Move the keys when a membership changes, not just the flag

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.

src/DodoSSH.Api/Features/Identity/IdentityService.cs

@@ -81,15 +81,25 @@ internal sealed class IdentityService(DodoDbContext database, IVaultAccessServic
         {
             var vault = access.Vault!;
 
-            // The grant must match both the current key generation and the exact identity key it
-            // was wrapped to. A grant left over from a superseded key is not merely stale — the
-            // client's current private key cannot open it, so offering it would produce a tag
-            // failure the user reads as data corruption.
-            var grant = grants.Find(g =>
-                g.VaultId == vault.Id
-                && g.KeyGeneration == vault.KeyGeneration
-                && key is not null
-                && g.RecipientKeyFingerprint.AsSpan().SequenceEqual(key.FingerprintSha256));
+            // The grant must match the exact identity key it was wrapped to. One left over from a
+            // superseded identity key is not merely stale — the client's current private key cannot
+            // open it, so offering it would produce a tag failure the user reads as data corruption.
+            var mine = grants
+                .Where(g => g.VaultId == vault.Id
+                    && key is not null
+                    && g.RecipientKeyFingerprint.AsSpan().SequenceEqual(key.FingerprintSha256))
+                .ToList();
+
+            var grant = mine.Find(g => g.KeyGeneration == vault.KeyGeneration);
+
+            // Everything older, oldest first. A rotation does not re-encrypt what is already stored —
+            // each item keeps the generation it was sealed under — so a client holding only the
+            // current key would read the vault's whole history as corrupt. See RekeyVaultRequest.
+            var prior = mine
+                .Where(g => g.KeyGeneration < vault.KeyGeneration)
+                .OrderBy(g => g.KeyGeneration)
+                .Select(g => new VaultKeyWrap((uint)g.KeyGeneration, g.WrappedKey))
+                .ToArray();
 
             summaries.Add(new VaultSummary(
                 VaultId: vault.Id,
@@ -103,7 +113,8 @@ internal sealed class IdentityService(DodoDbContext database, IVaultAccessServic
                 // re-wrap it; the client has to say so rather than showing an empty vault.
                 WrappedVaultKey: grant?.WrappedKey,
 
-                RekeyRequired: vault.RekeyRequired));
+                RekeyRequired: vault.RekeyRequired,
+                PriorKeyWraps: prior));
         }
 
         return summaries;

src/DodoSSH.Api/Features/Teams/TeamLog.cs

@@ -66,6 +66,21 @@ internal static partial class TeamLog
     internal static partial void GrantRevoked(
         ILogger logger, Guid vaultId, Guid recipientId, Guid actorId);
 
+    /// <remarks>
+    /// Warning, because a rotation is the one operation that changes what every other member's key is
+    /// worth: until each of them is wrapped the new generation, they hold the vault's history and
+    /// cannot read anything written since. An operator seeing members report an unreadable vault needs
+    /// this line and its timestamp to explain it.
+    /// </remarks>
+    [LoggerMessage(
+        EventId = 2115,
+        Level = LogLevel.Warning,
+        Message = "Rotated the key of vault {VaultId} to generation {KeyGeneration}, by {ActorId}. "
+            + "Earlier grants are kept so stored items stay readable; every other member needs the new "
+            + "generation wrapped to them before they can read anything written from now on.")]
+    internal static partial void VaultRekeyed(
+        ILogger logger, Guid vaultId, int keyGeneration, Guid actorId);
+
     [LoggerMessage(
         EventId = 2108,
         Level = LogLevel.Information,

src/DodoSSH.Api/Features/Teams/VaultGrantEndpoints.cs

@@ -114,6 +114,70 @@ internal sealed class IssueVaultGrantEndpoint(
     }
 }
 
+/// <summary>Moves this vault to a fresh key.</summary>
+/// <remarks>
+/// Gated on Share rather than on a rotation permission of its own. Rotating decides who can read what
+/// is written next, which is the same question sharing and withdrawing answer, and a fourth permission
+/// would be a distinction nobody administering a team would be able to explain.
+/// </remarks>
+internal sealed class RekeyVaultEndpoint(
+    ICurrentUserContext currentUser,
+    IVaultAccessService vaultAccess,
+    VaultGrantService grants)
+    : Endpoint<RekeyVaultRequest, Results<Ok<VaultSummary>, NotFound, ProblemHttpResult>>
+{
+    /// <inheritdoc />
+    public override void Configure()
+    {
+        Post("/api/v1/vaults/{vaultId:guid}/rekey");
+
+        Policies(Auth.EnrolledPolicy);
+
+        Description(b => b
+            .WithName("RekeyVault")
+            .WithSummary("Advances this vault's key generation, wrapped to the caller.")
+            .WithTags("Vaults"));
+    }
+
+    /// <inheritdoc />
+    public override async Task<Results<Ok<VaultSummary>, NotFound, ProblemHttpResult>> ExecuteAsync(
+        RekeyVaultRequest req,
+        CancellationToken ct)
+    {
+        var user = await currentUser.GetOrProvisionAsync(ct).ConfigureAwait(false);
+        var access = await vaultAccess
+            .ResolveAsync(user.Id, Route<Guid>("vaultId"), ct)
+            .ConfigureAwait(false);
+
+        if (!access.Granted || !access.Permissions.HasFlag(PermissionFlags.Read))
+        {
+            return TypedResults.NotFound();
+        }
+
+        if (!access.Permissions.HasFlag(PermissionFlags.Share))
+        {
+            return Problems.Coded(
+                StatusCodes.Status403Forbidden,
+                ProblemCodes.Forbidden,
+                "You do not have permission to share this vault, so you cannot rotate its key.");
+        }
+
+        try
+        {
+            var summary = await grants
+                .RekeyAsync(user, access.Vault!, (int)access.Permissions, req, ct)
+                .ConfigureAwait(false);
+
+            return TypedResults.Ok(summary);
+        }
+        catch (VaultGrantInvalidException exception)
+        {
+            return Problems.Coded(
+                StatusCodes.Status400BadRequest, ProblemCodes.InvalidVaultGrant, exception.Message);
+        }
+    }
+}
+
 /// <summary>Withdraws a member's key to this vault.</summary>
 /// <remarks>
 /// 404 for a member who holds no live grant, rather than a bland 204, for the reason device

src/DodoSSH.Api/Features/Teams/VaultGrantService.cs

@@ -152,6 +152,13 @@ internal sealed class VaultGrantService(
     }
 
     /// <summary>Lists who can open a vault.</summary>
+    /// <remarks>
+    /// One row per holder, not one per grant. A rotated vault holds several grants per member — one per
+    /// generation, which is what lets them read its history — and a listing that showed each of them
+    /// would answer "who can open this" with the same person three times. The row carries the best key
+    /// they hold: the live grant at the highest generation, or, for somebody whose access has been
+    /// withdrawn, the most recent grant they had, so the withdrawal is still visible.
+    /// </remarks>
     internal async Task<VaultGrantsResponse> ListGrantsAsync(
         Vault vault,
         CancellationToken cancellationToken)
@@ -163,13 +170,25 @@ internal sealed class VaultGrantService(
             .ToListAsync(cancellationToken)
             .ConfigureAwait(false);
 
+        var holders = grants
+            .GroupBy(g => g.RecipientUserId!.Value)
+            .Select(group => group
+                .OrderByDescending(g => g.RevokedAtUtc is null)
+                .ThenByDescending(g => g.KeyGeneration)
+                .First())
+
+            // The order the first grant of each holder was made in, so the list reads as the vault was
+            // shared rather than reshuffling itself every time somebody is re-wrapped.
+            .OrderBy(g => grants.Find(first => first.RecipientUserId == g.RecipientUserId)!.CreatedAtUtc)
+            .ToList();
+
         return new VaultGrantsResponse(
             VaultId: vault.Id,
             KeyGeneration: (uint)vault.KeyGeneration,
             RekeyRequired: vault.RekeyRequired,
             Grants:
             [
-                .. grants.Select(g => new VaultGrantSummary(
+                .. holders.Select(g => new VaultGrantSummary(
                     g.RecipientUserId!.Value,
                     g.RecipientUser?.Email,
                     g.RecipientUser?.DisplayName,
@@ -183,10 +202,18 @@ internal sealed class VaultGrantService(
 
     /// <summary>Wraps a vault key to another member.</summary>
     /// <remarks>
-    /// Re-issuing to a recipient who already holds a live grant replaces it in place rather than
-    /// inserting a second row, because the unique index permits exactly one live grant per recipient
-    /// per generation — and because the operation somebody is actually performing when they do this
-    /// is "wrap it again", after a rotation or a botched first attempt.
+    /// <para>
+    /// Re-issuing to a recipient who already holds a live grant <em>for that generation</em> replaces it
+    /// in place rather than inserting a second row, because the unique index permits exactly one live
+    /// grant per recipient per generation — and because the operation somebody is actually performing
+    /// when they do this is "wrap it again", after a botched first attempt.
+    /// </para>
+    /// <para>
+    /// A recipient may hold one grant per generation at once, and after a rotation they need to: an item
+    /// is sealed under whatever generation was current when it was written, so somebody given only the
+    /// newest key would find everything older unreadable. Which generations get wrapped is the sharing
+    /// client's decision — it is the only party that can tell which ones it holds.
+    /// </para>
     /// </remarks>
     internal async Task IssueGrantAsync(
         UserAccount actor,
@@ -199,10 +226,12 @@ internal sealed class VaultGrantService(
         var granterKey = await RequireCurrentKeyAsync(actor.Id, cancellationToken)
             .ConfigureAwait(false);
 
+        var generation = (int)request.KeyGeneration;
+
         var existing = await database.VaultKeyGrants
             .SingleOrDefaultAsync(
                 g => g.VaultId == vault.Id
-                    && g.KeyGeneration == vault.KeyGeneration
+                    && g.KeyGeneration == generation
                     && g.RecipientUserId == request.RecipientUserId
                     && g.RevokedAtUtc == null,
                 cancellationToken)
@@ -212,7 +241,7 @@ internal sealed class VaultGrantService(
         {
             Id = Guid.CreateVersion7(),
             VaultId = vault.Id,
-            KeyGeneration = vault.KeyGeneration,
+            KeyGeneration = generation,
             Kind = GrantKind.Member,
             RecipientUserId = request.RecipientUserId,
             CreatedAtUtc = clock.GetUtcNow(),
@@ -239,7 +268,7 @@ internal sealed class VaultGrantService(
         await database.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
 
         TeamLog.GrantIssued(
-            logger, vault.Id, vault.KeyGeneration, request.RecipientUserId, actor.Id);
+            logger, vault.Id, generation, request.RecipientUserId, actor.Id);
     }
 
     /// <summary>
@@ -270,7 +299,11 @@ internal sealed class VaultGrantService(
         RequireDigest(request.RecipientKeyFingerprint, "recipient key fingerprint");
         RequireDigest(request.KeyLogHead, "key log head");
 
-        if (request.KeyGeneration != (uint)vault.KeyGeneration)
+        // Any generation the vault has actually reached, not only the current one — sharing a rotated
+        // vault means handing over its history as well as its present. A generation ahead of the
+        // current one is refused: nothing is sealed under it, so the grant would open nothing, and
+        // accepting it would let a client move the vault forward without the transaction that does so.
+        if (request.KeyGeneration is 0 || request.KeyGeneration > (uint)vault.KeyGeneration)
         {
             throw new VaultGrantInvalidException(
                 $"This vault is at key generation {vault.KeyGeneration}. A grant for generation "
@@ -309,6 +342,170 @@ internal sealed class VaultGrantService(
         }
     }
 
