or using compromised keyfiles
that were necessary to mount the volume before the volume header was re-encrypted). Due to security reasons, we recommend that TrueCrypt volumes are not created/stored on devices (or in file systems) that utilize a wear-leveling mechanism (and that TrueCrypt is not used to encrypt any portions of such devices or filesystems).
If you decide not to follow this recommendation and you intend to use in-place encryption on a drive that utilizes wear-leveling mechanisms, make sure the partition/drive does not contain any sensitive data before you fully encrypt it (TrueCrypt cannot reliably perform secure in-place encryption of existing data on such a drive; however, after the partition/drive has been fully encrypted, any new data that will be saved to it will be reliably encrypted on the fly). That includes the following precautions: Before you run TrueCrypt to set up pre-boot authentication, disable the paging files and restart the operating system (you can enable the paging files after the system partition/drive has been fully encrypted). Hibernation must be prevented during the period between the moment when you start TrueCrypt to set up pre-boot authentication and the moment when the system partition/drive has been fully encrypted. However, note that even if you follow those steps, it is not guaranteed that you will prevent data leaks and that sensitive data on the device will be securely encrypted. For more information, see the sections Data Leaks, Paging File, Hibernation File, and Memory Dump Files.
If you need plausible deniability, you must not use TrueCrypt to encrypt any part of (or create encrypted containers on) a device (or file system) that utilizes a wear-leveling mechanism.
To find out whether a device utilizes a wear-leveling mechanism, please refer to documentation supplied with the device or contact the vendor/manufacturer.
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