Copyright © 2012, 2013 Free Software Foundation, Inc.
Permission is granted to copy, distribute and/or modify this document under the terms of the GNU Free Documentation License, Version 1.3 or any later version published by the Free Software Foundation; with no Invariant Sections, no Front-Cover Texts, and no Back-Cover Texts. A copy of the license is included in the section entitled “GNU Free Documentation License”. GNU fcrypt, a program to transparent and automatic encryption. Allow you to access encrypted data like unencrypted HD's.
GNU fcrypt is part of the GNU project, released under the aegis of GNU.
GNU fcrypt is on the fly encryption. Instead encrypting individual files. You can have entire file systems encrypted.
GNU fcrypt allows you to hide data from a attacker, who has physical access to your hard drive (eg. seizure or steal your computer). Without inserting the correct password, the data must be intelligible and indistinguishable from random data.
It simply writes your data in a encrypted file. Containing the partitions. It is not bootable. You can choose to remember a password, or you can let GNU fcrypt generate a random password.
If you choose to use a random password then the data will be lost when you disable GNU fcrypt or shutdown your computer(If you not have a swap partition). If you have a unencrypted swap partition the attacker can extract the password (random or not) from your hard disk. So if you want to hide data I recommend that you do not use a swap partition. Or use a encrypted swap partition using a random password. You can use GNU fcrypt to do so.
The goal of GNU fcrypt is that in many cases try to force you to reveal the password is useless. Because whenever the attacker try to force you to reveal a password, you can reveal passwords to data that you not want to hide. You can do it many times. The attacker should not be able to prove that you're hiding something from him.
If your computer is on 24 hours a day. So I recommend that you use a random password, this will allow you to hide data telling the truth.
Using GNU fcrypt you will have some resistance who try to censor the data that you have on your hard disk.
If something is not clear to you, or if you have any doubts. You can ask on the mailing lists of GNU fcrypt.
I recommend that you try things with GNU fcrypt, the extent that you learns it from this documentation.
It is used by governments to protect confidential data. When used with 128-bit keys can be broken by quantum computers. But it supports 256-bit key. GNU fcrypt uses it with 256-bit key.
AES is the successor of DES. That has
been broken by the small key size. This
algorithm is fast in hardware, but slow in
software. This is yet used with triple encryption
to protect data. This will fix the problem
of small key size and make it more safe.
But DES have a small block size and
triple-encryption is 3 time slower. libgcrypt
not recommend this algorithm for new protocols.
Then DES is not used on GNU fcrypt.
On Trisquel you can install the depedencies using:
sudo apt-get install linux-headers-`uname -r` libgcrypt-dev
It is recommeded to not uninstall the gcc and the linux-headers. If not GNU fcrypt can stop working after updating the Linux kernel.
To get the last released stable version of GNU fcrypt see:
If you is interested in GNU fcrypt (eg. new releases, security information or development). You can join to GNU fcrypt maling lists, see
Download the tarball and unpack it. This can be
done using GNU tar:
tar xzvf fcrypt-<version number>.tar.gz.
Then go to that directory using:
cd fcrypt-<version number>.
To get the development version of GNU fcrypt, use:
git clone git://git.savannah.gnu.org/fcrypt.git
Goto to it's directory:
Generate the configure file:
./configure && make all. If the command
was been completed, run(as root):
The default directory is /usr/local(because it you, may need install as root). But, you can use the directory you want. Read the file INSTALL(Comes in GNU fcrypt package), for more information.
If all dependencies were found (eg. the configure script finished), but you can not compile. Report this as a bug at firstname.lastname@example.org.
If you is using the GNU/Linux or GNU/Hurd operating systems and all dependencies, are installed and you still can not compile, run or GNU fcrypt not work well. Report this as a bug. But, try the latest released version before, maybe the bug has already been fixed.
GNU fcrypt not touch your real hard disks. Instead GNU fcrypt use a normal file and write encrypted data on it.
You can create partition's
fcrypt -c <some file name>
In this case <some file name> not need exists.
GNU fcrypt can create it.
You can use
If you need hide some data, in the case
the attacker force you to reveal the password.
In GNU/Linux after created. You can activate the partition
fcrypt -a <file name>.
Once activated, the partitions are accessed in
/dev/fcrypt[some letter] (eg. /dev/fcrypta).
Encryption is automatic and transparent, so you can access
as if you were accessing a volume that is not encrypted.
You can create a file system using mkfs and then
mount it using mount(eg.
mkfs.ext4 /dev/fcrypta and
mount /dev/fcrypta /mnt).
To deactivate GNU fcrypt. Make sure that no program is
using: /dev/fcrypt(It is not the same
/dev/fcrypt_info). If /dev/fcrypt is
mounted, umount it(eg.
