en	[[!meta title="Revocation of the Tails signing key"]]

en	This document proposes a mechanism for the distribution and activation of the revocation certificate of the Tails signing key.

en	Status

en	This document is obsolete.

en	It will be simplified or removed by [[!tails_gitlab tails/summit/-/issues/36]].

en	Goals

en	Covered by the current proposal:

en	Prevent any single individual from revoking our signing key.

en	Allow a coalition of people from the Board to revoke our signing key in case most of the people from the Board become unavailable.

en	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.

en	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.

en	People not from the Board shouldn't know how the shares are spread and who has them.

en	People in possession of a share of the revocation certificate of the signing key should have instructions on how to use it if needed.

en	Groups

en	We define four complementary groups of trusted people:

en	Group A: people from the Board themselves

en	Group B

en	Group C

en	Group D

en	All these people should have an OpenPGP key and understand what a revocation certificate is.

en	Cryptographic shares

en	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`.

en	The following combinations of people could get together and reassemble their shares to reconstruct a complete revocation certificate:

en	Three people from the Board: A{3}

en	Two people from the Board and one person not from the Board: A{2}+(B|C|D)

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

en	Three people not from the Board but from three different groups: (B+C+D){3}

en	We generate these shares:

en	N shares, one for each person from the Board

en	1 share for people in group B

en	1 share for people in group C

en	1 share for people in group D

en	Who knows what

en	People from the Board know the composition of each group

en	People not from the Board:

en	Are explained in which circumstances they should revoke the signing key

en	Are told to write to a certain contact email address if they decide to revoke the signing key

en	Are told that they need three different shares to reassemble the revocation certificate

en	Infrastructure

en	Everybody who owns a share is subscribed to a mailing list.

en	This mailing list is hosted on a trusted server different from boum.org to be more resilient than our usual communication channels.

en	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.

en	Changing the members of the groups B, C, or D

en	To add someone to a given group:

en	Request someone from that group to send her share to the new person in the group.

en	To remove someone from a given group:

en	Send new shares to everybody except to the person who is being removed.

en	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.

en	Let's hope that this doesn't happen very often :)

en	Expiry

en	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.

en	Invitation email

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

en	<pre> Subject: distribution

en	Hi,

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

en	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.

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

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

en	https://tails.net/doc/about/openpgp_keys/signing_key_revocation.

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

en	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.

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

en	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.

en	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.

en	Distribution of the shares

en	Each person from the Board, group A, has a *different* share, A1, A2, ..., An.

en	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.

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

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

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

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

en	How to store your share

en	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.

en	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.

en	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.

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

en	When to use your share

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

en	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.

en	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.

en	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.

en	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.

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

en	Further communications

en	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:

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

en	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.

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

en	https://tails.net/tails-email.key

en	So, can we count on you for this?

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

en	Thanks, and may the force be with you! </pre>

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

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

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

en	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.

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

en	Update email

en	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.

en	<pre> Subject: update

en	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.

en	Today, we are asking you to:

en	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.

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

en	https://tails.net/tails-email.key

en	Confirm whether you still have in your possession:

en	Your share of the revocation certificate.

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

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

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

en	address@example.org

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

en	We are also copying below a summary of the mechanism.

en	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?" </pre>

en	Adding a new member

en	Send the [[invitation email|signing_key_revocation#invitation]] to the new member.

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

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

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

en	Sharing email

en	<pre> Subject: sharing

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

en	Please tell us once you receive it.

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