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Filesystem security


A solid house needs a solid foundation, otherwise it will collapse. In Linux's case this is the ext2 (EXTended, version 2) filesystem. Pretty much your everyday standard UNIX-like filesystem. It supports file permissions (read, write, execute, sticky bit, suid, sgid and so on), file ownership (user, group, other), and other standard things. Some of its drawbacks are: no journaling, and especially no Access Control Lists, which are rumored to be in the upcoming ext3. On the plus side, Linux has excellent software RAID, supporting Level 0, 1 and 5 very well (RAID isn't security related, but it certainly is safety/stability related). There is an excellent HOWTO on file systems in regards to Linux http://www.penguin.cz/~mhi/fs/Filesystems-HOWTO/Filesystems-HOWTO.html.

So the attacker somehow manages to circumvent the physical security and console security you put in place and now has a root prompt. What can you do to protect sensitive files, and prevent the attacker from accessing or copying them? Keeping data on a server can help, even with local root access an attacker will still have to steal credentials to access the fileserver (assuming you use root_squash under NFS or have similarly secured your file sharing configuration). Of course there is still sensitive local information, and the solution to this is disk encryption. There are several disk encryption packages for Linux, most will let you create an encrypted volume and access it. The usual method is to create an encrypted file and mount it using a loopback mount so it appears as a disk to the system. There is then usually a kernel interface, or userland program that allows you to access it. These systems are good and will protect sensitive information, however for an additional level of security you should encrypt everything, data partitions, system partitions, and swap partitions. An attacker can usually find interesting bits of data in the swap partition, and if they can access system files they can replace your favorite text editor with one that creates unencrypted backup copies of the file in /tmp for later retrieval.


Basic file commands

The basic utilities to interact with files are: “ls”, “chown”, “chmod” and “find”. Others include "ln" (for creating links), "stat" (tells you about a file) and many more. As for creating and maintaining the file systems themselves, we have “fdisk” (good old fdisk), “mkfs” (MaKe FileSystem, which formats partitions), and “fsck” (FileSystem ChecK, which will usually fix problems). So, what is it we are trying to prevent hostile people (usually users, and or network daemons fed bad info) from doing? A Linux system can be easily compromised if access to certain files is gained, for example the ability to read a non-shadowed password file results in the ability to run the encrypted passwords against crack, easily finding weak password. This is a common goal of attackers coming in over the network (poorly written CGI scripts seem to be a favorite). Alternatively, if an attacker can write to the password file, he or she can seriously disrupt the system, or (arguably worse) get whatever level of access they want. These conditions are commonly caused by "tmp races", where a setuid program (one running with root privileges) writes temporary files, typically in /tmp, however far to many do not check for the existence of a file, thus allowing an attacker to make a hard link in /tmp pointing to the password file, and when the setuid program is run, kaboom, /etc/passwd is wiped out or possibly appended to. There are many more attacks similar to this, so how can we prevent them?

Simple: set the file system up correctly when you install. The two common directories that users have write access to are /tmp and /home, splitting these off onto separate partitions also prevents users from filling up any critical filesystem (a full / is very bad indeed). A full /home could result in users not being able to log in at all (this is why root’s home directory is in /root). Putting /tmp and /home on separate partitions is pretty much mandatory if users have shell access to the server, putting /etc, /var, and /usr on separate partitions is also a very good idea.

The primary tools for getting information about files and file systems are all relatively simple and easy to use. “df” (shows disk usage) will also show inode usage, “df –i” (inodes contain information about files such as their location on the disk drive, and you can run out of these before you run out of disk space if you have many small files. This results in error messages of "disk full" when in fact “df” will show there is free space (“df –i” however would show the inodes are all used). This is similar to file allocation entries in Windows, with vfat it actually stores names in 8.3 format, using multiple entries for long filenames, with a max of 512 entries per directory, to many long filenames, and the directory is 'full'. The “du” utility will tell you the size of directories, which is very useful for finding out where all that disk space has disappeared to, usage is “du” (lists everything in the current directory and below it that you have access to) or “du /dir/name”, optionally using “-s” for a summary which is useful for directories like /usr/src/linux. To gain information about specific files the primary tool is ls (similar to DOS's “dir” command), “ls” shows just file/dir names, “ls –l” shows information such as file perms, size and so on, and 'ls -la' shows directories and files beginning in “.”'s, typical for config files and directories (.bash_history, .bash_logout, etc.). The “ls” utility has a few dozen options for sorting based on size, date, in reverse order and so forth; “man ls” for all the details. For details on a particular file (creation date, last access, inode, etc) there is “stat”, which simply tells all the vital statistics on a given file(s), and is very useful to see if a file is in use/etc.

