TrueCrypt Volume Format Specification

The format of file-hosted volumes is identical to the format of partition/device-hosted volumes (however, the "volume header", or key data, for a system partition/drive is stored in the last 512 bytes of the first logical drive track). TrueCrypt volumes have no "signature" or ID strings. Until decrypted, they appear to consist of random data. Therefore, it is impossible to identify a TrueCrypt container or partition.

Free space on each TrueCrypt volume is filled with random data when the volume is created (if the options Quick Format and Dynamic are disabled). The random data is generated as follows: Right before TrueCrypt volume formatting begins, a temporary encryption key and a temporary secondary key (XTS mode) are generated by the random number generator (see the section Random Number Generator). The encryption algorithm that the user selected is initialised with the temporary keys. The encryption algorithm is then used to encrypt plaintext blocks generated by the random number generator. The encryption algorithm operates in XTS mode (see the section Hidden Volume). The resulting ciphertext blocks are used to fill (overwrite) the free space on the volume. The temporary keys are stored in RAM and are securely erased after formatting finishes.

The fields located at the byte #0 (salt) and #256 (master keys) contain random values generated by the random number generator (see the section Random Number Generator) during the volume creation process.

If a TrueCrypt volume hosts a hidden volume (within its free space), the header of the hidden volume is located at the byte #65536 of the host volume (the header of the host/outer volume is located at the byte #0 of the host volume – see the section Hidden Volume). If there is no hidden volume within a TrueCrypt volume, the bytes 65536–131071 of the volume (i.e., the area where the header of a hidden volume can reside) contain random data (see above for information on the method used to fill free volume space with random data when the volume is created). The layout of the header of a hidden volume is the same as the one of a standard volume (bytes 0–65535).

The maximum possible TrueCrypt volume size is 2⁶³ bytes (8,589,934,592 GB). However, due to security reasons (with respect to the 128-bit block size and the mode of operation), the maximum allowed volume size is 1 PB (1,048,576 GB).

Embedded Backup Headers

Each TrueCrypt volume created by TrueCrypt 6.0 or later contains an embedded backup header, located at the end of the volume (see above). The header backup is not a copy of the volume header because it is encrypted with a different header key derived using a different salt (see the section Header Key Derivation, Salt, and Iteration Count).

When the volume password and/or keyfiles are changed, or when the header is restored from the embedded (or an external) header backup, both the volume header and the backup header (embedded in the volume) are re-encrypted with different header keys (derived using newly generated salts – the salt for the volume header is different from the salt for the backup header). Each salt is generated by the TrueCrypt random number generator (see the section Random Number Generator).

For more information about header backups, see the subsection Tools > Restore Volume Header in the chapter Main Program Window.