[dm-crypt] Editing patch to LUKS on-disk format specification 1.2

Karl O. Pinc kop at meme.com
Sun Sep 4 20:13:55 CEST 2011


Hi,

Reading over 
http://cryptsetup.googlecode.com/svn-history/r500/wiki/LUKS-standard/
on-disk-format.pdf
LUKS On-Disk Format Specification
Version 1.2
I found a number of typos and, to me, some sentence
structure needing editing.  Attached is a patch of the output to:

ps2ascii on-disk-format.pdf on-disk-format.txt

In case it helps I'm also appending wdiff output.
(Look for the {+foo+} [-bar-] sorts of strings.)

I didn't start reading with an eye to making corrections,
these are just the ones I noticed.

I couldn't find the source document in the svn cryptsetup repo....

While reading I also noted, FWIW, that in Appendex A the constants
LUKS_DIGESTSIZE
LUKS_SALTSIZE
LUKS_NUMKEYS
are undefined.

(My only other comment is that I found the hyphens in the
pseudo code variable names to be quite distracting because
the look like minus signs.)

Regards,

Karl <kop at meme.com>
Free Software:  "You don't pay back, you pay forward."
                 -- Robert A. Heinlein

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LUKS On-Disk Format Specification

Version 1.2

Clemens Fruhwirth <clemens at endorphin.org>

April 11, 2011

Document History

Version Author Date Changes
1.0 clemens 22.01.2005 more clear distinction between raw data

and string data by adding a byte[] data
type for LUKS magic, salt and checksum
data.
1.0.1 clemens 15.01.2006 corrected the hash-spec length in Figure 1

from 64 to 32 bytes as implied by offset
calculation and all other assumptions in
this document.
1.1 clemens 18.02.2006 Added precise AFsplit specification. Removed lrw-plain mode spec as the LRW
standardization process is not about to
be finished any time soon; will be reintroduced when a normative documentation
is released by SISWG. Extended introduction text. Thanks to Sarah Dean for providing valuable feedback with respect to
the AFsplit specification.
1.1.1 clemens 08.12.2008 Clarify IV reference point for decrypt/encrypt. Thanks to Michael Gorven for this
suggestion.
1.2 mbroz 11.04.2011 Fix hash block size/digest size AF comment. Clarify master key digest iteration
count. Add XTS mode reference. Some
minor typo fixes. Add reference to NIST
SP 800-132.

1

1. OVERVIEW 2
Introduction
LUKS is short for "Linux Unified Key Setup". It has initially been developed
to remedy the unpleasantness a user experienced that arise from deriving the
encryption setup from changing user space, and forgotten command line arguments. The result of this changes are an unaccessible encryption storage. The
reason for this to happen was, a unstandardised way to read, process and set up
encryption keys, and if the user was unlucky, he upgraded to an incompatible
version of user space tools that needed a good deal of knowledge to use with
old encryption volumes, see [Fru03].

LUKS has been invented to standardise key setup. But the project became
bigger as anticipated, because standards creation involves decision making,
which in turn demands for a justification of these decision. An overspring of this
effort can be found as TKS1 [Fru04], a design model for secure key processing
from entropy-weak sources1. LUKS is also treaded extensivly in Chapters 5 and
6 in "New Methods in Hard Disk Encryption", which provides a theoretic base
for the security of PBKDF2 passwords and anti-forensic information splitting.
See [Fru05b].

LUKS is the proof-of-concept implementation for TKS1. In LUKS 1.0, the
implementation switched to TKS2, a varient of TKS1, introduced in [Fru05b].
Additionally to the security provided by the TKS1 model, LUKS gives the user
the ability to associate more than one password with an encrypted partition.
Any of these passwords can be changed or revoked in a secure manner.

This document specifies the structure, syntax and semantic of the partition
header and the key material. The LUKS design can be used with any cipher
or cipher mode, but for compatibility reasons, LUKS standarises cipher names
and cipher modes.

While the reference implementation is using dm-crypt, Linux' kernel facility
for bulk data encryption, it is not tied to it in any particular way.

Next to the reference implementation which works on Linux, there is a Windows implementation named FreeOTFE provided by Sarah Dean, see
http://www.freeotfe.org.

