forked from pool/util-linux
170 lines
6.6 KiB
Plaintext
170 lines
6.6 KiB
Plaintext
cryptoloop - a generic cryptographic loop device filter
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=======================================================
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To enable cryptoloop, you must fetch the cryptoloop-version.tar.gz source from
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your kernel.org mirror or directly from the non-US <http://www.kerneli.org>.
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Quickstart
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~~~~~~~~~~
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(if you have old encrypted volumes, please read the section about
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conversion below)
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you need a kernel compiled with
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*) loop device driver
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*) cryptographic support
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*) generic crypto loop filter
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*) one or more ciphers you want to use for encryption either as module or
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statically (if you build them as module, don't forget to insmod them!)
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Download the cryptoloop latest version tarball from the Internet and enter
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the untarred directory. The following commands will patch your kernel and
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copy the cryptoloop source in.
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$ make patch-kernel KDIR=/your/kernel/source LOOP=[iv or jari]
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Which will patch your kernel, and enable you to select cryptoloop in the
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Cryptography section of the kernel configuration. Note, you have to manually
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enable both CryptoAPI and cryptoloop in your kernel configuration.
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If you would rather build modules you can use the following commands.
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$ make modules KDIR=/your/kernel/source CDIR=/cryptoapi/source \
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LOOP=[iv or jari]
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$ make modules_install KDIR=/your/kernel/source
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You also need a util-linux patched package (see
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<http://www.kernel.org/pub/linux/kernel/people/hvr/> for some recent
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patches)
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Read on at <http://encryptionhowto.sourceforge.net/>
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How Loopback Encryption Works
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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Here is a small overview of how the loopback filesystem encryption works.
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For all the of the data to be encrypted on a drive we need to interrupt the
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disk write/read *after* the filesystem meta data is attached. Instead of hi-
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jacking the system call we pipe the the filesystem commands through a loopback
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device.
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One advantage of this is that you can either encrypt a device(hard drive) or
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a file already on a drive and mount it as a file system. This allows you to
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have encrypted folders on an unencrypted drive, useful if you don't want to
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encrypt everything.
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The loopback device fits in like this:
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device-->loop-->mountpoint
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The loop stage is actually divided into a couple stages, which is why the
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cryptoloop module is included. It splits the input and output from the
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loopback into pieces for the ciphers. Since they operate on a specific
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blocksize(i.e. 64-bits, 128-bits) the data must be chopped up for them.
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device-->cryptoloop-->loop-->mountpoint
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The cryptoloop module is also where the actual encryption and decryption of
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the data takes place.
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Some words about IV Calculation
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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There are some problems with the IV calculation up to recent 2.4.x
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linux kernels. It's been calculated more or less by
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IV = rel_sector / (blocksize >> 9) + (lo_offset / blocksize)
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The first problem which arises, is that data transfers are not always
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guaranteed to have a size of an integral multiple of the blocksize;
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This is a problem, because of CBC mode's property of needing the
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complete ciphertext block, i.e. you can't cipher or decipher only the
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2nd half of a block if you don't know the first half of it!
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Another problem which may be experienced is, when the soft blocksize
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changes, i.e. due to different medias, as CDROM block devices and
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alike, or when the filesystem layer sets the blocksize to some other
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size.
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But there's hope, as all transfer sizes are usually (except for the
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last block on file backed loop devices sometimes...) an integral
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multiple of 512 byte units (which is linux' atomic sector size);
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So the solution is to stick to a portable, uniform 512 byte based IV
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metric! Alas this can't be accomplished without modifying the loop
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driver, as the more granular IV metric can't be calculated from the
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current dynamic IV metric.
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This change also renders most previously used encrypted volumes
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unreadable, which need to be converted to the new IV metric.
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Converting from Older IV Metrics
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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The recommended procedure is as follow, in case you don't want to
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backup your old data in a conventional manner, and then restore it to
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a freshly created encrypted volume with the new IV-metric.
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The more valuable your data is, the more important it is, that you try
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the following procedure with some dummy data, before risking your real
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data! Cause if something goes wrong you'll have to keep both pieces...
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1) decrypt from within your old kernel version
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1.1) setup your encrypted loop device, and mount it (this is also
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important for making sure, the filesystem layer sets the soft block
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size accordingly!)
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1.2) unmount the filesystem (but leave the loop device setted up)
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1.3) dd if=/dev/loop0 of=/dev/<underlying blockdev/file> make sure
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the process does not get interrupted, otherwise you will have a hard
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time, reconstructing your data, since you are decrypting the data
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back to the same area where the ciphertext was.
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3) make sure the block/file contains the properly deciphered content!!
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2) boot the new kernel featuring the 512byte based IV
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follow the instructions for 'encrypting unencrypted volumes' below
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Encrypting Unencrypted Volumes
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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just as simple as that (please test this before trying on important data)
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1) set up a new encrypted loop device over the still unencrypted data
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2) dd if=/dev/<underlying blockdev/file> of=/dev/loop0
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Migrating from loop-AES to CryptoAPI
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~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
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If you want to move from using the loop-AES filesystem encryption to
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CryptoAPI it only requires a small tweak. After loading cryptoapi and loading
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the cipher-aes module use the following line when setting up your loopback
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device:
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$ losetup -e aes -k 256 -P sha512 /dev/loop0 /dev/<your drive>
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Then proceed to mount your loop device as you normall would. Here is the
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table for mapping loop-AES to cryptoapi losetup flags:
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loop-AES cryptoapi
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============================================================
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-e aes128 -H rmd160 -e aes -k 128
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-e aes128 -H sha256 -e aes -k 128 -P sha256
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-e aes128 -H sha384 -e aes -k 128 -P sha384
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-e aes128 -H sha512 -e aes -k 128 -P sha512
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-e aes256 -H rmd160 -e aes -k 256
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-e aes256 -H sha256 -e aes -k 256 -P sha256
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-e aes256 -H sha384 -e aes -k 256 -P sha384
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-e aes256 -H sha512 -e aes -k 256 -P sha512
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-e aes128 -e aes -k 128 -P sha256
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-e aes192 -e aes -k 192 -P sha384
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-e aes256 -e aes -k 256 -P sha512
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--
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$Id: cryptoloop.txt,v 1.3 2002/08/03 19:29:58 kyle Exp $
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