To manipulate files and folders we have the typical utilities like cp, mv, rm (CoPy, MoVe and ReMove), as well as tools for manipulating security information. chown is responsible for CHanging OWNership of files the user and group a given file belongs to (the group other is always other, similar to Novell or NT's 'everyone' group). chmod (CHange MODe) changes a files attributes, the basic ones being read, write and execute, as well there is setuid, setguid (set user and group id the program is run as to the ones that own it, often times root), sticky bit and so forth. With proper use of assigning users to groups, chmod and chown you can emulate ACL's to a degree, but it is far less flexible then Sun/AIX/NT's file permissions (although this is rumored for ext3). Please be especially careful with setuid/setguid as any problems in that program/script can be magnified greatly.

I thought I should also mention “find”. It find's files (essentially it will list files), and can also filter based on permissions/ownership (also on size, date, and several other criterion). A couple of quick examples for hunting down setuid/guid programs:

to find all setuid programs:

find / -perm +4000

to find all setgid programs:

find / -perm +2000

The biggest part of file security however is user permissions. In Linux a file is 'owned' by 3 separate entities, a User, a Group, and Other (which is everyone else). You can set which user owns a file and which group it belongs to by:

chown user:group object

where object is a file, directory, etc. If you want to deny execute access to all of the 3 owners simply: 

chmod x="" object

where x is a|g|u|o (All/Group/User/Other), force the permissions to be equal to "" (null, nothing, no access at all) and object is a file, directory, etc. This is by far the quickest and most effective way to rip out permissions and totally deny access to users/etc (="" forces it to clear it). Remember that root can ALWAYS change file perms and view/edit/run the file, Linux does not yet provide safety to users from root (which many would argue is a good thing). Also whoever owns the directory the object is in (be they a user/group/other with appropriate perms on the parent directory) can also potentially edit permissions (and since root owns / it can make changes that can traverse down the filesystem to any location).


Secure file deletion

One thing many of us forget is that when you delete a file, it is not actually gone. Even if you overwrite it, reformat the drive, or otherwise attempt to destroy it, chances are it can be recovered, and typically data recovery services only cost a few thousand dollars, so it might well be worth an attackers time and money to have it done. The trick is to scramble the data by repeatedly flipping the magnetic bits (a.k.a. the 1’s and 0’s) so that when finished no traces of the original data remain (i.e. magnetic bits still charged the same way they originally were). Two programs (both called wipe) have been written to do just this. An ideal solution for long term security is to encrypt data where possible, if keys are protected properly then an attacker will have a very difficult time recovering any data. This combined with good deletion techniques makes recovering data very difficult, of course there are still possibilities for data leakage (i.e. temporary files on unencrypted swap partitions).



Wipe is available http://wipe.sourceforge.net/.


fwip is available http://www.nb.net/~lbudney/linux/software/fwipe.html.


Access control lists (ACL’s)

One major missing component in Linux is a filesystem with Access Control Lists (ACL’s) instead of the standard User, Group, Other with it’s dozen or so permissions. ACL’s enable you to control access to the filesystem in a much more fine grained fashion, for example on a file you may want to grant the user “bob” full access, “mary” read, the groups sales “change”, the accounting group “read”, and nothing for everyone else . Under existing Linux permissions you could not do this. Hence the need for ACL’s. The ACL stuff is currently under kernel patches.



NSA SELinux is now based on the Linux Security Module (LSM) kernel patches and provides (among other things) access control lists.



Information is available here.


Critical system configuration files

Like any operating system Linux has a number of configuration files. Some of these are extremely critical, for example the password file. This section covers several of the more critical files, explains their layout, how they interact with the system, and so on. For example it is counter-intuitive that while the password file is probably the most critical file on the system, it must be readable by everyone, and there are several good reasons for this.


/etc/ directory

Typically the /etc/ directory contains the majority of the system and application configuration files and many critical startup scripts. Some applications when installed to the /usr/local/ directory will place their files in /usr/local/etc/ (or other locations), this directory should largely be treated in the same way as /etc/. If an attacker is able to change files chances are they will be able to elevate their privileges and ultimately gain root access since many startup scripts are run with root privileges.



