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Using OpenPGP subkeys in Debian development
What are keys?
In public key cryptography, a key is actually a pair: a public key, and a private key. You use the private key to digitally sign files, and others use the public key to verify the signature. Or, others use the public key to encrypt something, and you use the private key to decrypt it.
As long as only you have access to the private key, other people can rely on your digital signatures being made by you, and you can rely on nobody else being able to read messages encrypted for you.
GnuPG, the implementation used in Debian, picks the right key at any one time.
What are subkeys?
OpenPGP further supports subkeys, which are like the normal keys, except they're bound to a primary key pair. A subkey can be used for signing or for encryption. The really useful part of subkeys is that they can be revoked independently of the primary keys, and also stored separately from them.
In other words, subkeys are like a separate key pair, but automatically associated with your primary key pair.
GnuPG actually uses a signing-only key as the primary key, and creates an encryption subkey automatically. Without a subkey for encryption, you can't have encrypted e-mails with GnuPG at all. Debian requires you to have the encryption subkey so that certain kinds of things can be e-mailed to you safely, such as the initial password for your debian.org shell account.
Subkeys make key management easier. The primary key pair is quite important: it is the best proof of your identity online, at least for Debian, and if you lose it, you'll need to start building your reputation from scratch. If anyone else gets access to your private primary key or its private subkey, they can make everyone believe they're you: they can upload packages in your name, vote in your name, and do pretty much anything else you can do. This can be very harmful for Debian. You might dislike it as well. So you should keep all your private keys safe.
You should keep your private primary key very, very safe. However, keeping all your keys extremely safe is inconvenient: every time you need to sign a new package upload, you need to copy the packages onto suitable portable media, go into your sub-basement, prove to the armed guards that you're you by using several methods of biometric and other identification, go through a deadly maze, feed the guard dogs the right kind of meat, and then finally open the safe, get out the signing laptop, and sign the packages. Then do the reverse to get back up to your Internet connection for uploading the packages.
Subkeys make this easier: you already have an automatically created encryption subkey and you create another subkey for signing, and you keep those on your main computer. You publish the subkeys on the normal keyservers, and everyone else will use them instead of the primary keys for encrypting messages or verifying your message signatures. Likewise, you will use the subkeys for decrypting and signing messages.
You will need to use the primary keys only in exceptional circumstances, namely when you want to modify your own or someone else's key. More specifically, you need the primary private key:
- when you sign someone else's key or revoke an existing signature,
when you add a new UID or mark an existing UID as primary,
- when you create a new subkey,
- when you revoke an existing UID or subkey,
when you change the preferences (e.g., with setpref) on a UID,
- when you change the expiration date on your primary key or any of its subkey, or
- when you revoke or generate a revocation certificate for the complete key.
(Because each of these operation is done by adding a new self- or revocation signatures from the private primary key.)
Since each link of the Web of Trust is an endorsement of the binding between a public key and a user ID, OpenPGP certification signatures (from the signer's private primary key) are relative to a UID and are irrelevant for subkeys. In particular, subkey creation or revocation does not affect the reputation of the primary key. So in case your subkey gets stolen while your primary key remains safe, you can revoke the compromised subkey and replace it with a new subkey without having to rebuild your reputation and without reducing reputation of other people's keys signed with your primary key.
Unfortunately, GnuPG's user interface is not entirely fun to use. We'll take you through the necessary steps below.
These instructions assume you use one computer, and keep the primary keys on an encrypted USB flash drive, or preferably at least two (you should keep backups of your secret keys). We also assume you already have a key; if not, see http://keyring.debian.org/creating-key.html for instructions.
Make backups of your existing GnuPG files ($HOME/.gnupg). Keep them safe. If something goes wrong during the following steps, you may need this to return to a known good place.
umask 077; tar -cf $HOME/gnupg-backup.tar -C $HOME .gnupg
- Create a new subkey for signing.
Find your key ID: gpg --list-keys yourname
gpg --edit-key YOURPRIMARYKEYID
At the gpg> prompt: addkey
- This asks for your passphrase, type it in.
- Choose the "RSA (sign only)" key type.
- It would be wise to choose 4096 (or at least 2048) bit key size.
- Choose an expiry date (you can rotate your subkeys more frequently than the primary keys, or keep them for the life of the primary key, with no expiry).
Julian calendars can help calculate the exact number of days to a target calendar date
date +%j # command line to display current Julian date
date -d 2021-05-16 +%j # command to display specific Julian date
- GnuPG will (eventually) create a key, but you may have to wait for it to get enough entropy to do so.
