310 lines
13 KiB
Groff
310 lines
13 KiB
Groff
.\"
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.\" Copyright (c) 2002 Poul-Henning Kamp
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.\" Copyright (c) 2002 Networks Associates Technology, Inc.
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.\" All rights reserved.
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.\"
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.\" This software was developed for the FreeBSD Project by Poul-Henning Kamp
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.\" and NAI Labs, the Security Research Division of Network Associates, Inc.
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.\" under DARPA/SPAWAR contract N66001-01-C-8035 ("CBOSS"), as part of the
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.\" DARPA CHATS research program.
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.\"
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.\" Redistribution and use in source and binary forms, with or without
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.\" modification, are permitted provided that the following conditions
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.\" are met:
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.\" 1. Redistributions of source code must retain the above copyright
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.\" notice, this list of conditions and the following disclaimer.
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.\" 2. Redistributions in binary form must reproduce the above copyright
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.\" notice, this list of conditions and the following disclaimer in the
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.\" documentation and/or other materials provided with the distribution.
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.\"
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.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR AND CONTRIBUTORS ``AS IS'' AND
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.\" ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE
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.\" IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE
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.\" ARE DISCLAIMED. IN NO EVENT SHALL THE AUTHOR OR CONTRIBUTORS BE LIABLE
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.\" FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL
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.\" DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS
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.\" OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION)
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.\" HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
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.\" LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY
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.\" OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF
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.\" SUCH DAMAGE.
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.\"
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.\" $FreeBSD$
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.\"
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.Dd October 19, 2002
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.Os
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.Dt GBDE 4
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.Sh NAME
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.Nm gbde
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.Nd Geom Based Disk Encryption
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.Sh SYNOPSIS
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.Cd "options GEOM_BDE"
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.Sh DESCRIPTION
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.Bf -symbolic
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NOTICE:
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Please be aware that this code has not yet received much review
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and analysis by qualified cryptographers and therefore should be considered
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a slightly suspect experimental facility.
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.Pp
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We cannot at this point guarantee that the on-disk format will not change
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in response to reviews or bug-fixes, so potential users are advised to
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be prepared that
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.Xr dump 8 Ns / Ns Xr restore 8
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based migrations may be called for in the future.
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.Ef
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.Pp
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The objective of this facility is to provide a high degree of
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denial of access to the contents of a
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.Dq cold
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storage device.
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.Pp
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Be aware that if the computer is compromised while up and running
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.Em and
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the storage device is actively attached and opened with a valid
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pass-phrase, this facility offers no protection or denial of access
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to the contents of the storage device.
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.Pp
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If, on the other hand, the device is
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.Dq cold ,
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it should present an formidable
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challenge for an attacker to gain access to the contents in the absence of
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a valid pass-phrase.
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.Pp
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Four cryptographic barriers must be passed to gain access to the data,
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and only a valid pass-phrase will yield this access.
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.Pp
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When the pass-phrase is entered, it is hashed with SHA2 into a 512 bit
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.Dq key-material .
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This is a way of producing cryptographic usable keys from a typically
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.No all- Ns Tn ASCII
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pass-phrase of an unpredictable user-selected length.
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.Ss First barrier: the location of the \&"lock-sector".
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During initialization, up to four independent but mutually aware
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.Dq lock
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sectors are written to the device in randomly chosen
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locations.
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These lock-sectors contain the 2048 random bit master-key and a number
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of parameters of the layout geometry (more on this later).
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Since the entire device will contain isotropic data, there is no
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short-cut to rapidly determine which sequence of bytes contain a lock-sector.
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.Pp
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To locate a lock-sector, a small piece of data called the
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.Dq metadata
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and the key-material must be available.
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The key-material decrypts the
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metadata, which contains the byte offset on the device where the
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corresponding lock-sector is located.
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If the metadata is lost or unavailable but the key-material is at
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hand, it would be feasible to do a brute force scan where each byte offset
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of the device is checked to see if it contains the lock-sector data.
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.Ss Second barrier: decryption of the master-key using key-material.
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The lock-sector contains an encrypted copy of an architecture neutral
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byte-sequence which encodes the fields of the lock-structure.
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The order in which these fields are encoded is determined from the key-material.
