How to Use GPG in Linux to Encrypt Files

GPG includes the tools you need to use public key encryption and digital signatures on your Linux system. You can figure out how to use GPG gradually as you begin using encryption in Linux. The information you find here shows some of the typical tasks you can perform with GPG to protect your Linux system.

How to Generate the key pair with GPG in Linux

The steps for generating the key pairs are as follows:

1. Type gpg --gen-key.

   If you’re using GPG for the first time, it creates a .gnupg directory in your home directory and a file named gpg.conf in that directory. Then it asks what kind of keys you want:

   Please select what kind of key you want:
   (1) DSA and ElGamal (default)
   (2) DSA (sign only)
   (4) RSA (sign only)
   Your selection?

2. Press Enter for the default choice, which is good enough.

   GPG prompts you for the key size (the number of bits).

3. Press Enter again to accept the default value of 2,048 bits.

   GPG asks you when the keys expire. The default is to never expire.

4. If the default is what you want (and why not?), press Enter.
5. When GPG asks whether you really want the keys to never expire, press the Y key to confirm.

   GPG prompts you for your name, your email address, and a comment to make it easier to associate the key pair with your name.

6. Type each piece of requested information, and press Enter.
7. When GPG gives you a chance to change the information or confirm it, confirm by typing o and pressing Enter.

   GPG prompts you for a passphrase that protects your private key.

8. Type a long phrase that includes lowercase and uppercase letters, numbers, and punctuation marks — the longer the better — and then press Enter.

   Be careful to choose a passphrase that you can remember easily.

   GPG generates the keys. It may ask you to perform some work on the PC so that the random-number generator can generate enough random numbers for the key-generation process.

How to exchange keys using GPG in Linux

If you're an administrator, protecting your Linux system should always be at the top of your mind. To communicate with others, you have to give them your public key. You also have to get public keys from those who may send you a message (or when someone who might sign a file and you want to verify the signature). GPG keeps the public keys in your key ring. (The key ring is simply the public keys stored in a file, but the name sounds nice because everyone has a key ring in the real world, and these keys are keys of a sort.) To list the keys in your key ring, type

gpg --list-keys

To send your public key to someone or to place it on a website, you have to export the key to a file. The best way is to put the key in what GPG documentation calls ASCII-armored format, with a command like this:

gpg --armor --export [email protected] > kdulaneykey.asc

This command saves the public key in ASCII-armored format (which looks like garbled text) in the file named kdulaneykey.asc. You replace the email address with your email address (the one you used when you created the key) and replace the output filename with something different.

After you export the public key to a file, you can mail that file to others or place it on a website for use by others.

When you import a key from someone, you typically get it in ASCII-armored format as well. If you have a us-http://www.us-cert.gov/pgp/email.htmlin a file named uscertkey.asc, you import it into the key ring with the following command:

gpg --import uscertkey.asc

Use the gpg --list-keys command to verify that the key is in your key ring. Here’s what you might see when typing gpg --list-keys on the system:

/home/kdulaney/.gnupg/pubring.gpg
-----------------------------
pub 1024D/7B38A728 2018-08-28
uid Kristin Dulaney <[email protected]>
sub 2048g/3BD6D418 2018-08-28
pub 2048R/F0E187D0 2019-09-08 [expires: 2019-10-01]
uid US-CERT Operations Key <[email protected]>

The next step is checking the fingerprint of the new key. Type the following command to get the fingerprint of the US-CERT key:

gpg --fingerprint [email protected]

GPG prints the fingerprint, as follows:

pub 2048R/F0E187D0 2018-09-08 [expires: 2019-10-01]
Key fingerprint = 049F E3BA 240B 4CF1 3A76 06DC 1868 49EC F0E1 87D0
uid US-CERT Operations Key <[email protected]>

At this point, you need to verify the key fingerprint with someone at the US-CERT organization.

If you think that the key fingerprint is good, you can sign the key and validate it. Here’s the command you use to sign the key:

gpg --sign-key [email protected]

GPG asks for confirmation and then prompts you for your passphrase. After that, GPG signs the key.

Because key verification and signing are potential weak links in GPG, be careful about what keys you sign. By signing a key, you say that you trust the key to be from that person or organization.

How to sign a file with GPG in Linux

You may find signing files to be useful if you send a file to someone and want to assure the recipient that no one tampered with the file and that you did in fact send the file. GPG makes signing a file easy. You can compress and sign a file named message with the following command:

gpg -o message.sig -s message

To verify the signature, type

gpg --verify message.sig

To get back the original document, type

gpg -o message --decrypt message.sig

Sometimes, you don’t care about keeping a message secret, but you want to sign it to indicate that the message is from you. In such a case, you can generate and append a clear-text signature with the following command:

gpg -o message.asc --clearsign message

This command appends a clear-text signature to the text message. Here’s a typical clear-text signature block:

-----BEGIN PGP SIGNATURE-----
Version: GnuPG v1.4.2 (GNU/Linux)
iD8DBQFDEhAtaHWlHHs4pygRAhiqAJ9Qj0pPMgKVBuokDyUZaEYVsp6RIQCfaoBm
9zCwrSAG9mo2DXJvbKS3ri8=
=2uc/
-----END PGP SIGNATURE-----

When a message has a clear-text signature appended, you can use GPG to verify the signature with the following command:

gpg --verify message.asc
If you indeed signed the message, the last line of the output says that the signature is good.

Encrypting and decrypting documents with GPG in Linux

To encrypt a message meant for a recipient, you can use the --encrypt (or -e) GPG command. Here’s how you might encrypt a message for US-CERT by using its GPG key:

gpg -o message.gpg -e -r [email protected] message

The message is encrypted with the US-CERT public key (without a signature, but you can add the signature with the -s command).

When US-CERT receives the message.gpg file, the recipient must decrypt it by using US-CERT’s private key. Here’s the command that someone at US-CERT can use:

gpg -o message --decrypt message.gpg

Then GPG prompts for the passphrase to unlock the US-CERT private key, decrypts the message, and saves the output in the file named message.

If you want to encrypt a file that no one else has to decrypt, you can use GPG to perform symmetric encryption. In this case, you provide a passphrase to encrypt the file with the following GPG command:

gpg -o secret.gpg -c somefile

GPG prompts you for the passphrase and asks you to repeat the passphrase (to make sure that you didn’t mistype anything). Then GPG encrypts the file, using a key generated from the passphrase.

To decrypt a file encrypted with a symmetric key, type

gpg -o myfile --decrypt secret.gpg

GPG prompts you for the passphrase. If you enter the correct passphrase, GPG decrypts the file and saves the output (in this example) in the file named myfile.

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