# ADR 0009 — Team access: membership authorises, a grant unlocks

- Status: accepted
- Date: 2026-07-31
- Builds on: [ADR 0001](0001-e2ee-trust-model.md)

## Context

M3 makes vaults shareable. The obvious way to model that is one concept — "access" — with a role
attached, and to let the server hand it out. Every hosted competitor works that way, and it is what
the imported design drew: a members table with a role column, and a share button beside each item.

This architecture cannot implement that concept, and the interesting part of M3 was working out
what it can implement instead.

The server holds ciphertext and no keys. A vault key is 32 random bytes sealed to each member's
X25519 public key (`docs/crypto.md` §3), and only a client holding the plaintext key can produce a
seal for somebody else. So "give Bob access" decomposes into two operations that live on different
machines and cannot be performed by the same actor:

- deciding that the server will **serve** Bob this vault's rows, which is a database write; and
- **wrapping** the vault key to Bob's public key, which needs a client that already holds it.

The schema anticipated this — `team`, `team_membership`, `vault.team_id` and `vault_key_grant` have
existed since the first migration — but nothing had had to name the split.

## Decision

**Membership is authorisation. A grant is access. The product says so out loud.**

`VaultAccessService` resolves a team vault through `team_membership`, mapping the role to
`PermissionFlags` by a union with no Deny rules. That decides what the server serves and nothing
else. Whether the caller can read what it serves is decided by whether they hold a grant, which the
server records, cannot produce and cannot verify.

Four consequences, each of which is a place where a more reassuring design was rejected:

- **A member with no grant is a normal state, not an error.** `VaultSummary.WrappedVaultKey` is null
  and the vault appears in their list saying it is waiting for a key. Hiding it until a grant existed
  would have been tidier and would have implied the server was the thing granting access.
- **The roles are only the ones that are enforceable.** There is no `ConnectOnly`, despite the design
  asking for one and `TeamRole` having room. SSH terminates on the client, so opening a session needs
  the credential's plaintext on that machine; "may connect but may not read the key" cannot be
  enforced here, and shipping it as a role would have been a lie in a dropdown. `Connect` rides along
  with `Read` and is documented as an interface hint.
- **Sharing verifies the recipient's key against the append-only key log, or refuses.** A directory
  lookup is a claim by the server about a third party's public key; wrapping to an unverified claim
  hands the vault to whoever made it. `KeyLogAudit` reads the whole log, checks its hash chain from
  genesis, and refuses unless the offered key appears in it unchanged. There is no override flag,
  because a flag that exists gets used on the day the log is briefly unreachable.
- **Removal is named for what it does.** It revokes grants and flags the vault for rekey. It does not
  claim to reach anything already downloaded, and the interface says the remediation is rotating the
  credential — the same non-retroactive limit ADR 0001 records.

Two things were deliberately **not** built, and both are refusals rather than omissions:

- **The rekey itself.** Only a client holding the current vault key can re-wrap every item's data key
  under a new one. The server records that a rotation is owed and the interface reports it. M5.
- **Ownership transfer.** The owner cannot be demoted or removed, with its own problem code. Allowing
  it without a transfer would leave a team nobody can administer, recoverable only by an operator
  editing the database.

Two smaller choices, recorded because the alternative was written down first and rejected:

- **No `v_user_vault_permission` view.** ADR-adjacent notes and the old `VaultAccessService` remark
  both anticipated one. The rules turned out to be about sixteen lines of C# shared by the two
  methods that need them; a view would have moved the authorisation model into migrations, where a
  test cannot reach it without a container.
- **Host key trust stays vault-scoped to the personal vault.** Pins in a team vault are listed but
  not consulted at connect time. Consulting them would let any member with Write pre-approve a
  fingerprint that another member's client then trusts silently for a host in their *own* vault,
  which is a cross-boundary trust escalation. Scoping trust properly needs a scope on the SSH connect
  path (`IKnownHostStore.FindAsync` takes host, port and algorithm and knows nothing about vaults);
  until that exists, the safe direction is the narrow one, and the cost — approving a team host's key
  once per member per machine — is stated in the README rather than hidden.

## Consequences

The sharing graph is visible to the operator: who is in which team, which vaults exist, and who holds
a grant are all plaintext rows. That was already true of metadata generally (`docs/crypto.md` §10)
and is not made worse here, but it is now a graph rather than a list.

A malicious granter can seal garbage. The recipient detects it as a tag failure and the grant's
Ed25519 signature names who issued it — detectable and attributable, which is the most that is
achievable without the server holding a key.

The two-step model costs a step in the interface and buys the property the whole product is for. It
also makes a class of bug impossible: there is no code path on the server that could accidentally
grant read access to plaintext, because there is no plaintext on the server to grant.
