9.4. Encryption

9.4.1. General remarks Why should you encrypt data?

Encryption is synonym to secrecy. In the context of backups, encryption can be very useful, for instance if you need to leave your backed up data in a place where you can not control access, such as the server of your provider.

Apart from that, encryption can be applied to E-mails as well: normally, mail is not encrypted and it is often sent in the open over the netwerk or the Internet. If your message contains sensitive information, better encrypt it. GNU Privacy Guard

On Linux systems you will find GnuPG, the GNU Privacy Guard, which is a suite of programs that are compatible with the PGP (Pretty Good Privacy) tools that are commercially available.

In this guide we will only discuss the very simple usage of the encryption tools and show what you will need in order to generate an encryption key and use it to encrypt data for yourself, which you can then safely store in a public place. More advanced usage directions can be found in the man pages of the various commands.

9.4.2. Generate a key

Before you can start encrypting your data, you need to create a pair of keys. The pair consists of a private and a public key. You can send the public key to correspondents, who can use it to encrypt data for you, which you decrypt with your private key. You always keep the private key, never share it with somebody else, or they will be able to decrypt data that is only destined for you. Just to make sure that no accidents happen, the private key is protected with a password. The key pair is created using this command:

willy@ubuntu:~$ gpg --key-gen
gpg (GnuPG); Copyright (C) 2005 Free Software Foundation, Inc.
This program comes with ABSOLUTELY NO WARRANTY.
This is free software, and you are welcome to redistribute it
under certain conditions.  See the file COPYING for details.

gpg: directory `/home/willy.gnupg' created
gpg: new configuration file `/home/willy/.gnupg/gpg.conf' created
gpg: WARNING: options in `/home/willy/.gnupg/gpg.conf' are not yet
 active during this run
gpg: keyring `/home/willy/.gnupg/secring.gpg' created
gpg: keyring `/home/willy/.gnupg/pubring.gpg' created
Please select what kind of key you want:
    (1) DSA and Elgamal (default)
    (2) DSA (sign only)
    (5) RSA (sign only)
Your selection? 1
DSA keypair will have 1024 bits.
ELG-E keys may be between 1024 and 4096 bits long.
What keysize do you want? (2048) 4096
Requested keysize is 4096 bits
Please specify how long the key should be valid.
         0 = key does not expire
      <n>  = key expires in n days
      <n>w = key expires in n weeks
      <n>m = key expires in n month
      <n>y = key expires in n years
Key is valid for? (0) 0
Key does not expire at all
Is this correct? (y/N) y

You need a user ID to identify your key; the software constructs the
user ID from the Real Name, Comment and Email Address in this form:
    "Heinrich Heine (Der Dichter) <heinrichh@duesseldorf.de>"

Real name: Willy De Wandel
Email address: wdw@mvg.vl
Comment: Willem
You selected this USER-ID:
    "Willy De Wandel (Willem) <wdw@mvg.vl>"

Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O
You need a Passphrase to protect your secret key.

Now enetr your password. This can be a phrase, the longer, the better, the only condition is that you should be able to remember it at all times. For verification, you need to enter the same phrase again.

Now the key pair is generated by a program that spawns random numbers and that is, among other factors, fed with the activity data of the system. So it is a good idea to start some programs now, to move the mouse cursor or to type some random characters in a terminal window. That way, the chances to generate a number that contains lots of different digits will be much bigger and the key will be more difficult to crack.

9.4.3. About your key

When your key has been created, you will get a message about the fingerprint. This is a sequence of 40 hexadecimal numbers, which is so long that it is very, very hard to generate the same key twice, on any computer. You can be rather sure that this is a unique sequence. The short form of this key consists of your name, followed by the last 8 hexadecimal numbers.

You can get information about your key as follows:

willy@ubuntu:~$ gpg --list-keys
pub     1024D/BF5C3DBB 2006-08-08
uid                    Willy De Wandel (Willem) <wdw@mvg.vl>
sub     4096g/A3449CF7 2006-08-08

The key ID of this key is "BF5C3DBB". You can send your key ID and your name to a key server, so that other people can get this info about you and use it to encrypt data for you. Alternatively, you can send your public key directly to the people who need it. The public part of your key is the long series of numbers that you see when using the --export option to the gpg command:

gpg --export -a

However, as far is this guide is concerned, we assume that you only need your key in order to encrypt and decrypt data for yourself. Read the gpg man pages if you want to know more.

9.4.4. Encrypt data

Now you can encrypt a .tar archive or a compressed archive, prior to saving it to a backup medium or transporting it to the backup server. Use the gpg command like this:

gpg -e -r (part of) uid archive

The -e option tells gpg to encrypt, the -r option indicates who to encrypt for. Keep in mind that only only the user name(s) following this -r option will be able to decrypt the data again. An example:

willy@ubuntu:~$ gpg -e -r Willy /var/tmp/home-willy-20060808.tar

9.4.5. Decrypting files

Using the -d option, you can decrypt files that have been encrypted for you. The data will scroll over your screen, but an encrypted copy will remain on disk. So for file formats other than plain text, you will want to save the decrypted data, so that you can view them with the appropriate program. This is done using the -o option to the gpg command:

willy@ubuntu:~$ gpg -d -o /var/tmp/home-willy-decrypt.tar /var/tmp/home-willy-20060808.tar.gpg

You need a passphrase to unlock the secret key for
user: "Willy De Wandel (Willem) <wdw@mvg.vl>"
4096 ELG-E key, ID A3449CF7, created 2006-08-08 (main key ID BF5C3DBB)

gpg: encrypted with 4096-bit ELG-E key, ID A3449CF7, created 2006-08-08
        "Willy De Wandel (Willem) <wdw@mvg.vl>"

WarningNo password = no data

If you can not remember your password, the data is lost. Not even the system administrator will be able to decrypt the data. That is why a copy of important keys is sometimes kept in a sealed vault in a bank.