1 Overview
A rough overall disk layout follows:

LUKS phdr KM1 KM2 . . . KM8 bulk data

A LUKS partition starts with the LUKS partition header (phdr) and is
followed by key material (labelled KM1, KM2 . . . KM8 in figure). After the
key material, the bulk data is located, which is encrypted by the master key.
The phdr contains information about the used cipher, cipher mode, the key
length, a uuid and a master key checksum.

Also, the phdr contains information about the key slots. Every key slot
is associated with a key material section after the phdr. When a key slot
is active, the key slot stores an encrypted copy of the master key in its key
material section. This encrypted copy is locked by a user password. Supplying

1such as a user password

2. PREREQUISITES 3
this user password unlocks the decryption for the key material, which stores
the master key. The master key in turn unlocks the bulk data. For a key slot,
all parameters how to decrypt its key material with a given user password are
stored in the phdr (f.e. salt, iteration depth).

A partition can have as many user passwords as there are key slots. To
access a partition, the user has to supply only one of these passwords. If a
password is changed, the old copy of the master key encrypted by the old
password must be destroyed. Peter Gutmann has shown in [Gut96], how data
destruction shall be done to maximise the chance, that no traces are left on
the disk. Usually the master key comprises only 16 or 32 bytes. This small
amount of data can easily be remapped as a whole to a reserved area. This
action is taken by modern hard disk firmware, when a sector is likely to become
unreadable due to mechanical wear. The original sectors become unaccessible
and any traces of key data can't be purged if necessary.

To counter this problem, LUKS uses the anti-forensic information splitter
to artificially inflate the volume of the key, as with a bigger data set the probability that the whole data set is remapped drops exponentially. The inflated
encrypted master key is stored in the key material section. These sections are
labelled as "KMx" in the figure above.

2 Prerequisites
2.1 Block encryption system
Instead of using cipher implementations like AES or Twofish internally, LUKS
reuses the block encryption facility used for the bulk data. The following syntax
is used in the pseudocode:

enc-da t a = e n c r y p t ( c i p h e r -name , c i p h e r -mode , key , o r i g i n a l ,

o r i g i n a l -l e n g t h )
o r i g i n a l = d e c r y p t ( c i p h e r -name , c i p h e r -mode , key , enc-data ,

o r i g i n a l -l e n g t h )

If the encryption primitive requires a certain block size, incomplete blocks
are padded with zero. The zeros are stripped upon decryptions.2

2.2 Cryptographic hash
A cryptographic hash is necessary for the following two prerequisites. In
PBKDF2 a pseudo-random function is needed, and for AFsplitting a diffusion
function is needed. The pseudo-random function needs to be parameterisable,
therefore the hash function is used in a HMAC setup [BCK97].

The following syntaxes may omit the hash-spec parameter, because the
following pseudo code does not need a great variation of this parameter. The
parameter can be obtained from the partition header and will not change, once
initialised.

2These primitives are also used for key material en/decryption. The key material is
always aligned to sector boundaries. If the block size of the underlaying encryption primitive
is larger than one sector, the pseudocode of section 4.1 has to be changed respectively.

2. PREREQUISITES 4
2.3 PBKDF2
LUKS needs to process password from entropy-weak sources like keyboard
input. PKCS #5's password based key derive function 2 (PBKDF2) has been
defined for the purpose to enhance the security properties of entropy-weak
password, see [Kal97]. Therefore, LUKS depends on a working implementation
of PBKDF2. LUKS uses SHA1 per default as the pseudorandom function
(PRF) but any other hash function can be put in place by setting the hashspec field. In the pseudo code, the following syntax is used:

r e s u l t = PBKDF2( password ,

s a l t ,

i t e r a t i o n -count ,
d e r i v e d -key-l e n g t h )

Notice that the result of this function depends on the current setting of hashspec but the parameter has been omitted. Think of hash-spec as sort of an
environment variable.

2.4 AF-Splitter
LUKS uses anti-forensic information splitting as specified in [Fru05b]. The
underlaying diffusion function shall be SHA1 for the reference implementation,
but can be changed exactly as described in the remarks above. A C reference
implementation using SHA1 is available from [Fru05a].

s p l i {+t+} -m a t e r i a l = A F s p l i t ( u n s p l i t -m a t e r i a l , l e n g t h , s t r i p e s )
u n s p l i t -m a t e r i a l = AFmerge ( s p l i t -m a t e r i a l , l e n g t h , s t r i p e s )

Notice that the result of AFsplit, split-material, is stripes-times as large as
the original, that is length * stripes bytes. Notice that the length parameter is
the length of the original content and not the length of the split-material array.

