Esse documento propõe um mecanismo para a distribuição e ativação do certificado de revogação da chave de assinatura do Tails.

Objetivos

Cobertos pela proposta atual:

  • Prevenir um indivíduo de revogar nossa chave de assinatura.

  • Permitir um grupo de pessoas do tails@boum.org a revogarem nossa chave de assinatura no caso da maior parte das pessoas do tails@boum.org se tornem indisponíveis.

  • permitir um grupo de pessoas, não necessariamente do tails@boum.org, a revogarem nossa chave de assinatura no caso de todos, ou quase todos do tails@boum.org se tornem indisponíveis.

  • Tornar difícil que um grupo de pessoas que não sejam do tails@boum.org revoguem nossa chave de assinatura, a menos que todos ou quase todos do tails@boum.org se tornem indisponíveis.

  • Pessoas que não são do tails@boum.org não devem saber como compartilhar e espalhar quem tem elas.

  • Pessoas que possuem uma copia do certificado de revogação da chave de assinatura devem haver instruções sobre como usa-las caso seja necessário.

Grupos

Definimos quatro grupos complementares de pessoas confiáveis:

  • Grupo A: pessoas do tails@boum.org
  • Grupo B
  • Grupo C
  • Grupo D

Todas essas pessoas devem haver uma chave OpenPGP e entender o que é um certificado de revogação.

Compartilhamentos criptografados

We generate a revocation certificate of the signing key and split it into a number of cryptographic shares, using for example Shamir's secret sharing scheme implemented by gfshare.

As seguintes combinações de pessoas podem se juntar e remontar suas partes compartilhadas para reconstruir um certificado de revogação completo:

  • Três pessoas do tails@boum.org: A{3}
  • Duas pessoas do tails@boum.org e uma pessoa que não é do tails@boum.org: A{2}+(B|C|D)
  • Uma pessoa do tails@boum.org e duas pessoas que não sejam do tails@boum.org, mas de grupos diferentes: A+(B|C|D){2}
  • Três pessoas que não sejam do tails@boum.org mas de três grupos diferentes: (B + C + D){3}

Nós geramos essas partes:

  • N partes, uma para cada pessoa no tails@boum.org
  • 1 parte para pessoas no grupo B
  • 1 parte para pessoas no grupo C
  • 1 parte para pessoas no grupo D

Quem sabe o que

  • Pessoas do tails@boum.org sabem a composição de cada grupo
  • Pessoas que não são do tails@boum.org:
    • São informadas em quais circunstâncias elas devem revelar as chaves de assinatura
    • São convidadas a contatar um determinado endereço de email caso eles decidam revogar a chave de assinatura
  • São informadas que elas necessitam de três diferentes partes para reconstruir o certificado de revogação

Infraestrutura

  • Todos que sabem de uma parte estão inscritos em uma lista de de email.
  • A lista de email é hospedada em um servidor diferente de boum.org para ser mais resiliente que nossos canais de comunicação usuais.
  • Alguém hospedando a lista de emails é parte do grupo B, C, ou D, então eles podem ter certeza que a lista continua funcionando mesmo se nunca for usada.

Mudando os membros dos grupos B, C ou D

Para adicionar alguém à um determinado grupo:

  • Solicite que alguém desse grupo mande sua parte para a nova pessoa nesse grupo.

Para remover alguém de um determinado grupo:

  • Mande novas partes para todos, menos para a pessoa que está sendo removida.
  • Solicite a todos que deletem sua parte passada e rastreie essa. Uma vez que todos em 2 grupos entre B, C ou D tiverem deletado sua parte, torna-se impossível que eles remontem o certificado de revogação com o conjunto passado de partes.
  • Esperamos que isso não ocorra muito frequentemente :)

Expiry

There is no expiry date on revocation certificates. One way of cancelling the revocation power is to destroy all copies of shares of 2 groups amongst B, C, or D.

Invitation email

Someone who has a personal trust relationship with the person being invited sends this email.

Subject: distribution

Hi,

We want to propose you to be part of a distributed mechanism for the
revocation certificate of the Tails signing key.

The idea is to distribute cryptographic shares of this revocation
certificate to people that we trust. These cryptographic shares can be put
together to reassemble the revocation certificate and revoke the Tails
signing key. This may be needed in case something really bad happens to us
and we are not able to do the revocation ourselves.

Note: In all this document, 'us' refers to the set of people subscribed to
tails@boum.org which is a Schleuder mailing list.

You can read a complete description of the distribution mechanism on:

https://tails.boum.org/doc/about/openpgp_keys/signing_key_revocation.

The recipe is public and the only secret component is the list of people who
are in possession of the cryptographic material.

We are proposing this to you because we trust in both your technical
abilities to store your share in a safe place and manipulate it as
required. But also because we trust you as a human being to make informed
judgment on when to use your share and act only in the interest of Tails.

The bad things that could happen if the mechanism fails are:

A. The signing key is not revoked when it should be. This could allow
possible attackers to distribute malicious Tails images or publish malicious
information on our name.

