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BitLocker Drive Encryption (BDE) format specification

Summary

The BitLocker Drive Encryption (BDE) format is used by Microsoft Windows to encrypt volumes. This specification is based on available documentation and was enhanced by reverse engineering of the file format.

This document is intended as a working document for the BitLocker Drive Encryption (BDE) format specification. Which should allow existing Open Source forensic tooling to be able to process this volume type.

Document information

Author(s):

Joachim Metz <joachim.metz@gmail.com>

Abstract:

This document contains information about the BitLocker Drive Encryption (BDE) format

Classification:

Public

Keywords:

BitLocker Drive Encryption, BDE, Full Volume Encryption, FVE

License

Copyright (C) 2011-2018, Joachim Metz <joachim.metz@gmail.com>.
Permission is granted to copy, distribute and/or modify this document under the
terms of the GNU Free Documentation License, Version 1.3 or any later version
published by the Free Software Foundation; with no Invariant Sections, no
Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included
in the section entitled "GNU Free Documentation License".

Revision history

Version Author Date Comments

0.0.1

J.B. Metz

March 2011

Worked on initial version.

0.0.2

J.B. Metz

October 2011

Additional information.

0.0.3

J.B. Metz

May 2012

Updates for Windows 8 Consumer Preview.

0.0.4

J.B. Metz

June 2013

Additional information regarding Windows 8 with thanks to T. Duc Nguyen.

0.0.5

J.B. Metz

June 2013

Additional information regarding encryption method MSB with thanks to S. Hansen.

0.0.6

J.B. Metz

June 2014

Additional information regarding encryption method MSB with thanks to J. Van Tubbergh.

0.0.7

J.B. Metz

October 2014

Switched to asccidoc format.

0.0.8

J.B. Metz

December 2015

Additional information regarding AES-XTS encryption method with thanks to github user anocs.

0.0.9

J.B. Metz

September 2017

Additional information regarding AES-XTS encryption method.

0.0.10

J.B. Metz

August 2018

Additional information regarding volume header size.
Additional information regarding BitLocker To Go Application files with thanks to K. Sung.

0.0.11

J.B. Metz

December 2018

Corrected typos

1. Overview

BitLocker Drive Encrypting (BDE) is the volume encryption used by Microsoft Windows as of Vista. There are multiple versions of BitLocker Drive Encryption (BDE):

  • BitLocker Windows Vista

  • TODO: BitLocker Windows 2008

  • BitLocker Windows 7

  • BitLocker To Go

  • BitLocker Windows 8

  • BitLocker Windows 10

Both BitLocker Windows Vista and BitLocker Windows 7 are intended to encrypt NTFS volumes on fixed storage media, like harddisks. BitLocker To Go was introduced in Windows 7 and is intended to encrypt removable drives, with e.g. FAT file systems. NTFS volumes on removable drives are treated as NTFS volumes on fixed storage media.

The BitLocker identifier (GUID) is 4967d63b-2e29-4ad8-8399-f6a339e3d00.

BitLocker To Go uses 4967d63b-2e29-4ad8-8399-f6a339e3d01.

1.1. Metadata files

1.1.1. Windows Vista

In Windows Vista the System Volume Information folder of the unencrypted volume contain several file entries for the BitLocker metadata blocks:

  • FVE.{%GUID%}.[123] maps the blocks that contain the FVE metadata. Typically 16384 bytes of size.

The contents of the metadata files, on an unencrypted volume, consists of 0-byte values. It is assumed that these files are used to prevent the BitLocker metadata to be overwritten.

Note
EnCase (at least version 6.18) does not zero out these metadata areas.

1.1.2. Windows 7

In Windows 7 the System Volume Information folder of the unencrypted volume contain several file entries for the BitLocker metadata blocks:

  • FVE2.{%GUID%} maps the block that contains the encrypted volume header. Typically 8192 bytes of size.

  • FVE2.{%GUID%}.[123] maps the blocks that contain the FVE metadata. Typically 65536 bytes of size.