+    /// <summary>
+    /// Moves a vault to a fresh key generation, wrapped to the caller.
+    /// </summary>
+    /// <returns>The vault as the caller now sees it, at the generation this call created.</returns>
+    /// <remarks>
+    /// <para>
+    /// <b>What the server contributes is the moment, not the key.</b> It cannot generate a vault key, tell
+    /// that the one it is handed differs from the old one, or check that the caller held the old one at
+    /// all. What it can do — and what nothing else can — is advance the generation exactly once, so two
+    /// admins rotating the same vault at the same time do not both walk away believing they succeeded.
+    /// The stale one's generation is no longer one past the current, and it is refused.
+    /// </para>
+    /// <para>
+    /// <b>Earlier grants are left standing.</b> They are what the remaining members read the vault's
+    /// history with: an item carries the generation it was sealed under, and nothing here re-encrypts
+    /// items — only a client holding both keys could. The departed member is cut off by the revocation
+    /// that removal already performed, which takes every generation they held.
+    /// </para>
+    /// <para>
+    /// The rekey flag is cleared here rather than when the last member is re-wrapped, because it records
+    /// that a membership change left the vault owing a rotation, and the rotation is this. Who still
+    /// needs the new key is a different question, and the grant list answers it by generation.
+    /// </para>
+    /// </remarks>
+    internal async Task<VaultSummary> RekeyAsync(
+        UserAccount actor,
+        Vault vault,
+        int permissions,
+        RekeyVaultRequest request,
+        CancellationToken cancellationToken)
+    {
+        var key = await RequireRotatableAsync(actor, vault, request, cancellationToken)
+            .ConfigureAwait(false);
+
+        var now = clock.GetUtcNow();
+        var generation = (int)request.KeyGeneration;
+
+        AddSelfGrant(actor, vault, key, generation, request, now);
+
+        vault.KeyGeneration = generation;
+        vault.RekeyRequired = false;
+        vault.RekeyReason = RekeyReason.None;
+        vault.UpdatedAtUtc = now;
+
+        try
+        {
+            // One SaveChanges, so the row and the grant land together. The vault's xmin concurrency
+            // token is what makes the generation check above binding rather than advisory: a second
+            // rotation that read the same generation fails here instead of overwriting this one.
+            await database.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
+        }
+        catch (DbUpdateConcurrencyException)
+        {
+            // Reported as the same refusal the pre-check gives, because it is the same situation seen a
+            // moment later — and a 500 about a concurrency token would tell the user nothing they could
+            // act on. Retrying is safe: the caller generates a fresh key and reads the generation again.
+            throw new VaultGrantInvalidException(
+                "Somebody else rotated this vault while this rotation was being recorded. Read it again "
+                + "and rotate from the generation they left behind.");
+        }
+
+        TeamLog.VaultRekeyed(logger, vault.Id, generation, actor.Id);
+
+        var prior = await database.VaultKeyGrants
+            .Where(g => g.VaultId == vault.Id
+                && g.RecipientUserId == actor.Id
+                && g.KeyGeneration < generation
+                && g.State == GrantState.Active
+                && g.RevokedAtUtc == null)
+            .OrderBy(g => g.KeyGeneration)
+            .Select(g => new VaultKeyWrap((uint)g.KeyGeneration, g.WrappedKey))
+            .ToListAsync(cancellationToken)
+            .ConfigureAwait(false);
+
+        return new VaultSummary(
+            VaultId: vault.Id,
+            Name: vault.Name,
+            IsPersonal: false,
+            TeamId: vault.TeamId,
+            KeyGeneration: request.KeyGeneration,
+            Permissions: permissions,
+            WrappedVaultKey: request.WrappedVaultKey,
+            RekeyRequired: false,
+            PriorKeyWraps: prior);
+    }
+
+    /// <summary>Records the rotating client's grant for the generation it has just created.</summary>
+    private void AddSelfGrant(
+        UserAccount actor,
+        Vault vault,
+        UserKey key,
+        int generation,
+        RekeyVaultRequest request,
+        DateTimeOffset now) =>
+        database.VaultKeyGrants.Add(new VaultKeyGrant
+        {
+            Id = Guid.CreateVersion7(),
+            VaultId = vault.Id,
+            KeyGeneration = generation,
+            Kind = GrantKind.Member,
+            RecipientUserId = actor.Id,
+            RecipientKeyFingerprint = key.FingerprintSha256,
+            WrappedKey = request.WrappedVaultKey,
+            GranterUserId = actor.Id,
+            GranterKeyFingerprint = key.FingerprintSha256,
+
+            // No key log head, as every self-grant carries none: there is no third party whose key
+            // could have been substituted when you wrap something to yourself.
+            KeyLogHead = null,
+            Signature = request.GrantSignature,
+            State = GrantState.Active,
+            CreatedAtUtc = now,
+        });
+
+    /// <summary>
+    /// Everything that can be checked about a rotation before it is recorded.
+    /// </summary>
+    /// <returns>The caller's current identity key, which the new grant is filed against.</returns>
+    private async Task<UserKey> RequireRotatableAsync(
+        UserAccount actor,
+        Vault vault,
+        RekeyVaultRequest request,
+        CancellationToken cancellationToken)
+    {
+        if (vault.OwnerKind != VaultOwnerKind.Team || vault.TeamId is null)
+        {
+            throw new VaultGrantInvalidException(
+                "Only a team vault can be rotated. A personal vault has one reader, so a rotation "
+                + "would re-wrap a key to the same person and change nothing about who can read it.");
+        }
+
+        RequireWrappedKey(request.WrappedVaultKey);
+        RequireSignature(request.GrantSignature);
+
+        if (request.KeyGeneration != (uint)vault.KeyGeneration + 1)
+        {
+            throw new VaultGrantInvalidException(
+                $"This vault is at key generation {vault.KeyGeneration}, so the next one is "
+                + $"{vault.KeyGeneration + 1} and not {request.KeyGeneration}. Read the vault again — "
+                + "somebody else has rotated it since you last looked.");
+        }
+
+        var key = await RequireCurrentKeyAsync(actor.Id, cancellationToken).ConfigureAwait(false);
+
+        // Held now, not merely permitted. The new key has to be wrapped from the old one, and an
+        // account that cannot open the current generation cannot have done that — so a request from
+        // one is either a mistake or an attempt to strand every other member behind a key nobody has.
+        var holdsCurrent = await database.VaultKeyGrants
+            .AnyAsync(
+                g => g.VaultId == vault.Id
+                    && g.KeyGeneration == vault.KeyGeneration
+                    && g.RecipientUserId == actor.Id
+                    && g.State == GrantState.Active
+                    && g.RevokedAtUtc == null,
+                cancellationToken)
+            .ConfigureAwait(false);
+
+        return holdsCurrent
+            ? key
+            : throw new VaultGrantInvalidException(
+                "You hold no key to this vault at its current generation, so you cannot rotate it. Ask "
+                + "a member who does.");
+    }
+
     /// <summary>
     /// Withdraws a member's key grant.
     /// </summary>

src/DodoSSH.Api/Setup/EndpointRegistration.cs

@@ -59,10 +59,11 @@ internal static class EndpointRegistration
             typeof(ListVaultGrantsEndpoint),
             typeof(IssueVaultGrantEndpoint),
             typeof(RevokeVaultGrantEndpoint),
+            typeof(RekeyVaultEndpoint),
 
             // Registered as each feature lands:
             //   Identity   — key rotation, passphrase change
-            //   Vaults     — rekey, per-item ACLs
+            //   Vaults     — per-item ACLs
             //   Relay      — tickets and the WebSocket
             //   Audit, Admin
         });

src/DodoSSH.Client.Api/DodoSshApiClient.cs

@@ -198,6 +198,21 @@ public interface IVaultGrantApi
         IssueVaultGrantRequest request,
         CancellationToken cancellationToken);
 
+    /// <summary>
+    /// Advances this vault to a fresh key generation, wrapped to the caller.
+    /// </summary>
+    /// <returns>The vault at its new generation, with the caller's grants for the earlier ones.</returns>
+    /// <remarks>
+    /// The key is generated by the caller and sealed to itself; the server contributes the moment it
+    /// takes effect, which is the one part a client cannot decide on its own. Wrapping the new
+    /// generation to everybody else is a separate act, and it is the caller's — see
+    /// <see cref="IssueVaultGrantAsync"/>.
+    /// </remarks>
+    Task<VaultSummary> RekeyVaultAsync(
+        Guid vaultId,
+        RekeyVaultRequest request,
+        CancellationToken cancellationToken);
+
     /// <summary>
     /// Withdraws a member's key to this vault.
     /// </summary>
@@ -581,6 +596,18 @@ public sealed class DodoSshApiClient(HttpClient http, IAccessTokenProvider token
             JsonContent.Create(request, DodoSshJsonContext.Default.IssueVaultGrantRequest),
             cancellationToken);
 
+    /// <inheritdoc />
+    public Task<VaultSummary> RekeyVaultAsync(
+        Guid vaultId,
+        RekeyVaultRequest request,
+        CancellationToken cancellationToken) =>
+        SendAsync(
+            HttpMethod.Post,
+            string.Create(CultureInfo.InvariantCulture, $"/api/v1/vaults/{vaultId}/rekey"),
+            JsonContent.Create(request, DodoSshJsonContext.Default.RekeyVaultRequest),
+            DodoSshJsonContext.Default.VaultSummary,
+            cancellationToken);
+
     /// <inheritdoc />
     public Task<bool> RevokeVaultGrantAsync(
         Guid vaultId,

src/DodoSSH.Client.Session/AccountProvisioner.cs

@@ -196,5 +196,6 @@ public sealed class AccountProvisioner(
             summary.KeyGeneration,
             summary.Permissions,
             summary.WrappedVaultKey,
-            summary.RekeyRequired);
+            summary.RekeyRequired,
+            summary.PriorKeyWraps);
 }

src/DodoSSH.Client.Session/VaultSharing.cs

@@ -14,10 +14,58 @@ namespace DodoSSH.Client.Session;
 /// is the interesting outcome and the reason for it is the whole of what a user needs to see.
 /// </param>
 /// <param name="Message">One line for a person. Never contains key material.</param>
+/// <param name="Generations">
+/// How many generations of the vault key were wrapped. One for a vault that has never been rotated;
+/// more for one that has, because its older items are still sealed under the keys they were written
+/// with and a recipient given only the newest would find them unreadable.
+/// </param>
 public sealed record ShareOutcome(
     bool Shared,
     RecipientVerification Verification,
-    string Message);
+    string Message,
+    int Generations = 0);
+
+/// <summary>What sharing or rotating one vault did, named so a message can say which vault.</summary>
+/// <param name="VaultId">The vault.</param>
+/// <param name="Name">Its display name.</param>
+/// <param name="Outcome">What happened, when the attempt was made.</param>
+/// <param name="Failure">
+/// Why it was not, when it failed. Carried rather than thrown for the reason a per-vault sync report
+/// carries its own: one unreachable vault must not stop the others, and a vault that silently did not
+/// get the key is the outcome this whole design exists to make visible.
+/// </param>
+public sealed record VaultShareReport(
+    Guid VaultId,
+    string Name,
+    ShareOutcome? Outcome,
+    Exception? Failure)
+{
+    /// <summary>Whether a grant was recorded for this vault.</summary>
+    public bool Succeeded => Outcome is { Shared: true };
+}
+
+/// <summary>What rotating one vault did.</summary>
+/// <param name="VaultId">The vault.</param>
+/// <param name="Name">Its display name.</param>
+/// <param name="KeyGeneration">The generation it now holds, or zero if it was not rotated.</param>
+/// <param name="Shared">The members the new key was wrapped to.</param>
+/// <param name="NotShared">
+/// The members it was not, with the reason. A rotation that re-wrapped to nobody has locked the
+/// remaining members out of everything written from now on, which they must be told rather than left
+/// to discover.
+/// </param>
+/// <param name="Failure">Why the rotation itself did not happen, when it did not.</param>
+public sealed record VaultRekeyReport(
+    Guid VaultId,
+    string Name,
+    uint KeyGeneration,
+    IReadOnlyList<Guid> Shared,
+    IReadOnlyList<(Guid UserId, string Reason)> NotShared,
+    Exception? Failure)
+{
+    /// <summary>Whether the vault moved to a new key.</summary>
+    public bool Rotated => Failure is null && KeyGeneration > 0;
+}
 
 /// <summary>
 /// Sharing, from the side that holds the keys.
@@ -113,6 +161,13 @@ public sealed partial class VaultSession
     /// <see cref="VerifiedRecipient.Fingerprint"/> with them over a channel this server does not carry;
     /// that is the only step that closes the gap, and the outcome message says so.
     /// </para>
+    /// <para>
+    /// <b>Every generation this session holds is wrapped, not only the newest.</b> A rotation does not
+    /// re-encrypt what is already stored, so a vault that has been rotated twice holds items under three
+    /// keys — and a recipient handed only the current one would open the vault to find most of it
+    /// unreadable. This is also the only party that can do it: the server holds ciphertext it cannot
+    /// read, and the recipient holds nothing yet.
+    /// </para>
     /// </remarks>
     public async Task<ShareOutcome> ShareVaultAsync(
         IVaultGrantApi grants,
@@ -125,7 +180,7 @@ public sealed partial class VaultSession
         ArgumentNullException.ThrowIfNull(grants);
         ArgumentNullException.ThrowIfNull(directory);
 
-        if (!keyring.TryGet(vaultId, out var vaultKey, out var keyGeneration))
+        if (!keyring.TryGet(vaultId, out _, out _))
         {
             throw new VaultUnreadableException(vaultId);
         }
@@ -142,17 +197,271 @@ public sealed partial class VaultSession
         }
 
         var recipient = verification.Recipient!;
+        var generations = keyring.GenerationsHeld(vaultId);
 
-        await IssueAsync(grants, vaultId, vaultKey, keyGeneration, recipient, cancellationToken)
-            .ConfigureAwait(false);
+        // Oldest first, so an interruption leaves the recipient holding history without the present
+        // rather than the reverse. Both are incomplete; only one of them looks like a working vault
+        // that is quietly missing its recent items.
+        foreach (var generation in generations)
+        {
+            if (!keyring.TryGetAt(vaultId, generation, out var vaultKey))
+            {
+                continue;
+            }
+
+            await IssueAsync(grants, vaultId, vaultKey, generation, recipient, cancellationToken)
+                .ConfigureAwait(false);
+        }
 
         return new ShareOutcome(
             true,
             verification,
             "Shared. Check the fingerprint with them out of band — everything the client can verify on "
-            + "its own only proves this server has been consistent with itself.");
+            + "its own only proves this server has been consistent with itself.",
+            generations.Count);
     }
 
