GPG Quick Guide

Saman · Post by **Saman** » Wed Jun 16, 2010 6:19 am

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.
Code: Select all
```
gpg --gen-key
```
You’ll have to answer a bunch of questions:
1. What kind and size of key you want; the defaults are probably good enough.
2. 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.
3. Your real name and e-mail address; these are necessary for identifying your key in a larger set of keys.
4. A comment for your key, perhaps to distinquish a key used for special tasks like signing software releases. The comment can be empty.
5. 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.
Code: Select all
```
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.
Code: Select all
```
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.

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.
Code: Select all
```
# 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.
Code: Select all
```
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).

Code: Select all

# example Makefile for viewing/editing an encrypted file
GPGID = [email protected]
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:
Code: Select all
```
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.

Code: Select all

gpg --search-keys '[email protected]'   --keyserver hkp://subkeys.pgp.net

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

Code: Select all

$ 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 <[email protected]>
          1024 bit DSA key 8F54CA35, created 2000-11-10
(2)     Paul Heinlein <[email protected]>
          1024 bit DSA key 8F54CA35, created 2000-11-10
(3)     Paul Heinlein <[email protected]>
          1024 bit DSA key 8F54CA35, created 2000-11-10
(4)     Paul Heinlein <[email protected]>
          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 <[email protected]>"
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.
Code: Select all
```
gpg --encrypt --recipient '[email protected]' 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.

Code: Select all

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:

Code: Select all

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.

Code: Select all

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:

Code: Select 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:

Code: Select all

gpg --delete-key '[email protected]'