The password file is arguably the most critical system file in Linux (and most other UNIX's). It contains the mappings of username, user ID and the primary group ID that person belongs to. It may also contain the actual password however it is more likely (and much more secure) to use shadow passwords to keep the passwords in /etc/shadow. This file MUST be world readable, otherwise commands even as simple as ls will fail to work properly. The GECOS field can contain such data as the real name, phone number and the like for the user, the home directory is the default directory the user gets placed in if they log in interactively, and the login shell must be an interactive shell (such as bash, or a menu program) and listed in /etc/shells for the user to log in. The format is:


Passwords are stored utilizing a one way hash (the default hash used is crypt, newer distributions support MD5 which is significantly stronger). Passwords cannot be recovered from the encrypted result, however you can attempt to find a password by using brute force to hash strings of text and compare them, once you find a match you know you have the password. This in itself is generally not a problem, the problem occurs when users choose easily guessed passwords. The most recent survey results showed that %25 of passwords could be broken in under an hour, and what is even worse is that %4 of users choose their own name as the password. Blank fields in the password field are left empty, so you would see “::”, this is something that is critical for the first four fields (name, password, uid and gid).



The shadow file holes the username and password pairs, as well as account information such as expiry date, and any other special fields. This file should be protected at all costs and only the root user should have read permission to it. 



The groups file contains all the group membership information, and optional items such as group password (typically stored in gshadow on current systems), this file to must be world readable for the system to behave correctly. The format is:


A group may contain no members (i.e. it is unused), a single member or multiple members, and the password is optional (and typically not used).



Similar to the password shadow file, this file contains the groups, password and members. Again, this file should be protected at all costs and only the root user should have read permission to it.



This file (/etc/login.defs) allows you to define some useful default values for various programs such as useradd and password expiry. It tends to vary slightly across distributions and even versions, but typically is well commented and tends to contain sane default values.



The shells file contains a list of valid shells, if a user’s default shell is not listed here they may not log in interactively. See the section on telnetd for more information.



This file contains a list of tty’s that root can log in from. Console tty’s are usually /dev/tty1 through /dev/tty6. Serial ports (if you want to log in as root over a modem say) are /dev/ttyS0 and up typically. If you want to allow root to login via the network (a very bad idea, use sudo) then add /dev/ttyp1 and up (if 30 users login and root tries to login root will be coming from /dev/ttyp31). Generally you should only allow root to login from /dev/tty1, and it is advisable to disable the root account altogether, before doing this however please install sudo or program that allows root access to commands. 


File encryption

Do you have files on your computer that you wouldn't want your spouse to read, or perhaps your main competitor. Chances are if you use your computer for work or general usage the answer is yes. Also what happens if you want to send a file to someone, or let them download it from you, but you only have access to a public site (like a free web hosting company). The answer is to encrypt the file, and fire it off.



Pretty Good Privacy is available as a command line driven program for most UNIX platforms, and there are a variety of front end GUI programs for it. I would not recommend using PGP on a UNIX platform since a completely OpenSource, and compatible replacement is now available, in the form of GnuPG.



GnuPG is a GPL licensed (a.k.a. completely free in every respect), written in Germany (a very pro-crypto and pro-privacy country). Since it is available in full source code chances are it has been ported to your UNIX platform (and if not try compiling it, it might work). You can download GnuPG as a compressed tarball of source code, and there are links to a number of source and binary packages for various UNIX platforms. Once installed GnuPG behaves very similarly to PGP. The first thing you'll probably want to do is generate a new keypair, simply use the command "gpg --gen-key", it will create a ".gnupg" directory in which to store your keys, option files and so on and exit, you then run it again and it will lead you through the key creation process. Choosing the defaults during key generation is a pretty safe bet, although you may want to use a 2048 bit keysize (realistically if someone manages to crack 1024 bit keys, chances are they can get at your 2048 bit key, however if they are only trying to brute force it a longer key is a good way to reduce the chances of that). For personal keys the expiry is typically set to "0" (that is to say they do not expire), however if these keys are for corporate use, or for really sensitive information it is a good idea to expire keys and rotate them (every month, year, decade, whatever your security policy dictates). The most important thing when generating a key (in my opinion) is the passphrase. This is a string of characters which should consist of letters (upper and lower case) numbers and punctuation marks, the longer the better (I'd say the bare minimum is 10 characters). This controls access to the private key, which is used to sign items (and if compromised means an attacker could easily impersonate you), and to decrypt data (meaning an attacker could access all your data). Keep your private key secure! If an attacker gains access to this key they only have to brute force the passphrase, which is typically a lot weaker then a random 1024 bit (or longer) key. Worse yet they may steal your passphrase, with a keyboard sniffer or similar attack, resulting in a compromise of your key. If the attacker does not have access to your private key they will be forced to guess it, which takes a brutally long time (on average however, there is a chance they may guess the key correctly on their first try).

