A quick HOWTO for getting started with GnuPG.
A colleague at work once asked me how to get started using
GNU Privacy Guard. He had no experience with it
at all. Here’s a slightly expanded version of what I told him.
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
Generate a private key.
You’ll have to answer a bunch of questions:
What kind and size of key you want; the defaults are probably good enough.
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.
Your real name and e-mail address; these are necessary for identifying your key in a larger set of keys.
A comment for your key, perhaps to distinquish a key used for special tasks like signing software releases. The comment can be empty.
A passphrase. Whatever you do, don’t forget it! Your key, and all your encrypted files, will be useless if you do.
Generate an ASCII version of your public key.
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.
You might also want to register your key with public keyservers so that others can retrieve your key without having to contact you directly.
gpg --send-keys 'Your Name' --keyserver hkp://subkeys.pgp.net
Encrypting files for your personal use is quite easy.
Encrypt a file called
foo.txt. The argument to the
option should be the all or part of the name you used when generating
your private key.
# the long version gpg --encrypt --recipient 'Your Name' foo.txt # using terse options 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
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
gpg --output foo.txt --decrypt foo.txt.gpg
If you have an encrypted file that you think you’ll want to edit on a regular basis, you might consider using the gnupg.vim plugin for transparently editing gpg-encrypted files.
A more DIY approach can use
make to automate the process of viewing
and editing your encrypted file. 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). The example assumes that
foo.txt is the name of the unencrypted version of your file.
# example Makefile for viewing/editing an encrypted file GPGID = email@example.com 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
The really cool thing about GnuPG is that you can safely encrypt files for others using publicly available keys.
Import your friend’s key, which you might have received via e-mail or on
a floppy. If the file is named
key.asc, then just use the
option to add it to your keyring:
gpg --import key.asc
That’s it! You can verify the import using the
Alternatively, you might be able to find your friend’s key on a public keyserver.
gpg --search-keys 'firstname.lastname@example.org' \ --keyserver hkp://subkeys.pgp.net
Here’s what a session looks like when someone searches for my key.
$ 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 > 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.
Once you’ve got the other person’s public key, encrypt a file using it.
gpg --encrypt --recipient 'email@example.com' 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.
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.
gpg --output foo.txt --decrypt foo.txt.gpg
GnuPG can come in handy when you want to be assured that the file you’ve just downloaded is the one its creator wants you to have. The OpenVPN developers, for instance, release GnuPG signatures for all their downloads.
To verify a file using its detached signature, you must first have
imported the signer’s public key. Assume we’ve downloaded
crucial.tar.gz and the developers have also released a signature file,
crucial.tar.gz.asc. Once you’re confident that you have the
developers’ public key in your local keyring, then the verification step
gpg --verify crucial.tar.gz.asc crucial.tar.gz
Creating a detached signature is similarly easy. The following example
will create a signature for
gpg --armor --detach-sign your-file.zip
People who have imported your public key into their keyrings can then verify that their version of your file is identical to theirs.
After a while, you will probably have several keys in your ring. It’s easy to list them all:
Should you lose trust in or contact with a person with a key in your ring, you’ll want to delete it:
gpg --delete-key 'firstname.lastname@example.org'
To move beyond these simple instructions, consult the GnuPG Documentation.