The contents of the metadata files, on an unencrypted volume, consists of 0-byte values. It is assumed that these files are used to prevent the BitLocker metadata to be overwritten.

Note
EnCase (at least version 6.18) does not zero out these metadata areas.

1.1.3. To Go

BitLocker To Go uses a hybrid volume that has a encrypted and an unencrypted part. The unencrypted part contains various files. Application files for the BitLocker To Go helper application; which can also be found in C:\Windows\BitLockerDiscoveryVolumeContents\

  • "COV 0000. BL" maps the block that contains the BitLocker To Go GUID and the offsets to the metadata. Typically 32768 bytes of size.

  • "COV 0000. ER" maps the encrypted data.

  • "PAD 0000. PD" maps padding.

  • "PAD 0000. NG" unknown. Typically 0 bytes of size.

Note
It has been observed that the "COV 0000. ER" and "PAD 0000. NG" files can be split in multiple 4294934528 byte (4 GiB - 32768) on a FAT32 volume, such as "COV 0001. ER", "COV 0002. ER", …​ or "PAD 0001. NG", …​
Note
It has been suggested that the purpose of the "PAD 0000. NG" are meant to fill the root directory with entries so that no new files may be created on the volume.

2. Keys

To encrypt storage media BitLocker uses different kind of keys.

2.1. Volume Master Key (VMK)

The Volume Master Key (VMK) is 256-bit of size and is stored in multiple FVE Volume Master Key (VMK) structures. The VMK is stored encrypted with either the recovery key, external key, or the TPM.

It is also possible that the VMK is stored unencrypted which is referred to as clear key.

2.2. Full Volume Encryption Key (FVEK)

The Full Volume Encryption Key (FVEK) is stored encrypted with the Volume Master Key (VMK). The size of the FVEK is dependent on the encryption method used:

  • For AES 128-bit the key is 128-bit of size

  • For AES 256-bit the key is 256-bit of size

When Elephant Diffuser is used the key data of the structure that hold the FKEV is always 512-bit of size. The First 256-bit are reserved for the FVEK and the other 256-bit for the TWEAK key. Only 128-bit of the 256-bits are used when the encryption method is AES 128-bit.

2.3. TWEAK key

The TWEAK is stored encrypted with the Volume Master Key (VMK). The size of the TWEAK key is dependent on the encryption method used:

  • For AES 128-bit the key is 128-bit of size

  • For AES 256-bit the key is 256-bit of size

The TWEAK key is only present when Elephant Diffuser is used. The TWEAK key is stored in the key data of the structure that hold the Full Volume Encryption Key (FVEK) is always 512-bit of size. The First 256-bit are reserved for the FVEK and the other 256-bit for the TWEAK key. Only 128-bit of the 256-bits are used when the encryption method is AES 128-bit.

2.4. Recovery key

BitLocker provides for a recovery (or numerical) password to unlock the encrypted data. The recovery password is used to determine a recovery key.

Example recovery password:

471207-278498-422125-177177-561902-537405-468006-693451

A valid recovery password consists of 48 digits where every number is dividable by 11 with a remainder of 0. The result of a division by 11 of a number is a 16-bit value. The individual 16-bit values make up a 128-bit key.

The corresponding recovery key is calculated using the following approach, written partially in pseudo C:

Initialize a structure consisting of:
uint8_t last_sha256[ 32 ];
uint8_t initial_sha256[ 32 ];
uint8_t salt[ 16 ];
uint64_t count;

Initialize both the last SHA256 and the count to 0.

Calculate the SHA256 of the 128-bit key and update the initial SHA256 value.

The salt is stored on disk in the stretch key which is stored in the recovery key protected Volume Master Key (VMK).

Loop for 1048576 (0x100000) times:

  • calculate the SHA256 of the structure and update the last SHA256 value

  • increment the count by 1

The last SHA256 value contains the 256-bit key which is recovery key that can unlock the recovery key protected Volume Master Key (VMK).

2.5. Clear key

The clear key is an unprotected 256-bit key stored on the volume to decrypt the VMK. It is used when the encrypted volume is being decrypted.

