Public Access
jaap-jan
d5b1a73182
Adding somebody to a team granted them nothing readable and removing them
rotated nothing. Both were honest — the interface said so in as many words — and
both left the actual work to a button somebody had to remember to press, on a
machine that happened to hold the key. Adding now wraps every team vault this
machine can open to the new member, and removing revokes their grants and moves
each of those vaults to a fresh key that goes to whoever is left.
The rotation is where the design had to be decided rather than written. A vault
key is per generation and an item carries the generation it was sealed under, so
advancing the vault and withdrawing the old grants would make everything already
stored unreadable to everybody, including whoever pressed the button. So earlier
grants are kept: a member holds one per generation, /me serves them as
PriorKeyWraps, and VaultKeyring holds a key per generation — the newest for
writing, the item's own for reading, chosen per item on every read path. Sharing
issues one grant per generation held, because a recipient handed only the current
key would open the vault to find most of it undecryptable; revocation takes every
generation, because leaving the history behind leaves them able to read
everything written before the rotation.
The bump itself is one server transaction. POST /vaults/{id}/rekey must name
exactly current + 1 and the vault's xmin token makes that binding, so two admins
rotating at once do not both walk away believing they succeeded — the second is
refused and told to read the vault again. The server contributes the moment and
no cryptography: it cannot generate the key, cannot tell that the one it is
handed differs from the old one, and checks that the caller held the old one the
only way it can, by requiring a live grant at the current generation.
What this does not do is re-encrypt what is already stored, and the product says
so rather than the reassuring version: everything written from the rotation
onwards is unreadable to the person who left, and nothing about the past changes.
That half is deferred and is safe to add incrementally precisely because a vault
at mixed generations stays readable. ADR 0010 records the alternatives — revoking
the old grants, chaining each key under its successor, re-sealing every item in
one request against a server that caps a push at 500 operations — and why each
was rejected.
Two things fell out of the change rather than being asked for. The grant listing
would have shown a member once per generation, so it now returns one row per
holder carrying the best key they hold, which is what makes a row below the
vault's generation mean "still owed the new key". And MarkUnreadable gives up the
write target as well as reporting: a client whose vault was rotated elsewhere
would otherwise have gone on sealing items under its superseded key — readable to
its author, unreadable to everybody else, with nothing to show for it.
139 lines
9.7 KiB
Markdown
139 lines
9.7 KiB
Markdown
# ADR 0009 — Team access: membership authorises, a grant unlocks
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- Status: accepted
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- Date: 2026-07-31
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- Builds on: [ADR 0001](0001-e2ee-trust-model.md)
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## Context
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M3 makes vaults shareable. The obvious way to model that is one concept — "access" — with a role
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attached, and to let the server hand it out. Every hosted competitor works that way, and it is what
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the imported design drew: a members table with a role column, and a share button beside each item.
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This architecture cannot implement that concept, and the interesting part of M3 was working out
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what it can implement instead.
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The server holds ciphertext and no keys. A vault key is 32 random bytes sealed to each member's
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X25519 public key (`docs/crypto.md` §3), and only a client holding the plaintext key can produce a
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seal for somebody else. So "give Bob access" decomposes into two operations that live on different
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machines and cannot be performed by the same actor:
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- deciding that the server will **serve** Bob this vault's rows, which is a database write; and
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- **wrapping** the vault key to Bob's public key, which needs a client that already holds it.
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The schema anticipated this — `team`, `team_membership`, `vault.team_id` and `vault_key_grant` have
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existed since the first migration — but nothing had had to name the split.
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## Decision
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**Membership is authorisation. A grant is access. The product says so out loud.**
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`VaultAccessService` resolves a team vault through `team_membership`, mapping the role to
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`PermissionFlags` by a union with no Deny rules. That decides what the server serves and nothing
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else. Whether the caller can read what it serves is decided by whether they hold a grant, which the
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server records, cannot produce and cannot verify.
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Five consequences, each of which is a place where a more reassuring design was rejected:
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- **A member with no grant is a normal state, not an error.** `VaultSummary.WrappedVaultKey` is null
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and the vault appears in their list saying it is waiting for a key. Hiding it until a grant existed
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would have been tidier and would have implied the server was the thing granting access.
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- **The roles are only the ones that are enforceable.** There is no `ConnectOnly`, despite the design
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asking for one and `TeamRole` having room. SSH terminates on the client, so opening a session needs
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the credential's plaintext on that machine; "may connect but may not read the key" cannot be
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enforced here, and shipping it as a role would have been a lie in a dropdown. `Connect` rides along
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with `Read` and is documented as an interface hint.
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- **Sharing verifies the recipient's key against the append-only key log, or refuses.** A directory
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lookup is a claim by the server about a third party's public key; wrapping to an unverified claim
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hands the vault to whoever made it. `KeyLogAudit` reads the whole log, checks its hash chain from
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genesis, and refuses unless the offered key appears in it unchanged. There is no override flag,
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because a flag that exists gets used on the day the log is briefly unreachable.
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- **Removal is named for what it does.** It revokes grants and flags the vault for rekey. It does not
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claim to reach anything already downloaded, and the interface says the remediation is rotating the
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credential — the same non-retroactive limit ADR 0001 records.