+    /// <summary>
+    /// Moves a vault to a fresh key and hands it to the members who are left.
+    /// </summary>
+    /// <param name="grants">The grant calls.</param>
+    /// <param name="directory">The directory and the key log that makes it checkable.</param>
+    /// <param name="vaultId">The vault to rotate.</param>
+    /// <param name="recipients">
+    /// Who should hold the new key. The caller's own id may be in here and is ignored: this session
+    /// wrapped the new key to itself as part of the rotation.
+    /// </param>
+    /// <param name="cancellationToken">Cancellation token.</param>
+    /// <remarks>
+    /// <para>
+    /// <b>Two acts, and only the first is atomic.</b> The generation advances in one server transaction,
+    /// so there is no moment at which two clients disagree about which key is current. Wrapping it to
+    /// each remaining member is a separate call per member, each verified against the key log the same
+    /// way an ordinary share is — and any of them can fail. A member who was missed holds the vault's
+    /// history and cannot read anything written since, which the report says so the interface can too.
+    /// </para>
+    /// <para>
+    /// <b>What a rotation is worth, stated honestly.</b> Nothing already stored is re-encrypted — only a
+    /// client holding both keys could, and that is deferred work. So this does not take back what the
+    /// departed member already has, and it does not re-seal the vault's history against the key they may
+    /// have kept. What it does is make everything written from now on unreadable to them. Retroactive
+    /// revocation is not achievable; rotate the credentials themselves. See ADR 0001.
+    /// </para>
+    /// </remarks>
+    public async Task<VaultRekeyReport> RekeyVaultAsync(
+        IVaultGrantApi grants,
+        IDirectoryApi directory,
+        Guid vaultId,
+        IReadOnlyList<Guid> recipients,
+        CancellationToken cancellationToken)
+    {
+        ObjectDisposedException.ThrowIf(disposed, this);
+        ArgumentNullException.ThrowIfNull(grants);
+        ArgumentNullException.ThrowIfNull(directory);
+        ArgumentNullException.ThrowIfNull(recipients);
+
+        if (!keyring.TryGet(vaultId, out _, out var keyGeneration))
+        {
+            throw new VaultUnreadableException(vaultId);
+        }
+
+        var name = Vaults.FirstOrDefault(vault => vault.VaultId == vaultId)?.Name ?? "this vault";
+        var summary = await RotateAsync(grants, vaultId, keyGeneration, cancellationToken)
+            .ConfigureAwait(false);
+
+        var shared = new List<Guid>();
+        var missed = new List<(Guid UserId, string Reason)>();
+
+        foreach (var recipient in recipients.Distinct().Where(id => id != Profile.UserId))
+        {
+            try
+            {
+                var outcome = await ShareVaultAsync(
+                        grants, directory, vaultId, recipient, cancellationToken)
+                    .ConfigureAwait(false);
+
+                if (outcome.Shared)
+                {
+                    shared.Add(recipient);
+                }
+                else
+                {
+                    missed.Add((recipient, outcome.Message));
+                }
+            }
+            catch (Exception exception) when (exception is not OperationCanceledException)
+            {
+                // One member's key being unusable — never enrolled, rotated their identity key mid-call
+                // — is not a reason to leave the rest of the team without the new one.
+                missed.Add((recipient, exception.Message));
+            }
+        }
+
+        return new VaultRekeyReport(
+            vaultId, name, summary.KeyGeneration, shared, missed, Failure: null);
+    }
+
+    /// <summary>Generates the next vault key, records it, and takes it into the keyring.</summary>
+    /// <remarks>
+    /// The key is adopted only after the server has accepted the rotation. The other order would leave
+    /// this session sealing items under a generation the vault never reached, and every one of them
+    /// would be unreadable to everybody including its author at the next unlock.
+    /// </remarks>
+    private async Task<VaultSummary> RotateAsync(
+        IVaultGrantApi grants,
+        Guid vaultId,
+        uint keyGeneration,
+        CancellationToken cancellationToken)
+    {
+        var generation = keyGeneration + 1;
+        var vaultKey = VaultKeys.Create();
+        var now = clock.GetUtcNow();
+
+        try
+        {
+            var wrapped = VaultKeys.WrapTo(
+                vaultKey, bundle.EncryptionPublicKey, vaultId, generation);
+
+            var fingerprint = DshCrypto.ComputeFingerprint(
+                bundle.EncryptionPublicKey, bundle.SigningPublicKey);
+
+            var canonical = GrantStatementCodec.Encode(
+                vaultId,
+                generation,
+                GrantPurpose.Member,
+                granteeUserId: Profile.UserId,
+                granteeKeyFingerprint: fingerprint,
+                wrappedKey: wrapped,
+                granterUserId: Profile.UserId,
+                granterKeyFingerprint: fingerprint,
+
+                // Absent, as in every self-grant: there is no third party whose key could have been
+                // substituted when you wrap something to yourself.
+                keyLogHead: default,
+                grantedAt: now);
+
+            var summary = await grants.RekeyVaultAsync(
+                    vaultId,
+                    new RekeyVaultRequest(
+                        KeyGeneration: generation,
+                        WrappedVaultKey: wrapped,
+                        GrantSignature: GrantStatementCodec.Sign(bundle.SigningKey, canonical),
+                        GrantedAt: now),
+                    cancellationToken)
+                .ConfigureAwait(false);
+
+            var stored = ToStored(summary);
+
+            await Vault.UpsertAsync(stored, cancellationToken).ConfigureAwait(false);
+
+            keyring.Adopt(vaultId, vaultKey, summary.KeyGeneration);
+
+            Vaults = await Vault.ListAsync(cancellationToken).ConfigureAwait(false);
+
+            return summary;
+        }
+        catch
+        {
+            // Never reached the keyring, so this is the only thing that can release it.
+            CryptographicOperations.ZeroMemory(vaultKey);
+            throw;
+        }
+    }
+
+    /// <summary>
+    /// Hands every team vault this session can open to one member.
+    /// </summary>
+    /// <returns>One report per vault, in the order they were attempted.</returns>
+    /// <remarks>
+    /// What "adding somebody to a team" means in full. Membership is a server-side authorization change
+    /// and takes effect at once; a key is a cryptographic act only a machine holding one can perform, so
+    /// this is the half that has to happen here. A vault this session cannot open is skipped rather than
+    /// failed — somebody else holds its key, and this client has nothing to wrap.
+    /// </remarks>
+    public async Task<IReadOnlyList<VaultShareReport>> ShareTeamVaultsAsync(
+        IVaultGrantApi grants,
+        IDirectoryApi directory,
+        Guid teamId,
+        Guid recipientUserId,
+        CancellationToken cancellationToken)
+    {
+        ObjectDisposedException.ThrowIf(disposed, this);
+        ArgumentNullException.ThrowIfNull(grants);
+        ArgumentNullException.ThrowIfNull(directory);
+
+        var reports = new List<VaultShareReport>();
+
+        foreach (var vault in TeamVaults(teamId))
+        {
+            try
+            {
+                var outcome = await ShareVaultAsync(
+                        grants, directory, vault.VaultId, recipientUserId, cancellationToken)
+                    .ConfigureAwait(false);
+
+                reports.Add(new VaultShareReport(vault.VaultId, vault.Name, outcome, null));
+            }
+            catch (Exception exception) when (exception is not OperationCanceledException)
+            {
+                reports.Add(new VaultShareReport(vault.VaultId, vault.Name, null, exception));
+            }
+        }
+
+        return reports;
+    }
+
+    /// <summary>
+    /// Rotates every team vault this session can open, handing each new key to the members who remain.
+    /// </summary>
+    /// <returns>One report per vault, in the order they were attempted.</returns>
+    /// <remarks>
+    /// What "removing somebody from a team" means in full, and the reason it is per vault rather than
+    /// per team: a key belongs to a vault, and a client can only rotate the ones it can currently open.
+    /// A vault it cannot is left alone and stays flagged for rekey, which is the honest state — somebody
+    /// who holds its key has to finish the job.
+    /// </remarks>
+    public async Task<IReadOnlyList<VaultRekeyReport>> RekeyTeamVaultsAsync(
+        IVaultGrantApi grants,
+        IDirectoryApi directory,
+        Guid teamId,
+        IReadOnlyList<Guid> recipients,
+        CancellationToken cancellationToken)
+    {
+        ObjectDisposedException.ThrowIf(disposed, this);
+        ArgumentNullException.ThrowIfNull(grants);
+        ArgumentNullException.ThrowIfNull(directory);
+        ArgumentNullException.ThrowIfNull(recipients);
+
+        var reports = new List<VaultRekeyReport>();
+
+        foreach (var vault in TeamVaults(teamId))
+        {
+            try
+            {
+                reports.Add(
+                    await RekeyVaultAsync(
+                            grants, directory, vault.VaultId, recipients, cancellationToken)
+                        .ConfigureAwait(false));
+            }
+            catch (Exception exception) when (exception is not OperationCanceledException)
+            {
+                reports.Add(new VaultRekeyReport(
+                    vault.VaultId, vault.Name, KeyGeneration: 0, [], [], exception));
+            }
+        }
+
+        return reports;
+    }
+
+    /// <summary>The team's vaults this session actually holds a current key for.</summary>
+    /// <remarks>
+    /// Materialised before the loops above use it, because both of them write to <see cref="Vaults"/>
+    /// through the vault store — and a rotation part-way through a lazily evaluated sequence would be
+    /// enumerating a list that has been replaced underneath it.
+    /// </remarks>
+    private List<StoredVault> TeamVaults(Guid teamId) =>
+        [.. Vaults.Where(vault => vault.TeamId == teamId && keyring.CanRead(vault.VaultId))];
+
     /// <summary>
     /// Re-reads which vaults the server says are reachable, and opens any that have become readable.
     /// </summary>
@@ -179,14 +488,18 @@ public sealed partial class VaultSession
 
         foreach (var vault in Vaults)
         {
-            if (keyring.CanRead(vault.VaultId))
-            {
-                continue;
-            }
+            // Attempted even for a vault that already opens, because the answer can have grown: a
+            // rotated vault arrives with a new current generation, and a vault shared by somebody who
+            // holds more of its history arrives with wraps this session did not have. Admitting is
+            // idempotent, so the only thing an unconditional call costs is the unwrap it skips.
+            var readable = keyring.CanRead(vault.VaultId);
 
             if (keyring.TryAdmit(bundle, vault))
             {
-                admitted++;
+                if (!readable)
+                {
+                    admitted++;
+                }
             }
             else
             {
@@ -289,5 +602,6 @@ public sealed partial class VaultSession
             summary.KeyGeneration,
             summary.Permissions,
             summary.WrappedVaultKey,
-            summary.RekeyRequired);
+            summary.RekeyRequired,
+            summary.PriorKeyWraps);
 }

src/DodoSSH.Client.Shell/ViewModels/TeamsViewModel.cs

@@ -607,33 +607,134 @@ internal sealed partial class TeamsViewModel(
 
             InviteEmail = string.Empty;
 
+            // Before the reload, so the vault list this screen redraws already shows what they can
+            // open. The sharing is what makes the membership worth anything, and doing it here rather
+            // than leaving a SHARE KEY button to be pressed is the difference between adding a
+            // colleague and adding a colleague who then waits for somebody to notice.
+            var shared = await ShareWithAsync(server, team, member, cancellationToken)
+                .ConfigureAwait(true);
+
             await ReloadAsync(cancellationToken).ConfigureAwait(true);
 
-            Status = Describe(member);
+            Status = Describe(member, shared);
         }).ConfigureAwait(true);
     }
 