B. The signing key is revoked when it should not have been. This would
prevent people from verifying our images with OpenPGP until we publish a new
signing key and build trust in it.

Distribution of the shares
==========================

Each person from tails@boum.org, group A, has a *different* share, A1, A2,
..., An.

On top of this, we defined three complementary groups, B, C, and D of
trusted people who have a close relationship with Tails but different
interests and different access to information about us. You are part of one
of these groups.

Everybody in group B has an *identical* share B.

Everybody in group C has an *identical* share C.

Everybody in group D has an *identical* share D.

Three different shares are needed to reassemble the revocation
certificate. For example, shares A1, A2, and A3, or shares A1, B, and C.

How to store your share
=======================

Please keep your share in an encrypted storage and make it as hard as you
can for untrusted people to get a copy of it.

You can rename the file as long as you keep the number in the file name of
your share as it is needed to use the share.

Feel free to back up the file but we might also request you to delete it at
some point and you should be able to know whether you still have a copy of
it or not. It is all-right to lose your share as long as you tell us that
you have lost it. It is actually worse to still have a copy of the share
"somewhere" while thinking that you don't, than to lose it by mistake.

Don't hesitate to ask us if you need clarification on the technical aspects
of this.

When to use your share
======================

Everybody in possession of a share is subscribed to a mailing list.

If someone in possession of a share gets to learn about a very bad event
that happened to many of us and really thinks that we are not capable of
revoking the Tails signing key ourselves anymore, then this person should
write to the mailing list explaining why she thinks that the signing key
needs to be revoked.

Yes, there is no mathematically proven algorithm for this and here is where
your judgment as a human being is needed. The description of the very bad
event should be checked or backed by enough people to be plausible.

People on the list who are also convinced that the signing key should be
revoked share their shares until they have 3 different shares. Then they can
assemble the revocation certificate and publish it to revoke the signing
key.

Keep in mind that we could still revoke the signing key ourselves as long as
three of us are able to communicate and gather their shares. So we only need
your help if no more than two of us are still able to communicate.

Unless you really want to start the key revocation process, do not write to
this mailing list.

Further communications
======================

In case we need to communicate with you about this revocation mechanism in
the future, we will always do it through the tails@boum.org mailing list. We
might do so for example to:

  - Ask you to send your share to a new member of your group.

  - Ask you to delete your share. This could be needed to cancel the power
    of others people's share: as long as enough of you delete their shares,
    the few people that might not delete them would end up with unusable
    shares.

The tails@boum.org mailing list has its own OpenPGP key, which is signed by
the Tails signing key itself:

    https://tails.boum.org/tails-email.key

So, can we count on you for this?

If you answer positively, we will send you your share and subscribe you to
the mailing list.

Thanks, and may the force be with you!

Keeping the members of the groups B, C, and D up-to-date

At least every 2 years, we make sure that the mechanism still works:

  1. We review internally the list of members of each group and decide possible additions to, and removals from, each group.

  2. We write to every individual member of each group to ask them to check that they still have their share and the number in the file name.

  3. We log in to the administration interface of the mailing list, make sure that it still exists, and is configured correctly.

Update email

We send these emails through tails@boum.org to avoid the need for a personal trust relationship between the person sending the mail and the recipient. We don't send shares from groups B, C, or D with tails@boum.org by doing so.

Subject: update

Hi,

Some years ago, you agreed to be part of a distributed mechanism for the
revocation certificate of the Tails signing key and we sent you a
cryptographic share of this revocation certificate.

Today, we are asking you to:

1. Verify the authenticity of this email, either by verifying that it is
   signed by the tails@boum.org mailing or by talking directly to someone
   from tails@boum.org.

   The tails@boum.org mailing list has its own OpenPGP key, which is
   signed by the Tails signing key itself:

   https://tails.boum.org/tails-email.key

2. Confirm whether you still have in your possession:

   - Your share of the revocation certificate.

   - The number NNN in the file name of your share.

     The file was named tails-signing-key-revocation-cert.asc.NNN, where
     NNN is a 3 digit number.

For the record, the address of the mailing list that you should write to in
case you want to assemble the revocation certificate is:

    address@example.org

Do not write to this mailing list unless you really want to start the key
revocation process.

We are also copying below a summary of the mechanism.

XXX: Copy the invitation email:
XXX: - Include "You can read a complete description of the distribution mechanism on:"
XXX: - Stop before "So, can we count on you for this?"

Adding a new member

  1. Send the invitation email to the new member.

  2. If they agree, ask someone else from the same group to send them their share of the key.

    Unfortunately, this reveals some membership information to these two people.

  3. Ask the new member to confirm the reception of their share.

Sharing email

We send these emails through tails@boum.org to avoid the need for a personal trust relationship between the person sending the mail and the recipient. We don't send shares from groups B, C, or D with tails@boum.org by doing so.

Subject: sharing

Hi,

We asked someone else from your group to send you a copy of your share.

Please tell us once you receive it.

The address of the mailing list that you should write to in case you want to
assemble the revocation certificate is:

    address@example.org

Do not write to this mailing list unless you really want to start the key
revocation process.

Thanks, and may the force be with you!