Signing files is useful if you want to distribute a file to someone, and be able to prove that you sent it, and it was not tampered with. Internally GnuPG takes a hash sum (such as MD5 or SHA1) of the file (basically it reduces the file to a shorter, unique string of data) which it then encrypts with your private key, generating a signature. This signature can then be decrypted with your public key, resulting in possession of the hash sum of the file, simply take the hash sum of the file in question, and if the they match, then obviously the file is what it claims to be. This signature file can be a binary file, or converted into text (for example signing email, or distributing file signatures via email). To sign a file with gpg simply use :

$ gpg -b file

which will create a detached signature of the file.

To verify the signature use "gpg --verify file.sig file". If all is well you should see something like:

$ gpg --verify file.sig file
gpg: Signature made Sat 15 Jan 2000 05:23:31 AM MST using DSA key ID 47D0D9A8
gpg: Good signature from "Kurt Seifried <seifried@seifried.org>"

If someone has fiddled with the file or signature you will see something like:

$ gpg --verify file.sig file
gpg: Signature made Sat 15 Jan 2000 05:23:31 AM MST using DSA key ID 47D0D9A8
gpg: BAD signature from "Kurt Seifried <seifried@seifried.org>"

Encrypting files is also relatively simple, a person uses your public key to run the data through a one way algorithm which results in a seemingly random mishmash of data, you can then use your private key to recover what the original data was, thus decrypting it. To encrypt a file to someone you first need their public key, you can download it from their homepage (if they have it online of course), or you can go to a public key server, of which there are many:

http://pgp.ai.mit.edu/ - PGP key server
http://www.keyserver.net/ - OpenPGP key server

Once you have their key it is simply a matter of signing and encrypting the file (just encrypting the file is rare as there is no proof of who the data is from, unless you use some other method, like physically handing them a floppy disk with the encrypted file). The following is an example of me signing a file and encrypting it with my public key:

$ gpg -s -e file

You need a passphrase to unlock the secret key for
user: "Kurt Seifried <seifried@seifried.org>"
1024-bit DSA key, ID 47D0D9A8, created 2000-01-15

You did not specify a user ID. (you may use "-r")

Enter the user ID: seifried@seifried.org

The user ID can either be the key ID (such as: 47D0D9A8), the email address associated with the key (kurt@seifried.org) or the name (not recommended as these are not unique, there are many John Smith's). You will end up with a "file.gpg" that is binary, if you wish to send the file via email it is advisable to use the "-a" ("--armor") option which will result in "file.asc" and is ASCII text, so you can read it straight into an email, or print it out, mail it, and let them OCR and decrypt it at their end. To decrypt a file sent to you simply:

$ gpg --decrypt file.asc

You need a passphrase to unlock the secret key for
user: "Kurt Seifried <seifried@seifried.org>"
1024-bit ELG-E key, ID 47D0D9A8, created 2000-01-15 (main key ID 39B0D9A8)

and it will display the file (hopefully a text file) to your screen, followed by the veracity of the signature (if you have the persons public key):

gpg: Signature made Sat 15 Jan 2000 06:06:19 AM MST using DSA key ID 47D0D9A8
gpg: Good signature from "Kurt Seifried <seifried@seifried.org>"

if you want to save the decrypted file simply use "--output filename" and it will dump the content to "filename". You can also use shell commands such as "|" or ">" to further mangle the output (this is useful if you have automated systems such as a reporting mechanism which sends encrypted emails to a central repository).


Filesystem encryption

Filesystem encryption is an important security consideration if the data stored on the machine is of sufficient value or sensitivity. No matter how well you secure a system and tightly control file permissions if someone steals the machine or boots it into single user mode or from a floppy disk they can trivially bypass most protection methods, except for file encryption. Unfortunately most encrypted file systems will "leak" information, temporary files, swap partitions and so on can all end up with clear text copies of the files you want to protect with encryption. The best solution to deal with this problem is of course to encrypt everything, unfortunately this is very difficult and results in a lot of over head. Another solution (although less optimal) is to periodically wipe the clear space on your harddrive (i.e. the space where deleted files remain), as well as cleaning out temporary files and swap partitions, of course finding all of these is never simple.



