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GnuPG

Last modified: 2016-12-30

Generating a key pair

First, you must generate a (public-private) key pair. Running the following command and using the defaults should do.

$ gpg --gen-key

Even better, we can improve the key stretching with better algorithms and more rounds by using the following command instead.

$ gpg --gen-key \
$ --s2k-cipher-algo AES256 \
$ --s2k-digest-algo SHA512 \
$ --s2k-mode 3 \
$ --s2k-count 65000000

You can verify that the private key was indeed generated with the chosen algorithms by running

$ gpg --delete-secret-key morten@alas.dk

and checking for the lines containing

iter+salt S2K, algo: 9, SHA1 protection, hash: 10

In this context, algo: 9 refers to AES-256 and hash: 10 is SHA-512.

Changing your passphrase

It may occasionally be useful to change your passphrase. This is accomplished by the commands below. Notice that this changes your private key, so you will need to export it again.

$ gpg --edit-key Your-Key-ID-Here
gpg> passwd
gpg> save

Backing up your key pair

First, export your public key. You can share this publicly (hence the name). If you have a dotfiles repository on [Github](https://github.com, I suggest it in there.

$ gpg --export --armor > your@email-public.asc

Next, export your secret key. Be careful not transfer this over an insecure channel. I suggest putting it on a pen drive and storing it in a secure location. Note, if your key is protected by a passphrase (as it should be!), the exported file will also be.

$ gpg --export-secret-keys --armor > your@email-secret.asc

As an additional precaution and to avoid bit rot, you should also print your key to paper. I suggest storing it as a QR code. Fortunately, this is easy to do using paperkey (for compression) and qrencode (to generate the QR code).

$ gpg --export-secret-keys | paperkey --output-type raw \
| base64 | qrencode -o secret.png

We use base64 since some QR code encoders and decoders have trouble with binary data. You can use ZBar to read the QR code.

$ zbarimg --raw secret.png | base64 -d | paperkey --pubring \
your@email-public.asc --input-type=raw > your@email-secret.gpg

At this point, you should print the QR code of your secret key onto paper. Verify that you are able to reconstruct your private key by either photographing or scanning the QR code and using ZBar as above. In my experience, ZBar has trouble with excessively large resolutions (such as you would get from modern high resolution cameras or scanners), so you may have to resize the resulting image down to a more reasonable size (say, 800x800). Remember to crop the image before resizing. Finally, I noticed that it can sometimes help to rotate the image if ZBar still refuses to recognize the QR code.

$ convert -rotate 90 secret.png secret.png

Before you start using GnuPG, you should verify that everything went smoothing by deleting your key pair and importing it again. The secret key contains a copy of your public key, so it's only necessary to import that.

$ gpg --delete-secret-key KEYID
$ gpg --delete-key KEYID
$ gpg --import your@email-secret.gpg

Also notice that applications like KWalletManager probably will not accept your key before you trust it. You can fix this on the command-line or, more easily, using KGpg.

Replacing ssh-agent with gpg-agent

It can be a little annoying having to maintain keys and remember passphrases for both PGP and SSH. Luckily, GnuPG can be configured to store your SSH keys, so you only have to remember the passphrase for your PGP key. This is accomplished by replacing ssh-agent with gpg-agent.

The following assumes Debian/jessie. If you are using a different distribution, things may vary. First, you must disable ssh-agent by commenting the line that says use-ssh-agent in /etc/X11/Xsession.options. Next you must tell gpg-agent to emulate ssh-agent.

$ echo enable-ssh-support >> ~/.gnupg/gpg-agent.conf

Now gpg-agent will start automatically the next time you log into X. The final step is to add your SSH key to GnuPG.

$ ssh-add

You may want to verify that your key was added successfully.

$ ssh-add -l
4096 18:d8:e7:97:87:31:d7:4f:a1:0e:e5:e3:2d:98:7c:95 /home/mortenbh/.ssh/id_rsa (RSA)

You will only have to add your key one time; gpg-agent remembers your key across reboots. Whenever SSH needs your key, gpg-agent will ask for your PGP passphrase.