When D is the unsplit material, H is a diffusion function, and n is the stripe
number, AFsplit returns s1; s2 : : : sn where s1 : : : sn-1 are randomly chosen
while sn is computed according to:

d0 = 0 (1)
dk = H(dk-1 \Phi  sk) (2)
sn = dn-1 \Phi  D (3)

To reverse the process, AFmerge computes dn-1 and recovers D from:

D = dn-1 \Phi  sn (4)

2.4.1 H1
H1 is a hash function with an underlaying hash function P .3 H1 can operate
on a variable amount of data, hence it is constructed for hash extension. The
underlaying hash function is SHA1, we use it solely in LUKS. We use |P | to
denote the digest size of P , for SHA1 it is 160 bit.

3H1's function definition stems from an implementation error that I'm responsible for.
Do not try to analyse it, the structure given here is specified according to this implementation
error and hence is a mistake itself. H2 is the correct hash extension as originally envisioned.

3. THE PARTITION HEADER 5

The input to H1(d), namely d, is partitioned into individual data [-junks.-] {+hunks.+}
The partitioning [-repeataly-] {+repeatedly+} takes a data vector with the size |P | as di with the
[-finally-]
{+final+} block (possibly shorter than |P |) dn. The transformation happens as
follows:

pi = P (i || di) (5)
The end of the last block pn is cropped, so that its length is |dn|. The integer
i has to be delivered to the hash as an unsigned 32-bit integer in big-endien
format.

2.4.2 H2
All remarks for H1 apply, except

pi = P (i || d) (6)
Notice the missing subscript of d in contrast to (5). This version will be used
in future LUKS revisions.4

3 The partition header
3.1 Version 1
The LUKS partition header has the layout as described in Figure 1. It starts
at sector 0 of the partition. LUKS uses 3 primitive data types in its header,

* unsigned integer, 16 bit, stored in big endian

* unsigned integer, 32 bit, stored in big endian

* char[], a string stored as null terminated sequence of 8-bit characters5

* byte[], a sequence of bytes, treated as binary.
Further, there is an aggregated data type key slot, which elements are described
in Figure 2.

A reference definition as C struct for phdr is available in the appendix.

3.2 Forward compatibility
LUKS' forward compatibility centers around the on-disk format. Future versions are required to be able to correctly interpret older phdr versions. Future
versions are not required to be able to generate old versions of the phdr.

4The transition has not happend yet. It is likely that the transition will occour in
conjunction with a version nummer bump to Version 2. Do not use H2 until then.

5also known as C string

3. THE PARTITION HEADER 6

start offset field name length data type description

0 magic 6 byte[] magic for LUKS partition header, see
LUKS MAGIC
6 version 2 uint16 t LUKS version
8 cipher-name 32 char[] cipher name specification
40 cipher-mode 32 char[] cipher mode specification
72 hash-spec 32 char[] hash specification
104 payload-offset 4 uint32 t start offset of the bulk

data (in sectors)
108 key-bytes 4 uint32 t number of key bytes
112 mk-digest 20 byte[] master key checksum

from PBKDF2
132 mk-digest-salt 32 byte[] salt parameter for master key PBKDF2
164 mk-digest-iter 4 uint32 t iterations parameter for master key
PBKDF2
168 uuid 40 char[] UUID of the partition
208 key-slot-1 48 key slot key slot 1
256 key-slot-2 48 key slot key slot 2

. . . . . . . . . . . . . . .
544 key-slot-8 48 key slot key slot 8
592 total phdr size

Figure 1: PHDR layout

offset field name length data type description

0 active 4 unit32 t state of keyslot, enabled/disabled
4 iterations 4 uint32 t iteration parameter for

PBKDF2
8 salt 32 byte[] salt parameter for

PBKDF2
40 key-material-offset 4 uint32 t start sector of key material
44 stripes 4 uint32 t number of anti-forensic

stripes

Figure 2: key slot layout

4. LUKS OPERATIONS 7

A LUKS implementation encountering a newer phdr version should not try
to interpret it, and return an error. Of course, an error should be returned, if
the phdr's magic is not present.

