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Revocación de la clave de firma de Tails

Revocación de la clave de firma de Tails

Este documento propone un mecanismo para la distribución y activación del certificado de revocación de la clave de firma de Tails.

Objetivos

Cubiertos por la propuesta actual:

Impedir que una sola persona pueda revocar nuestra clave de firma.
Allow a coalition of people from the Board to revoke our signing key in case most of the people from the Board become unavailable.
Allow a coalition of people, not necessarily from the Board, to revoke our signing key in case everybody or almost everybody from the Board becomes unavailable.
Make it hard for a coalition of people not from the Board to revoke our signing key unless everybody or almost everybody from the Board becomes unavailable.
People not from the Board shouldn't know how the shares are spread and who has them.
People in possession of a share of the revocation certificate of the signing key should have instructions on how to use it if needed.

Grupos

Definimos cuatro grupos complementarios de gente confiable:

Group A: people from the Board themselves
Grupo B
Grupo C
Grupo D

Todas estas personas tienen que tener una clave OpenPGP y entender lo que es un certificado de revocación.

Porciones criptográficas

Generamos un certificado de revocación de la clave de firma y la dividimos en un número de porciones criptográficas, usando por ejemplo el esquema para compartir secretos de Shamir implementado por gfshare.

Las siguientes combinaciones de gente podrían juntarse y rearmar sus porciones para reconstruir un certificado de revocación completo:

Three people from the Board: A{3}
Two people from the Board and one person not from the Board: A{2}+(B|C|D)
One person from the Board, and two people not from the Board but from two different groups: A+(B|C|D){2}
Three people not from the Board but from three different groups: (B+C+D){3}

Generamos estas porciones:

N shares, one for each person from the Board
Una porción para las personas en el grupo B
Una porción para las personas en el grupo C
Una porción para las personas en el grupo D

Quién sabe qué

People from the Board know the composition of each group
People not from the Board:
- Are explained in which circumstances they should revoke the signing key
- Are told to write to a certain contact email address if they decide to revoke the signing key
- Are told that they need three different shares to reassemble the revocation certificate

Infraestructura

Toda persona que tenga una porción está suscrita a una lista de correo.
Esta lista de correo está alojada en un servidor de confianza distinto de boum.org para ser más estable que nuestros canales de comunicación habituales.
Someone hosting the mailing list is part of group B, C, or D so they can ensure that the list keeps working even if it never used.

Cambiar los miembros de los grupos B, C o D

Para agregar a alguien a un grupo determinado:

Pedirle a alguien de ese grupo que le envíe su porción a la nueva persona del grupo.

Para remover a alguien de un grupo:

Enviar nuevas porciones a todo el mundo, excepto a la persona que se está removiendo.
Request everybody to delete their previous share and track this. Once everybody in 2 groups amongst B, C, or D have deleted their share, it becomes impossible for them to reassemble the revocation certificate with the previous set of shares.
Esperemos que esto no pase tan a menudo :)

Expiración

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.

Asunto: distribución

Hola,

Te proponemos que seas parte de un mecanismo distribuido para el certificado
de revocación de la clave de firma de Tails.

La idea es distribuir porciones criptográficas de este certificado de
revocación a gente en la que confiamos. Estas porciones criptográficas
pueden juntarse para recrear el certificado de revocación y revocar la clave
de firma de Tails. Esto podría ser necesario en caso de que nos pase algo
malo y no podamos revocarla por nuestra cuenta.

Note: In all this document, 'us' refers to the Board.

Puedes leer una descripción completa del mecanismo de distribución en:

https://tails.net/doc/about/openpgp_keys/signing_key_revocation/index.es.html.

La receta es pública y el único componente secreto es la lista de gente que
tiene el material criptográfico.

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.

Las cosas malas que podrían pasar si el mecanismo falla son:

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.

Distribución de las porciones
==============================

Each person from the Board, 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.

Todas las personas en el grupo B tienen *la misma* porción B.

Todas las personas en el grupo C tienen *la misma* porción C.

Todas las personas en el grupo D tienen *la misma* porción 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.

Cuándo usar tu porción
========================

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.

Siguientes comunicaciones
=========================

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.net/tails-email.key

Entonces, ¿Podemos contar contigo para esto?

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:

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

Hola,

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 the Board.

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

   https://tails.net/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

Send the invitation email to the new member.
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.
Ask the new member to confirm the reception of their share.

Sharing email

Subject: sharing

Hola,

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!