LibreCrypt: Open-Source disk encryption for Windows

Technical Details: LibreCrypt Critical Data Block (CDB) Layout (CDB Format ID 2)

NOTE: This CDB layout is *_obsolete_; all new volumes should use the latest CDB format.*

OBSOLETE This format is used by older versions of FreeOTFE, but not by LibreCrypt. LibreCrypt supports only (CDB Format ID 4)

A LibreCrypt container critical data block consists of "CDL" bits of data. The following table describes the high-level layout of a LibreCrypt CDB:

Critical data block: Password salt Encrypted block: <TD>Random padding #3 </TD> Check MAC Volume details block: <TD>Volume IV length</TD> CDB format ID Volume flags Encrypted partition image length Master key length Master key Requested drive letter Volume IV Random padding #2 Random padding #1

Color key:

Color	Encryption used
	Red items are not encrypted
	Blue items are encrypted with the user's chosen cypher together with a "critical data key" derived from the user's password, salt, and the user's chosen hash algorithm

Seem intimidating? Read on, and all will become clear... When broken down into its component parts, the CDB structure is reasonably straightforward to understand.

Note: Throughout this document, the following definitions apply:

CDL

  <TD>Critical Data Length (in bits)  This is defined as 4096 bits.</TD>

</TR>
<TR>
  <TD>_MML_</TD>

  <TD>Maximum MAC Length (in bits)  This is defined as 512 bits.</TD>

</TR>

  <TD>Salt length (in bits)  This is the user specified salt length, as specified by the user when the CDB is created</TD>

</TR>
<TR>
  <TD>**cbs**</TD>

  <TD>Cypher Block Size (in bits)  The block size of the cypher used to encrypt the volume</TD>

</TR>
<TR>
  <TD>**cks**</TD>

  <TD>Cypher Key Size (in bits)  The key size of the cypher used to encrypt the volume.  If the cypher accepts variable length keysizes, this is set to a user-specified value up to a maximum of 512.</TD>

</TR>
<TR>
  <TD>**ml**</TD>

  <TD>MAC length (in bits)  This is the length of MAC generated</TD>

</TR>

Variable	Definition
sl

Breakdown: CDB layout

Password salt

Encrypted block

Random padding #1

Item name	Size (in bits)	Description
Password salt	sl (User specified to a max 512)	This data is used together with the user's password to derive the "critical data key". This key is then used to encrypt/decrypt the "Encrypted block".
Encrypted block	If cbs>8 then: ((CDL - sl) div cbs) * cbs If cbs<=8 then: (CDL - sl) This size is referred to as "leb"	This block contains the actual key which is used to encrypt/decrypt the encrypted partition image. See below for further breakdown.
Random padding #1	((CDL- sl) - leb)	Random "padding" data. Required to pad out any remaining, unused, bits in the "critical data block"
_Total size:_	CDL

Breakdown: Encrypted block layout

  <TD>Random padding #3</TD>

Check MAC

Volume details block

As described above, this entire block is encrypted using the user's password, salt, and chosen hash and cypher algorithms.

As this block is encrypted, its length (in bits) must be a multiple of the cypher's blocksize.

<TR>
  <TD>Random padding #3</TD>

  <TD>**MML **- **ml**</TD>

  <TD>Random "padding" data. Required to pad out the check MAC to a predetermined number of bits.</TD>
</TR>

Item name	Size (in bits)	Description
Check MAC	ml _Up to a maximum of **MML **bits _	This is the MAC of the plaintext version of the "Volume details block". If hk is zero or undefined, then this hash will be either truncated to *MML *bits, or right-padded with 0 bits up to a maximum of *MML *bits
Volume details	leb* - *MML **	This stores the details of how to encrypt/decrypt the encrypted partition.
_Total size:_	leb

Breakdown: Volume details block layout

  <TD>Volume IV</TD>

CDB format ID

Volume flags

Encrypted partition image length

Master key length

Master key

Requested drive letter

Volume IV length

Random padding #2

Finally, we reach the details that the critical data block was designed to protect. All of the items within this block have bit order: MSB first.