2.6. Startup key

The startup key (or external key) is stored in a file named {%GUID%}.BEK. The GUID in the filename equals the key identifier in the BitLocker metadata.

There can be multiple startup keys for a single BitLocker volume. Each key is identified a by a different key identifier.

2.7. User key

BitLocker To Go provides for a user password (or passphrase) to unlock the encrypted data. The user password is used to determine a user key.

Check this: the password can be maximal 49 characters in size.

Convert the user password into a UTF16 little-endian string.

Initialize a structure consisting of:

uint8_t last_sha256[ 32 ];
uint8_t initial_sha256[ 32 ];
uint8_t salt[ 16 ];
uint64_t count;

Initialize both the last SHA256 and the count to 0.

Calculate the SHA256 of the user password.

Calculate the SHA256 of the SHA256 of the user password, and set it as the initial SHA256 value.

The salt is stored on disk in the stretch key which is stored in the user key (or password) protected Volume Master Key (VMK).

Loop for 1048576 (0x100000) times:

  • calculate the SHA256 of the structure and update the last SHA256 value

  • increment the count by 1

The last SHA256 value contains the 256-bit key which is user key that can unlock the user key (or password) protected Volume Master Key (VMK).

3. Encryption methods

BitLocker uses different kind of encryption methods. To encrypt the sector data it either uses AES-CBC with or without Elephant Elephant Diffuser. To encrypt the key data BitLocker uses AES-CCM.

3.1. AES-CBC

Both encryption and decryption use:

  • AES-CBC with FVEK decryption of sector data

The initialization vector of the AES-CBC is the sector offset AES-ECB encrypted with the FVEK stored as a 16-byte little-endian value. The sector offset is the offset of the sector relative from the start of the volume.

3.2. AES-CBC with Elephant Diffuser

Encryption:

  • XOR with sector key

  • Elephant Elephant Diffuser A

  • Elephant Elephant Diffuser B

  • AES-CBC with FVEK

Decryption:

  • AES-CBC with FVEK

  • Elephant Elephant Diffuser B

  • Elephant Elephant Diffuser A

  • XOR with sector key

The initialization vector of the AES-CBC is the sector offset AES-ECB encrypted with the FVEK stored as a 16-byte little-endian value. The sector offset is the offset of the sector relative from the start of the volume.

The sector key 32-byte of size and contains:

  • the lower 16-byte contain a little-endian version of the offset of the sector, relative from the start of the volume, AES-ECB encrypted with the TWEAK key

  • the upper 16-byte contain a 16-byte little-endian version of the offset of the sector, relative from the start of the volume, with the most upper bit set (or upper byte set to 0x80) AES-ECB encrypted with the TWEAK key

3.3. AES-CCM

The key data is encrypted using AES-CCM with an initialization vector of 0.

3.4. AES-XTS

The FKEV contains both XTS keys.

Both encryption and decryption use:

  • AES-XTS with FVEK decryption of sector data

The initialization vector of the AES-XTS is the sector number stored as a 16-byte little-endian value. The sector number is the offset of the sector relative from the start of the volume divided by the sector size.

3.5. Elephant Diffuser

The Elephant Diffuser A and B variants are described in [FERGUSON06].

3.6. Virtual sector(s)

In BitLocker the certain sector(s) of the encrypted storage media are handled in a specific manner. These are sectors to store:

  • the unencrypted volume header

  • the BitLocker metadata

3.6.1. BitLocker Windows Vista

In BitLocker Windows Vista the first sector of the unencrypted volume header sector is reconstructed by replacing values in the BitLocker Volume header, namely

  • replacing the "File system signature" with "NTFS\x20\x20\x20\x20"

  • replacing the "FVE metadata block 1 cluster block number" with the "MTF mirror cluster block number"

The 15 sectors directly following the first sector are also unencrypted.

The sectors that contain the BDE metadata are shown as empty sectors; containing 0-byte values.

Note
EnCase (at least version 6.18) does not zero out these metadata areas.