+    /// <summary>
+    /// Wraps every team vault this machine can open to somebody who has just been added.
+    /// </summary>
+    /// <returns>What to tell the user about the keys, or null when there was nothing to say.</returns>
+    /// <remarks>
+    /// <para>
+    /// Skipped outright for an account with no identity key: there is nothing to wrap to, and a
+    /// refusal per vault would bury that one fact under a list. Their row says so, and adding them was
+    /// still worth doing.
+    /// </para>
+    /// <para>
+    /// A failure here is reported and never thrown. The membership has already been recorded on the
+    /// server and is not undone by a key that could not be wrapped — so the honest outcome is "they are
+    /// in the team, and this vault still needs sharing", which is a state somebody can act on.
+    /// </para>
+    /// </remarks>
+    private async Task<string?> ShareWithAsync(
+        IVaultServer server,
+        TeamRowViewModel team,
+        TeamMemberSummary member,
+        CancellationToken cancellationToken)
+    {
+        if (!member.IsEnrolled)
+        {
+            return null;
+        }
+
+        if (session() is not { } open)
+        {
+            // Distinguished from holding no keys, because the two lead somewhere different: this one is
+            // fixed by unlocking, and the other by asking somebody who holds the vault.
+            return "Nothing was shared with them — a vault key is wrapped on an unlocked machine, and "
+                + "this keychain is locked.";
+        }
+
+        var reports = await open
+            .ShareTeamVaultsAsync(
+                server.Grants, server.Directory, team.TeamId, member.UserId, cancellationToken)
+            .ConfigureAwait(true);
+
+        if (reports.Count == 0)
+        {
+            return null;
+        }
+
+        var shared = reports.Where(report => report.Succeeded).ToList();
+        var refused = reports.Where(report => !report.Succeeded).ToList();
+
+        var sentence = shared.Count > 0
+            ? $"Shared {VaultCount(shared.Count)} with them: {Join(shared.Select(r => r.Name))}."
+            : null;
+
+        if (refused.Count == 0)
+        {
+            return sentence;
+        }
+
+        // Named one by one rather than counted. Each of these is a vault somebody now expects them to
+        // be able to open, and which one it is decides who has to fix it.
+        var reasons = refused.Select(report =>
+            $"'{report.Name}' ({report.Failure?.Message ?? report.Outcome?.Message})");
+
+        return (sentence is null ? string.Empty : sentence + " ")
+            + $"Could not share {Join(reasons)}.";
+    }
+
     /// <summary>
     /// What just happened to the account that was added, and what is still owed them.
     /// </summary>
     /// <remarks>
-    /// Both branches say out loud that nothing readable was granted, because the single most common
-    /// misunderstanding this design invites is that adding somebody gave them the vault. The unenrolled
-    /// branch says more, and has to: their row will sit in the list saying it holds no key, and without
-    /// this somebody would read that as the addition having half-failed rather than as a colleague who
-    /// has not finished setting their machine up. It is also the one case where SHARE KEY cannot be the
-    /// next step, so pointing at it would be pointing at a button that will refuse.
+    /// <para>
+    /// The enrolled branch reports what the keys did, because that is the half of "adding somebody"
+    /// that this machine performs and the half that can partly fail. A vault that could not be wrapped
+    /// is named there rather than left to be noticed when they say they cannot open it.
+    /// </para>
+    /// <para>
+    /// The unenrolled branch says more, and has to: their row will sit in the list saying it holds no
+    /// key, and without this somebody would read that as the addition having half-failed rather than as
+    /// a colleague who has not finished setting their machine up. Nothing was shared with them and
+    /// nothing could have been — there is no key to wrap to — so the membership is all there is yet.
+    /// </para>
     /// </remarks>
-    private static string Describe(TeamMemberSummary member)
+    private static string Describe(TeamMemberSummary member, string? shared)
     {
         var who = member.Email ?? member.DisplayName ?? "the account";
 
-        return member.IsEnrolled
-            ? $"Added {who} as a member. They cannot read anything yet — select a vault below and "
-                + "share its key."
-            : $"Added {who} as a member. They have no key yet, so their row says so and no vault can "
-                + "be shared with them until they finish signing in on their own machine. The "
+        if (!member.IsEnrolled)
+        {
+            return $"Added {who} as a member. They have no key yet, so their row says so and no vault "
+                + "can be shared with them until they finish signing in on their own machine. The "
                 + "membership is real in the meantime.";
+        }
+
+        return shared is null
+            ? $"Added {who} as a member. This machine holds no team vault key to give them — select a "
+                + "vault below and press SHARE KEY from one that does."
+            : $"Added {who} as a member. {shared}";
+    }
+
+    /// <summary>"1 vault" or "3 vaults", for a sentence that has to read either way.</summary>
+    private static string VaultCount(int count) =>
+        string.Create(CultureInfo.CurrentCulture, $"{count} vault{(count == 1 ? string.Empty : "s")}");
+
+    /// <summary>Joins names into a phrase a person would say, rather than a comma-separated list.</summary>
+    private static string Join(IEnumerable<string> parts)
+    {
+        var list = parts.ToList();
+
+        return list.Count switch
+        {
+            0 => string.Empty,
+            1 => list[0],
+            2 => $"{list[0]} and {list[1]}",
+            _ => string.Join(", ", list.Take(list.Count - 1)) + " and " + list[^1],
+        };
     }
 
     /// <summary>
@@ -917,7 +1018,14 @@ internal sealed partial class TeamsViewModel(
         }).ConfigureAwait(true);
     }
 
-    /// <summary>Removes a member, revoking every vault key grant they hold from this team.</summary>
+    /// <summary>
+    /// Removes a member, revoking their grants and rotating the vaults they could read.
+    /// </summary>
+    /// <remarks>
+    /// The removal and the rotation are separate acts and only the first is the server's. Nothing here
+    /// undoes the removal if the rotation fails, and nothing waits for it: the membership change is what
+    /// stops them fetching anything more, and it has already happened by then.
+    /// </remarks>
     [RelayCommand]
     private async Task RemoveMemberAsync(CancellationToken cancellationToken)
     {
@@ -928,22 +1036,110 @@ internal sealed partial class TeamsViewModel(
             return;
         }
 
+        // Read before the removal, because afterwards this list no longer contains them — and it is the
+        // list of who the new key goes to.
+        var remaining = Members
+            .Where(row => row.UserId != member.UserId)
+            .Select(row => row.UserId)
+            .ToList();
+
         await RunAsync(async () =>
         {
             await server.Teams
                 .RemoveTeamMemberAsync(team.TeamId, member.UserId, cancellationToken)
                 .ConfigureAwait(true);
 
+            var rotated = await RotateAfterRemovalAsync(server, team, remaining, cancellationToken)
+                .ConfigureAwait(true);
+
             await ReloadAsync(cancellationToken).ConfigureAwait(true);
 
             // The honest sentence, not the reassuring one. See ADR 0001: revocation is not retroactive,
-            // and a message implying otherwise is the one thing this screen must not say.
-            Status = $"Removed {member.Name}. They can no longer fetch this team's vaults, and anything "
-                + "they had already downloaded is still on their machine — rotate the credentials that "
-                + "matter.";
+            // and a message implying otherwise is the one thing this screen must not say. The rotation
+            // is described in the same breath for the same reason — it decides what happens next, not
+            // what already happened.
+            Status = $"Removed {member.Name}. {rotated} Anything they had already downloaded is still "
+                + "on their machine — rotate the credentials that matter.";
         }).ConfigureAwait(true);
     }
 
+    /// <summary>
+    /// Rotates every team vault this machine can open, handing each new key to the members who remain.
+    /// </summary>
+    /// <returns>What to tell the user about the keys. Never null — something always happened.</returns>
+    /// <remarks>
+    /// A vault this machine cannot open is not rotated and is not counted as a failure here: its key
+    /// belongs to somebody else, the server has flagged it as owing a rekey, and the vault row says so
+    /// until one of them does it.
+    /// </remarks>
+    private async Task<string> RotateAfterRemovalAsync(
+        IVaultServer server,
+        TeamRowViewModel team,
+        IReadOnlyList<Guid> remaining,
+        CancellationToken cancellationToken)
+    {
+        if (session() is not { } open)
+        {
+            return "Their key grants are withdrawn, so they can fetch nothing more. Unlock your "
+                + "keychain to rotate the vault keys themselves.";
+        }
+
+        var reports = await open
+            .RekeyTeamVaultsAsync(
+                server.Grants, server.Directory, team.TeamId, remaining, cancellationToken)
+            .ConfigureAwait(true);
+
+        if (reports.Count == 0)
+        {
+            return "Their key grants are withdrawn, so they can fetch nothing more. This machine holds "
+                + "no key to any of this team's vaults, so there was nothing here to rotate.";
+        }
+
+        var rotated = reports.Where(report => report.Rotated).ToList();
+        var failed = reports.Where(report => !report.Rotated).ToList();
+
+        var sentences = new List<string>();
+
+        if (rotated.Count > 0)
+        {
+            // Says what a rotation is and is not worth, because the word promises more than it can
+            // deliver: from here on they cannot read this vault, and what is already in it was sealed
+            // under the key they used to hold.
+            sentences.Add(
+                $"Rotated {VaultCount(rotated.Count)} — {Join(rotated.Select(r => r.Name))} — so nothing "
+                + "written from now on is readable to them.");
+
+            // The members who did not get the new key. They are still in the team and can still write,
+            // but until somebody wraps it to them they will find the vault stops updating.
+            // Distinct by id rather than by name, because two accounts can share a display name and
+            // collapsing them would tell somebody one person is owed a key when two are.
+            var missed = rotated
+                .SelectMany(report => report.NotShared.Select(entry => entry.UserId))
+                .Distinct()
+                .Select(Name)
+                .ToList();
+
+            if (missed.Count > 0)
+            {
+                sentences.Add(
+                    $"The new key did not reach {Join(missed)} — press SHARE KEY for them, or they "
+                    + "will stop seeing changes.");
+            }
+        }
+
+        if (failed.Count > 0)
+        {
+            sentences.Add(
+                $"Could not rotate {Join(failed.Select(r => $"'{r.Name}' ({r.Failure?.Message})"))}.");
+        }
+
+        return string.Join(" ", sentences);
+    }
+
+    /// <summary>What to call a member in a sentence, from the list this screen already has.</summary>
+    private string Name(Guid userId) =>
+        Members.FirstOrDefault(row => row.UserId == userId)?.Name ?? userId.ToString();
+
     /// <summary>Opens the name-a-vault form, aimed at the selected team.</summary>
     [RelayCommand]
     private void NewVault() => ArmNewVault(SelectedTeam?.TeamId);
@@ -1196,8 +1392,16 @@ internal sealed partial class TeamsViewModel(
                 .ShareVaultAsync(server.Grants, server.Directory, vault.VaultId, member.UserId, cancellationToken)
                 .ConfigureAwait(true);
 
+            // The generation count is said out loud when there is more than one, because it is the
+            // answer to a question somebody will have about a rotated vault: whether the person they
+            // just shared it with can see what was in it before the rotation.
+            var history = outcome.Generations > 1
+                ? $" All {outcome.Generations} generations of the key were wrapped, so they can read "
+                    + "what was in the vault before it was last rotated."
+                : string.Empty;
+
             Status = outcome.Shared
-                ? $"Shared '{vault.Name}' with {member.Name}. {outcome.Message}"
+                ? $"Shared '{vault.Name}' with {member.Name}. {outcome.Message}{history}"
                 : $"Did not share '{vault.Name}': {outcome.Message}";
         }).ConfigureAwait(true);
     }

src/DodoSSH.Client.Shell/ViewModels/VaultViewModel.cs

@@ -7025,7 +7025,12 @@ internal sealed partial class VaultViewModel(
 
         if (report.RekeyRequired)
         {
-            notes.Add("this keychain was rekeyed and your access needs re-issuing");
+            // What is readable and what is not, because the two differ and the difference is the whole
+            // of what somebody in this state needs to know: the keys they hold still open everything
+            // written before the rotation, and nothing written since.
+            notes.Add(
+                "this keychain was rekeyed — you can still read what was here, and need the new key "
+                + "before you can see anything written since");
         }
 
         return replayed + "Synchronised, but: " + string.Join("; ", notes) + ".";

src/DodoSSH.Client.Storage/CacheRows.cs

@@ -159,6 +159,31 @@ internal sealed class CachedVaultRow
     public DateTimeOffset UpdatedAtUtc { get; set; }
 }
 
+/// <summary>
+/// A vault key this user holds for a generation the vault has moved past.
+/// </summary>
+/// <remarks>
+/// <para>
+/// A table rather than a column, because there is one of these per rotation and the vault row has one
+/// of everything else. The current generation's wrap stays on <see cref="CachedVaultRow"/>: it is what
+/// unlocking needs, and burying it in a child table would make the common case the awkward one.
+/// </para>
+/// <para>
+/// Cached for the reason the current wrap is. An item keeps the generation it was sealed under, so a
+/// machine that came back from a rotation with only the newest key would read everything written
+/// before it as corrupt — offline, with no way to ask for the rest.
+/// </para>
+/// </remarks>
+internal sealed class CachedVaultKeyWrapRow
+{
+    public Guid VaultId { get; set; }
+
+    public uint KeyGeneration { get; set; }
+
+    /// <summary>The vault key at this generation, sealed to this user's X25519 key.</summary>
+    public byte[] WrappedKey { get; set; } = [];
+}
+
 /// <summary>
 /// The last state of an item that the server confirmed.
 /// </summary>

src/DodoSSH.Client.Storage/ClientCacheContext.cs

@@ -123,7 +123,8 @@ public sealed class ClientCacheContext(DbContextOptions<ClientCacheContext> opti
             entity.Property(row => row.SealedRefreshToken).IsRequired();
         });
 
-    private static void ConfigureVaults(ModelBuilder modelBuilder) =>
+    private static void ConfigureVaults(ModelBuilder modelBuilder)
+    {
         modelBuilder.Entity<CachedVaultRow>(entity =>
         {
             entity.ToTable("vault");
@@ -132,6 +133,18 @@ public sealed class ClientCacheContext(DbContextOptions<ClientCacheContext> opti
             entity.Property(row => row.Name).IsRequired();
         });
 
