airsigner

Automates signing OpenPGP public keys when using offline, airgapped computer.

Why not caff and pius?

The idea was to minimize the amount of trusted code that needs to run on the offline computer. caff is a 2000 lines perl script while pius is an entire project full of various options.

air signer on the other hand is just a simple 40 lines Bash script. The savings come from the fact that air signer uses new GnuPG features (such as --quick-sign-key) and that it doesn't send the e-mails leaving these tasks to other software.

Online computer:

First, convert KSP list into line format:

cat ksp-fosdem2020.txt | ./online/convert-ksp-list > offline/converted.txt

Then, edit converted.txt and remove lines that you don't want to sign. Don't edit anything.

Now, we'll need keys of all these people. Make sure you have them in your keyring (e.g. by importing the KSP keyring with gpg --import).

Then run:

cat offline/converted.txt | ./online/export-keys

That will export all keys as files with gpg extension.

Copy offline directory and all exported gpg files to offline computer.

Offline computer:

Customize inner-1.eml because that's what your recipients will see.

Note: Currently outer-1.eml also needs to be modified adjusting From header.

Carefully read and customize offline/sign-and-create-emails as this will take the converted list, import keys, sign them, export into an encrypted e-mails and remove signed keys.

Check once again that converted.txt contains the list of keys and identities you want to sign.

Then run it with:

cat converted.txt | ./sign-and-create-emails

Copy all eml files to online computer.

Online computer:

Check eml files for e-mail addresses. If they are missing you need to add them manually.

Then send e-mails with:

git send-email --suppress-cc=all --quiet --confirm=never 001_0.eml

This assumes that you have git send-email configured. It's also possible to use any other sending mechanism.

Transfer file format

The transfer file format is structured in a way that is both human-readable and machine-readable using the following, line-based format:

IDNTF FULL-KEY-FINGERPRINT FULL-USER-ID-TO-SIGN

The fields:

• IDNTF is a 5 character identifier for the line that must be unique in the file. Only ASCII letters, numbers and characters - and _ are allowed in this field.
• FULL-KEY-FINGERPRINT is uppercased full version 4 key fingerprint.
• FULL-USER-ID-TO-SIGN is a complete User ID string that should be signed.

There is a single mandatory space between IDNTF and FULL-KEY-FINGERPRINT and between FULL-KEY-FINGERPRINT and FULL-USER-ID-TO-SIGN. Line endings should be a single LF character.

The tool signs only one User ID at a time so if one needs to sign multiple eachUser ID one should be listed on a separate line.

An example:

001_0 7231473218429462833203212349200321874398 John Doe
001_1 7231473218429462833203212349200321874398 John Doe <[email protected]>
001_2 7231473218429462833203212349200321874398 John Doe <[email protected]>
001_3 7231473218429462833203212349200321874398 John Doe <[email protected]>
001_4 7231473218429462833203212349200321874398 John Doe <[email protected]>
002_0 2789357304602357629846520457238491000461 Charlie Anderson <[email protected]>

This file will cause the offline signer to sign 5 User IDs of key 7231473218429462833203212349200321874398 and one User ID of key 2789357304602357629846520457238491000461. Each signature will go into a separate file named using the identifier field (e.g. first one will have 001_0 in the name).

It is customary to structure the identifer format to represent which User IDs belong to the same key. Thus 001_2 represents third User ID of key 001. This is not strictly necessary and tools should not require that. This is purely for human convenience.

The tools should fail loud as soon as format error has been detected.