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Team Box Auditor

Background

Auditable and cryptographically provable team membership, provided by team sigchains, is an important security requirement that guards against the server being able to inject “ghost users” into Keybase teams.

Whenever a team admin adds or removes users, this statement is signed into the sigchain and all the teams’ members can see it and their clients make sure the statement is signed by a team admin. Additionally, when a team member is removed, the admin rotates the team’s secret key so the removed team member doesn’t have access to the secret keys used to decrypt and sign new messages and files.

However, there are cases where we want to rotate the team’s keys due to an action, but the user doing that action is unable to rotate the team themselves.

  • A team member leaves the team
  • An implicit admin leaves the only parent team they’re a member of
  • A team member (or implicit admin) revokes a device (which rotates the user’s per-user key)
  • A team member (or implicit admin) resets their account
  • A team member (or implicit admin) deletes their account

For example, if a reader decides to leave a team, an admin has to come back later and rotate the team’s keys after the fact. Similarly, a user who is resetting their account will likely not have the keys needed to rotate the team before they reset, so the team needs to be rotated by an admin later.

In these cases, the CLKR mechanism has the server signal the team’s admins that someone left or has new keys, and so the admin can rotate the team’s keys properly, and encrypt them for the right users and their current keys.

Problem

However, CLKR is entirely driven by the server. While it is cryptographically impossible for the server to trick clients into adding arbitrary members, the server can, for example, omit to tell the team admins about a team member’s device revocation.

Imagine a Keybase user whose laptop was stolen by an attacker. Even if the user revokes their that laptop’s keys from another device, all the teams the user is in still have their per-team keys encrypted for the old per-user key. If a malicious server fails to issue CLKRs, and also cooperates with the attacker to provide encrypted team data, the attacker could read new team messages and files.

Solution

Thus, we introduce the client Box Auditor, which audits the team boxes of every team a user is in. Note that 1-on-1 and group chats are also driven by the teams mechanism behind the scenes, so these chats are audited as well.

The box auditor is a background process that randomly selects teams the user is in, loads the sigchains of all of its members, and verifies that the team’s boxes are currently keyed for the right user keys. If not, the auditor rotates the team’s keys. By the security of the Keybase Merkle tree, the server cannot lie, rollback, or equivocate about a user’s per-user key (or a team’s membership), since it’s signed into their sigchain.

Note that the box auditor is designed in such a way that it is impossible for it to be “tricked” by a malicious server attempting to stop these audits. Any possible error coming back from the server during the audit, including network errors and authentication errors, are untrusted by the client, which schedules another audit to happen again in the near future. If a single team fails to audit successfully enough times, the team is jailed and the user is notified whenever that team is accessed until it passes an audit. This way, a malicious server cannot stop audits by pretending that the client’s requests were not well-formed, or pretending to be down.

Try it out

You don’t need to do anything in particular for the auditor to protect your teams: the background process does everything automatically.

But if you wanted to manually run an audit, Keybase client 4.0.0 ships with a command line interface to the box auditor.

To audit the keybase team,

keybase audit box --audit --team keybase

To audit all known teams in the local cache, including 1-on-1 chats and group chats,

keybase audit box --audit-all-known-teams

Note that if you are in a subteam but not in a parent team of that subteam, this command will list an audit of that parent team as failing. This is intentional; see the next section for details.

Implementation details

The critical functionality we need is to get the per-user keys of all of a team’s members at a given Merkle root (call this a box summary).

The function calculateSummaryAtMerkleSeqno works like this:

  1. Load the team’s sigchain
  2. Construct a map of Merkle checkpoints. If we’re getting a historical summary, for each active member, the checkpoint is the Merkle root when they were last given a box: at the last team rotation, or when they were added to the team (if a rotation has not happened since then). If we’re getting a current summary, for each active member, the checkpoint is the latest Merkle root available.
  3. Traverse the global Merkle tree at the Merkle checkpoints to find the last link of the user’s sigchain at the time the team’s per-team-key was keyed for them
  4. For each user, infer the per-user key generation from the user’s sigchain and the information above.
  5. Construct a box summary, which is a map from a user ID and an eldest seqno (i.e., 1 or the sigchain link number of the last time they reset their account) to their per-user key generation.

Once we have this primitive, we can get the summary at the Merkle root of the last rotation and compare it with the summary at the current Merkle root. If these differ, rotate and try again later. Do the same if there was any other error from the server.

If the team failed to audit more than 6 times, the team goes into the box audit jail.

When jailed, any attempt to load the team via the regular team loader or the fast team loader results in a re-audit. If that re-audit fails, the client repeatedly warns the user that the audit has failed. Once we are sure that the auditor is working as intended, we may update the client to start refusing to load the team entirely instead.

Auditing and retrying are handled as automated background tasks while the Keybase service is running.

Note that the box auditor is currently under by a server-controlled feature flag while it is in testing and we make sure everything works properly. In the next release, this flag will be removed so the server is unable to turn off the auditor unilaterally.

There are some details that the client has to be careful about.

  • Implicit admins, who are not in the team but admins of parent teams, are also keyed for the team’s per-team key, so the auditor needs to check their per-user keys as well.
  • Open teams are not sent CLKRs since the admins auto-accept any request, so these are not audited. Similarly, readers aren’t allowed to rotate teams, so they don’t audit teams either.
  • Clients sign the last seen Merkle root into team sigchain links. However, they must be careful to pick this Merkle root before they do a team rotation, or otherwise, a user could revoke a device in between the time of the key boxing and picking the Merkle root. If this race happens, an auditor looking back would infer that the admin rotated for the new per-user key, but in reality, the admin keyed for the old per-user key.
  • The auditor needs to know which teams the user is in in order to decide to audit them. We can’t trust the server with giving us this information, which may maliciously hide teams that it doesn’t want the user to audit. So we simply use the local cache of all loaded teams in order to select which teams to audit.
  • An audit can permanently fail for legitimate reasons. Imagine that you’ve joined a team, so it’s in your local team cache, but then later you are removed from the team. Now when we try to do an audit, it fails because we are not in the team, and so don’t have permissions to see the team sigchain. At the same time, the server can’t prove we aren’t in the team, since it won’t show us the team’s sigchain. Because of this, we can’t trust the server’s word that we aren’t really in the team, so the client just schedules a reaudit. Eventually, the team audit will fail 6 times for the same (legitimate) reason and enter the jail. However, this is OK because the user never needs to load the team, since they aren’t in the team anyway. If they are readded to the team later, the reaudit during team load will then pass.

Conclusion

The team box auditor provides strong guarantees that a team’s secret keys are rekeyed after team member device revocations even in the event of a malicious server attempting to surreptitiously avoid being audited.