Save the key: save
- You can repeat this, and create an "RSA (encrypt only)" subkey as well, if you like or if you need to. As mentioned above, keep in mind that the default option when initially creating a new keypair is to create an encryption subkey, so you probably have one already. In any case, for Debian, just the signing key is sufficient.
Now copy $HOME/.gnupg to your USB drives.
- Here comes the tricky part: you need to remove the private primary key.
If you are using GnuPG 2.1 or later, all you have to do is to delete the file $HOME/.gnupg/private-keys-v1.d/KEYGRIP.key, where KEYGRIP is the "keygrip" of the primary key which can be found by running gpg2 --with-keygrip --list-key YOURPRIMARYKEYID. (The private part of each key pair has a keygrip, hence this command lists one keygrip for the primary key and one for each subkey.) Note however that if the keyring has just been migrated to the new format, then the now obsolete $HOME/.gnupg/secring.gpg file might still contain the private primary key: thus be sure to delete that file too if it is not empty.
Unfortunately GnuPG <2.1 does not provide a convenient way to do remove the private primary key. Instead, we need to export the subkey, remove the private key, and import the subkey back.
Export the subkeys: gpg --output secret-subkeys --export-secret-subkeys YOURPRIMARYKEYID. Alternatively, specify the subkey IDs each followed with an exclamation mark to choose which subkeys to export: gpg --output secret-subkeys --export-secret-subkeys SUBKEYID! [SUBKEYID! ..]
Remove your primary secret key: gpg --delete-secret-keys YOURPRIMARYKEYID
Import the subkeys back: gpg --import secret-subkeys
Remove the file containing the private subkeys: rm secret-subkeys
Verify that gpg -K shows a sec# instead of just sec for your private key. That means the secret key is not really there. (See also the presence of a dummy OpenPGP packet in the output of gpg --export-secret-keys YOURPRIMARYKEYID | gpg --list-packets.)
Change the passphrase protecting the subkeys: gpg --edit-key YOURPRIMARYKEYID passwd. This way if your everyday passphrase is compromised, the private primary key will remain safe from someone with access to the backup: the private key material on the backup, including the private primary key, are protected by the old passphrase.
Your computer is now ready for normal use.
When you need to use the primary keys, mount the encrypted USB drive, and set the GNUPGHOME environment variable:
export GNUPGHOME=/media/something gpg -K
or use the --homedir command-line argument:
gpg --homedir=/media/something -K
The latter command should now list your private key with sec and not sec#.
At this point, you have a subkey, and you need to send it to the Debian keyserver, if your key is already in the Debian keyring, and the general keyserver network:
gpg --keyserver hkp://keyring.debian.org --send-key YOURPRIMARYKEYID gpg --keyserver hkp://pool.sks-keyservers.net --send-key YOURPRIMARYKEYID
The upload to the Debian key server only works if your primary public key is in the DD or DM keyrings already: the Debian key server accepts updates to existing keys, but not new keys. New keys are added by the keyring maintainers manually. Updates to keys further need a manual update to be added to the actual keyring used by Debian's servers, which usually happens about once a month. (See https://salsa.debian.org/debian-keyring/keyring/commits/master to see if your subkey has been added.)
So it may take 1 month, to make the new ?SubKeys get updated to Debian online servers.
(First time your key gets added to the Debian keyrings: manual, when you get accepted as DD or DM. After that, uploading subkeys to key server: automatic. Copying updates from key server to the Debian keyrings: manual, once a month.)
After this, you should be able to upload packages to Debian using the subkey, rather than the primary key. The subkey will inherit the web-of-trust status of the primary key pair.
If disaster strikes, and you need to revoke the subkey for whatever reason, do the following:
- Mount the encrypted USB drive.
gpg --edit-key YOURPRIMARYKEYID
At the gpg> prompt, list the keys (list), select the unwanted one (key 123), and generate a revocation certificate (revkey), then save.
- Send the updated key to the keyservers as above.
Multiple Subkeys per Machine vs. One Single Subkey for All Machines
One might be tempted to have one subkey per machine so that you only need to exchange the potentially compromised subkey of that machine. In case of a single subkey used on all machines, it needs to be exchanged on all machines in case of a compromising.
But this only works for signing subkeys. If you have multiple encryption subkeys, gpg is said to encrypt only for the most recent encryption subkey and not for all known and not revoked encryption subkeys.