+        // No foreign key to the vault row, deliberately. The two are written by the same store in the
+        // same call, and a cascade would make "which of these two tables is authoritative" a question
+        // the schema answers rather than the code — while buying nothing, since a wrap for a vault this
+        // machine can no longer see is removed by the same pass that removes the vault.
+        modelBuilder.Entity<CachedVaultKeyWrapRow>(entity =>
+        {
+            entity.ToTable("vault_key_wrap");
+            entity.HasKey(row => new { row.VaultId, row.KeyGeneration });
+            entity.Property(row => row.WrappedKey).IsRequired();
+        });
+    }
+
     private static void ConfigureItems(ModelBuilder modelBuilder) =>
         modelBuilder.Entity<CachedItemRow>(entity =>
         {

src/DodoSSH.Client.Storage/Migrations/20260803202241_AddVaultKeyWrapHistory.Designer.cs

@@ -0,0 +1,465 @@
+// <auto-generated />
+using System;
+using DodoSSH.Client.Storage;
+using Microsoft.EntityFrameworkCore;
+using Microsoft.EntityFrameworkCore.Infrastructure;
+using Microsoft.EntityFrameworkCore.Migrations;
+using Microsoft.EntityFrameworkCore.Storage.ValueConversion;
+
+#nullable disable
+
+namespace DodoSSH.Client.Storage.Migrations
+{
+    [DbContext(typeof(ClientCacheContext))]
+    [Migration("20260803202241_AddVaultKeyWrapHistory")]
+    partial class AddVaultKeyWrapHistory
+    {
+        /// <inheritdoc />
+        protected override void BuildTargetModel(ModelBuilder modelBuilder)
+        {
+#pragma warning disable 612, 618
+            modelBuilder.HasAnnotation("ProductVersion", "10.0.10");
+
+            modelBuilder.Entity("DodoSSH.Client.Storage.CachedItemRow", b =>
+                {
+                    b.Property<Guid>("VaultId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("vault_id");
+
+                    b.Property<int>("EntityType")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("entity_type");
+
+                    b.Property<Guid>("EntityId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("entity_id");
+
+                    b.Property<byte>("AadVersion")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("aad_version");
+
+                    b.Property<long>("ChangeSequence")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("change_sequence");
+
+                    b.Property<Guid?>("DataKeyId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("data_key_id");
+
+                    b.Property<bool>("IsDeleted")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("is_deleted");
+
+                    b.Property<uint>("KeyGeneration")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("key_generation");
+
+                    b.Property<byte[]>("Payload")
+                        .HasColumnType("BLOB")
+                        .HasColumnName("payload");
+
+                    b.Property<byte[]>("ProtectedFields")
+                        .HasColumnType("BLOB")
+                        .HasColumnName("protected_fields");
+
+                    b.Property<long>("UpdatedAtUtc")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("updated_at_utc");
+
+                    b.Property<int>("Version")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("version");
+
+                    b.Property<byte[]>("WrappedDataKey")
+                        .HasColumnType("BLOB")
+                        .HasColumnName("wrapped_data_key");
+
+                    b.HasKey("VaultId", "EntityType", "EntityId")
+                        .HasName("pk_item");
+
+                    b.HasIndex("VaultId", "ChangeSequence")
+                        .HasDatabaseName("ix_item_vault_id_change_sequence");
+
+                    b.HasIndex("VaultId", "EntityType")
+                        .HasDatabaseName("ix_item_vault_id_entity_type");
+
+                    b.ToTable("item", (string)null);
+                });
+
+            modelBuilder.Entity("DodoSSH.Client.Storage.CachedVaultKeyWrapRow", b =>
+                {
+                    b.Property<Guid>("VaultId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("vault_id");
+
+                    b.Property<uint>("KeyGeneration")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("key_generation");
+
+                    b.Property<byte[]>("WrappedKey")
+                        .IsRequired()
+                        .HasColumnType("BLOB")
+                        .HasColumnName("wrapped_key");
+
+                    b.HasKey("VaultId", "KeyGeneration")
+                        .HasName("pk_vault_key_wrap");
+
+                    b.ToTable("vault_key_wrap", (string)null);
+                });
+
+            modelBuilder.Entity("DodoSSH.Client.Storage.CachedVaultRow", b =>
+                {
+                    b.Property<Guid>("VaultId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("vault_id");
+
+                    b.Property<bool>("Hidden")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("hidden");
+
+                    b.Property<bool>("IsPersonal")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("is_personal");
+
+                    b.Property<uint>("KeyGeneration")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("key_generation");
+
+                    b.Property<string>("Name")
+                        .IsRequired()
+                        .HasColumnType("TEXT")
+                        .HasColumnName("name");
+
+                    b.Property<int>("Permissions")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("permissions");
+
+                    b.Property<bool>("RekeyRequired")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("rekey_required");
+
+                    b.Property<Guid?>("TeamId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("team_id");
+
+                    b.Property<long>("UpdatedAtUtc")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("updated_at_utc");
+
+                    b.Property<byte[]>("WrappedVaultKey")
+                        .HasColumnType("BLOB")
+                        .HasColumnName("wrapped_vault_key");
+
+                    b.HasKey("VaultId")
+                        .HasName("pk_vault");
+
+                    b.ToTable("vault", (string)null);
+                });
+
+            modelBuilder.Entity("DodoSSH.Client.Storage.ConflictRow", b =>
+                {
+                    b.Property<Guid>("Id")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("id");
+
+                    b.Property<bool>("Acknowledged")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("acknowledged");
+
+                    b.Property<byte[]>("Detail")
+                        .IsRequired()
+                        .HasColumnType("BLOB")
+                        .HasColumnName("detail");
+
+                    b.Property<long>("DetectedAtUtc")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("detected_at_utc");
+
+                    b.Property<Guid>("EntityId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("entity_id");
+
+                    b.Property<int>("EntityType")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("entity_type");
+
+                    b.Property<int>("Kind")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("kind");
+
+                    b.Property<Guid>("VaultId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("vault_id");
+
+                    b.HasKey("Id")
+                        .HasName("pk_conflict");
+
+                    b.HasIndex("VaultId", "Acknowledged")
+                        .HasDatabaseName("ix_conflict_vault_id_acknowledged");
+
+                    b.HasIndex("VaultId", "EntityType", "EntityId")
+                        .HasDatabaseName("ix_conflict_vault_id_entity_type_entity_id");
+
+                    b.ToTable("conflict", (string)null);
+                });
+
+            modelBuilder.Entity("DodoSSH.Client.Storage.OutboxRow", b =>
+                {
+                    b.Property<long>("Sequence")
+                        .ValueGeneratedOnAdd()
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("sequence");
+
+                    b.Property<byte>("AadVersion")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("aad_version");
+
+                    b.Property<byte?>("AncestorAadVersion")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("ancestor_aad_version");
+
+                    b.Property<Guid?>("AncestorDataKeyId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("ancestor_data_key_id");
+
+                    b.Property<uint?>("AncestorKeyGeneration")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("ancestor_key_generation");
+
+                    b.Property<byte[]>("AncestorPayload")
+                        .HasColumnType("BLOB")
+                        .HasColumnName("ancestor_payload");
+
+                    b.Property<byte[]>("AncestorProtectedFields")
+                        .HasColumnType("BLOB")
+                        .HasColumnName("ancestor_protected_fields");
+
+                    b.Property<int?>("AncestorVersion")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("ancestor_version");
+
+                    b.Property<byte[]>("AncestorWrappedDataKey")
+                        .HasColumnType("BLOB")
+                        .HasColumnName("ancestor_wrapped_data_key");
+
+                    b.Property<int>("Attempts")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("attempts");
+
+                    b.Property<Guid?>("DataKeyId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("data_key_id");
+
+                    b.Property<Guid>("EntityId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("entity_id");
+
+                    b.Property<int>("EntityType")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("entity_type");
+
+                    b.Property<int?>("ExpectedVersion")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("expected_version");
+
+                    b.Property<bool>("IsParked")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("is_parked");
+
+                    b.Property<uint>("KeyGeneration")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("key_generation");
+
+                    b.Property<string>("LastError")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("last_error");
+
+                    b.Property<int>("Operation")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("operation");
+
+                    b.Property<Guid>("OperationId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("operation_id");
+
+                    b.Property<byte[]>("Payload")
+                        .HasColumnType("BLOB")
+                        .HasColumnName("payload");
+
+                    b.Property<byte[]>("ProtectedFields")
+                        .HasColumnType("BLOB")
+                        .HasColumnName("protected_fields");
+
+                    b.Property<long>("QueuedAtUtc")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("queued_at_utc");
+
+                    b.Property<Guid>("VaultId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("vault_id");
+
+                    b.Property<byte[]>("WrappedDataKey")
+                        .HasColumnType("BLOB")
+                        .HasColumnName("wrapped_data_key");
+
+                    b.HasKey("Sequence")
+                        .HasName("pk_outbox");
+
+                    b.HasIndex("OperationId")
+                        .IsUnique()
+                        .HasDatabaseName("ix_outbox_operation_id");
+
+                    b.HasIndex("VaultId", "EntityType", "EntityId")
+                        .IsUnique()
+                        .HasDatabaseName("ix_outbox_vault_id_entity_type_entity_id");
+
+                    b.HasIndex("VaultId", "IsParked", "Sequence")
+                        .HasDatabaseName("ix_outbox_vault_id_is_parked_sequence");
+
+                    b.ToTable("outbox", (string)null);
+                });
+
+            modelBuilder.Entity("DodoSSH.Client.Storage.RememberedSignInRow", b =>
+                {
+                    b.Property<int>("Id")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("id");
+
+                    b.Property<byte[]>("SealedRefreshToken")
+                        .IsRequired()
+                        .HasColumnType("BLOB")
+                        .HasColumnName("sealed_refresh_token");
+
+                    b.Property<long>("UpdatedAtUtc")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("updated_at_utc");
+
+                    b.HasKey("Id")
+                        .HasName("pk_remembered_sign_in");
+
+                    b.ToTable("remembered_sign_in", null, t =>
+                        {
+                            t.HasCheckConstraint("ck_remembered_sign_in_singleton", "id = 1");
+                        });
+                });
+
+            modelBuilder.Entity("DodoSSH.Client.Storage.SyncStateRow", b =>
+                {
+                    b.Property<Guid>("VaultId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("vault_id");
+
+                    b.Property<string>("Cursor")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("cursor");
+
+                    b.Property<uint>("KeyGeneration")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("key_generation");
+
+                    b.Property<long?>("LastPulledAtUtc")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("last_pulled_at_utc");
+
+                    b.Property<long?>("LastPushedAtUtc")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("last_pushed_at_utc");
+
+                    b.Property<long>("ServerTimeSkewMs")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("server_time_skew_ms");
+
+                    b.HasKey("VaultId")
+                        .HasName("pk_sync_state");
+
+                    b.ToTable("sync_state", (string)null);
+                });
+
+            modelBuilder.Entity("DodoSSH.Client.Storage.UnlockMaterialRow", b =>
+                {
+                    b.Property<int>("Id")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("id");
+
+                    b.Property<Guid?>("DeviceId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("device_id");
+
+                    b.Property<byte[]>("DeviceWrappedPrivateKey")
+                        .HasColumnType("BLOB")
+                        .HasColumnName("device_wrapped_private_key");
+
+                    b.Property<string>("DisplayName")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("display_name");
+
+                    b.Property<string>("Email")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("email");
+
+                    b.Property<string>("Issuer")
+                        .IsRequired()
+                        .HasColumnType("TEXT")
+                        .HasColumnName("issuer");
+
+                    b.Property<string>("KdfAlgorithm")
+                        .IsRequired()
+                        .HasColumnType("TEXT")
+                        .HasColumnName("kdf_algorithm");
+
+                    b.Property<int>("KdfMemoryKibibytes")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("kdf_memory_kibibytes");
+
+                    b.Property<int>("KdfParallelism")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("kdf_parallelism");
+
+                    b.Property<int>("KdfPasses")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("kdf_passes");
+
+                    b.Property<byte[]>("KdfSalt")
+                        .IsRequired()
+                        .HasColumnType("BLOB")
+                        .HasColumnName("kdf_salt");
+
+                    b.Property<uint>("KeyGeneration")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("key_generation");
+
+                    b.Property<string>("ServerUrl")
+                        .IsRequired()
+                        .HasColumnType("TEXT")
+                        .HasColumnName("server_url");
+
+                    b.Property<string>("Subject")
+                        .IsRequired()
+                        .HasColumnType("TEXT")
+                        .HasColumnName("subject");
+
+                    b.Property<long>("UpdatedAtUtc")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("updated_at_utc");
+
+                    b.Property<Guid>("UserId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("user_id");
+
+                    b.Property<byte[]>("WrappedPrivateKey")
+                        .IsRequired()
+                        .HasColumnType("BLOB")
+                        .HasColumnName("wrapped_private_key");
+
+                    b.HasKey("Id")
+                        .HasName("pk_unlock_material");
+
+                    b.ToTable("unlock_material", null, t =>
+                        {
+                            t.HasCheckConstraint("ck_unlock_material_singleton", "id = 1");
+                        });
+                });
+#pragma warning restore 612, 618
+        }
+    }
+}

src/DodoSSH.Client.Storage/Migrations/20260803202241_AddVaultKeyWrapHistory.cs

@@ -0,0 +1,35 @@
+using System;
+using Microsoft.EntityFrameworkCore.Migrations;
+
+#nullable disable
+
+namespace DodoSSH.Client.Storage.Migrations
+{
+    /// <inheritdoc />
+    public partial class AddVaultKeyWrapHistory : Migration
+    {
+        /// <inheritdoc />
+        protected override void Up(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.CreateTable(
+                name: "vault_key_wrap",
+                columns: table => new
+                {
+                    vault_id = table.Column<Guid>(type: "TEXT", nullable: false),
+                    key_generation = table.Column<uint>(type: "INTEGER", nullable: false),
+                    wrapped_key = table.Column<byte[]>(type: "BLOB", nullable: false)
+                },
+                constraints: table =>
+                {
+                    table.PrimaryKey("pk_vault_key_wrap", x => new { x.vault_id, x.key_generation });
+                });
+        }
+
+        /// <inheritdoc />
+        protected override void Down(MigrationBuilder migrationBuilder)
+        {
+            migrationBuilder.DropTable(
+                name: "vault_key_wrap");
+        }
+    }
+}

src/DodoSSH.Client.Storage/Migrations/ClientCacheContextModelSnapshot.cs

@@ -83,6 +83,27 @@ namespace DodoSSH.Client.Storage.Migrations
                     b.ToTable("item", (string)null);
                 });
 