3.6.2. BitLocker Windows 7 and To Go

Both BitLocker Windows 7 and To Go store an encrypted version of the unencrypted first sectors in a specific location. This location is defined in the FVE Volume header block. It is commonly 8192 bytes an size, entailing the first 16 sectors.

The sectors that contain the encrypted volume header and the BDE metadata are shown as empty sectors; containing 0-byte values.

Note
EnCase (at least version 6.18) does not zero out these metadata areas.

3.6.3. BitLocker Windows 10

In later versions of Bitlocker Windows 10 the FVE Volume header block no longer is present. The number of volume header sectors in the FVE metadata block header can be used to determine the volume header size. It is commonly 8192 bytes an size, entailing the first 16 sectors.

4. Volume header

4.1. BitLocker Windows Vista

The BitLocker Windows Vista volume header is similar to NTFS volume header. The differences have been emphasized in bold. The volume header is 512 bytes of size and consists of:

Offset Size Value Description

0

3

"\xeb\x52\x90"

Boot entry point

3

8

"-FVE-FS-"

File system signature

11

2

Bytes per sector

13

1

Sectors per cluster block

14

2

0x00

Reserved Sectors

16

1

0x00

Number of File Allocation Tables (FATs)

17

2

0

Root directory entries

19

2

Total number of sectors (16-bit)

21

1

Media descriptor

22

2

0x00

Sectors Per File Allocation Table (FAT)

24

2

0x3f

Sectors per track

26

2

Number of heads

28

4

Number of hidden sectors

32

4

0x00

Total number of sectors (32-bit)

36

1

0x80

Unknown (Disc unit number)

37

1

0x00

Unknown (Flags)

38

1

0x80

Unknown (BPB version signature byte)

39

1

0x00

Unknown (Reserved)

40

8

Total number of sectors (64-bit)

48

8

Master File Table (MFT) cluster block number

56

8

FVE metadata block 1 cluster block number

64

1

MFT entry size

65

3

Unknown

68

1

Index entry size

69

3

Unknown

72

8

NTFS volume serial number

80

4

0x00

Checksum

84

426

Bootcode

510

2

0x55 0xaa

Sector signature

Note
The number of sectors can be 1 less then the value indicated in the partition table.

4.2. BitLocker Windows 7 and later

The BitLocker Windows 7 (and later) volume header less similar to NTFS volume header than the BitLocker Windows Vista volume header. The differences between the versions have been emphasized in bold. The volume header is 512 bytes of size and consists of:

Offset Size Value Description

0

3

"\xeb\x58\x90"

Boot entry point

3

8

"-FVE-FS-"

File system signature

11

2

Bytes per sector

13

1

Sectors per cluster block

14

2

0x00

Reserved Sectors

16

1

0x00

Number of File Allocation Tables (FATs)

17

2

0

Root directory entries

19

2

Total number of sectors (16-bit)

21

1

Media descriptor

22

2

0x00

Sectors Per File Allocation Table (FAT)

24

2

0x3f

Sectors per track

26

2

Number of heads

28

4

Number of hidden sectors
Contains the volume start sector number

32

4

0x00

Total number of sectors (32-bit)

36

4

0x1fe0

Sectors per file allocation table

40

2

FAT Flags (Only used during a conversion from a FAT12/16 volume.)

42

2

Version (Defined as 0)

44

4

Cluster number of root directory start

48

2

0x0001

Sector number of FS Information Sector

50

2

0x0006

Sector number of a copy of this boot sector (0 if no backup copy exists)

52

12

Reserved

64

1

0x80

Physical Drive Number (see FAT12/16 BPB at offset 0x24)

65

1

Reserved (see FAT12/16 BPB at offset 0x25)

66

1

0x29

Extended boot signature. (see FAT12/16 BPB at offset 0x26)

67

4

Volume serial number

71

11

"NO NAME\x20\x20\x20\x20"

Volume label

82

8

"FAT32\x20\x20\x20"

File system signature

90

70

Bootcode

160

16

BitLocker identifier
contains a GUID

176

8

FVE metadata block 1 offset
Contains an offset relative to the start of the volume

184

8

FVE metadata block 2 offset
Contains an offset relative to the start of the volume

192

8

FVE metadata block 3 offset
Contains an offset relative to the start of the volume

200

307

Unknown (part of bootcode)

507

3

Unknown

510

2

0x55 0xaa

Sector signature

Note
The number of sectors can be 1 less then the value indicated in the partition table.

