[转贴]为交换分区加密

风戬

在llinuxfromscratch网站上找到的一篇好文，本计划翻译的，但最近忙于考试没时间，大家就还是看e文版吧

AUTHOR: Jerome Pinot <ngc891@gmail.com>;

DATE: 2005-04-14

LICENSE: GNU Free Documentation License Version 1.2

SYNOPSIS: Encrypting swap partition

DESCRIPTION:
Here is a way to enable disk encryption on HLFS system. Aim is to provide native
swap encryption and possibility to encrypt the root partition later, with
third-part software.

ATTACHMENTS:
* http://ngc891.blogdns.net/projects/hlfs/hlfs-461-eswap-1.patch
* http://www.linuxfromscratch.org/patches/downloads/util-linux/util-linux-2.12q-loop_AES-3.0c.patch
* http://www.linuxfromscratch.org/patches/downloads/linux/linux-2.6.11.7-loop_AES-3.0c.patch
* http://www.linuxfromscratch.org/patches/downloads/gnupg/gnupg-1.4.1-loop_AES-3.0c.patch

PREREQUISITES:
This hint is written for HLFS but can be easily applied to LFS. You should have
some basic knowledge about devices and using swap.

HINT:

I. About encrypting disk
------------------------

A Linux system already provides some basic security environment including users
and groups, passwords, permissions,
and now access control via SELinux project.

However, this could be not enough in some cases like attackers with physical
access. If someone can physically access your hard drive and mount it in an
other computer, he overpasses the system and can read whatever he wants.

The problem is the same with the swap partition. It stores short lifetime data
including most of the things you have just done with the computer and that
didn't fit in the RAM. The system continously overwrite this partition and there
is no easy structure inside but an attacker could seek in for passwords and
other data you just typed.

One protection against this kind of attack is swap encryption. It means cipher
your data with an algorithm, so you need a passphrase and/or a key to access
clearly to it. Even, some ciphers like AES provide you "plausible deniability".
It means there is no way to know that the partition is actually encrypted
because it looks just like trash, like an empty partition. So, there is no
more problem for th
e system being overpassed by physical access.

Encrypting your partitions could increase drastically your data security.

II. How to do?
--------------

There is several ways to encrypt disk on Linux, including cryptoloop, dm-crypt,
loop-AES and StegFS.

StegFS is a special encrypted file system. It's sounds really great but is still
under development and needs big modifications of the base system.

Cryptoloop was a special loop device included in the kernel that provides access
to encrypted device by loopback. Everybody was happy to have such an easy way to
access encrypted device, but unfortunately, it was found that cryptoloop has a
flaw and cannot be trust. If you can find cryptoloop in some linux distribution,
it was actually removed from the official Linux kernel source code [1].

dm-crypt is an encrypted device mapper created to replace cryptoloop [2]. You
can find it in the official source, under the device mapper sub-section. It is
supposed to avoid the flaw of cryptoloop, but actually, it fails.
You can find
on the web more informations about that [3]. So even if it's available natively
in the kernel it should be avoid for more security.

So there is loop-AES. It is stable and modular and needs few modifications to
the base system. It consists of patches to apply to the kernel (2.4 and 2.6) and
some utilities. Using multi-key with loop-AES avoids the flaw of cryptoloop and
dm-crypt. It uses the AES algorithm which is known to be one of the strongest
available. Moreover, there is already an LFS hint for encrypting root partition
using loop-AES [4].

You can find loop-AES here:
http://sourceforge.net/projects/loop-aes/

[1] http://lwn.net/Articles/67216/
[2] http://kerneltrap.org/node/2433
[3] http://mareichelt.de/pub/texts.cryptoloop.php
[4] http://linuxfromscratch.org/~devine/erfs-howto.html

III. Encrypting swap
--------------------

It's a matter of applying 2 patches and changing a little the /etc/fstab file.
The easiest way is to patch the svn version of the book like this:

-- Optiona
l --

wget http://ngc891.blogdns.net/projects/hlfs/hlfs-461-eswap-1.patch
cd HLFS
patch -Np1 -i ../hlfs-461-eswap-1.patch

-- Optional --

Unfortunatly, it can be out of date so you have choice to follow the other way:

1. First you need to apply the util-linux-2.12q-loop_AES-3.0c.patch to the
util-linux before building it during chapter 6. This patch enables the use of
mount, umount, and swapon for encrypted devices.

$ patch -Np1 -i ../util-linux-2.12q-loop_AES-3.0c.patch

2. You need to change the line about swap file in the /etc/fstab (chapter 7)

from:        /dev/[yyy]        swap        swap        pri=1        0        0
to:        /dev/[yyy]        swap        swap        sw,loop=/dev/loop7,encryption=AES128,pri=1        0        0

The swap will be mount using a loopback device with a multikeys AES encryption.
It uses the last loop device so you will be able to use from /dev/loop0 to
/dev/loop6 for other purpose.

3. Finally, you must patch your kernel source before "make menuconfig" in
chapter 7:

patch -Np1 -i ../linux-2.6.11.7-loop_AES-3.0c.patch

Then, during "make me
nuconfig", you MUST select loop-AES under loop item of the
block sub-section or your swap partition may not be available. You should enable
BLK_DEV_LOOP_AES and BLK_DEV_LOOP_KEYSCRUB.

IV. Setting up third-part software
----------------------------------

You can find the following packages in http://ngc891.blogdns.net/projects/hlfs/packages

1. GnuPG 1.4.1

patch -Np1 -i ../gnupg-1.4.1-loop_AES-3.0c.patch

sed -e 's/^CFLAGS .*$/& -pie -fpie/' -i `find . -name Makefile.in`

./configure --prefix=/usr --enable-static-rnd=linux \
--libexecdir=/usr/lib --enable-noexecstack &&
make && make install

2. Sharutils 4.3.78

We need sharutils for uuencode to convert randon binary data from /dev/urandom
to random ascii data for keys generation.

sed -e 's/^CFLAGS .*$/& -pie -fpie/' -i `find . -name Makefile.in` &&
./configure --prefix=/usr &&
make && make install

3. Aespipe 2.3b

Add some flags and build aespipe:

sed -e 's/^LINK .*$/& -nointl/' -i Makefile.in
sed -e '10,0s/^/CFLAGS+=-pie -fpie\n&/' -i Make
file.in &&
./configure --prefix=/usr &&
make && make install

ACKNOWLEDGEMENTS:
Thanks to the author of loop-AES, Jari Ruusu