Seven core contributors and one board member met in Scotland, the birthplace
of F-Droid, for the first in-person F-Droid team meeting. One of the most
pressing tasks we needed to take care of was setting up a
contributor-controlled backup of all of our signing keys. The requirements
made it necessary to have a lengthy, in-person, consensus-driven planning
session. We found no good documentation of such a procedure, so we’re going
out on a limb here and publishing the general outline of our process. This
process was informally audited by multiple people with varying expertise
before the public key was used to encrypt anything.

F-Droid manages secret signing keys for thousands of apps. Someone who has
control over those keys could create malicious app releases that could be
transparently installed as updates. On top of that,
Android
does
not
make it
easy
to rotate to new signing keys, unlike TLS or Signal. So these keys are very important to protect. They are also very
important to backup, since the Android OS uses the signing key combined with
the Application ID as the unique identifier to represent each installed
app. This meeting gave us the perfect opportunity to create a new backup
process that ensures that at least 4 trusted community members must be
physically present in order to decrypt the backup of all the keys. First,
we started with the requirements:

• Regular backups as new keys get added.
• Strong, proven encryption for all backups.
• Minimum of four participants to decrypt.
• Specific technical experience should not be required to be a participant.
• The components should be physically spread out across jurisdictions.
• Access should minimized as much as possible (e.g. the signing server
  maintainer does not need access to the backups).
• As low a stress level as possible for each participant.
• Each participant should be free to hand over their component if they are
  forced to, without jeopardizing the encryption.

Then we mapped out who was present:

• Seven core contributors.
• One board member.
• One trusted external witness.
• No one else.

From that, we built the process:

• Three roles for physical control: operator of the signing server,
  encrypted backup holder, passphrase shard holder.
• Each person only takes on a single role, for example, shard holders do not
  have access to the signing keys or the encrypted backup.
• The encryption key used for the data is public key cryptography.
• The private key was generated on a one-time-use, in-memory, disposable
  TAILS session.
• The private key was sharded using Shamir’s Secret
  Sharing.
• We used an implementation of Shamir’s Secret Sharing that has been
  maintained for over 15 years. The installation that was used was
  confirmed via reproducible builds.
• Each shard was written to a removeable storage device bought in a store
  with cash without pre-order or registration.
• Each removeable storage device with a shard was physically handed each
  shard holder to pack in a tamper-evident envelope as the others observed.
• Backups must not be transmitted over the internet, only exchanged via
  in-person meetings between people who know each other.
• All involved sat around a table for the duration of the ceremony. The
  security profile of in-person discussions is drastically easier to manage
  than secure online discussions.
• All present were observers of each step of the process, and verbally
  confirmed their agreement.
• The operators of the signing server and the holders of the backup data
  verified each others’ identities via F-Droid networks, PGP Web of Trust,
  and checks of government issued IDs.
• Pieces from a minimum of two jurisdictions are required to decrypt: EU,
  Europe non-EU, US.

One important factor in reliable backups is regular updates. New apps are
constantly being added, and those usually get a new signing key assigned.
So we needed a system where it was easy to update the backup data while
involving as few people as possible. An operator of the signing server
receives the public key to encrypt the backups via in-person exchange with a
holder of the backups. The holders of the backup data receives the
encrypted backups from an operator of the signing server via in-person
exchange.

Holding such important secrets also brings some unavoidable stresses to the
people holding them. One key design goal was to create a protocol that did
not add to the stress of any existing operators. Furthermore, we aimed to
keep the individual stress as low as possible for all roles in this
protocol. That makes it possible to empower volunteer contributors without
overburdening them.

For restoring, we agreed that it should happen in an in-person meeting. The
process requires three shard holders meet with one encrypted backup holder,
then the results need be given to a signing server operator. Requiring an
in-person meeting could delay the restore process, but the added trust
seemed worth it. So this is the default process. We could still switch to
partially online process if the need arises. That would require the
agreement of five participants.

We believe this is a secure and reliable backup procedure for very sensitive
data. We welcome further scrutiny and plan to update the procedure as
needed in a future meeting.

(This meeting was paid for by the FFDW-DVD grant.)