I’ve been playing with GnuPG for a while, trying to find the right setup for me. I think I might have found it, so this is an attempt to document that before I forget everything.

The Perfect Keypair

Following instructions by Alex Cabal I created a master keypair, which I stored on a USB key and keep secure. I also created a day-to-day subkey for that key which will be stored on my laptop and used for decrypting and signing messages sent to me.

One pretty important (for me) change: I made sure that all keys has an expiry date set. It’s pretty trivial to extend the expiry date on a key or subkey so there’s really no reason not to set one.

It’s not totally clear from the provided instructions how you’re meant to reconsititute the master keypair for signing. This is how I did it:

mkdir master-keypair
GNUPGHOME=master-keypair gpg \
  --allow-secret-key-import \
  --import \
    /path/to/master-keypair-public-key \
    /path/to/master-keypair-private-key

The directory master-keypair now has the original master keypair in it. To use it continue to point GNUPGHOME to it. Mostly though, I only mount it to sign keys using Caff.

Caff

Once you’ve got a working keypair you probably want to do some key signing, or to get your own key signed. Maybe you’ve even attended a keysign party. You could sign all those keys by hand and email the results to the individuals, but it’s a bit of a bother and can be quite error prone to do it all manually.

Caff automates away a lot of the fuss associated with signing other people’s PGP keys. It’s a bit annoying to get setup though, and assumes things like a local mail relay that isn’t avaialble on my computer.

I’ve got Docker though, and that’s pretty easy to get setup using boot2docker. Docker will let me run Caff and provide it the environment it wants. There’s even a pre-built Docker image for Caff!

Of course, it would be too easy if that worked perfectly with my setup. Since I’m using the protected master keypair I need to get that into the Docker container instead of my day-to-day keypair. I also need to customise the .caffrc to provide a different keyserver since I couldn’t get the default to work.

To mount the master keypair into the Docker container, I’ve modified the caff script to look like this:

#!/bin/bash

# Make sure the master keypair is available on the USB stick
USB_KEY_PATH="/Volumes/Survivor/master-keypair"
if [ ! -d "$USB_KEY_PATH" ]; then
  echo "Have you mounted your master keypair?"
  echo "I'm expecting it on ${USB_KEY_PATH}"
  exit 1
fi

# Create a temporary directory for the master keypair to be restored to
# Note:
# 1) I keep the keypair as backup certificates, so I can recreate the keypair
#    fresh each time.
# 2) I can't just mount the USB key as a volume because boot2docker is running
#    docker and that's in a separate VM running under VirtualBox.
GNUPGHOME=`mktemp -d .gnupg/XXXXXXXXXXXXXXXXXXXXXX`
env GNUPGHOME=$GNUPGHOME \
  gpg \
    --allow-secret-key-import \
    --import $USB_KEY_PATH/craig@barkingiguana.com-{public,private}-key.bak

# Run the Caff docker image and sign the key(s).
docker run -it --rm \
    --name caff \
    -e OWNER='Craig R Webster' \
    -e EMAIL='craig@barkingiguana.com' \
    -e KEYS='78782A6576E09768' \
    -v /Users/craig/.gnupg/caffrc:/home/user/.caffrc:ro \
    -v /Users/craig/.gnupg/caff-ssmtp.conf:/etc/ssmtp/ssmtp.conf:ro \
    -v /Users/craig/$GNUPGHOME:/home/user/.gnupg \
    tianon/caff "$@"

# Clean up afterwards
rm -rf $GNUPGHOME

Note that running caff restores the master keypair and makes it available to the docker container. I also make .gnupg/caff-ssmtp.conf available, this contains my ssmtp.conf for sending through GMail, and I mount my customised caffrc.

How to sign keys

Retrieve the media containing your master keypair from your ultra secure safe location, maybe at the bottom of a volcano or your super secret bank deposit box in Zurich. Mount it in your machine then run Caff as normal:

caff 78782A6576E09768

This will guide you through signing the keys you provide with the command and sending them by email to the recipients.

When all signatures have been sent, unmount the master keypair and return it to the locked guarded room in the basement behind the leopards and the barbed wire. Remember to turn off the lights and turn on the lasers on your way out.

Importing signatures received from Caff

Caff sends one email to each identity in the key that you’re signing. Each email will be encrypted to that key. This ensures that before they can apply your signature to their key:

  1. That the person whose key you’re signing really has access to that email address.
  2. They really have access to the secret key.

After decrypting each email will look like the following:

Hi,

Please find attached the user id
  Their Name <the-identity@email.address.com>
of your key [key_id] signed by me.

If you have multiple user ids, I sent the signature for each user id
separately to that user id's associated email address. You can import
the signatures by running each through `gpg --import`.

Note that I did not upload your key to any keyservers. If you want this
new signature to be available to others, please upload it yourself.
With GnuPG this can be done using
  gpg --keyserver pool.sks-keyservers.net --send-key [key_id]

If you have any questions, don't hesitate to ask.

Regards,
Craig R Webster

Each email will have an attachment, the actual signature.

To import the signature, save the attachemnt to disk and do this:

gpg --import 0xSIGNEDKEYID.1.signed-by-0xSIGNEDBYID.asc

Success will look something like this:

gpg: key SIGNEDKEYID: "Their Name <the-identity@email.address.com>” 1 new signature
gpg: Total number processed: 1
gpg:         new signatures: 1
gpg: 3 marginal(s) needed, 1 complete(s) needed, PGP trust model
gpg: depth: 0  valid:   3 signed:   1  trust: 0-, 0q, 0n, 0m, 0f, 3u
gpg: depth: 1  valid:   1  signed:   0  trust: 1-, 0q, 0n, 0m, 0f, 0u
gpg: next trustdb check due at 2015-07-28

They can then choose to publish their new signatures to a keyserver like so:

gpg --keyserver pgp.mit.edu --send-key SIGNEDKEYID
written by
Craig
published
2015-03-02
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