Then simply interrupt GNU fcrypt. Once stopped GNU
fcrypt will not process any more requests. Programs
that try to write/read data from /dev/fcrypt
will get a Input/Output error. New opened
programs can no longer open /dev/fcrypt,
these programs will get the error:
Device or resource busy.
If you want to finalize GNU fcrypt, I recommend
you to not use
kill -9 or
Instead you can use the simply
killall(without the -9).
Because, when correctly finalized, GNU fcrypt,
will write the remaining data to disk and
delete all it's allocated data from memory.
fcrypt-pwgen is a small program for generating keys
from passwords, that is part of GNU fcrypt. It's job is to
generate random-looking sequences from possible human
typed passwords(this is called key derivation).
Even if the attacker gets the resulting sequence, him
will not be able to recover the original password. It
is useful for example if you have accounts on many web
sites, and want to access each account without exposing,
your passwords to the another account.
If you access these sites with similar passwords will expose your password to other accounts. But if you use very different passwords, may be difficult to remember.
You can save these passwords to disk. But this may expose them to other users of the computer, or even in the case of seizure or stolen.
A way to avoid this problem is using disk encryption, such that GNU fcrypt. But sometimes you not have enough privileges to setup a fcrypt device.
Then you can use a very visible randomly generated salt file, that remains in your computer and use it to generate secure, distinct passwords for very similar passwords, without exposing your original password.
Note that using this program to generate passwords to be used with fcrypt, will be useless and may reduce the security. The fcrypt program implement key-derivation.
fcrypt-pwgen generate keys such that,
the attacker are unable to generate the
password(that you typed), from the key generated
by the program.
It is possible to compute the key from the passwords. Then someone can store these pre-computed keys together with the password. Then use this to easier get the password that you typed from the password. Using a salt file avoid that. If someone not have the salt, then will be unable to pre-compute the salt from the password, because the salt have the same size that the generate key. See GNU fcrypt and Quantum computers. Then the attacker will not be able to pre-compute the key from the password. The salt file not need be secret. But if it is public will give the attacker a advantage to pre-compute the password.
Now I will describe the options showed when you type
fcrypt-pwgen --help. This command will show
the shortcut to these options.
This number is readed as a
float: Can be
fcrypt-pwgenask to your typing the password twice. Because it is less likely to generate password for a typo. But if you use this option
fcrypt-pwgenwill not ask to you retype.
The default is 10.
If you use only this option will
be displayed only special characters
and letters. But this option can
be combined with
If you use only this option will
be displayed only special characters
and numbers. But this option can
be combined with
Often when you try to delete a file from disk, it isn't really deleted. Instead the information about it's existence(eg. what sectors it is using), but not the file itself. Because it, have thousands of programs, that undelete your data from disk.
If you want to really delete data you need to replace
it by something else. The program
shred -u <filename>,
included in the GNU/Linux and GNU/Hurd(coreutils package)
securely delete your files.
But if you have deleted your files in the wrong way then
you can fill your hard disc several times with random data.
You can copy your secret files to a encrypted file system
see Quick start. Then fill your hard disk with random
dd if=/dev/urandom of=/mnt/random_data). You
can delete /mnt/random_data after it.
Maybe a powerfull atackers can see what has recorded
previously in your hard disk. Even if you replace it
with something else. You can use
shred <device name>
to securely erase your hard disk. Maybe you want to use:
shred --iterations=5 <device name>. To be more
resistant to these attackers.
A example of unbreakable encryption is one time pad. That is, the data is XORed with a perfectly random key of the same size. Many others are very hard to compute, but not unbreakable. A big enough computer can break them. But, not exists a big enough computer to break a 256 bit key by brute force yet.
This encryption is unbreakable, because symmetric encryption is not deterministic. The only way to know if I have the correct password is using some knowledge about the unencrypted content's. If the password is much smaller than the content. Eg a single character password and a 9 characters plaintext(the unencrypted content). Then it is unlikely(but not impossible) that I get HELLO EVA randomly by using a wrong password. Then I have the correct password. But if the key have the same size that the plaintext, and each key generate a different plaintext. Then I can get all the possible phrases with 9 characters. Then if I get the phrase HELLO EVA, that means nothing. Because have the same chance of being to be HELLO BOB or SICAIBJV, if I use a perfectly randomly password.
But with some public key encryption (like RSA) it is very different, the private key is deterministic from the public key. But it is very hard to compute.
A big enough Quantum computer can broken these public key encryption in logarithm time. Then these public key encryption will be unsafe.
But, the symmetric encryption used by GNU fcrypt is much more secure, the best known algorithm to run on a quantum computer. Takes 2 ^ (n/2) attempts to break a password of n bits. Then 128 bits key will be unsafe, but, it is not a problem to GNU fcrypt, because it uses at least 512 bit keys. Using 256 bit ciphers. Note however that the key length may be larger.
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