BitLocker Drive Encryption (BDE) format

The BitLocker Drive Encryption (BDE) format is used by Microsoft Windows to encrypt volumes.

Overview

There are multiple versions of BitLocker Drive Encryption (BDE):

• BitLocker Windows Vista, Windows 7 and 10; used to encrypt volumes on fixed storage media, like harddisks, which typically contain NTFS file systems.
• BitLocker To Go; introduced in Windows 7; used to encrypt removable drives, which typically contain FAT file systems.
• BitLocker Used Disk Space Only encryption; used to encrypt only the used space of volumes, which presumably was introduced in Windows 8.
• BitLocker Encrypt-on-Write (EOW), which presumably was introduced in Windows 10 (1511).

Note that Windows treats NTFS volumes on removable drives are treated as NTFS volumes on fixed storage media.

Characteristics

Identifiers

BitLocker is known to use the following identifiers:

• 4967d63b-2e29-4ad8-8399-f6a339e3d001, for BitLocker and BitLocker To Go
• 92a84d3b-dd80-4d0e-9e4e-b1e3284eaed8, for BitLocker Used Disk Space Only encryption

Metadata files

BitLocker exposes various files in the “\System Volume Information” directory of the unencrypted volume that correspond to the BitLocker metadata areas.

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 that not all tools zero out the metadata areas.

BitLocker Windows Vista

In BitLocker Windows Vista the “\System Volume Information” directory contains the following BitLocker related files:

• “FVE.{e40ad34d-dae9-4bc7-95bd-b16218c10f72}.[123]” maps a metadata block; typically 16384 bytes in size.

BitLocker Windows Windows 7 and later

In BitLocker Windows 7 and later the “\System Volume Information” directory contains the following BitLocker related files:

• “FVE2.{09cf57b8-9e6c-43d4-ae1f-0408882a397d}.[1-6]”, maps an Encrypt-on-Write (EOW) block map area, used by “Used Disk Space Only encryption”.
• “FVE2.{24e6f0ae-6a00-4f73-984b-75ce9942852d}” maps the block that contains the encrypted boot record; typically 8192 bytes in size.
• “FVE2.{93de4bce-e958-48b6-9fac-602d0294e5ef}.[12]”, maps Encrypt-on-Write (EOW) descriptor, used by “Used Disk Space Only encryption”.
• “FVE2.{aff97bac-a69b-45da-aba1-2cfbce434750}.[12]”, introduced in Windows 8, typically 512 bytes in size (possibly 1 sector in size?).
• “FVE2.{c9ca54a3-6983-46b7-8684-a7e5e23499e3}.[1-6]”, maps an Encrypt-on-Write (EOW) relocation log area, used by “Used Disk Space Only encryption”.
• “FVE2.{da392a22-cae0-4f0f-9a30-b8830385d046}”, introduced in Windows 10, typically 65536 bytes in size.
• “FVE2.{e40ad34d-dae9-4bc7-95bd-b16218c10f72}.[1-3]” maps a metadata block; typically 65536 bytes in size.

BitLocker 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 in size.
• “COV 0000. ER” maps the encrypted data.
• “PAD 0000. PD” maps padding.
• “PAD 0000. NG” unknown; typically 0 bytes in size.

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”, …

The “PAD 0000. NG” are presumaly used to fill the root directory with entries so that no new files may be created on the volume.

Keys

To encrypt storage media BitLocker uses different kind of keys.

Volume Master Key (VMK)

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

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

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.

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.

Recovery password

BitLocker provides for a recovery (or numerical) password to unlock the encrypted data. The recovery password is used to determine a recovery password key. Not to be confused with the “recovery key” key protector.

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 password 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 SHA-256 and the count to 0.

Calculate the SHA-256 of the 128-bit key and update the initial SHA-256 value.

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

Loop for 1048576 (0x100000) times:

• calculate the SHA-256 of the structure and update the last SHA-256 value
• increment the count by 1

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

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.