4 LUKS operations
4.1 Initialisation
The initialisation process takes a couple of parameters. First and most important, the master key. This key is used for the bulk data. This key must be
created from an entropy strong (random) source, as the overcoming of entropy
weak keys is one of LUKS' main objectives. For the following remarks, the
pseudo code is available as Figure 3.

Further, the user specifices the cipher setup details that are stored in the
cipher-name and cipher-mode fields. Although no LUKS operation manipulates these two strings, it is likely that the LUKS implementation will have
to convert it into something suitable for the underlaying cipher system, as the
interface is not likely to be as ideal as described in Section 2.1.

The overall disk layout depends on the length of the key material sections
following the phdr. While the phdr is always constant in size, the key material
section size depends on the length of the master key and the number of stripes
used by the anti-forensic information splitter. The exact disk layout is generated by computing the size for the phdr and a key material section in sectors
rounded up. Then the disk is filled sector-wise by phdr first, and following
key material section 1 till key material section 8. After the eight key material
section, the bulk data starts.

After determining the exact key layout and boundaries between phdr, key
material and bulk data, the key material locations are written into the key
slot entries in the phdr. The information about the bulk data start is written
into the payload-offset field of the phdr. These values will not change during
the lifetime of a LUKS partition and are simply cached for safety reasons as a
miscalculation of these values can cause data corruption [-(f.i.-] {+(e.g.+} an incorrect start
of the bulk data can overwrite key material, same is true in reverse).

The master key is checksummed, so a correct master key can be detected.
To future-proof the checksumming, a hash is not only applied once but multiple
times. In fact, the PBKDF2 primitive is reused. The master key is feed into
the PBKDF2 process as if it were a user password. After the iterative hashing,
the random chosen salt, the iteration count6 and result are stored in the phdr.

Although everything is correctly initialised up to this point, the initialisation process should not stop here. Without an active key slot the partition is
useless. At least one key slot should be activated from the master key still in
memory.

6Master key iteration count was set to 10 in previous revisions of LUKS. For new devices
the iteration count should be determined by benchmarking with suggested minimum of 1000
iterations.

4. LUKS OPERATIONS 8
masterKeyLength = d e f i n e d by u s e r
masterKey = g e n e r a t e random v e c t o r , l e n g t h : masterKeyLeng th

phdr . magic = LUKS MAGIC
phdr . v e r s i o n = 1
phdr . c i p h e r -name = as s u p p l i e d by u s e r
phdr . c i p h e r -mode = as s u p p l i e d by u s e r
phdr . key-b y t e s = masterKey
phdr . mk-d i g e s t -s a l t = g e n e r a t e random v e c t o r ,

l e n g t h : LUKS SALTSIZE

// benchmarked a c c o r d i n g t o u s e r i n p u t
// ( i n o l d e r v e r s i o n s f i x e d t o 10)

phdr . mk-d i g e s t -i t e r a t i o n -co unt = as above

phdr . mk-d i g e s t = PBKDF2( masterKey ,

phdr . mk-d i g e s t -s a l t ,
phdr . mk-d i g e s t -i t e r a t i o n -count ,
LUKS DIGESTSIZE )
s t r i p e s = LUKS STRIPES o r u s e r d e f i n e d

// i n t e g e r d i v i s i o n s , r e s u l t rounded down :

b a s e O f f s e t = ( s i z e o f phdr )/ SECTOR SIZE + 1
k e y M a t e r i a l S e c t o r s = ( s t r i p e s * masterKeyLength )/ SECTOR SIZE + 1

f o r each k e y s l o t i n phdr a s ks {

ks . a c t i v e = LUKS KEY DISABLED
ks . s t r i p e s = s t r i p e s
ks . key-m a t e r i a l -o f f s e t = b a s e O f f s e t

b a s e O f f s e t = b a s e O f f s e t + k e y M a t e r i a l S e c t o r s
}

phdr . pa y loa d -o f f s e t = b a s e O f f s e t
phdr . uu i d = g e n e r a t e u u i d

w r i t e phdr t o d i s k

Figure 3: Pseudo code for partition initialisation

4. LUKS OPERATIONS 9
4.2 Adding new passwords
To add a password to a LUKS [-partition,-] {+partition+} one has to possess an
unencrypted copy of the master [-key. Either this is, because the initialisation process is-] {+key; either initialization must+} still
{+be+} in [-progress,-] {+progress+} or the [-user has supplied-] {+master key must be recovered using+} a [-correct-] {+valid+}
password [-for-] {+to+} an existing key [-slot,
which master key could therefore be recovered. This-] {+slot.  The latter+} operation is sketched in
Figure 4.

