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 publicly available work on the format and was enhanced by analyzing test data.

This document is intended as a working document of the data format specification for the libbde project.

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) 2011-2022, 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
0.0.12	J.B. Metz	February 2019	Additional information regarding BitLocker Used Disk Space Only encryption format with thanks to M. Bulhakov.
0.0.13	J.B. Metz	December 2020	Additional information BEK file format.
0.0.14	J.B. Metz	June 2021	Additional information about unencrypted volume.
0.0.15	J.B. Metz	January 2022	Changes to formatting.
0.0.16	J.B. Metz	February 2022	Additional information about metadata version 3.

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-f6a339e3d001.

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

BitLocker Used Disk Space Only encryption uses 92a84d3b-dd80-4d0e-9e4e-b1e3284eaed8.

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 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.

2.3. 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 FVEK 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
DOS version 2.0 BIOS parameter block (BPB)
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)
DOS version 3.4 BIOS parameter block (BPB)
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)
NTFS version 8.0 BIOS parameter block (BPB) or extended BPB
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
DOS version 2.0 BIOS parameter block (BPB)
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)
DOS version 3.4 BIOS parameter block (BPB)
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)
Unknown
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		Unknown (Reserved)
64	1	0x80	Physical Drive Number (see FAT12/16 BPB at offset 0x24)
65	1		Unknown (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
DOS version 2.0 BIOS parameter block (BPB)
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)
DOS version 3.4 BIOS parameter block (BPB)
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)
Unknown
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		Unknown (Reserved)
64	1	0x80	Physical Drive Number (see FAT12/16 BPB at offset 0x24)
65	1		Unknown (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		Unknown (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		Unknown (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

The FVE metadata entry is variable of size and consists of:

Offset	Size	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	Version Seen: 1 and 3
8	…	Data

Note: version is typically 1 but 3 has been seen for VMK FVE metadata entry in combination with clear key.

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 (FVEK)
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 (FVEK)?

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	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	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	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

0x0500		VMK protected with TPM and PIN

0x0800		VMK protected with recovery password

0x2000		VMK protected with password

Notes

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	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 more bytes in size and consists of:

Offset	Size	Description
0	8	Block offset
8	8	Block size
Unknown additional data
16	2	Unknown (number of entries?)
18	2	Unknown (size of additional data?)
28	…	Unknown (array of 14 byte sized entries)
…	2	Unknown (empty values)

Offset	Size	Description
0	8	Unknown (seen combination of block offset + block size)
8	4	Unknown
12	2	Unknown

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.

7. Notes

Seen on an unencrypted BDE volume (password was set shorted then 8 chars to bdetest).

7.1. FVE-EOW block

The FVE-EOW block has …

Offset	Size	Value	Description
0	8	"FVE-EOW\x00"	Signature

7.2. FVE-EOWBM block

The FVE-EOWBM block has …

Offset	Size	Value	Description
0	10	"FVE-EOWBM\x00"	Signature
10	2		Block size Size of the block including the signature and size value
12	4		Unknown

7.3. FVE-EOWBR block

The FVE-EOWBR block has …

Offset	Size	Value	Description
0	10	"FVE-EOWBR\x00"	Signature
10	2		Block size Size of the block including the signature and size value
12	4		Unknown

7.4. OLRDHEVF2 block

The OLRDHEVF2 block has …

Offset	Size	Value	Description
0	10	"OLRDHEVF2\x00"	Signature
10	2		Unknown

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

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