TODO check highlighted values

4.3. BitLocker To Go

BitLocker To Go on an NTFS volume is similar to BitLocker Windows 7. The BitLocker Windows To Go volume header for a FAT volume is similar to FAT32 volume header. The differences have been emphasized in bold. The volume header is 512 bytes of size and consists of:

Offset Size Value Description

0

3

"\xeb\x58\x90"

Boot entry point

3

8

"MSWIN4.1"

Signature

11

2

Bytes per sector

13

1

Sectors per cluster block

14

2

0x00

Reserved Sectors

16

1

0x00

Number of File Allocation Tables (FATs)

17

2

0

Root directory entries

19

2

Total number of sectors (16-bit)

21

1

Media descriptor

22

2

0x00

Sectors Per File Allocation Table (FAT)

24

2

0x3f

Sectors per track

26

2

Number of heads

28

4

Number of hidden sectors

32

4

Total number of sectors (32-bit)

36

4

0x1f0e

Sectors per file allocation table

40

2

FAT Flags (Only used during a conversion from a FAT12/16 volume.)

42

2

Version (Defined as 0)

44

4

Cluster number of root directory start

48

2

0x0001

Sector number of FS Information Sector

50

2

0x0006

Sector number of a copy of this boot sector (0 if no backup copy exists)

52

12

Reserved

64

1

0x80

Physical Drive Number (see FAT12/16 BPB at offset 0x24)

65

1

Reserved (see FAT12/16 BPB at offset 0x25)

66

1

0x29

Extended boot signature. (see FAT12/16 BPB at offset 0x26)

67

4

Volume serial number

71

11

"NO NAME\x20\x20\x20\x20"

Volume label

82

8

"FAT32\x20\x20\x20"

File system signature

90

334

Bootcode

424

16

BitLocker identifier
contains a GUID

440

8

FVE metadata block 1 offset
Contains an offset relative to the start of the volume

448

8

FVE metadata block 2 offset
Contains an offset relative to the start of the volume

456

8

FVE metadata block 3 offset
Contains an offset relative to the start of the volume

464

46

Unknown

510

2

0x55 0xaa

Sector signature

TODO check highlighted values

5. FVE metadata block

A BitLocker volume contains 3 FVE metadata blocks. Each FVE metadata block consists of:

  • a block header

  • a metadata header

  • an array of metadata entries

  • padding (0-byte values) (seen in Windows 8)

5.1. FVE metadata block header

5.1.1. FVE metadata block header version 1 - Windows Vista

The FVE metadata block header version 1 is 64 bytes of size and consists of:

Offset Size Value Description

0

8

"-FVE-FS-"

Signature

8

2

Size

10

2

1

Version

12

2

Unknown
0x04 commonly

14

2

Unknown
0x04 commonly

16

16

0

Unknown (empty values)

32

8

FVE metadata block 1 offset
Contains an offset relative to the start of the volume

40

8

FVE metadata block 2 offset
Contains an offset relative to the start of the volume

48

8

FVE metadata block 3 offset
Contains an offset relative to the start of the volume

56

8

MFT mirror cluster block number

5.1.2. FVE metadata block header version 2 – Windows 7 and later

The FVE metadata block header version 2 is 64 bytes of size and consists of:

Offset Size Value Description

0

8

"-FVE-FS-"

Signature

8

2

Size

10

2

2

Version

12

2

Unknown
0x04 commonly
0x05 in partial decrypted volume (protection status?)