Assuming we have a good copy of the master key in [-memory,-] {+memory+} the next [-step
is-] {+steps
are+} to fetch a salt from a random [-source,-] {+source+} and [-the choice of-] {+to choose+} a password iteration
count7. This information is written into a free - that is disabled - key slot of
the phdr.

The user password is entered and processed by PBKDF2. The master key is
then split by the AFsplitter into a number of stripes. The number of stripes is
determined by the stripes field already stored in the key slot. The split result is
written into the key material section, but encrypted. The encryption uses the
same cipher setup as the bulk data (cipher type, cipher mode, ...), but while
for the bulk data the master key is used, the key material section is keyed by
the result of the PBKDF2.

4.3 Master key recovery
To access the payload bulk data, the master key has to be recovered. Compare
the pseudo code in Figure 5.

First, the user supplies a password. Then the password is processed by
PBKDF2 for every active key slot individually and an attempt is made to
recover the master key. The recovery is successful, when a master key candidate
correctly checksums against the master key checksum stored in the phdr. Before
this can happen, the master key candidate is read from storage, decrypted and
after decryption processed by the anti-forensic information splitter in reverse
gear, that is AFmerge.

When the checksumming of the master key succeeds for one key slot, the
correct user key was given and the partition is successfully opened.

4.4 Password revocation
The key material section is wiped according to Peter Gutmann's data erasure
principals [Gut96]. To wipe the sectors containing the key material, start from
the sector as recorded in key slot's key-material-offset field, and proceed for
phdr.key-bytes * ks.stripes bytes.

7The iteration count should be determined by benchmarking with suggested minimum
of 1000 iterations.

4. LUKS OPERATIONS 10
masterKey = must be a v a i l a b l e , e i t h e r be c a u s e i t i s s t i l l i n

memory from i n i t i a l i s a t i o n o r b e c a u s e i t has been

r e c o v e r e d by a c o r r e c t pas swo rd
masterKeyLength = phdr . key-b y t e s

e m p t y K e y S l o t I nd e x = f i n d i n a c t i v e key s l o t i n d e x i n phdr by

s c a n n i n g t he k e y s l o t . a c t i v e f i e l d f o r
LUKS KEY DISABLED .

k e y s l o t ks = phdr . k e y s l o t s [ e m p t y K e y S l o t In de x ]
PBKDF2-I t e r a t i o n s P e r S e c o n d = benchmark s ys tem

ks . i t e r a t i o n -count = PBKDF2-I t e r a t i o n s P e r S e c o n d *

i n t e [-nt-] {+n d+} e d P a s s w o r d C h e c k i n g T i m e ( i n s e c o n d s )

ks . s a l t = g e n e r a t e random v e c t o r , l e n g t h : LUKS SALTSIZE

s p l i t K e y = A F s p l i t ( masterKey , // s o u r c e

masterKeyLength , // s o u r c e l e n g t h

ks . s t r i p e s ) // number o f s t r i p e s

s p l i t K e y L e n g t h = ma sterKeyLength * ks . s t r i p e s
pwd = r e a d pas sw or d from u s e r i n p u t
pwd-PBKDF2ed = PBKDF2( password ,

ks . s a l t ,
ks . i t e r a t i o n -count
masterKeyLength ) // key s i z e i s t h e same

// a s f o r t h e b u l k dat a

e nc r y p t e dK ey = e n c r y p t ( phdr . c i p h e r -name , // c i p h e r name

phdr . c i p h e r -mode , // c i p h e r mode
pwd-PBKDF2ed , // key

s p l i t K e y , // c o n t e n t
s p l i t K e y L e n g t h ) // c o n t e n t l e n g t h

w r i t e t o p a r t i t i o n ( e nc ry pte dK ey , // s o u r c e

ks . key-m a t e r i a l -o f f s e t , // s e c t o r number

s p l i t K e y L e n g t h ) // l e n g t h i n b y t e s

ks . a c t i v e = LUKS KEY ACTIVE // mark key as a c t i v e i n phdr
updat e k e y s l o t ks i n phdr

