For those of you who don't know, GPG is a tool that can be used to encrypt and decrypt files and messages through the PGP protocol.
I was recently teaching someone how to use GPG, and I realised that there aren't many tutorials online that show you how to use GPG's command line interface, so here goes (this tutorial assumes that you already have GPG installed).
First, you'll want to generate a key for yourself:
gpg --gen-key
You'll be asked to enter a few details. Don't forget these details.
Now before I go any further, let me explain the basics about how PGP works. You generate two keys for yourself: a private key, and a public key. Your private key should be kept safe and no one else except you should have it. Your public key, however, can be sent out to anyone you want. When you want to send a message to someone, you need to have their public key. Their public key is used by you to encrypt the file or message that you want to send to them. After you have encrypted the message, only they can decrypt it with their private key (that's why you only keep your private key to yourself, else anyone could decrypt your messages). Simple, right?
Now, let's take a look at your keys:
To list your public keys:
gpg --list-keys
To list your private keys:
gpg --list-secret-keys
Now, let's say your name is John Doe, and you want to send a message to Jane Doe. This is how you would do it (note that all names used must be the names you see when listing the keys).
First, export your public key:
gpg --export --armor youremail@example.com > publickey.asc
Example: gpg --export --armor john@gmail.com > mypublickey.asc
or
gpg --export --armor yourname > publickey.asc
Example: gpg --export --armor John Doe > mypublickey.asc
Send this file to Jane Doe. Get her to do the same.
To import someone else's public key:
gpg --import publickey.asc
Now that you've imported Jane Doe's key, let's send her an encrypted message.
To encrypt a file to send to Jane Doe:
gpg --encrypt --recipient receiversname filename.txt
Example: gpg --encrypt --recipient Jane Doe secretmessage.txt
or, if the previous command doesn't work:
gpg -e -u "sender's name (you)" -r "name of the receiver's key" filename.txt
Example: gpg -e -u "John Doe" -r "Jane Doe" secretmessage.txt
This will create a file called secretmessage.txt.pgp. Send this to Jane Doe.
Now Jane has received your file. This is how she decrypts it:
To decrypt to command line (meaning that you'll only see the message in the command line, and it won't be saved decrypted to your hard drive):
gpg --decrypt filename.txt.gpg
To decrypt to disk:
gpg filename.txt.gpg
Done!