GPG Quick Start

Your Key

Private and public keys are at the heart of gpg’s encryption and decryption processes. The best first step is to create a key pair for yourself.

1.Generate a private key.

[root@host]# gpg --gen-key

You’ll have to answer a bunch of questions:

a. What kind and size of key you want; the defaults are probably good enough.
b. How long the key should be valid. You can safely choose a non-expiring key for your own use. If you plan to use a key for public signing, you might want to consider a yearly expiration.
c. Your real name and e-mail address; these are necessary for identifying your key in a larger set of keys.
d. A comment for your key, perhaps to distinquish a key used for special tasks like signing software releases. The comment can be empty.
e. A passphrase. Whatever you do, don’t forget it! Your key, and all your encrypted files, will be useless if you do.

2.Generate an ASCII public version of your private key.

[root@host]# gpg --armor --output pubkey.txt --export 'Your Name'

You can freely distribute this file by sending it to friends, posting it on your web site, or whatever.

3.You might also want to register your key with public keyservers so that others can retrieve your key without having to contact you directly.

[root@host]# gpg --send-keys 'Your Name' --keyserver hkp://subkeys.pgp.net 

Encrypting a file for personal use

Encrypting files for your personal use is quite easy.

1. Encrypt a file called foo.txt. The argument to the --recipient option should be the all or part of the name you used when generating your private key.

# the long version

[root@host]# gpg --encrypt --recipient 'Your Name' foo.txt

# using terse options

[root@host]# gpg -e -r Name foo.txt 

The encrypted version of the file will by default be named foo.txt.gpg. You can modify that behavior using the --output (-o) option.

2.Decrypt the encrypted file. You’ll be asked to provide the passphrase you used when generating your private key. If you don’t use the --output option, the contents of the encrypted file will be sent to standard output.

[root@host]# gpg --output foo.txt --decrypt foo.txt.gpg 

If foo.txt is a file that you think you’ll want to edit and/or view on a regular basis, you might consider using make to reduce the amount of typing you’ll have to do. Here’s an example Makefile (to use it, you’ll need to make sure that the leading whitespace in the targets is composed of Tabs, not ordinary spaces).

# example Makefile for viewing/editing an encrypted file
GPGID = you@your.address
FILEPLAIN = foo.txt
FILECRYPT = $(FILEPLAIN).gpg
GPG = gpg
RM = /bin/rm -i
VI = vim
all:
         @echo ""
         @echo "usage:"
         @echo ""
         @echo "* make view -- to see $(FILEPLAIN)"
         @echo "* make edit -- to edit $(FILEPLAIN)"
         @echo ""

edit:

         @umask 0077;
         $(GPG) --output $(FILEPLAIN) --decrypt $(FILECRYPT)
         @$(VI) $(FILEPLAIN)
         @umask 0077;
         $(GPG) --encrypt --recipient $(GPGID) $(FILEPLAIN)
         @$(RM) $(FILEPLAIN)

view:

         @umask 0077; $(GPG) --decrypt $(FILECRYPT) | less

Encrypting a file for someone else

The really cool thing about GnuPG is that you can safely encrypt files for others using publicly available keys.

1.Import your friend’s key, which you might have received via e-mail. If the file is named key.asc, then just use the --import option to add it to your keyring:

[root@host]# gpg --import key.asc 

That’s it! You can verify the import using the --list-keys option.

2. Alternatively, you might be able to find your friend’s key on a public keyserver.

[root@host]# gpg --search-keys 'myfriend@his.isp.com' --keyserver hkp://subkeys.pgp.net

Here’s what a session looks like when someone searches for my key.

[root@host]# gpg --search-keys heinlein@madboa
gpgkeys: WARNING: this is an *experimental* HKP interface!
gpgkeys: searching for "heinlein@madboa" from HKP server subkeys.pgp.net
Keys 1-5 of 5 for "heinlein@madboa"
(1) Paul Heinlein
1024 bit DSA key 8F54CA35, created 2000-11-10
(2) Paul Heinlein
1024 bit DSA key 8F54CA35, created 2000-11-10
(3) Paul Heinlein
1024 bit DSA key 8F54CA35, created 2000-11-10
(4) Paul Heinlein
1024 bit DSA key 8F54CA35, created 2000-11-10
(5) [user attribute packet]
1024 bit DSA key 8F54CA35, created 2000-11-10
Enter number(s), N)ext, or Q)uit > 1
gpgkeys: WARNING: this is an *experimental* HKP interface!
gpg: key 8F54CA35: public key "Paul Heinlein " imported
gpg: Total number processed: 1
gpg: imported: 1

You’ll note that my key has four different e-mail addresses attached to it. That’s perfectly normal.

3.Once you’ve got the other person’s public key, encrypt a file using it.

[root@host]# gpg --encrypt --recipient 'myfriend@his.isp.net' foo.txt 

You’ll end up with a file called foo.txt.gpg that you can send as an e-mail attachment or make available for downloading via ftp or the web.

Decrypting a file from someone else

If someone sends you an encrypted file, the file has typically been encrypted using your public key. Decrypting it is no different than decrypting a file you’ve encrypted for your own use.

[root@host]# gpg --output foo.txt --decrypt foo.txt.gpg

For further reading

To move beyond these simple instructions, consult the GnuPG Documentation at http://www.gnupg.org/documentation/

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