External key

The external key is used by the “startup key” and “recovery key” key protectors. The 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 external keys for a single BitLocker encrypted volume. Each key is identified a by a different key identifier.

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.

TODO: check if the password can be maximal 49 characters in size.

Convert the user password into a UCS-2 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 SHA-256 and the count to 0.

Calculate the SHA-256 of the user password.

Calculate the SHA-256 of the SHA-256 of the user password, and set it as the initial SHA-256 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 SHA-256 of the structure and update the last SHA-256 value
• increment the count by 1

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

Encryption methods

BitLocker uses different kind of encryption methods. To encrypt the sector data it either uses AES-CBC (Cipher Block Chaining) with or without Elephant Elephant Diffuser or AES-XTS (XEX-based tweaked-codebook mode with ciphertext stealing).

To encrypt the key data BitLocker uses AES-CCM (Counter with cipher block chaining message authentication code (CBC-MAC)).

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.

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

AES-CCM

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

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.

Elephant Diffuser

The Elephant Diffuser A and B variants are described in “AES-CBC + Elephant diffuser - A Disk Encryption Algorithm for Windows Vista”.

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 boot record
• the BitLocker metadata

BitLocker Windows Vista

In BitLocker Windows Vista the first sector of the unencrypted boot recored sector is reconstructed by replacing values in the BitLocker boot record, namely

• replacing the “File system signature” with “NTFS\x20\x20\x20\x20”
• replacing the “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.

BitLocker Windows 7 and To Go

Both BitLocker Windows 7 and To Go store an encrypted version of the unencrypted boot record in a specific location. This location is defined in the metadata area descriptors.

The encrypted boot record is commonly 8192 bytes an size, entailing the first 16 sectors.

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

BitLocker Windows 10

In later versions of Bitlocker Windows 10 the metadata area descriptors is not always present.

The number of boot record sectors in the metadata block header can be used to determine the boot record size.

The encrypted boot record is commonly 8192 bytes an size, entailing the first 16 sectors.

Boot record

BitLocker Windows Vista

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

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

BitLocker Windows 7 and later

The BitLocker Windows 7 boot record for a NTFS volume is similar to a FAT32 boot record. The differences have been emphasized in bold. The boot record is 512 bytes of size and consists of:

BitLocker To Go

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

BitLocker Used Disk Space Only encryption

The BitLocker Used Disk Space Only encryption boot record for a NTFS volume is similar to a BitLocker Windows 7 and later boot record. The differences have been emphasized in bold. The boot record is 512 bytes of size and consists of:

Note that Windows 10 version for Bitlocker without “Used Disk Space Only encryption” can also contain Encrypt-on-Write structures within the metadata blocks, these might be remnants of of encrypting the volume.

Metadata block

A BitLocker encrypted volume contains 3 metadata blocks. Which is typically:

• 12384 bytes in size for BitLocker Windows Vista;
• 65536 bytes in size for BitLocker Windows 7 and later.

A metadata block consists of:

• a (metadata) block header
• a metadata header
• an array of metadata entries
• padding (0-byte values)

Metadata block header

Metadata block header version 1 - Windows Vista

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

Metadata block header version 2 – Windows 7 and later

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

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

Metadata header

The metadata header is 48 bytes in size and consists of:

Note that it is currently unknown what the copy of encryption method value is used for.

Encryption methods

Metadata entry

The metadata entry is of variable size and consists of:

Metadata entry types

Note that older versions of BitLocker use a locale dependent date format in the description, such as “MM/DD/YYYY”. Recent versions of BitLocker use “YYYY-MM-DD”.