Figure 4: Pseudo code for key creation

4. LUKS OPERATIONS 11
r e a d phdr from d i s k
check f o r c o r r e c t LUKS MAGIC and c o m p a t i b l e v e r s i o n number

masterKeyLength = phdr . key-b y t e s
pwd = r e a d pas sw or d from u s e r i n p u t

f o r each a c t i v e k e y s l o t i n phdr do a s k s {

pwd-PBKDF2ed = PBKDF2( pwd , ks . s a l t , ks . i t e r a t i o n -count

masterKeyLength )
r e a d from p a r t i t i o n ( encr y ptedK ey , // d e s t i n a t i o n

ks . key-m a t e r i a l -o f f s e t , // s e c t o r number
masterKeyLength * ks . s t r i p e s ) // number o f b y t e s

s p l i t K e y = d e c r y p t ( phdr . c i p h e r S p e c , // c i p h e r s p e c .

pwd-PBKDF2ed , // key

encr y pte dK ey , // c o n t e n t

e n c r y p t e d ) // c o n t e n t l e n g t h

m a s t e r K ey Ca nd id a t e = AFmerge ( s p l i t K e y , ma s ter k ey Leng th ,

ks . s t r i p e s )

MKCandidate-PBKDF2ed = PBKDF2( m as ter Key Candidate ,

phdr . mk-d i g e s t -s a l t ,
phdr . mk-d i g e s t -i t e r ,
LUKS DIGEST SIZE )
i f e q u a l ( MKCandidate-PBKDF2ed , phdr . mk-d i g e s t ) {

b r e a k l o o p and r e t u r n m a s t e r K ey Ca nd id a t e a s

c o r r e c t m a st er key
}
}

r e t u r n e r r o r , pas swo rd do es not match any k e y s l o t

Figure 5: Pseudo code for master key recovery

5. CONSTANTS 12
4.5 Password changing
The password changing is a synthetic operating of "master key recovery", "new
password adding", and "old password revocation".

5 Constants
All strings and characters are to be encoded in ASCII.

Symbol Value Description

LUKS MAGIC {'L','U','K','S',

0xBA,0xBE}

partition header starts
with magic
LUKS DIGESTSIZE 20 length of master key

checksum
LUKS SALTSIZE 32 length of the PBKDF2

salts
LUKS NUMKEYS 8 number of key slots
LUKS KEY DISABLED 0x0000DEAD magic for disabled

key slot in keyblock[i].active
LUKS KEY ENABLED 0x00AC71F3 magic for enabled

key slot in keyblock[i].active
LUKS STRIPES 4000 number of stripes for

AFsplit. See [Fru05b]
for rationale.

BIBLIOGRAPHY 13
Bibliography
[BCK97] Mihir Bellare, Ran Canetti, and Hugo Krawczyk. The HMAC

papers. http://www.cs.ucsd.edu/users/mihir/papers/hmac.html,
1996-1997.

[Fru03] Clemens Fruhwirth. Cryptoloop 2.4.22

to cryptoloop 2.5.x migration guide.
http://clemens.endorphin.org/Cryptoloop_Migration_Guide, 2003.

[Fru04] Clemens Fruhwirth. TKS1 - An anti-forensic, two level, and iterated

key setup scheme. http://clemens.endorphin.org/publications,
2004.

[Fru05a] Clemens Fruhwirth. Fruhwirth's Cryptography Website.

http://clemens.endorphin.org/cryptography, 2005.

[Fru05b] Clemens Fruhwirth. New methods in hard disk encryption.

http://clemens.endorphin.org/publications, 2005.

[Gut96] Peter Gutmann. Secure Deletion of Data

from Magnetic and Solid-State Memory.
http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html,
1996.

[Kal97] Burt Kaliski. RFC 2898; PKCS #5: Password-Based Cryptography

Specification Version 2.0. http://www.faqs.org/rfcs/rfc2898.html,
1996-1997.

[TBBC10] Meltem Snmez Turan, Elaine Barker, William Burr, and

Lily Chen. Recommendation for password-based key derivation, part 1: Storage applications. NIST SP 800-132,
http://csrc.nist.gov/publications/nistpubs/800-132/nist-sp800-132.pdf,
2010.