14

2

Unknown copy
0x04 commonly
0x01 in partial decrypted volume

16

8

Encrypted volume size
Contains the number of bytes

24

4

Unknown

28

4

Number of volume header sectors
Contains the number of sectors

32

8

FVE metadata block 1 offset
Contains an offset relative to the start of the volume

40

8

FVE metadata block 2 offset
Contains an offset relative to the start of the volume

48

8

FVE metadata block 3 offset
Contains an offset relative to the start of the volume

56

8

Volume header offset
Contains an offset relative to the start of the volume

When decrypting BitLocker will decrypt from the back to the front. The encrypted volume size therefore contains the number of bytes of the volume that are still encrypted (or need to be decrypted).

5.2. FVE metadata header (version 1)

The FVE metadata header (version 1) is 48 bytes of size and consists of:

Offset Size Value Description

0

4

Metadata size
Size of the data in the FVE metadata including this size value itself

4

4

1

Version

8

4

48

Metadata header size

12

4

Metadata size copy

16

16

Volume identifier
Contains a GUID

32

4

Next nonce counter

36

4

Encryption method
See section: Encryption methods
It is currently unknown what the upper 16-bit is used for the MSB has been seen to be used or is this value actually 2x 16-bit values.

40

8

Creation time
Contains a FILETIME

5.2.1. Encryption methods

Value Identifier Description

0x0000

Unknown (Not encrypted/External Key)

0x1000

Unknown (Stretch key)

0x1001

Unknown (Stretch key)

0x2000

Unknown (AES-CCM 256 bit encryption)

0x2001

Unknown (AES-CCM 256 bit encryption)

0x2002

Unknown (AES-CCM 256 bit encryption)

0x2003

Unknown (AES-CCM 256 bit encryption)

0x2004

Unknown (AES-CCM 256 bit encryption)

0x2005

Unknown (AES-CCM 256 bit encryption)

0x8000

AES-CBC 128-bit encryption with Elephant Diffuser

0x8001

AES-CBC 256-bit encryption with Elephant Diffuser

0x8002

AES-CBC 128-bit encryption

0x8003

AES-CBC 256-bit encryption

0x8004

AES-XTS 128-bit encryption

0x8005

Unknown (AES-XTS 256-bit encryption)

5.3. FVE metadata entry (version 1)

The FVE metadata entry (version 1) is variable of size and consists of:

Offset Size Value Description

0

2

Entry size
Size of the data in the FVE metadata entry including this size value itself

2

2

Entry type

4

2

Value type

6

2

1

Version

8

…​

Data

5.3.1. FVE metadata entry types

Value Identifier Description

0x0000

None, entry is a property

0x0002

Volume Master Key (VMK)

0x0003

Full Volume Encryption Key (FKEV)

0x0004

Validation

0x0006

Startup key

0x0007

Description (Drive label)
Contains computer name, volume name and date
Is the date format dependent on the locale MM/DD/YYYY?

0x000b

Unknown
Backup of the Full Volume Encryption Key (FKEV)?

0x000f

Volume header block

5.3.2. FVE metadata value types

Value Identifier Description

0x0000

Erased

0x0001

Key

0x0002

Unicode string
UTF-16 little-endian with end of string character

0x0003

Stretch Key

0x0004

Use Key

0x0005

AES-CCM encrypted key

0x0006

TPM encoded key

0x0007

Validation

0x0008

Volume master key

0x0009

External key

0x000a

Update

0x000b

Error

0x000f

Offset and size
Contains a tuple of 2 x 64-bit values

5.4. FVE key

The FVE Stretch encrypted key has value type 0x0001. It is variable in size and consists of:

Offset Size Value Description

0

4

Encryption method
See section: Encryption methods

4

…​

Key data

5.5. FVE Stretch encrypted key

The FVE Stretch encrypted key has value type 0x0003. It is variable in size and consists of:

Offset Size Value Description

0

4

Encryption method
See section: Encryption methods

4

16

Salt

20

…​

FVE metadata entry
Contains an AES-CCM encrypted key

5.6. FVE AES-CCM encrypted key

The FVE AES-CCM encrypted key has value type 0x0005. It is variable in size and consists of:

Offset Size Value Description

0

8

Nonce date and time
Contains a FILETIME

8

4

Nonce counter

12

…​

AES-CCM encrypted data

5.6.1. Unencrypted data

The unencrypted data is variable of size and consist of:

