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GnuPG

Some useful commands

Create and manage keys

  • gpg --gen-key generates a new key
  • gpg --edit-key <ID> edits that key
    • trust handle trustness of that key
    • <number> selects an uid
      • deluid removes it
      • primary sets it as primary uid
    • key <number> selects a key
      • delkey removes it
    • addkey adds a new key
    • sign signs that key
    • fpr prints its fingerprint to check by phone for example
    • addphoto allows you to add a photo to your key
    • setpref SHA512 SHA384 SHA256 SHA224 AES256 AES192 AES CAST5 ZLIB BZIP2 ZIP Uncompressed strength hash prefs
  • gpg --gen-revoke <ID> generates a revocation certificate

Sharing keys

List

  • gpg --list-keys lists all keys
  • gpg -k lists all keys too
  • gpg --list-secret-keys list all secret keys
  • gpg -K list your private keys

Export

  • gpg --export -a <ID> > public.key exports that user's public key
  • gpg --export-secret-key -a <ID> > private.key exports the private key

Import

  • gpg --import public.key adds the public key to your public key ring

Delete

  • gpg --delete-key <ID> deletes that public key
  • gpg --delete-secret-key <ID> deletes that secret key
  • gpg --delete-secret-and-public-keys <ID> deletes both

Encrypt data

  • gpg -e -u <sender ID> -r <receiver ID> file creates file.gpg encrypted

Decrypt data

  • gpg -o file -d file.gpg decripts file.gpg into file

Sign data

  • gpg -s textfile signs a file with your key, without encrypt it

Subkeys

Subkeys are bound to a master key pair, so you can export a stub of your keys, use them to sign or encrypt documents, and if they are compromised, your master key will not be there, so you can revoke them easily and create new keys. Really useful with security smart cards.

(data from wiki.debian.org/Subkeys)

Create a new subkey for signing

First of all you should consider to make a backup of your ~/.gnupg folder.

$gpg --edit-key YOURID
gpg> addkey
# Type your passphrase
# Choose RSA (sign only)
# Choose 4096 (or 2048) bit key size
# Choose (or not) expiry date
gpg> save
# Repeat it for generate a RSA (encrypt only) if you want
# Now we need to export subkeys, remove private key and import subkeys back.
$gpg --export-secret-subkeys ID > secret-subkeys.txt
$gpg --delete-secret-key ID
$gpg import secret-subkeys.txt
$gpg -K
# You will see your master key as "sec#"
# Optionally change the passphrase protecting the subkeys
$gpg --edit-key ID paswd

Yubikey

Now I'm going to show how to install a Yubikey smart card and generate keys for the yubikey smartcard. I'm using the Yubikey NEO.

(data from https://www.2realities.com)

Installation in Ubuntu

sudo add-apt-repository ppa:yubico/stable
sudo apt-get update && sudo apt-get install yubikey-personalization-guide
  yubikey-neo-manager yubikey-personalization pcscd scdaemon gnupg2 pcsc-tools
# I want to use gpg2 so to keep things easier
sudo ln -s /usr/bin/gpg2 /usr/local/bin/gpg
# Enable gnupg agents
echo "use-agent" >> ~/.gnupg/gpg.conf
echo "enable-ssh-support" >> ~/.gnupg/gpg-agent.conf

You should enable CCID mode of Yubikey with neo manager.

Creation of keys

# Lets work on a new .gnupg folder in our encrypted USB for example
mkdir /media/user/usb/gpghome /media/user/usb/armor-baks
chmod 700 /media/user/usb/gpghome /media/user/usb/armor-baks
export GNUPGHOME=/media/user/usb/gpghome
cat <<EOF > $GNUPGHOME/gpg.conf
default-preference-list SHA512 SHA384 SHA256
cert-digest-algo SHA512
use-agent
EOF

# generate new master key
gpg --gen-key # generate a key of 4096 bits

# generate revocation cert
gpg --output $GNUPGHOME/../armor-backs/revocation-cert --gen-revoke <ID>

# backup of master key
gpg -a --export-secret-keys > $GNUPGHOME/../armor-baks/masterkeys.txt

# Create subkeys for yubikey
gpg --expert --edit-key <ID>

gpg> addkey
# RSA sign only, 2048 bits
gpg> addkey
# RSA encrypt only, 2048 bits
gpg> addkey
# RSA set your own capabilities (8)
# Select Sign first, then Encrypt, then Authenticate, to keep
# only Authenticate as current allowed actions
# now "q", 2048
gpg> save

#backup subkeys
gpg -a --export-secret-subkeys > $GNUPGHOME/../armor-baks/subkeys.txt

#configure yubikey
gpg --card-edit
gpg/card> admin
# select 3, and change admin pin, by default it is 12345678 I think,
# and user pin is 123456, after that "q"
gpg/card> name # sets a name
gpg/card> lang
gpg/card> url # of your public.key shared in a keystore like pgp.mit.edu
gpg/card> sex
gpg/card> login
gpg/card> quit

#move subkeys to smartcard
gpg --edit-key <ID>
gpg> toggle
gpg> key 1
gpg> keytocard #select 1, signature
gpg> key 1 #to deselect it
gpg> key 2
gpg> keytocard #select 2, encryption
gpg> key 2
gpg> key 3
gpg> keytocard #select 3, authentication
gpg> save

#last backup
gpg -a --export-secret-keys > $GNUPGHOME/../armor-baks/masterkeys-stubs.txt
gpg -a --export-secret-subkeys > $GNUPGHOME/../armor-baks/subkeys-stubs.txt
gpg -a --export > $GNUPGHOME/../armor-baks/publickey.txt

#import the subkeys-stubs in other computers to be able to use them
gpg --import < subkeys-stubs.txt

Remember we made everything in an encrypted USB (or it is a good practice) so you should go back to your environment with export $GNUPGHOME=~/.gnupg and maybe import your subkeys-stubs.txt

Also, if using gpg 2.1 the authentication key that we've created before can be used for ssh, so theoretically with ssh-add -L you could see it imported. You can add your old keys to gpg key store just with ssh-add $folder/id_rsa