+            modelBuilder.Entity("DodoSSH.Client.Storage.CachedVaultKeyWrapRow", b =>
+                {
+                    b.Property<Guid>("VaultId")
+                        .HasColumnType("TEXT")
+                        .HasColumnName("vault_id");
+
+                    b.Property<uint>("KeyGeneration")
+                        .HasColumnType("INTEGER")
+                        .HasColumnName("key_generation");
+
+                    b.Property<byte[]>("WrappedKey")
+                        .IsRequired()
+                        .HasColumnType("BLOB")
+                        .HasColumnName("wrapped_key");
+
+                    b.HasKey("VaultId", "KeyGeneration")
+                        .HasName("pk_vault_key_wrap");
+
+                    b.ToTable("vault_key_wrap", (string)null);
+                });
+
             modelBuilder.Entity("DodoSSH.Client.Storage.CachedVaultRow", b =>
                 {
                     b.Property<Guid>("VaultId")

src/DodoSSH.Client.Storage/StoredTypes.cs

@@ -71,6 +71,11 @@ public sealed record StoredUnlockMaterial(
 /// <param name="Permissions">Effective permissions, as a flags value.</param>
 /// <param name="WrappedVaultKey">The vault key sealed to this user. Null while awaiting re-wrap.</param>
 /// <param name="RekeyRequired">Whether a membership change has left this vault needing a rekey.</param>
+/// <param name="PriorKeyWraps">
+/// The same key at every generation before <paramref name="KeyGeneration"/> that this user still holds
+/// a grant for. Empty for a vault that has never been rotated, and what makes one that has readable
+/// back to its first item.
+/// </param>
 public sealed record StoredVault(
     Guid VaultId,
     string Name,
@@ -79,7 +84,8 @@ public sealed record StoredVault(
     uint KeyGeneration,
     int Permissions,
     byte[]? WrappedVaultKey,
-    bool RekeyRequired)
+    bool RekeyRequired,
+    IReadOnlyList<VaultKeyWrap>? PriorKeyWraps = null)
 {
     /// <summary>
     /// The <c>Write</c> bit of <see cref="Permissions"/>.

src/DodoSSH.Client.Storage/VaultStore.cs

@@ -1,3 +1,4 @@
+using DodoSSH.Contracts;
 using Microsoft.EntityFrameworkCore;
 
 namespace DodoSSH.Client.Storage;
@@ -25,7 +26,9 @@ public sealed class VaultStore(IDbContextFactory<ClientCacheContext> contexts, T
             .ToListAsync(cancellationToken)
             .ConfigureAwait(false);
 
-        return [.. rows.Select(ToStored)];
+        var wraps = await ReadWrapsAsync(context, cancellationToken).ConfigureAwait(false);
+
+        return [.. rows.Select(row => ToStored(row, wraps.GetValueOrDefault(row.VaultId, [])))];
     }
 
     /// <summary>Reads one vault.</summary>
@@ -39,7 +42,19 @@ public sealed class VaultStore(IDbContextFactory<ClientCacheContext> contexts, T
             .SingleOrDefaultAsync(r => r.VaultId == vaultId, cancellationToken)
             .ConfigureAwait(false);
 
-        return row is null ? null : ToStored(row);
+        if (row is null)
+        {
+            return null;
+        }
+
+        var wraps = await context.Set<CachedVaultKeyWrapRow>()
+            .AsNoTracking()
+            .Where(w => w.VaultId == vaultId)
+            .OrderBy(w => w.KeyGeneration)
+            .ToListAsync(cancellationToken)
+            .ConfigureAwait(false);
+
+        return ToStored(row, [.. wraps.Select(ToWrap)]);
     }
 
     /// <summary>
@@ -81,10 +96,20 @@ public sealed class VaultStore(IDbContextFactory<ClientCacheContext> contexts, T
             }
 
             Apply(row, vault, now);
+
+            await ApplyWrapsAsync(context, vault, cancellationToken).ConfigureAwait(false);
         }
 
         context.RemoveRange(existing.Values);
 
+        // The wraps of a vault that is gone from the list go with it. They are keys to something this
+        // machine can no longer fetch, and keeping them would be keeping key material for a vault the
+        // user has been told they no longer have.
+        foreach (var dropped in existing.Keys)
+        {
+            await RemoveWrapsAsync(context, dropped, cancellationToken).ConfigureAwait(false);
+        }
+
         await context.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
     }
 
@@ -116,6 +141,8 @@ public sealed class VaultStore(IDbContextFactory<ClientCacheContext> contexts, T
 
         Apply(row, vault, clock.GetUtcNow());
 
+        await ApplyWrapsAsync(context, vault, cancellationToken).ConfigureAwait(false);
+
         await context.SaveChangesAsync(cancellationToken).ConfigureAwait(false);
     }
 
@@ -182,7 +209,66 @@ public sealed class VaultStore(IDbContextFactory<ClientCacheContext> contexts, T
         row.UpdatedAtUtc = now;
     }
 
-    private static StoredVault ToStored(CachedVaultRow row) =>
+    /// <summary>
+    /// Replaces one vault's earlier-generation wraps with what the server reported.
+    /// </summary>
+    /// <remarks>
+    /// Deleted and re-inserted rather than merged. There are a handful of these per vault at most, the
+    /// server's list is authoritative, and a merge would have to decide what a wrap present here and
+    /// absent there means — which is "that grant was revoked", and the answer to that is to drop it.
+    /// </remarks>
+    private static async Task ApplyWrapsAsync(
+        ClientCacheContext context,
+        StoredVault vault,
+        CancellationToken cancellationToken)
+    {
+        await RemoveWrapsAsync(context, vault.VaultId, cancellationToken).ConfigureAwait(false);
+
+        foreach (var wrap in vault.PriorKeyWraps ?? [])
+        {
+            context.Add(new CachedVaultKeyWrapRow
+            {
+                VaultId = vault.VaultId,
+                KeyGeneration = wrap.KeyGeneration,
+                WrappedKey = wrap.WrappedKey,
+            });
+        }
+    }
+
+    private static async Task RemoveWrapsAsync(
+        ClientCacheContext context,
+        Guid vaultId,
+        CancellationToken cancellationToken)
+    {
+        var stale = await context.Set<CachedVaultKeyWrapRow>()
+            .Where(w => w.VaultId == vaultId)
+            .ToListAsync(cancellationToken)
+            .ConfigureAwait(false);
+
+        context.RemoveRange(stale);
+    }
+
+    private static async Task<Dictionary<Guid, IReadOnlyList<VaultKeyWrap>>> ReadWrapsAsync(
+        ClientCacheContext context,
+        CancellationToken cancellationToken)
+    {
+        var rows = await context.Set<CachedVaultKeyWrapRow>()
+            .AsNoTracking()
+            .OrderBy(row => row.KeyGeneration)
+            .ToListAsync(cancellationToken)
+            .ConfigureAwait(false);
+
+        return rows
+            .GroupBy(row => row.VaultId)
+            .ToDictionary(
+                group => group.Key,
+                group => (IReadOnlyList<VaultKeyWrap>)[.. group.Select(ToWrap)]);
+    }
+
+    private static VaultKeyWrap ToWrap(CachedVaultKeyWrapRow row) =>
+        new(row.KeyGeneration, row.WrappedKey);
+
+    private static StoredVault ToStored(CachedVaultRow row, IReadOnlyList<VaultKeyWrap> priorWraps) =>
         new(
             row.VaultId,
             row.Name,
@@ -191,5 +277,6 @@ public sealed class VaultStore(IDbContextFactory<ClientCacheContext> contexts, T
             row.KeyGeneration,
             row.Permissions,
             row.WrappedVaultKey,
-            row.RekeyRequired);
+            row.RekeyRequired,
+            priorWraps);
 }

src/DodoSSH.Client.Sync/ItemReconciler.cs

@@ -144,14 +144,19 @@ internal sealed class ItemReconciler<TSecret>(
     {
         ArgumentNullException.ThrowIfNull(pending);
 
-        if (!keyring.TryGet(vaultId, out var vaultKey, out var generation) || pending.Payload is null)
+        if (!keyring.TryGet(vaultId, out var vaultKey, out var generation)
+            || pending.Payload is null
+            || !keyring.TryGetAt(vaultId, pending.Payload.KeyGeneration, out var queuedKey))
         {
             return "This item has no usable vault key.";
         }
 
+        // Opened under the generation it was queued at and re-sealed under the current one. Those
+        // differ whenever a rotation lands between an offline edit and its push, and re-sealing is
+        // the point: what goes back to the server has to be readable by everybody holding the new key.
         var local = kind.TryOpen(
             pending.Payload,
-            vaultKey.Span,
+            queuedKey.Span,
             pending.EntityId,
             SyncVersions.NextVersion(pending.ExpectedVersion));
 
@@ -253,8 +258,16 @@ internal sealed class ItemReconciler<TSecret>(
 
         var (local, remoteSecret, vaultKey, generation) = opened.Value;
 
-        var ancestor = kind.TryOpen(
-            pending.Ancestor.Payload, vaultKey.Span, remote.EntityId, pending.Ancestor.Version);
+        // The ancestor is the version the server last confirmed, so it carries its own generation —
+        // typically the oldest of the three when a rotation has happened since.
+        var ancestor =
+            keyring.TryGetAt(vaultId, pending.Ancestor.Payload.KeyGeneration, out var ancestorKey)
+                ? kind.TryOpen(
+                    pending.Ancestor.Payload,
+                    ancestorKey.Span,
+                    remote.EntityId,
+                    pending.Ancestor.Version)
+                : null;
 
         if (ancestor is null)
         {
@@ -313,10 +326,11 @@ internal sealed class ItemReconciler<TSecret>(
         }
 
         var local = pending.Payload is null
+            || !keyring.TryGetAt(vaultId, pending.Payload.KeyGeneration, out var queuedKey)
             ? null
             : kind.TryOpen(
                 pending.Payload,
-                vaultKey.Span,
+                queuedKey.Span,
                 remote.EntityId,
                 SyncVersions.NextVersion(pending.ExpectedVersion));
 
@@ -419,21 +433,26 @@ internal sealed class ItemReconciler<TSecret>(
     {
         if (!keyring.TryGet(vaultId, out var vaultKey, out var generation)
             || pending.Payload is null
-            || remote.Payload is null)
+            || remote.Payload is null
+            || !keyring.TryGetAt(vaultId, pending.Payload.KeyGeneration, out var queuedKey)
+            || !keyring.TryGetAt(vaultId, remote.Payload.KeyGeneration, out var remoteKey))
         {
             await ParkAsync(vaultId, remote.EntityId, pending, report, cancellationToken)
                 .ConfigureAwait(false);
             return null;
         }
 