Offset Size Value Description

0

16

Message Authentication Code (MAC)

Key container

16

4

Size
Does not include the size of the MAC

20

2

1

Version

22

2

Unknown

24

4

Encryption method
See section: Encryption methods

28

…​

Unencrypted key data

5.7. FVE TPM encoded key

The FVE TPM encoded key has value type 0x0006. It is variable in size and consists of:

TODO – this structure has not been analyzed yet

5.8. FVE Validation

The FVE Validation has value type 0x0007. It is variable in size and consists of:

TODO – this structure has not been analyzed yet

5.9. FVE Volume Master Key (VMK)

The FVE Volume Master Key has value type 0x0008. It is variable in size and consists of:

Offset Size Value Description

0

16

Key identifier
Contains a GUID

16

8

Last modification date and time
Contains a FILETIME

24

2

Unknown

26

2

Protection type
See section: Key protection types

28

…​

Properties
Contains an array of FVE metadata entries where the entry type is set to 0.

The available properties depend on the VMK type.

The clear key protected VMK consists of:

  • key (with 256-bit of key data)

  • AES-CCM encrypted key

The recovery key protected VMK consists of:

  • optional description string containing "DiskPassword\x00"

  • stretch key

  • AES-CCM encrypted key

The startup key protected VMK consists of:

  • optional description string containing "ExternalKey\x00"

  • stretch key

  • AES-CCM encrypted key

The password protected VMK consists of:

  • optional description string containing "ExternalKey\x00"

  • stretch key

  • AES-CCM encrypted key

5.9.1. Key protection types

Value Identifier Description

0x0000

VMK protected with clear key
(Basically this is an unprotected VMK)

0x0100

VMK protected with TPM

0x0200

VMK protected with startup key

0x0800

VMK protected with recovery password

0x2000

VMK protected with password

Notes

It has been suggested that 0x0500 represents VMK protected with TPM and PIN

Key protector types defined by the GetKeyProtectorType function documenation

0 Unknown or other protector type
1 Trusted Platform Module (TPM)
2 External key
3 Numerical password
4 TPM And PIN
5 TPM And Startup Key
6 TPM And PIN And Startup Key
7 Public Key
8 Passphrase
9 TPM Certificate
10 CryptoAPI Next Generation (CNG) Protector

5.10. FVE External Key

The FVE External Key has value type 0x0009. It is variable in size and consists of:

Offset Size Value Description

0

16

Key identifier
Contains a GUID

16

8

Last modification date and time
Contains a FILETIME

24

…​

Properties
Contains an array of FVE metadata entries where the entry type is set to 0.

The available properties:

  • optional description string containing "ExternalKey\x00"

  • key

5.11. FVE Volume header block

The FVE Volume header block has value type 0x000f. It is 16 or 52 byte in size and consists of:

Offset Size Value Description

0

8

Block offset

8

8

Block size
Added in Windows 8

16

8

Unknown

24

8

Unknown

32

12

Unknown (empty values)

44

4

Unknown (sector size?)

48

4

Unknown (sector size?)

The FVE Volume header block seems to have been introduced in Windows 7. It specifies the location in the encrypted volume where the unencrypted volume header is stored.

The FVE Volume header block is commonly 8192 bytes in size for Windows 7 and 5365760 bytes for a BitLocker To Go.

6. BitLocker External Key (BEK) file

A BitLocker External Key (BEK) file is commonly 156 bytes of size and consists of:

  • a file header

  • an array of metadata entries

6.1. BEK file header (version 1)

The BEK file header is similar to the FVE metadata header (version 1). The BEK file header (version 1) is 48 bytes of size and consists of:

Offset Size Value Description

0

4

Metadata size
Size of the remaining data in the file including this size value itself

4

4

1

Version

8

4

48

Metadata header size

12

4

Metadata size copy

16

16

Volume identifier
Contains a GUID

32

4

Next nonce counter

36

4

Encryption method
See section: Encryption methods

40

8

Creation time
Contains a FILETIME

The key identifier in the file must match the key identifier in the FVE Volume Master Key (VMK).