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The encoded bytestream is encrypted with 256bit AES in CBC mode.
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.Ss Third barrier: decryption of the sector key.
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For each sector, an MD5 hash over a
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.Dq salt
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from the lock-sector and the sector number is used to
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.Dq cherry-pick
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a subset of the master key,
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which hashed together with the sector offset through MD5 produces the
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.Dq kkey ,
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the key which encrypts the sector key.
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.Ss Fourth barrier: decryption of the sector data.
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The actual payload of the sector is encrypted with 128 bit AES in CBC mode
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using a single-use random bits key.
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.Ss Examining the reverse path
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Assuming an attacker knows an amount of plaintext and has managed to
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locate the corresponding encrypted sectors on the device, gaining access
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to the plaintext context of other sectors is a daunting task:
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.Pp
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First he will have to derive from the encrypted sector and the known plain
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text the sector key(s) used.
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At the time of writing, it has been speculated that it could maybe be
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possible to break open AES in only 2^80 operations; even so, that is still
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a very impossible task.
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.Pp
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Armed with one or more sector keys, our patient attacker will then go
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through essentially the same exercise, using the sector key and the
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encrypted sector key to find the key used to encrypt the sectorkey.
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.Pp
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Armed with one or more of these
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.Dq kkeys ,
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our attacker has to
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run them backwards through MD5.
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Even though he knows that the input to MD5 was 24 bytes and has the value
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of 8 of these bytes from the sector number, he is still faced with 2^128
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equally likely possibilities.
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.Pp
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Having successfully done that, our attacker has successfully discovered
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up to 16 bytes of the master-key, but is still unaware which 16 bytes,
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and in which other sectors any of these known bytes contribute to the kkey.
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.Pp
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To unravel the last bit, the attacker has to guess the 16 byte random-bits
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salt stored in the lock-sector to recover the indexes into the masterkey.
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.Pp
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Any attacker with access to the necessary machine power to even attempt
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this attack will be better off attempting to brute-force the pass-phrase.
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.Ss Positive denial facilities
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Considering the infeasibility of the above attack,
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gaining access to the pass-phrase will be of paramount importance for an
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attacker,
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and a number of scenarios can be imagined where undue pressure will be
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applied to an individual to divulge the pass-phrase.
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.Pp
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A
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.Dq Blackening
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feature provides a way for the user, given a moment of
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opportunity, to destroy the master-key in such a way that the pass-phrase
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will be acknowledged as good but access to the data will still be
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denied.
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.Ss A practical analogy
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For persons who think cryptography is only slightly more interesting than
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watching silicon sublimate the author humbly offers this analogy to the
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keying scheme for a protected device:
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.Pp
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Imagine an installation with a vault with walls of several hundred meters
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thick solid steel.
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This vault can only be feasibly accessed using the
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single key, which has a complexity comparable to a number with 600 digits.
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.Pp
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This key exists in four copies, each of which is stored in one of
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four small safes, each of which can be opened
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with unique key which has a complexity comparable to an 80 digit
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number.
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.Pp
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In addition to the masterkey, each of the four safes also contains
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the exact locations of all four key-safes which are located in
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randomly chosen places on the outside surface of the vault where they
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are practically impossible to detect when they are closed.
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.Pp
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Finally, each safe contains four switches which are wired to a bar
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of dynamite inside each of the four safes.
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.Pp
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In addition to this, a keyholder after opening his key-safe is
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also able to install a copy of the master-key and re-key any of
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key-safes (including his own).
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.Pp
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In normal use, the user will open the safe for which he has the key,
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take out the master-key and access the vault.
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When done, he will lock up the master-key in the safe again.
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.Pp
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If a keyholder-X for some reason distrusts keyholder-Y, she
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has the option of opening her own safe, flipping one of the switches
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and detonating the bar of dynamite in safe-Y.
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This will obliterate the master-key in that safe and thereby deny
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keyholder-Y access to the vault.
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.Pp
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Should the facility come under attack, any of the keyholders can detonate
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all four bars of dynamite and thereby make sure that access to the
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vault is denied to everybody, keyholders and attackers alike.