APPENDIX A. PHDR AS C STRUCT 14
A PHDR as C struct
#d e f i n e LUKS MAGIC L 6
#d e f i n e LUKS CIPHERNAME L 32
#d e f i n e LUKS CIPHERMODE L 32
#d e f i n e LUKS HASHSPEC L 32
#d e f i n e UUID STRING L 40

s t r u c t l u k s p h d r {

c h a r magic [ LUKS MAGIC L ] ;

u i n t 1 6 t v e r s i o n ;
c h a r cipherName [ LUKS CIPHERNAME L ] ;
c h a r cipherMode [ LUKS CIPHERMODE L ] ;
c h a r hashSpec [ LUKS HASHSPEC L ] ;

u i n t 3 2 t p a y l o a d O f f s e t ;
u i n t 3 2 t k e y B y t e s ;
c h a r mkDigest [ LUKS DIGESTSIZE ] ;
c h a r m k D i g e s t S a l t [ LUKS SALTSIZE ] ;

u i n t 3 2 t m k D i g e s t I t e r a t i o n s ;
c h a r u u i d [ UUID STRING L ] ;

s t r u c t {

u i n t 3 2 t a c t i v e ;

/* parameters f o r PBKDF2 p r o c e s s i n g */

u i n t 3 2 t p a s s w o r d I t e r a t i o n s ;
c h a r p a s s w o r d S a l t [ LUKS SALTSIZE ] ;

/* parameters f o r AF s t o r e / l o a d */

u i n t 3 2 t k e y M a t e r i a l O f f s e t ;
u i n t 3 2 t s t r i p e s ;
} k e y b l o c k [ LUKS NUMKEYS ] ;
} ;

B Cipher and Hash specification registry
Even if the cipher-name and cipher-mode strings are not interpreted by any
LUKS operation, they must have the same meaning for all implementations
to achieve compatibility among different LUKS-based implementations. LUKS
has to ensure that the underlaying cipher system can utilise the cipher name
and cipher mode strings, and as these strings might not always be native to the
cipher system, LUKS might need to map them into something appropriate.

Valid cipher names are listed in Table 1.
Valid cipher modes are listed in Table 2. By contract, cipher modes using
IVs and tweaks must start from the all-zero IV/tweak. This applies for all
calls to the encrypt/decrypt primitives especially when handling key material.
Further, these IVs/tweaks cipher modes usually cut the cipher stream into
independent blocks by reseeding tweaks/IVs at sector boundaries. The all-zero

APPENDIX B. CIPHER AND HASH SPECIFICATION REGISTRY 15

cipher name normative document
aes Advanced Encryption Standard - FIPS PUB 197
twofish Twofish: A 128-Bit Block Cipher http://www.schneier.com/paper-twofish-paper.html
serpent http://www.cl.cam.ac.uk/~rja14/serpent.html
cast5 RFC 2144
cast6 RFC 2612

Table 1: Valid cipher names

mode description
ecb The cipher output is used directly.
cbc-plain The cipher is operated in CBC mode. The CBC chaining

is cut every sector, and reinitialised with the sector number
as initial vector (converted to 32-bit and to little-endian).
This mode is specified in [Fru05b], Chapter 4.
cbc-essiv:hash The cipher is operated in ESSIV mode using hash for

generating the IV key for the original key. For instance,
when using sha256 as hash, the cipher mode spec is "cbcessiv:sha256". ESSIV is specified in [Fru05b], Chapter 4.
xts-plain64 http://grouper.ieee.org/groups/1619/email/pdf00086.pdf,

plain64 is 64-bit version of plain initial vector

Table 2: Valid cipher modes
hash-spec string normative document
sha1 RFC 3174 - US Secure Hash Algorithm 1 (SHA1)
sha256 SHA variant according to FIPS 180-2
sha512 SHA variant according to FIPS 180-2
ripemd160 http://www.esat.kuleuven.ac.be/~bosselae/ripemd160.html

Table 3: Valid hash specifications

IV/tweak requirement for the first encrypted/decrypted block is equivalent to
the requirement that the first block is defined to rest at sector 0.

Table 3 lists valid hash specs for hash-spec field. A compliant implementation does not have to support all cipher, cipher mode or hash specifications.


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