6.2. BEK metadata entry (version 1)

The format of a BEK metadata entry (version 1) is similar to the format of a FVE metadata entry (version 1).

The metadata in a BEK file consists of an FVE external key, which contains 256-bits of unprotected key data.

The identifier of the VMK should match the identifier in the BEK file header.

Appendix A: References

[FERGUSON06]

Title: AES-CBC + Elephant diffuser - A Disk Encryption Algorithm for Windows Vista

Author(s):

Niels Ferguson

Date:

August 2006

URL:

http://download.microsoft.com/download/0/2/3/0238acaf-d3bf-4a6d-b3d6-0a0be4bbb36e/bitlockercipher200608.pdf

[KUMAR08]

Title: Bitlocker and Windows Vista

Author(s):

Nitan Kumar, Vipin Kumar

Date:

May 19, 2008

URL:

http://www.nvlabs.in/nvbit_bitlocker_white_paper.pdf

[KORNBLUM09]

Title: Implementing BitLocker Drive Encryption for Forensic Analysis

Author(s):

Jesse Kornblum

Date:

2009

URL:

http://jessekornblum.com/publications/di09.pdf

[KORNBLUM10]

Title: BitLocker To Go

Author(s):

Jesse Kornblum

Date:

2010

URL:

http://jessekornblum.com/presentations/dodcc10-1.pdf

[MSDN]

Title: BitLocker Drive Encryption Overview

URL:

http://technet.microsoft.com/en-us/library/cc732774.aspx

Title: Win32_EncryptableVolume class

URL:

https://docs.microsoft.com/en-us/windows/desktop/SecProv/win32-encryptablevolume

Title: GetKeyProtectorType method of the Win32_EncryptableVolume class

URL:

https://docs.microsoft.com/en-us/windows/desktop/SecProv/getkeyprotectortype-win32-encryptablevolume

Appendix B: GNU Free Documentation License

Version 1.3, 3 November 2008 Copyright © 2000, 2001, 2002, 2007, 2008 Free Software Foundation, Inc. http://fsf.org/

Everyone is permitted to copy and distribute verbatim copies of this license document, but changing it is not allowed.

0. PREAMBLE

The purpose of this License is to make a manual, textbook, or other functional and useful document "free" in the sense of freedom: to assure everyone the effective freedom to copy and redistribute it, with or without modifying it, either commercially or noncommercially. Secondarily, this License preserves for the author and publisher a way to get credit for their work, while not being considered responsible for modifications made by others.

This License is a kind of "copyleft", which means that derivative works of the document must themselves be free in the same sense. It complements the GNU General Public License, which is a copyleft license designed for free software.

We have designed this License in order to use it for manuals for free software, because free software needs free documentation: a free program should come with manuals providing the same freedoms that the software does. But this License is not limited to software manuals; it can be used for any textual work, regardless of subject matter or whether it is published as a printed book. We recommend this License principally for works whose purpose is instruction or reference.

1. APPLICABILITY AND DEFINITIONS

This License applies to any manual or other work, in any medium, that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License. Such a notice grants a world-wide, royalty-free license, unlimited in duration, to use that work under the conditions stated herein. The "Document", below, refers to any such manual or work. Any member of the public is a licensee, and is addressed as "you". You accept the license if you copy, modify or distribute the work in a way requiring permission under copyright law.

A "Modified Version" of the Document means any work containing the Document or a portion of it, either copied verbatim, or with modifications and/or translated into another language.

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  11. For any section Entitled "Acknowledgements" or "Dedications", Preserve the Title of the section, and preserve in the section all the substance and tone of each of the contributor acknowledgements and/or dedications given therein.

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11. RELICENSING

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An MMC is "eligible for relicensing" if it is licensed under this License, and if all works that were first published under this License somewhere other than this MMC, and subsequently incorporated in whole or in part into the MMC, (1) had no cover texts or invariant sections, and (2) were thus incorporated prior to November 1, 2008.

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