Previously, in Getting Started with GnuPG, I explained how to import a public key to encrypt a file and verify a signature. Now learn how to create your own GPG key pair, add an email address, and export the public key.
Creating a GPG keypair
To receive an encrypted file that only you can open, you first need to create a key pair and then share your public key. Creating the key pair is similar to creating ssh keys in that you choose a key size, specify an identifier, and set a passphrase.
gpg command has three options for creating a key pair:
--quick-generate-keyoption requires you to specify the USER-ID field on the command line and optionally an algorithm, usage, and expire date. It implements defaults for all other options.
--generate-keyoption prompts for the real name and email fields before asking for a confirmation to proceed. In addition to creating the key, it also stores a revocation certificate.
--full-generate-keyoption, demonstrated below, provides a dialog for all options.
full generate options can also be used in a batch mode as documented in the
Let's describe the options on the
full generate option:
$ gpg --full-generate-key Please select what kind of key you want: (1) RSA and RSA (default) (2) DSA and Elgamal (3) DSA (sign only) (4) RSA (sign only) Your selection?
The first question is what kind of key algorithm you want. Defaults are that for a reason. Unless you have a company policy that specifies otherwise, choose the default of RSA and RSA for your multi-use or email exchange key pair.
RSA keys may be between 1024 and 4096 bits long. What keysize do you want? (2048)
Next is the key size. Longer is not always better, but I would definitely go with 2048 or 4096. The Fedora and Red Hat security keys we imported in the last article are both 4096 in length.
Please specify how long the key should be valid. 0 = key does not expire <n> = key expires in n days <n>w = key expires in n weeks <n>m = key expires in n months <n>y = key expires in n years Key is valid for? (0)
Check company policies for how long the key should be valid. Then consider your security habits as well. Notice the default is "does not expire." I usually go with years for an email key. For signing keys, I think about the expected lifetime of the objects I am signing. If you don't expire the key, it is never automatically revoked even if the private key is compromised. If you do expire the key, you need a plan to update and rotate keys before the expiration. You are asked to confirm your selection before continuing.
The next set of prompts constructs the identity.
GnuPG needs to construct a user ID to identify your key. Real name: Best User Email address: firstname.lastname@example.org Comment: Best Company You selected this USER-ID: "Best User (Best Company) <email@example.com>" Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit?
The Real name is the name of a person, company, or product. Email address is the contact email for the key, and the optional Comment can identify a company, use, or version. You can use the
gpg --list-keys command to view some of the identities for imported keys. Here are a few examples:
Red Hat, Inc. (Product Security) <firstname.lastname@example.org> Fedora (32) <email@example.com> Fedora (iot 2019) <firstname.lastname@example.org> Fedora EPEL (8) <email@example.com> Susan Lauber (Lauber System Solutions, Inc.) <firstname.lastname@example.org>
After confirming the settings, you are prompted for a passphrase for the private key. The
gpg command requires an agent for this, so you may find that you need to be logged in directly as the user. If you are on a graphical desktop such as GNOME, the agent may be a graphical pop-up box. Once completed, the key information is displayed on the screen.
Additionally, a lot of random bytes are needed to generate the key. A newly installed virtual machine may not have enough entropy. You can check the available entropy on a system by viewing a
$ cat /proc/sys/kernel/random/entropy_avail 3859
If the value is less than 3000, you may need to generate more entropy. Besides the keyboard and mouse activity that is suggested in the output of the
gpg command, additional entropy sources can be configured with the rng-tools package. A Red Hat knowledge article explains how to configure rngd to use the
/dev/urandom device for additional entropy.
Editing a GPG key
Occasionally you need to edit a key. You can change expiration dates and passwords, sign or revoke keys, and add and remove emails and photos.
$ gpg --edit-key email@example.com gpg>
At the subprompt,
help or a
? lists the available edit commands.
To add an email address, you will actually add a USER-ID value.
gpg> adduid Real name: Best User Email address: firstname.lastname@example.org Comment: Student account You selected this USER-ID: "Best User (Student account) <email@example.com>" Change (N)ame, (C)omment, (E)mail or (O)kay/(Q)uit? O
You can use
list to show the identities,
uid to select an identity, and
deluid to delete an identity. The
quit command exits the edit utility and prompts you to save your changes.
After adding a new USER-ID, both identities are shown when listing the key.
$ gpg --list-keys gpg: checking the trustdb gpg: marginals needed: 3 completes needed: 1 trust model: pgp gpg: depth: 0 valid: 1 signed: 0 trust: 0-, 0q, 0n, 0m, 0f, 1u gpg: next trustdb check due at 2021-04-23 /home/bestuser/.gnupg/pubring.kbx --------------------------------- pub rsa2048 2020-04-23 [SC] [expires: 2021-04-23] CC1795E6F83B091A7B813A6D94F45C144CD3559D uid [ultimate] Best User (Student account) <firstname.lastname@example.org> uid [ultimate] Best User (Best Company) <email@example.com> sub rsa2048 2020-04-23 [E] [expires: 2021-04-23]
Export the public key to share with others
For others to send you encrypted messages that can only be decrypted with your private key, you must first share your public key. Use the
--export option to export the key from the keyring to a file. In most cases, you will want to make sure the key file does not contain any binary characters so it can be displayed on a web page. The
--armor option encodes the output to plain text. The
--output option saves the output to a specified file instead of displaying it to standard out on the screen.
$ gpg --export --armor --output bestuser-gpg.pub
To allow other people a method of verifying the public key, also share the fingerprint of the public key in email signatures and even on business cards. The more places it appears, the more likely others will have a copy of the correct fingerprint to use for verification.
$ gpg --fingerprint
Publishing your exported GPG public key and fingerprint on your web site is a simple way to share the key. The key can also be shared on public keyservers, which also work with email program plugins.
GnuPG can help you better secure your communications and ensure that files originate from where you believe they should. Consider using it the next time you are sharing important files.
If you're interested in more on how entropy and random number generation play a role in cryptography, and how this plays out in the real world, check out these resources:
- Understanding random number generators, and their limitations, in Linux
- Entropy in RHEL-based cloud instances
- Understanding the Red Hat Enterprise Linux random number generator interface
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