Introduction

Beginner's Tutorial

System Encryption

 Supported Systems

 Hidden Operating System

 Rescue Disk

Plausible Deniability

 Hidden Volume

  Protection of Hidden Vol.

  Security Requirements

 Hidden Operating System

Parallelization

Pipelining

Hardware Acceleration

Encryption Algorithms

 AES

 Serpent

 Twofish

 Cascades

Hash Algorithms

 RIPEMD-160

 SHA-512

 Whirlpool

Technical Details

 Notation

 Encryption Scheme

 Modes of Operation

 Header Key Derivation

 Random Number Gen.

 Keyfiles

 Volume Format Spec.

 Standards Compliance

 Source Code

TrueCrypt Volume

 Creating New Volumes

 Favorite Volumes

 System Favorite Volumes

Main Program Window

 Program Menu

 Mounting Volumes

Supported Systems

Portable Mode

Keyfiles

Tokens & Smart Cards

Language Packs

Hot Keys

Security Model

Security Requirements

 Data Leaks

  Paging File

  Hibernation File

  Memory Dump Files

 Unencrypted Data in RAM

 Physical Security

 Malware

 Multi-User Environment

 Authenticity and Integrity

 New Passwords & Keyfiles

 Password/Keyfile Change

 Trim Operation

 Wear-Leveling

 Reallocated Sectors

 Defragmenting

 Journaling File Systems

 Volume Clones

 Additional Requirements

Command Line Usage

Backing Up Securely

Miscellaneous

 Use Without Admin Rights

 Sharing over Network

 Background Task

 Removable Medium Vol.

 TrueCrypt System Files

 Removing Encryption

 Uninstalling TrueCrypt

 Digital Signatures

Troubleshooting

Incompatibilities

Issues and Limitations

License

Future Development

Acknowledgements

Version History

References

   

Technical Details >  Encryption Scheme Search

Disclaimers





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Encryption Scheme

When mounting a TrueCrypt volume (assume there are no cached passwords/keyfiles) or when performing pre-boot authentication, the following steps are performed:

  1. The first 512 bytes of the volume (i.e., the standard volume header) are read into RAM, out of which the first 64 bytes are the salt (see TrueCrypt Volume Format Specification). For system encryption (see the chapter System Encryption), the last 512 bytes of the first logical drive track are read into RAM (the TrueCrypt Boot Loader is stored in the first track of the system drive and/or on the TrueCrypt Rescue Disk).

  2. Bytes 65536–66047 of the volume are read into RAM (see the section TrueCrypt Volume Format Specification). For system encryption, bytes 65536–66047 of the first partition located behind the active partition* are read into RAM (see the section Hidden Operating System). If there is a hidden volume within this volume (or within the partition behind the active partition), we have read its header at this point; otherwise, we have just read random data (whether or not there is a hidden volume within it has to be determined by attempting to decrypt this data; for more information see the section Hidden Volume).

  3. Now TrueCrypt attempts to decrypt the standard volume header read in (1). All data used and generated in the course of the process of decryption are kept in RAM (TrueCrypt never saves them to disk). The following parameters are unknown** and have to be determined through the process of trial and error, i.e., by testing all possible combinations of the following:
    1. PRF used by the header key derivation function (as specified in PKCS #5 v2.0; see the section Header Key Derivation, Salt, and Iteration Count), which can be one of the following:

      HMAC-SHA-512, HMAC-RIPEMD-160, HMAC-Whirlpool.

      A password entered by the user (to which one or more keyfiles may have been applied – see the section Keyfiles) and the salt read in (1) are passed to the header key derivation function, which produces a sequence of values (see the section Header Key Derivation, Salt, and Iteration Count) from which the header encryption key and secondary header key (XTS mode) are formed. (These keys are used to decrypt the volume header.)

    2. Encryption algorithm: AES-256, Serpent, Twofish, AES-Serpent, AES-Twofish-Serpent, etc.

    3. Mode of operation:  XTS, LRW (deprecated/legacy), CBC (deprecated/legacy)

    4. Key size(s)
  4. Decryption is considered successful if the first 4 bytes of the decrypted data contain the ASCII string "TRUE", and if the CRC-32 checksum of the last 256 bytes of the decrypted data (volume header) matches the value located at byte #8 of the decrypted data (this value is unknown to an adversary because it is encrypted – see the section Header Key Derivation, Salt, and Iteration Count). If these conditions are not met, the process continues from (3) again, but this time, instead of the data read in (1), the data read in (2) are used (i.e., possible hidden volume header). If the conditions are not met again, mounting is terminated (wrong password, corrupted volume, or not a TrueCrypt volume).

  5. Now we know (or assume with very high probability) that we have the correct password, the correct encryption algorithm, mode, key size, and the correct header key derivation algorithm. If we successfully decrypted the data read in (2), we also know that we are mounting a hidden volume and its size is retrieved from data read in (2) decrypted in (3).

  6. The encryption routine is reinitialized with the primary master key*** and the secondary key (XTS mode), which are retrieved from the decrypted volume header (see the section TrueCrypt Volume Format Specification). These keys can be used to decrypt any sector of the volume, except the volume header area (or the key data area, for system encryption), which has been encrypted using the header keys. The volume is mounted.

See also the section Modes of Operation and the section Header Key Derivation, Salt, and Iteration Count and also the chapter Security Model.




* If the size of the active partition is less than 256 MB, then the data is read from the second partition behind the active one (Windows 7 and later, by default, do not boot from the partition on which they are installed).
** These parameters are kept secret not in order to increase the complexity of an attack, but primarily to make TrueCrypt volumes unidentifiable (indistinguishable from random data), which would be difficult to achieve if these parameters were stored unencrypted within the volume header. Also note that if a non-cascaded encryption algorithm is used for system encryption, the algorithm is known (it can be determined by analyzing the contents of the unencrypted TrueCrypt Boot Loader stored in the first logical drive track or on the TrueCrypt Rescue Disk).
*** The master keys were generated during the volume creation and cannot be changed later. Volume password change is accomplished by re-encrypting the volume header using a new header key (derived from a new password).





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