A colleague at work once asked me how to get started using gpg, the GNU Privacy Guard. He had no experience with it at all. Here’s a slightly expanded version of what I told him.

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.

Generate a private key.

gpg --gen-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.

Keyserver Registration

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

First up, you need to identify your key’s ID or fingerprint. The former is easier to use, the latter a bit more secure.

In the example below,

• the ID is 8F54CA35; it’s on the line marked pub;
• the fingerprint is 00E5 2D6D 91C0 20D0 F596 2CC5 1E36 9C62 8F54 CA35.

[~]$ gpg --fingerprint heinlein
pub   1024D/8F54CA35 2000-11-10
      Key fingerprint = 00E5 2D6D 91C0 20D0 F596  2CC5 1E36 9C62 8F54 CA35
uid                  Paul Heinlein <heinlein@madboa.com>
uid                  [jpeg image of size 3853]
uid                  Paul Heinlein (Galois, Inc.) <heinlein@galois.com>
sub   1024g/6088B91E 2000-11-10

To send them, you’ll need to locate a public key server. MIT’s is well known, but there are others.

# using ID (GnuPG versions 1 and 2)
gpg --keyserver pgp.mit.edu --send-keys '8F54CA35'
# using fingerprint (GnuPG version 2 and higher)
gpg --keyserver pgp.mit.edu \
    --send-keys '00E5 2D6D 91C0 20D0 F596  2CC5 1E36 9C62 8F54 CA35'

ASCII Version

You may also want to generate an ASCII version of your public key for distribution by e-mail or posting on a web site.

gpg --armor --output pubkey.txt --export 'Your Name'

Encrypting a file for personal use

Encrypting files for your personal use is quite easy.

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
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 --output (-o) option.

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.

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 = 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.

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 --import option to add it to your keyring:

gpg --import key.asc

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

Alternatively, you might be able to find your friend’s key on a public keyserver. Here’s what a session looks like when someone searches for my key.

[~]$ gpg --keyserver pool.sks-keyservers.net --search-keys 'paul heinlein'
gpg: searching for "paul heinlein" from hkp server pool.sks-keyservers.net
(1)  Paul Heinlein <heinlein@madboa.com>
       1024 bit RSA key 8F54CA35, created: 2014-06-16 (revoked)
(2)  Paul Heinlein <heinlein@ohsu.edu>
     Paul Heinlein <heinlein@madboa.com>
     Paul Heinlein <heinlein@cse.ogi.edu>
     Paul Heinlein (Galois, Inc.) <heinlein@galois.com>
       1024 bit DSA key 8F54CA35, created: 2000-11-10
Keys 1-2 of 2 for "paul heinlein".  Enter number(s), N)ext, or Q)uit > Q
[~]$

A few notes on this:

• The first result (marked “revoked”) was the result of a failed test I conducted. You can ignore it.
• My key has several e-mail addresses attached to it. That’s perfectly normal.
• In the example above, I chose the Q)uit option. Had I pressed 1 or 2, that key would have been downloaded and added to my local keyring.

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

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.

gpg --output foo.txt --decrypt foo.txt.gpg

Detached Signatures

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 is easy:

gpg --verify crucial.tar.gz.asc crucial.tar.gz

Creating a detached signature is similarly easy. The following example will create a signature for your-file.zip called your-file.zip.asc.

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.

Basic Key Management

After a while, you will probably have several keys in your ring. It’s easy to list them all:

gpg --list-keys

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 'myfriend@his.isp.com'

For further reading

To move beyond these simple instructions, consult the GnuPG Documentation.

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