Security Precautions Pertaining to Hidden Volumes

If you use a hidden TrueCrypt volume, you must follow these security precautions:

• If an adversary has access to a (dismounted) TrueCrypt volume at several points over time, he may be able to determine which sectors of the volume are changing. If you change the contents of a hidden volume (e.g., create/copy new files to the hidden volume or modify/delete/rename/move files stored on the hidden volume, etc.), the contents of sectors (ciphertext) in the hidden volume area will change. After being given the password to the outer volume, the adversary might demand an explanation why these sectors changed. Your failure to provide a plausible explanation might indicate the existence of a hidden volume within the outer volume.
  
  Note that the issue described above may also arise, for example, in the following cases:
  
  
  • The file system in which you store a file-hosted TrueCrypt container has been defragmented and a copy of the TrueCrypt container (or of its fragment) remains in the free space on the host volume (in the defragmented file system). To prevent this, do one of the following:
    • Use a partition/device-hosted TrueCrypt volume instead of file-hosted.
    • Securely erase free space on the host volume (in the defragmented file system) after defragmenting.
    • Do not defragment file systems in which you store TrueCrypt volumes.
      
      
  • A file-hosted TrueCrypt container is stored in a journaling file system (such as NTFS). A copy of the TrueCrypt container (or of its fragment) may remain on the host volume. To prevent this, do one the following:
    • Use a partition/device-hosted TrueCrypt volume instead of file-hosted.
    • Store the container in a non-journaling file system (for example, FAT32).
      
      
  • A TrueCrypt volume resides on a device that utilizes a wear-leveling mechanism (e.g. some USB flash drives). A copy of (a fragment of) the TrueCrypt volume may remain on the device. For more information on wear-leveling, see the section Wear-Leveling in the chapter Security Precautions.

• Make sure that Quick Format is disabled when encrypting a partition/device within which you intend to create a hidden volume.
  
  
• On Windows, make sure you have not deleted any files within a volume within which you intend to create a hidden volume (the cluster bitmap scanner does not detect deleted files).
  
  
• On Linux or Mac OS X, if you intend to create a hidden volume within a file-hosted TrueCrypt volume, make sure that the volume is not sparse-file-hosted (the Windows version of TrueCrypt verifies this and disallows creation of hidden volumes within sparse files).
  
  
• When a hidden volume is mounted, the operating system and third-party applications may write to non-hidden volumes (typically, to the unencrypted system volume) unencrypted information about the data stored in the hidden volume (e.g. filenames and locations of recently accessed files, databases created by file indexing tools, etc.), or the data itself in an unencrypted form (temporary files, etc.), or unencrypted information about the filesystem residing in the hidden volume (which might be used e.g. to identify the filesystem and to determine whether it is the filesystem residing in the outer volume). Therefore, the following guidelines and precautions must be followed:
  
  
  • Windows: Create a hidden operating system (for information on how to do so, see the section Hidden Operating System) and mount hidden volumes only when the hidden operating system is running.
    
    Note: When a hidden operating system is running, TrueCrypt ensures that all local unencrypted filesystems and non-hidden TrueCrypt volumes are read-only (i.e. no files can be written to such filesystems or TrueCrypt volumes).* Data is allowed to be written to filesystems within hidden TrueCrypt volumes.
    
    
  • Linux: Download or create a "live CD" version of your Linux operating system (i.e. a "live" Linux system entirely stored on and booted from a CD/DVD) that ensures that any data written to the system volume is written to a RAM disk. Mount hidden volumes only when such a "live CD" system is running. During the session, only filesystems that reside in hidden TrueCrypt volumes may be mounted in read-write mode (outer or unencrypted volumes/filesystems must be mounted as read-only or must not be mounted/accessible at all). If you cannot use such a "live CD" version of the operating system or if you are not able to ensure that applications and the standard version (as opposed to a "live CD" version) of your operating system do not write the above types of sensitive data to non-hidden volumes (or filesystems), you should not mount or create hidden TrueCrypt volumes under Linux.
    
    
  • Mac OS X: If you are not able to ensure that applications and the operating system do not write the above types of sensitive data to non-hidden volumes (or filesystems), you should not mount or create hidden TrueCrypt volumes under Mac OS X.
    
    
• If you use an operating system residing within a hidden volume (see the section Hidden Operating System), then, in addition to the above precautions, you must follow these security precautions:
  
  
  • You should use the decoy operating system as frequently as you use your computer. Ideally, you should use it for all activities that do not involve sensitive data. Otherwise, plausible deniability of the hidden operating system might be adversely affected (if you revealed the password for the decoy operating system to an adversary, he could find out that the system is not used very often, which might indicate the existence of a hidden operating system on your computer). Note that you can save data to the decoy system partition anytime without any risk that the hidden volume will get damaged (because the decoy system is not installed in the outer volume).
    
    
  • If the operating system requires activation, it must be activated before it is cloned (cloning is part of the process of creation of a hidden operating system — see the section Hidden Operating System) and the hidden operating system (i.e. the clone) must never be reactivated. The reason is that the hidden operating system is created by copying the content of the system partition to a hidden volume (so if the operating system is not activated, the hidden operating system will not be activated either). If you activated or reactivated a hidden operating system, the date and time of the activation (and other data) might be logged on a Microsoft server (and on the hidden operating system) but not on the decoy operating system. Therefore, if an adversary had access to the data stored on the server or intercepted your request to the server (and if you revealed the password for the decoy operating system to him), he might find out that the decoy operating system was activated (or reactivated) at a different time, which might indicate the existence of a hidden operating system on your computer.
    
    
  • The computer may be connected to a network (including the internet) only when the decoy operating system is running. When the hidden operating system is running, the computer should not be connected to any network, including the internet (one of the most reliable ways to ensure it is to unplug the network cable, if there is one). Note that if data is downloaded from or uploaded to a remote server, the date and time of the connection, and other data, are typically logged on the server. Various kinds of data are also logged on the operating system (e.g. Windows auto-update data, application logs, error logs, etc.) Therefore, if an adversary had access to the data stored on the server or intercepted your request to the server (and if you revealed the password for the decoy operating system to him), he might find out that the connection was not made from within the decoy operating system, which might indicate the existence of a hidden operating system on your computer.
    
    Also note that similar issues would affect you if there were any filesystem shared over a network under the hidden operating system (regardless of whether the filesystem is remote or local). Therefore, when the hidden operating system is running, there must be no filesystem shared over a network (in any direction).
    

In addition to the above precautions, you must follow the security precautions listed in the following chapters:

• Security Precautions
  
  
• How to Back Up Securely