Metadata value types

Key encrypted key (KEK)

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

Stretch encrypted key

The stretch encrypted key has value type 0x0003 and is variable in size and consists of:

AES-CCM encrypted key

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

Unencrypted (AES-CCM encrypted) data

The unencrypted (AES-CCM encrypted) data is of variable size and consist of:

TPM encoded key

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

TODO: complete section

Validation

The validation has value type 0x0007 and is variable in size and consists of:

TODO: complete section

Volume Master Key (VMK)

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

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 external 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

The recovery password key protected VMK consists of:

• optional description string containing “DiskPassword\x00”
• Stretch Key
• AES-CCM encrypted key

The TPM protected VMK consists of:

• optional description string containing “TPM Protection\x00”
• TPM encoded key

Key protector types

TODO: SID (or Active Directory account or group) protector

External Key

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

The available properties:

• optional description string containing “ExternalKey\x00”
• key

Metadata area descriptors

The metadata area descriptors has value type 0x000f and is 16 or more bytes in size and consists of:

The metadata area descriptors seems to have been introduced in Windows 7. They specify the location, within the encrypted volume, where certain metadata is stored, such as the unencrypted boot record.

The unencrypted boot record is commonly 8192 bytes in size for BitLocker Windows 7 (and later) and 5365760 bytes for BitLocker To Go.

Unknown flags

BitLocker External Key (BEK) file

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

• a file header
• an array of metadata entries

BEK file header

The BEK file header is similar to the metadata header. The BEK file header is 48 bytes in size and consists of:

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

BEK metadata entry

The format of a BEK metadata entry is similar to the format of a metadata entry.

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

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

Encrypt-on-Write (EOW)

Checksums use a CRC-32 with the polynominal 0xedb88320 and initial value 0.

Encrypt-on-Write (FVE-EOW) descriptor

The Encrypt-on-Write (FVE-EOW) descriptor is variable of size and consists of:

Note that Encrypt-on-Write descriptor 1 and 2 are copies and should reference the same block map areas.

Encrypt-on-Write block map area

The Encrypt-on-Write block map area is variable of size and consists of:

• Encrypt-on-Write block map (FVE-EOWBM)
• One or more Encrypt-on-Write block (map) records

Note that “FVE2.{09cf57b8-9e6c-43d4-ae1f-0408882a397d}.[1-6]” masks the Encrypt-on-Write block map area, with a 4096 aligment size.

Encrypt-on-Write block map (FVE-EOWBM)

The Encrypt-on-Write block map (FVE-EOWBM) is variable of size, stored as a (physical) sector, and consists of:

Encrypt-on-Write block (map) record (FVE-EOWBR)

The Encrypt-on-Write block (map) record (FVE-EOWBR) is variable of size, stored as one or more (physical) sectors, and consists of:

Encrypt-on-Write relocation log area

The Encrypt-on-Write Encrypt-on-Write relocation log area is variable of size and consists of:

• Encrypt-on-Write relocation log area (OLRDHEVF2) header
• Encrypt-on-Write relocation log entries
  • Encrypt-on-Write relocation log entry descriptor
  • encrypted sector data or 0-byte values
  • backup of Encrypt-on-Write relocation log entry descriptor
• backup of Encrypt-on-Write relocation log area (OLRDHEVF2) header
• Unknown (empty values)

Note that “FVE2.{c9ca54a3-6983-46b7-8684-a7e5e23499e3}.[1-6]” masks the Encrypt-on-Write relocation log area, with a 4096 aligment size.

Encrypt-on-Write relocation log area (OLRDHEVF2) header

The Encrypt-on-Write relocation log area (OLRDHEVF2) header is variable of size, stored as a (logical) sector, and consists of:

Encrypt-on-Write relocation log entry descriptor

The Encrypt-on-Write relocation log entry descriptor is variable of size, stored as a (logical) sector, and consists of:

Note that an Encrypt-on-Write relocation log entry descriptor can be empty (filled with 0-byte values).

Encrypt-on-Write encrypted sector data

• 16 bytes of encrypted sector data per sector? decryption sector key is the corresponding volume region offset

TODO: determine if 32 is a compression factor, given 16 x 32 = 512

References

• AES-CBC + Elephant diffuser - A Disk Encryption Algorithm for Windows Vista, by N. Ferguson