+        // Each side under its own generation. The two genuinely differ after a rotation: what the
+        // server holds was sealed before it, and the queued edit after — or the other way round, for a
+        // client that rotated while this one was offline.
         var local = kind.TryOpen(
             pending.Payload,
-            vaultKey.Span,
+            queuedKey.Span,
             remote.EntityId,
             SyncVersions.NextVersion(pending.ExpectedVersion));
 
         var remoteSecret = kind.TryOpen(
-            remote.Payload, vaultKey.Span, remote.EntityId, remote.Version);
+            remote.Payload, remoteKey.Span, remote.EntityId, remote.Version);
 
         if (local is null || remoteSecret is null)
         {

src/DodoSSH.Client.Sync/VaultItemRepository.cs

@@ -82,7 +82,10 @@ internal sealed class VaultItemRepository<TSecret>(
         Guid vaultId,
         CancellationToken cancellationToken)
     {
-        if (!keyring.TryGet(vaultId, out var vaultKey, out _))
+        // TryGet rather than CanRead, which answers false for a disposed keyring where this has to
+        // throw: a locked session being read from is a caller holding something it should have let go
+        // of, and the exception is what says so.
+        if (!keyring.TryGet(vaultId, out _, out _))
         {
             throw new VaultUnreadableException(vaultId);
         }
@@ -104,31 +107,17 @@ internal sealed class VaultItemRepository<TSecret>(
         {
             if (pendingByEntity.Remove(item.EntityId, out var local))
             {
-                AddPending(listed, ref unreadable, vaultKey, local);
+                AddPending(listed, ref unreadable, vaultId, local);
                 continue;
             }
 
-            if (item.IsDeleted || item.Payload is null)
-            {
-                continue;
-            }
-
-            var opened = kind.TryOpen(item.Payload, vaultKey.Span, item.EntityId, item.Version);
-
-            if (opened is null)
-            {
-                unreadable++;
-                continue;
-            }
-
-            listed.Add(new VaultItem<TSecret>(
-                item.EntityId, opened.Secret, item.Version, false, false, opened.IsReadOnly));
+            AddMirrored(listed, ref unreadable, vaultId, item);
         }
 
         // Whatever is left has no mirror row yet: items created here and not yet accepted.
         foreach (var local in pendingByEntity.Values)
         {
-            AddPending(listed, ref unreadable, vaultKey, local);
+            AddPending(listed, ref unreadable, vaultId, local);
         }
 
         return new ItemListing<TSecret>(listed, unreadable);
@@ -211,7 +200,7 @@ internal sealed class VaultItemRepository<TSecret>(
         // the payload was sealed at, which is why the pending and mirror cases differ: a pending payload
         // holds the version the server will assign, and a mirror row holds the one it has.
         var before = IsAudited
-            ? Open(vaultKey, entityId, pending, ancestor)
+            ? Open(vaultId, entityId, pending, ancestor)
             : null;
 
         await outbox.QueueAsync(
@@ -327,15 +316,10 @@ internal sealed class VaultItemRepository<TSecret>(
         PendingOperation? pending,
         CancellationToken cancellationToken)
     {
-        if (!keyring.TryGet(vaultId, out var vaultKey, out _))
-        {
-            return null;
-        }
-
         var ancestor = await MirrorAncestorAsync(vaultId, entityId, cancellationToken)
             .ConfigureAwait(false);
 
-        return Open(vaultKey, entityId, pending, ancestor)?.Label;
+        return Open(vaultId, entityId, pending, ancestor)?.Label;
     }
 
     /// <summary>
@@ -348,23 +332,29 @@ internal sealed class VaultItemRepository<TSecret>(
     /// at the version the server <em>will</em> assign, and a mirror row holds the one it has.
     /// </remarks>
     private TSecret? Open(
-        ReadOnlyMemory<byte> vaultKey,
+        Guid vaultId,
         Guid entityId,
         PendingOperation? pending,
         StoredAncestor? ancestor)
     {
         if (pending is { Operation: SyncOperation.Upsert, Payload: { } queued })
         {
-            return kind.TryOpen(
-                queued,
-                vaultKey.Span,
-                entityId,
-                SyncVersions.NextVersion(pending.ExpectedVersion))?.Secret;
+            return keyring.TryGetAt(vaultId, queued.KeyGeneration, out var queuedKey)
+                ? kind.TryOpen(
+                    queued,
+                    queuedKey.Span,
+                    entityId,
+                    SyncVersions.NextVersion(pending.ExpectedVersion))?.Secret
+                : null;
         }
 
-        return ancestor is null
-            ? null
-            : kind.TryOpen(ancestor.Payload, vaultKey.Span, entityId, ancestor.Version)?.Secret;
+        if (ancestor is null
+            || !keyring.TryGetAt(vaultId, ancestor.Payload.KeyGeneration, out var vaultKey))
+        {
+            return null;
+        }
+
+        return kind.TryOpen(ancestor.Payload, vaultKey.Span, entityId, ancestor.Version)?.Secret;
     }
 
     /// <summary>
@@ -401,10 +391,43 @@ internal sealed class VaultItemRepository<TSecret>(
         }
     }
 
+    /// <summary>Adds one row of the server's mirror to a listing, or counts it as unreadable.</summary>
+    private void AddMirrored(
+        List<VaultItem<TSecret>> listed,
+        ref int unreadable,
+        Guid vaultId,
+        StoredItem item)
+    {
+        if (item.IsDeleted || item.Payload is null)
+        {
+            return;
+        }
+
+        // The generation the item names, not the vault's current one. A rotated vault holds items
+        // written under two or three keys at once, and a list that assumed the newest would report
+        // everything older as unreadable.
+        if (!keyring.TryGetAt(vaultId, item.Payload.KeyGeneration, out var vaultKey))
+        {
+            unreadable++;
+            return;
+        }
+
+        var opened = kind.TryOpen(item.Payload, vaultKey.Span, item.EntityId, item.Version);
+
+        if (opened is null)
+        {
+            unreadable++;
+            return;
+        }
+
+        listed.Add(new VaultItem<TSecret>(
+            item.EntityId, opened.Secret, item.Version, false, false, opened.IsReadOnly));
+    }
+
     private void AddPending(
         List<VaultItem<TSecret>> listed,
         ref int unreadable,
-        ReadOnlyMemory<byte> vaultKey,
+        Guid vaultId,
         PendingOperation local)
     {
         if (local.Operation == SyncOperation.Delete)
@@ -419,6 +442,14 @@ internal sealed class VaultItemRepository<TSecret>(
             return;
         }
 
+        // A queued change is sealed under whatever generation was current when it was queued, which is
+        // not necessarily the current one: a rotation can land between an offline edit and its push.
+        if (!keyring.TryGetAt(vaultId, local.Payload.KeyGeneration, out var vaultKey))
+        {
+            unreadable++;
+            return;
+        }
+
         var version = SyncVersions.NextVersion(local.ExpectedVersion);
         var opened = kind.TryOpen(local.Payload, vaultKey.Span, local.EntityId, version);
 

src/DodoSSH.Client.Sync/VaultKeyring.cs

@@ -15,6 +15,13 @@ namespace DodoSSH.Client.Sync;
 /// "the keys exist only while unlocked" a property of the code and not of everyone's discipline.
 /// </para>
 /// <para>
+/// <b>A vault has a key per generation, and this holds every one it was granted.</b> A rotation does not
+/// re-encrypt what is already stored — each item keeps the generation it was sealed under — so reading a
+/// rotated vault means opening items under two or three different keys, chosen per item rather than per
+/// vault. Writing uses the newest, which is what <see cref="TryGet"/> answers; reading an item asks for
+/// the generation that item names, which is <see cref="TryGetAt"/>.
+/// </para>
+/// <para>
 /// A grant that will not open is not an error: it means the vault has been rekeyed and this client's
 /// grant has not been re-wrapped yet, or the grant was fabricated. Both leave the vault temporarily
 /// unreadable and both are reported rather than thrown, so one bad grant does not take the other vaults
@@ -23,7 +30,7 @@ namespace DodoSSH.Client.Sync;
 /// </remarks>
 public sealed class VaultKeyring : IDisposable
 {
-    private readonly Dictionary<Guid, byte[]> keys = [];
+    private readonly Dictionary<Guid, Dictionary<uint, byte[]>> keys = [];
     private readonly Dictionary<Guid, uint> generations = [];
     private bool disposed;
 
@@ -51,6 +58,12 @@ public sealed class VaultKeyring : IDisposable
         {
             foreach (var vault in vaults)
             {
+                // The history first, and never conditional on the current generation opening. A member
+                // who has been rotated past but not yet re-wrapped can still read everything written
+                // before the rotation, and dropping those keys because the newest grant is missing
+                // would turn "you cannot see the last hour's changes" into "the vault is empty".
+                keyring.OpenPriorWraps(bundle, vault);
+
                 if (vault.WrappedVaultKey is null)
                 {
                     // The server said so itself: a grant awaiting re-wrap after a rekey.
@@ -70,8 +83,7 @@ public sealed class VaultKeyring : IDisposable
                     continue;
                 }
 
-                keyring.keys[vault.VaultId] = key;
-                keyring.generations[vault.VaultId] = vault.KeyGeneration;
+                keyring.Adopt(vault.VaultId, key, vault.KeyGeneration);
             }
 
             keyring.Unopened = unopened;
@@ -94,26 +106,49 @@ public sealed class VaultKeyring : IDisposable
     /// </param>
     /// <param name="keyGeneration">The generation this key is for.</param>
     /// <remarks>
-    /// Creating a team vault is the only case: the client generates the key, wraps it to itself and
-    /// sends the wrap, so the plaintext is already here and unwrapping the server's copy back would be
-    /// a round trip to learn something this process just chose. Adopting it also means the new vault is
-    /// usable immediately rather than at the next unlock, which is what somebody who just pressed
-    /// "create" expects.
+    /// <para>
+    /// Two cases, and they are the same operation: creating a team vault, and rotating one. Both
+    /// generate the key here, wrap it to this user and send the wrap, so the plaintext is already in
+    /// this process and unwrapping the server's copy back would be a round trip to learn something it
+    /// just chose. Adopting it also means the vault is usable immediately rather than at the next
+    /// unlock, which is what somebody who has just pressed a button expects.
+    /// </para>
+    /// <para>
+    /// This generation becomes the one writes are sealed under. A key for a generation the vault has
+    /// moved <em>past</em> goes in through <see cref="AdoptPrior"/> instead, which is not the same
+    /// operation: it makes old items readable and must not walk the write target backwards.
+    /// </para>
     /// </remarks>
     public void Adopt(Guid vaultId, byte[] vaultKey, uint keyGeneration)
     {
         ObjectDisposedException.ThrowIf(disposed, this);
         ArgumentNullException.ThrowIfNull(vaultKey);
 
-        if (keys.TryGetValue(vaultId, out var previous))
-        {
-            CryptographicOperations.ZeroMemory(previous);
-        }
+        Store(vaultId, vaultKey, keyGeneration);
 
-        keys[vaultId] = vaultKey;
-        generations[vaultId] = keyGeneration;
+        Promote(vaultId, keyGeneration);
+    }
 
-        Unopened = [.. Unopened.Where(id => id != vaultId)];
+    /// <summary>
+    /// Takes a vault key for a generation the vault has already moved past.
+    /// </summary>
+    /// <param name="vaultId">The vault.</param>
+    /// <param name="vaultKey">
+    /// The plaintext key. <b>The keyring takes ownership</b>, exactly as <see cref="Adopt"/> does.
+    /// </param>
+    /// <param name="keyGeneration">The superseded generation this key opens.</param>
+    /// <remarks>
+    /// Holding one of these is what lets a rotated vault be read at all: items are not re-encrypted by a
+    /// rotation, so everything written before it is still sealed under the key it was written with.
+    /// Nothing is ever <em>written</em> under one, which is why this does not touch the current
+    /// generation and does not make an otherwise unreadable vault readable.
+    /// </remarks>
+    public void AdoptPrior(Guid vaultId, byte[] vaultKey, uint keyGeneration)
+    {
+        ObjectDisposedException.ThrowIf(disposed, this);
+        ArgumentNullException.ThrowIfNull(vaultKey);
+
+        Store(vaultId, vaultKey, keyGeneration);
     }
 
     /// <summary>
@@ -132,13 +167,23 @@ public sealed class VaultKeyring : IDisposable
         ArgumentNullException.ThrowIfNull(bundle);
         ArgumentNullException.ThrowIfNull(vault);
 
+        // Attempted whatever happens to the current generation, and before it. A share of a vault that
+        // has been rotated since it was created arrives as a current wrap plus its history, and the
+        // history is not a consolation prize — without it the recipient sees a vault full of items that
+        // will not decrypt.
+        OpenPriorWraps(bundle, vault);
+
         if (vault.WrappedVaultKey is null)
         {
             return false;
         }
 
-        if (keys.ContainsKey(vault.VaultId) && generations[vault.VaultId] == vault.KeyGeneration)
+        if (Held(vault.VaultId, vault.KeyGeneration) is not null)
         {
+            // Already open at this generation. Promoted rather than returned early, because a vault
+            // that was rotated and re-granted arrives here with a generation this keyring has been
+            // treating as historic, and it is now the one writes belong under.
+            Promote(vault.VaultId, vault.KeyGeneration);
             return true;
         }
 