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- **Ownership is sole, so handing it over is one write and not a role change.** If membership
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authorises, the owner's membership is the last authority in the team, and a transfer that stopped
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halfway would leave nobody with the standing to finish it — owned twice if the promotion went first,
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owned by nobody if the demotion did, and in either case recoverable only by an operator editing the
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database. So `POST /teams/{id}/owner` promotes the recipient and demotes the outgoing owner to
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**admin** in one transaction, `ChangeRoleAsync` refuses `Owner` outright, and the recipient must
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already be an active member — handing a team to an id supplied once is the same mistake as adding
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somebody straight to the owner role. Demoting rather than removing is the deliberate half: removing
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them would revoke their vault key grants and flag every team vault for rekey, which is a far larger
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act than the one being asked for, and somebody handing over a team is usually staying in it.
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One thing is deliberately **not** built, and it is a refusal rather than an omission:
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- **The rekey itself.** Only a client holding the current vault key can re-wrap every item's data key
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under a new one. The server records that a rotation is owed and the interface reports it. M5.
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> **Superseded 2026-08-03 by [ADR 0010](0010-vault-key-rotation.md).** Rotation now ships, and it
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> turned out to divide differently than this paragraph assumed: advancing the generation is one
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> server transaction and is not the same act as re-wrapping the items, which is still outstanding.
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> Removing a member rotates the vaults the removing client can open and hands the new key to whoever
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> is left.
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Two smaller choices, recorded because the alternative was written down first and rejected:
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- **No `v_user_vault_permission` view.** ADR-adjacent notes and the old `VaultAccessService` remark
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both anticipated one. The rules turned out to be about sixteen lines of C# shared by the two
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methods that need them; a view would have moved the authorisation model into migrations, where a
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test cannot reach it without a container.
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- **Host key trust stays vault-scoped to the personal vault.** Pins in a team vault are listed but
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not consulted at connect time. Consulting them would let any member with Write pre-approve a
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fingerprint that another member's client then trusts silently for a host in their *own* vault,
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which is a cross-boundary trust escalation. Scoping trust properly needs a scope on the SSH connect
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path (`IKnownHostStore.FindAsync` takes host, port and algorithm and knows nothing about vaults);
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until that exists, the safe direction is the narrow one, and the cost — approving a team host's key
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once per member per machine — is stated in the README rather than hidden.
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### An invitation is membership decided before there is an account to hold it
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A membership names an account: `team_membership.user_id` is not nullable and carries a foreign key, so
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somebody who has never signed in here has nothing for that row to point at. `MembershipStatus.Invited`
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has existed since the first migration and is still never written — not as an oversight, but because a
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membership waiting for a person is the one shape this model cannot store. An invitation is therefore its
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own record, `team_invitation`, held against an **address**, and it becomes an ordinary active membership
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the moment an account with that address signs in.
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That extends the model rather than bending it. An invitation grants nothing readable and cannot be a
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step towards sharing, because there is no account and so no public key to wrap a vault to. It moves the
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first half of the split earlier and leaves the second half exactly where it was.
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Three decisions inside it belong here, because each had a more convenient alternative:
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- **The claim requires `email_verified` on the access token, and nothing relaxes it.** This is the whole
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of the security boundary. Membership is authorisation, so an invitation that could be taken by anybody
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able to obtain a token asserting somebody else's address is a way into a team — the same attack
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`OidcOptions.AllowEmailLinking` exists to refuse, arriving by another door and deserving the same bar.
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An unverified or absent claim claims nothing and logs a warning, which is the only signal an operator
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gets that their provider is not sending it. There is deliberately no setting to trust an unverified
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address: a flag that exists is a flag somebody turns on for the afternoon their provider is
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misconfigured, and this is the one it must not be possible to turn on.
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- **Nothing is sent, and the product says so rather than implying a mail path it has not got.** There is
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no token and no link — the row is a standing instruction, and telling the invitee to go and sign in
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happens over a channel this server does not carry. A link nobody can deliver would be worse than none.
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The compensation, such as it is, is real: an invitation that is not a bearer credential is one that
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cannot be forwarded, intercepted or replayed.
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- **An address that already has an account here is accepted rather than refused.** Refusing and pointing
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at the directory would have been tidier, and would have turned the endpoint into an oracle for which
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addresses have accounts on this deployment, answerable by anybody willing to create a team first. Only
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an address already belonging to a member of *this* team is refused, and that is a fact the caller can
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already read off the members table, so naming it leaks nothing.
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## Consequences
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The sharing graph is visible to the operator: who is in which team, which vaults exist, and who holds
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a grant are all plaintext rows. That was already true of metadata generally (`docs/crypto.md` §10)
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and is not made worse here, but it is now a graph rather than a list. Invitations widen it by one
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edge — an address that has been invited is on the graph before its owner has ever been here — which is
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the same class of fact and worth naming rather than leaving to be noticed.
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A malicious granter can seal garbage. The recipient detects it as a tag failure and the grant's
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Ed25519 signature names who issued it — detectable and attributable, which is the most that is
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achievable without the server holding a key.
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The two-step model costs a step in the interface and buys the property the whole product is for. It
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also makes a class of bug impossible: there is no code path on the server that could accidentally
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grant read access to plaintext, because there is no plaintext on the server to grant.
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