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Should the facility fall to the enemy, and a keyholder be forced to apply
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his personal key, he can do so in confidence that the contents of his safe
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will not yield access to the vault, and the enemy will hopefully realize
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that applying further pressure on the personnel will not give access to
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the vault.
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.Pp
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The final point to make here is that it is perfectly possible to
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make a detached copy of any one of these keys, including the master
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key, and deposit or hide it as one sees fit.
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.Ss Steganography support
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When the device is initialized, it is possible to restrict the encrypted
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data to a single contiguous area of the device.
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If configured with care, this area could masquerade as some sort of
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valid data or as random trash left behind by the systems operation.
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.Pp
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This can be used to offer a plausible deniability of existence, where
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it will be impossible to prove that this specific area of the device
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is in fact used to store encrypted data and not just random junk.
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.Pp
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The main obstacle in this is that the output from any encryption algorithm
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worth its salt is so totally random looking that it stands out like a sore
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thumb amongst practically any other sort of data which contains at least
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some kind of structure or identifying byte sequences.
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.Pp
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Certain file formats like ELF contain multiple distinct sections, and it
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would be possible to locate things just right in such a way that a device
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contains a partition with a file system with a large executable,
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.Pq Dq "a backup copy of my kernel"
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where a non-loaded ELF section is laid out
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consecutively on the device and thereby could be used to contain a
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.Nm
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encrypted device.
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.Pp
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Apart from the ability to instruct
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.Nm
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which those sectors are, no support is provided for creating such a setup.
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.Ss Deployment suggestions
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For personal use, it may be wise to make a backup copy of the masterkey
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or use one of the four keys as a backup.
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Fitting protection of this key is up to yourself, your local circumstances and
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your imagination.
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.Pp
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For company or institutional use, it is strongly advised to make a copy
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of the master-key and put it under whatever protection you have at your
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means.
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If you fail to do this, a disgruntled employee can deny you access to
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the data
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.Dq "by accident" .
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(The employee can still intentionally deny access by applying another
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encryption scheme to the data, but that problem has no technical solution.)
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.Ss Cryptographic strength
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This section lists the specific components which contribute to the cryptographic
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strength of
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.Nm .
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.Pp
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The payload is encrypted with AES in CBC mode using a 128 bit random
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single-use key
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.Pq Dq "the skey" .
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AES is well documented.
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.Pp
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No IV is used in the encryption of the sectors, the assumption being
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that since the key is random bits and single-use, an IV adds nothing to the
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security of AES.
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.Pp
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The random key is produced with
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.Xr arc4rand 9
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which is believed to do a respectable job at producing unpredictable bytes.
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.Pp
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The skey is stored on the device in a location which can be derived from
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the location of the encrypted payload data.
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The stored copy is encrypted with AES in CBC mode using a 128 bit key
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.Pq Dq "the kkey"
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derived
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from a subset of the master key chosen by the output of an MD5 hash
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over a 16 byte random bit static salt and the sector offset.
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Up to 6.25% of the masterkey (16 bytes out of 2048 bits) will be selected
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and hashed through MD5 with the sector offset to generate the kkey.
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.Pp
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Up to four copies of the master-key and associated geometry information
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is stored on the device in static randomly chosen sectors.
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The exact location inside the sector is randomly chosen.
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The order in which the fields are encoded depends on the key-material.
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The encoded byte-stream is encrypted with AES in CBC mode using 256 bit
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key-material.
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.Pp
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The key-material is derived from the user-entered pass-phrase using
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512 bit SHA2.
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.Pp
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No chain is stronger than its weakest link, which usually is poor pass-phrases.
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.Sh SEE ALSO
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.Xr gbde 8
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.Rs
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.%A Poul-Henning Kamp
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.%T "Making sure data is lost: Spook-strength encryption of on-disk data"
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.%R "Refereed paper, NORDU2003 conference"
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.Re
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.Sh HISTORY
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This software was developed for the
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.Fx
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Project by
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.An Poul-Henning Kamp
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and NAI Labs, the Security Research Division of Network Associates, Inc.\&
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under DARPA/SPAWAR contract N66001-01-C-8035
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.Pq Dq CBOSS ,
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as part of the
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DARPA CHATS research program.
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.Sh AUTHORS
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.An "Poul-Henning Kamp" Aq phk@FreeBSD.org
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