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H i d d e n   V o l u m e
It may happen that you are forced by somebody to reveal the password to an encrypted volume. There are many situations where you cannot refuse to reveal the password (for example, due to extortion). Using a so-called hidden volume allows you to solve such situations without revealing the password to your volume.

The principle is that a TrueCrypt volume is created within another TrueCrypt volume (within the free space on the volume). Even when the outer volume is mounted, it should be impossible to prove whether there is a hidden volume within it or not*, because free space on any TrueCrypt volume is always filled with random data when the volume is created** and no part of the (dismounted) hidden volume can be distinguished from random data. Note that TrueCrypt does not modify the file system (information about free space, etc.) within the outer volume in any way.

 

The layout of a standard TrueCrypt volume before and after a hidden volume was created within it.

 

The password for the hidden volume must be substantially different from the password for the outer volume. To the outer volume, (before creating the hidden volume within it) you should copy some sensitive-looking files that you actually do NOT want to hide. These files will be there for anyone who would force you to hand over the password. You will reveal only the password for the outer volume, not for the hidden one. Files that really are sensitive will be stored on the hidden volume.

The user can mount the hidden volume the same way as a standard TrueCrypt volume. TrueCrypt first attempts to decrypt the standard volume header using the entered password. If it fails, it loads the area of the volume where a hidden volume header can be stored (i.e. bytes 65536–131071, which contain solely random data when there is no hidden volume within the volume) to RAM and attempts to decrypt it using the entered password. Note that hidden volume headers cannot be identified, as they appear to consist entirely of random data. If the header is successfully decrypted (for information on how TrueCrypt determines that it was successfully decrypted, see the section Encryption Scheme in the documentation), the information about the size of the hidden volume is retrieved from the decrypted header (which is still stored in RAM), and the hidden volume is mounted (its size also determines its offset).




Hidden Operating System

As of TrueCrypt 6.0, it is possible to create and boot an operating system residing in a hidden volume (see the section Hidden Operating System in the documentation).




Protection of Hidden Volumes Against Damage

As of TrueCrypt 4.0, it is possible to write data to an outer volume without risking that a hidden volume within it will get damaged (overwritten).

When mounting an outer volume, the user can enter two passwords: One for the outer volume, and the other for a hidden volume within it, which he wants to protect. In this mode, TrueCrypt does not actually mount the hidden volume. It only decrypts its header and retrieves information about the size of the hidden volume (from the decrypted header). Then, the outer volume is mounted and any attempt to save data to the area of the hidden volume will be rejected (until the outer volume is dismounted).

Note that TrueCrypt never modifies the filesystem (e.g., information about allocated clusters, amount of free space, etc.) within the outer volume in any way. As soon as the volume is dismounted, the protection is lost. When the volume is mounted again, it is not possible to determine whether the volume has used hidden volume protection or not. The hidden volume protection can be activated only by users who supply the correct password (and/or keyfiles) for the hidden volume (each time they mount the outer volume). For more details, please see the section Protection of Hidden Volumes Against Damage in the documentation.


Further information may be found in the chapter Hidden Volume in the documentation.





* Provided that all the instructions in the TrueCrypt Volume Creation Wizard have been followed and provided that the requirements and precautions listed in the subsection Security Requirements and Precautions Pertaining to Hidden Volumes (chapter Plausible Deniability) in the documentation are followed.
** Provided that the options Quick Format and Dynamic are disabled. For information on the method used to fill free volume space with random data, see chapter Technical Details, section TrueCrypt Volume Format Specification in the documentation.





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