TCFS is a kernel level data encryption utility, similar to CFS. It however has several advantages over CFS; as it is implemented at the kernel level it is significantly faster. It is tightly integrated with NFS meaning you can server data securely on a local machine, or across the network. It decrypts data on the client machine, so when used over a network the password/etc is never passed over the network. The only catch is that it has not yet been ported to the latest 2.2 kernels (2.2.16 and 2.2.17 only). You can get TCFS http://tcfs.dia.unisa.it/.



BestCrypt is a disk encryption program available for Windows and Linux. The nice thing is you can create an encrypted container (a file that is then mounted as a filesystem), and use it in Windows or in Linux (as long as it resides on a partition accessible to both, so putting it on your Windows partition is fine since Linux reads almost all Windows partition types). BestCrypt consists of some kernel modules (so your kernel will need to support loadable kernel modules obviously, and it helps if you are using tools like depmod, modprobe and the kernel module loader), and a userspace utility called "bctool". This program is however officially in "beta testing" for Linux, and probably should not be used for critical data (if it is, make sure you have backups). After testing BestCrypt for Linux I am satisfied that even though the software is officially beta, it is probably stable enough for most users, however your mileage may vary, all sales final, and don't blame me for any lost data. The only real problem with BestCrypt is a severe lack of documentation, while there is a man page that explains basic options, there is not a single example of how to create and mount a container (I suspect the release will have documentation, their Windows version documentation is quite good, a half meg helpfile). You need to download the software first, available as a source tarball, and source rpm (very easy to install on an RPM based system). Simply download either one, I would recommend the source rpm if you can. You can get it http://www.jetico.com/.

# rpm -Uvh BestCrypt-0.3b-1.src.rpm 
BestCrypt ##################################################
# cd /usr/src/redhat/SPECS
# rpm -ba bcrypt.spec 

followed by a lot of text while it unpacks, compiles and assembles the source RPM and binary RPM. You should then have a:


Simply install the binary RPM with a:

# rpm -Uvh /usr/src/redhat/RPMS/i386/BestCrypt-0.3b-1.i386.rpm 
BestCrypt ##################################################

If you do not have an RPM based system, or the source RPM doesn't work for you, compiling the source code directly from it's tarball should be possible. Simply download the file, unpack it to an appropriate place (such as /usr/local/src) and issue the commands:

# make
# make install

And you should be up and running. The first step is to create a container (a file that is encrypted and mounted as a partition):

# bctool new -a blowfish -s 10M file
Enter password:
Verify password: 

You can of course use the "gost" or "des" algorithms, I would not recommend them as gost is less tested then the "twofish" and "blowfish" algorithms that BestCrypt supports, and single des is to easy to brute force. The next step is to format the container, you'll probably want to use MS-DOS if sharing with Windows (i.e. a dual boot Linux and Windows machine), or if just Linux then ext2 is a good bet. You can also specify the size, if you make it so small this can be a problem, but because it is a file and not a true partition you can easily create a new, larger file, move all the data to it and use it instead of the older smaller one.

# bctool format -t ext2 file
Enter password:
mke2fs 1.15, 18-Jul-1999 for EXT2 FS 0.5b, 95/08/09
Filesystem label=
OS type: Linux
Block size=1024 (log=0)
Fragment size=1024 (log=0)
2560 inodes, 10238 blocks
511 blocks (4.99%) reserved for the super user
First data block=1
2 block groups
8192 blocks per group, 8192 fragments per group
1280 inodes per group
Superblock backups stored on blocks: 

Writing inode tables: done 
Writing superblocks and filesystem accounting information: done

Once the file is formatted you should be able to mount it:

# bctool mount file /root/crypt/
Enter password:
# df
Filesystem	1k-blocks      Used Available Use% Mounted on
/dev/hda1         3122956     70596   2893720   2% /
/dev/hda2         2917360     24224   2744940   1% /crypto
/root/file	     9909        13      9385   0% /root/crypt

As you can see it is mounted as a part of the filesystem, just like a floppy disk would be for example. Remember to control access to the directory hosting the encrypted files carefully, no matter how good the encryption, if you have it set world readable you won't have gained any security. Also remember that as a user, root owns the / and can take ownership of any file or directory and see what's in it. Alternatively if an attacker gains root access they can log your keystrokes (or terminal traffic) and gain your password (and access to your files). As always your security is only as good as the weakest link.