<TR>
  <TD>Requested drive letter</TD>
  <TD>8</TD>

  <TD>The drive letter the volume should be normally be mounted as.  Set to 0x00 if there is no particular drive letter the volume should be mounted as (i.e. mount using the first available drive letter).</TD>
</TR>

  <TD> This will be set to the length of the Volume IV **in bits**.  If the cypher's blocksize is &gt;= 0, this will be set to the cypher's blocksize.  Otherwise, this will be set to 0.</TD>

</TR>
<TR>
  <TD>Volume IV</TD>
  <TD>If (**cbs** &gt; 0), then:_  cbs        _If (**cbs** &lt;= 0), then  0_        _       </TD>
  <TD>This is set to the random data generated when the volume was created. When each sector of the encrypted partition is encrypted/decrypted, this value will be XORd with any (hashed or unhashed) sector ID before being used as the sector IV.  This guarantees that every sector within the encrypted partition has a non-predictable IVs.</TD>

</TR>

<TR>
  <TD>Random padding #2</TD>
  <TD>****_  _</TD>
  <TD>Random "padding" data. Required to pad out the encrypted block to a multiple of **bs**, and to increase the size of this block to the maximum length that can fit within the "critical data block".</TD>
</TR>

Item name	Size (in bits)	Description
CDB format ID	8	This is a version ID which identifies the layout of the remainder of the volume details block When this layout format is used, this will always be set to 2.
Volume flags	32	Bitmap flagging various items. Bit - Description 0 - Use different IVs for each sector 0 = Use NULL IV for all sectors 1 = Use sector ID (possibly hashed; see bit 3) as IV 1 - Sector ID zero is at the start of the file 0 = Sector ID zero is at the start of the encrypted data 1 = Sector ID zero is at the start of the host volume file 2 - (unused) 3 - Hash sector ID before use (only valid if bit 0 is set) 0 = Use sector ID as sector IV - do not hash before using 1 = Hash sector ID before using as sector IV
Encrypted partition image length	64	This stores the length *of the encrypted partition image *in bytes.
Master key length	32	This will be set to the length of the master key in bits.
Master key	_cks _	This is set to the random data generated when the volume was created; and is the en/decryption key used to encrypt the encrypted partition image
Volume IV length	32
_Total size:_	(leb** - MML)

The design of the critical data layout eliminates the need for the cypher/hash used to be stored anywhere, denying an attacker this information and increasing the amount of work required to attack a volume file.

The "password salt" appears before the "encrypted block", and no indication of the length of salt used is stored anywhere in order to prevent an attacker from even knowing where the "encrypted block" starts within the CDB.

The "Check MAC" is limited to 512 bits. This is limited for practical reasons as some kind of limit is required if the critical data block is to be of a predetermined size. See section on mounting volume files for how multiple matching MACs are handled.

The "Password salt" is (fairly arbitrarily) limited to 512 bits. Again, this is primarily done for practical reasons.

Although at time of writing (March 2005) this limit to the length of salt used should be sufficient, the format of the critical data block (with included layout version ID) does allow future scope for modification in order to allow the critical data block to be extended (e.g. from 4096 to 8192 bits), should this limit be deemed inadequate..

The "Encrypted block" does contain a certain amount of data that may be reasonably guessed by an attacker (e.g. the CDB format ID), however this would be of very limited use to an attacker launching a "known plaintext" attack as the amount of this data is minimal, and as with pretty much any transparant encryption system the encrypted partition image can reasonably expected to contain significantly more known plaintext than the CDB anyway (e.g. the partition's boot sector)

CDB Encryption

The encrypted data block within a CDB is encrypted using:

* A key derived from the user's password * A NULL IV

The key used for this encryption/decryption depends on the CDB format used to create the CDB.

For older (CDB format 1) volumes, the key is derived as follows:

* The user's password is appended to the salt bits * The result is hashed with the user's choice of hash algorithm * If the cypher used has a fixed keysize, this hash value generated is truncated/right padded with NULLs until it is the same length as the cypher's keysize

For newer (CDB format 2) volumes, the key is derived as follows:

* The user's password is passed through "n" iterations (where "n" is user specified) of PKCS#5 PBKDF2 using HMAC as the PRF, which is turn employs the user's choice of hash algorithm. In doing so, the user's password is supplied as the password to PBKDF2, and the salt bits are used as the PBKDF2 salt.

The manner in which the check bytes within a CDB are calculated depends on the CDB format used.

For older (CDB format 1) volumes, the check bytes are calculated by simply hashing the volume details block with the user's choice of hash algorithm.

For newer (CDB format 2) volumes, the check bytes are calculated by passing the volume details block through HMAC with the user's choice of hash algorithm. In doing so, the derived key used to encrypt/decrypt the CDB is used as the HMAC key.