PPDD is similar to BestCrypt, but instead of creating a file, encrypting that and mounting it, it actually uses a partition which is encrypted and mounted using the PPDD driver, because of this it can do a few additional things BestCrypt can't. If you only want to encrypt a few directories then I advise compiling PPDD as a kernel module, but if you want to encrypt the entire file system (including what you boot from) you will need to compile PPDD directly into the kernel (although as of 1.0 it's not to hard). Unless you have a GPL only policy I would recommend using BestCrypt if you are new to this (it is easier to install and use, and you can buy support). PPDD does have one enormous advantage over BestCrypt however, you can encrypt all of the system, including the boot drive and swap partition, making it ideal for situations such as laptops with sensitive data and minimizing the risk (to zero if need be) of accidentally leaving sensitive data in an unencrypted location (such as the swap file, /tmp, and so on) so if you need a higher security level I would recommend PPDD over BestCrypt (simply because you can encrypt everything). Another advantage of PPDD is that is uses two passwords instead of just one for each encrypted filesystem, so you can give one administrator one password, and another administrator the other password, meaning no single person can gain access to the data. Unfortunately as of the writing of this chapter PPDD is not available for kernel 2.2.13 or 2.2.14, so you will have to run the older 2.2.12 kernel (which is the stock kernel on many distributions in any case). You can get PPDD http://linux01.gwdg.de/~alatham/.

Download PPDD, and unpack it in a suitable location, such as /usr/local/src/, there are several files you should read, most notable the README file, and once done install I would recommend reading the PPDDHow.txt file. Installation is rather simply with:

#make check_linux
#make trial_patch
#make apply_patch
#make devices

This will first test the kernel source to make sure it's the right version and so on, then it will test the patches, then apply the patches proper, and then create the devices needed (similar to what BestCrypt does). At this point you need to recompile your kernel, first make sure you go into the configuration (via make config or make menuconfig or make xconfig), and enable the PPDD driver (in the Block devices section). Then save the config file and recompile the kernel as your normally would. Once that is done you will have to install the new kernel (copy it to /boot typically, edit lilo.conf and rerun lilo). Once you have rebooted you will want to build the tools for PPDD and install them with:

#make install

At this point you should be ready to use it, however I would recommend running the tests with:

#make test

They take a while to run, but it will save frustration later on if something is broken. Using PPDD is relatively simple, there are a number of utilities for creating, managing, encrypting file systems, and so on. You will also want to set the permissions and ownership on the /dev/xxxx that contains your encrypted data so that only root has access to it, PPDD will complain otherwise

#chown root:root /dev/hda3
#chmod ugo-a /dev/hda3
#ppddinit /dev/ppdd0 /dev/hda3
#ppddsetup -s /dev/ppdd0 /dev/hda3
#mke2fs -b 1024 /dev/ppdd0
#mount /dev/ppdd0 /crypt

At this point you should have a directory called /crypt which is /dev/hda3 (although on df and the like it will show up as /dev/ppddx). I will cover how to encrypt you entire filesystem with PPDD, at a later date however (it is extensively documented though).


Hiding data

One issue many people forget that is the very act of encrypting data can draw attention. For example if a corporate administrator scanned workstations for files ending in .pgp, and you were the only one with files such as that.... Unfortunately most of these programs leave traces and are detectable.



StegHide hides data in files such as sound and picture files where not all of the bits in a byte are used. Since the data is encrypted it will appear random, and proving that the data is actually there is difficult. The only downside is to store a one megabyte file you need a sound/picture file of several megabytes, which can be cumbersome (but hard drives and high speed access are becoming cheap so it's a moot point). You can get StegHide http://www.stego.com/.



Steganographic File System actually hides data on your harddrive, making it difficult to prove that it even exists. This can be very useful as the attacker first has to find the data, let alone break the strong encryption used to protect it. You can get StegFS http://ban.joh.cam.ac.uk/~adm36/StegFS/.



OutGuess hides data in image files, meaning you can send files in a way that won't attract to much attention (and can't really be proved either). You can get it http://www.outguess.org/.



RubberHose is available http://www.rubberhose.org/.



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