@@ -157,14 +202,26 @@ public sealed class VaultKeyring : IDisposable
 
     /// <summary>Records that a vault cannot be read, so the interface can say so.</summary>
     /// <remarks>
+    /// <para>
     /// The counterpart of <see cref="TryAdmit"/> for the case where the grant did not open. Kept
     /// explicit rather than inferred from the absence of a key, because "no key" is also what a vault
     /// this session has never heard of looks like.
+    /// </para>
+    /// <para>
+    /// <b>It also gives up the write target, and that is the load-bearing half.</b> The usual way to
+    /// reach here is another client having rotated the vault: this session still holds the previous
+    /// generation's key and it is no longer the current one. Going on treating it as current would seal
+    /// new items under a superseded key — readable here, unreadable to everybody else, and with nothing
+    /// to show the author that anything was wrong. The keys themselves are kept, because the items
+    /// already written under them are still readable through <see cref="TryGetAt"/>.
+    /// </para>
     /// </remarks>
     public void MarkUnreadable(Guid vaultId)
     {
         ObjectDisposedException.ThrowIf(disposed, this);
 
+        generations.Remove(vaultId);
+
         if (!Unopened.Contains(vaultId))
         {
             Unopened = [.. Unopened, vaultId];
@@ -172,20 +229,27 @@ public sealed class VaultKeyring : IDisposable
     }
 
     /// <summary>
-    /// Borrows a vault's key.
+    /// Borrows a vault's current key: the one new items are sealed under.
     /// </summary>
     /// <remarks>
     /// The returned memory is the keyring's own buffer, not a copy, and is zeroed when the keyring is
     /// disposed. Callers must not retain it past the operation they borrowed it for.
+    /// <para>
+    /// False for a vault this session holds only the history of — one rotated past a grant that has not
+    /// been re-wrapped yet. That is deliberate: writing under a superseded key would produce an item
+    /// nobody else could read, and the honest answer is that the vault is not writable until the new
+    /// key arrives.
+    /// </para>
     /// </remarks>
     public bool TryGet(Guid vaultId, out ReadOnlyMemory<byte> vaultKey, out uint keyGeneration)
     {
         ObjectDisposedException.ThrowIf(disposed, this);
 
-        if (keys.TryGetValue(vaultId, out var key))
+        if (generations.TryGetValue(vaultId, out var current)
+            && Held(vaultId, current) is { } key)
         {
             vaultKey = key;
-            keyGeneration = generations[vaultId];
+            keyGeneration = current;
             return true;
         }
 
@@ -194,8 +258,52 @@ public sealed class VaultKeyring : IDisposable
         return false;
     }
 
-    /// <summary>Whether this vault can be read at all.</summary>
-    public bool CanRead(Guid vaultId) => !disposed && keys.ContainsKey(vaultId);
+    /// <summary>
+    /// Borrows the key one particular generation of a vault was sealed under.
+    /// </summary>
+    /// <param name="vaultId">The vault.</param>
+    /// <param name="keyGeneration">The generation the item names.</param>
+    /// <param name="vaultKey">The key, borrowed on the same terms as <see cref="TryGet"/>.</param>
+    /// <returns>Whether this session holds that generation.</returns>
+    /// <remarks>
+    /// What every read goes through, because an item names the generation it was sealed under and a
+    /// rotated vault holds items from more than one. False means that item is unreadable here and says
+    /// nothing about the rest of the vault — which is why a caller counts it rather than failing.
+    /// </remarks>
+    public bool TryGetAt(Guid vaultId, uint keyGeneration, out ReadOnlyMemory<byte> vaultKey)
+    {
+        ObjectDisposedException.ThrowIf(disposed, this);
+
+        if (Held(vaultId, keyGeneration) is { } key)
+        {
+            vaultKey = key;
+            return true;
+        }
+
+        vaultKey = default;
+        return false;
+    }
+
+    /// <summary>
+    /// Every generation of one vault's key that this session holds, oldest first.
+    /// </summary>
+    /// <remarks>
+    /// Read when sharing: a recipient given only the newest key would find the vault's history
+    /// undecryptable, so the sharing client wraps each of these in turn. It is the only party that can
+    /// — the server holds ciphertext, and the recipient holds nothing yet.
+    /// </remarks>
+    public IReadOnlyList<uint> GenerationsHeld(Guid vaultId)
+    {
+        ObjectDisposedException.ThrowIf(disposed, this);
+
+        return keys.TryGetValue(vaultId, out var held) ? [.. held.Keys.Order()] : [];
+    }
+
+    /// <summary>Whether this vault can be read and written at its current generation.</summary>
+    public bool CanRead(Guid vaultId) =>
+        !disposed
+        && generations.TryGetValue(vaultId, out var current)
+        && Held(vaultId, current) is not null;
 
     /// <inheritdoc />
     public void Dispose()
@@ -207,14 +315,70 @@ public sealed class VaultKeyring : IDisposable
 
         disposed = true;
 
-        foreach (var key in keys.Values)
+        foreach (var held in keys.Values)
         {
-            CryptographicOperations.ZeroMemory(key);
+            foreach (var key in held.Values)
+            {
+                CryptographicOperations.ZeroMemory(key);
+            }
         }
 
         keys.Clear();
         generations.Clear();
     }
+
+    /// <summary>Opens whatever superseded generations this vault came with.</summary>
+    /// <remarks>
+    /// A wrap that will not open is skipped rather than reported. It means one historic grant is
+    /// unusable — the items under that generation stay unreadable and are counted as such where they
+    /// are listed — and it is not a reason to refuse the generations that did open.
+    /// </remarks>
+    private void OpenPriorWraps(UserSecretBundle bundle, StoredVault vault)
+    {
+        foreach (var wrap in vault.PriorKeyWraps ?? [])
+        {
+            if (Held(vault.VaultId, wrap.KeyGeneration) is not null)
+            {
+                continue;
+            }
+
+            var key = VaultKeys.TryUnwrap(
+                bundle.EncryptionKey, wrap.WrappedKey, vault.VaultId, wrap.KeyGeneration);
+
+            if (key is not null)
+            {
+                AdoptPrior(vault.VaultId, key, wrap.KeyGeneration);
+            }
+        }
+    }
+
+    private byte[]? Held(Guid vaultId, uint keyGeneration) =>
+        keys.TryGetValue(vaultId, out var held) && held.TryGetValue(keyGeneration, out var key)
+            ? key
+            : null;
+
+    private void Store(Guid vaultId, byte[] vaultKey, uint keyGeneration)
+    {
+        if (!keys.TryGetValue(vaultId, out var held))
+        {
+            held = [];
+            keys[vaultId] = held;
+        }
+
+        if (held.TryGetValue(keyGeneration, out var previous))
+        {
+            CryptographicOperations.ZeroMemory(previous);
+        }
+
+        held[keyGeneration] = vaultKey;
+    }
+
+    private void Promote(Guid vaultId, uint keyGeneration)
+    {
+        generations[vaultId] = keyGeneration;
+
+        Unopened = [.. Unopened.Where(id => id != vaultId)];
+    }
 }
 
 /// <summary>Thrown when an operation needs a vault key the keyring does not hold.</summary>

src/DodoSSH.Contracts/DodoSshJsonContext.cs

@@ -54,6 +54,7 @@ namespace DodoSSH.Contracts;
 [JsonSerializable(typeof(IReadOnlyList<TeamInvitationSummary>))]
 [JsonSerializable(typeof(CreateTeamVaultRequest))]
 [JsonSerializable(typeof(IssueVaultGrantRequest))]
+[JsonSerializable(typeof(RekeyVaultRequest))]
 [JsonSerializable(typeof(VaultGrantsResponse))]
 [JsonSerializable(typeof(KeyLogPage))]
 [JsonSerializable(typeof(SyncPullRequest))]

src/DodoSSH.Contracts/Enrollment.cs

@@ -209,6 +209,16 @@ public sealed record MeResponse(
 /// must complete it.
 /// </param>
 /// <param name="RekeyRequired">Whether a membership change has left this vault needing a rekey.</param>
+/// <param name="PriorKeyWraps">
+/// Generations before <paramref name="KeyGeneration"/> that this caller still holds a grant for.
+/// <para>
+/// Empty for a vault that has never been rotated, which is nearly all of them. It is not empty after
+/// one, and it has to be served: an item is sealed under the generation in force when it was written,
+/// so a client that held only the current key would find every item older than the rotation
+/// undecryptable. See <c>VaultGrantService.RekeyAsync</c> for why old grants are kept rather than
+/// revoked.
+/// </para>
+/// </param>
 public sealed record VaultSummary(
     Guid VaultId,
     string Name,
@@ -217,4 +227,14 @@ public sealed record VaultSummary(
     uint KeyGeneration,
     int Permissions,
     byte[]? WrappedVaultKey,
-    bool RekeyRequired);
+    bool RekeyRequired,
+    IReadOnlyList<VaultKeyWrap>? PriorKeyWraps = null);
+
+/// <summary>A vault key sealed to one recipient, at one generation.</summary>
+/// <remarks>
+/// Only ever the caller's own. <c>VaultGrantSummary</c> deliberately carries no wrap: serving every
+/// member's sealed key to every member would widen what a stolen access token yields for nothing.
+/// </remarks>
+/// <param name="KeyGeneration">The generation this wrap opens.</param>
+/// <param name="WrappedKey">The vault key sealed to the caller's X25519 key. Opaque.</param>
+public sealed record VaultKeyWrap(uint KeyGeneration, byte[] WrappedKey);

src/DodoSSH.Contracts/PublicAPI.Unshipped.txt

@@ -359,6 +359,19 @@ DodoSSH.Contracts.RegisterDeviceResponse.EnrolledAt.get -> System.DateTimeOffset
 DodoSSH.Contracts.RegisterDeviceResponse.EnrolledAt.init -> void
 DodoSSH.Contracts.RegisterDeviceResponse.Equals(DodoSSH.Contracts.RegisterDeviceResponse? other) -> bool
 DodoSSH.Contracts.RegisterDeviceResponse.RegisterDeviceResponse(System.Guid DeviceId, System.DateTimeOffset EnrolledAt) -> void
+DodoSSH.Contracts.RekeyVaultRequest
+DodoSSH.Contracts.RekeyVaultRequest.<Clone>$() -> DodoSSH.Contracts.RekeyVaultRequest!
+DodoSSH.Contracts.RekeyVaultRequest.Deconstruct(out uint KeyGeneration, out byte[]! WrappedVaultKey, out byte[]! GrantSignature, out System.DateTimeOffset GrantedAt) -> void
+DodoSSH.Contracts.RekeyVaultRequest.Equals(DodoSSH.Contracts.RekeyVaultRequest? other) -> bool
+DodoSSH.Contracts.RekeyVaultRequest.GrantedAt.get -> System.DateTimeOffset
+DodoSSH.Contracts.RekeyVaultRequest.GrantedAt.init -> void
+DodoSSH.Contracts.RekeyVaultRequest.GrantSignature.get -> byte[]!
+DodoSSH.Contracts.RekeyVaultRequest.GrantSignature.init -> void
+DodoSSH.Contracts.RekeyVaultRequest.KeyGeneration.get -> uint
+DodoSSH.Contracts.RekeyVaultRequest.KeyGeneration.init -> void
+DodoSSH.Contracts.RekeyVaultRequest.RekeyVaultRequest(uint KeyGeneration, byte[]! WrappedVaultKey, byte[]! GrantSignature, System.DateTimeOffset GrantedAt) -> void
+DodoSSH.Contracts.RekeyVaultRequest.WrappedVaultKey.get -> byte[]!
+DodoSSH.Contracts.RekeyVaultRequest.WrappedVaultKey.init -> void
 DodoSSH.Contracts.RelayConfiguration
 DodoSSH.Contracts.RelayConfiguration.<Clone>$() -> DodoSSH.Contracts.RelayConfiguration!
 DodoSSH.Contracts.RelayConfiguration.Deconstruct(out bool Enabled, out System.Uri? WebSocketUrl) -> void
@@ -705,9 +718,18 @@ DodoSSH.Contracts.VaultGrantSummary.RevokedAt.init -> void
 DodoSSH.Contracts.VaultGrantSummary.State.get -> DodoSSH.Contracts.VaultGrantState
 DodoSSH.Contracts.VaultGrantSummary.State.init -> void
 DodoSSH.Contracts.VaultGrantSummary.VaultGrantSummary(System.Guid RecipientUserId, string? Email, string? DisplayName, uint KeyGeneration, DodoSSH.Contracts.VaultGrantState State, System.Guid GranterUserId, System.DateTimeOffset CreatedAt, System.DateTimeOffset? RevokedAt) -> void
+DodoSSH.Contracts.VaultKeyWrap
+DodoSSH.Contracts.VaultKeyWrap.<Clone>$() -> DodoSSH.Contracts.VaultKeyWrap!
+DodoSSH.Contracts.VaultKeyWrap.Deconstruct(out uint KeyGeneration, out byte[]! WrappedKey) -> void
+DodoSSH.Contracts.VaultKeyWrap.Equals(DodoSSH.Contracts.VaultKeyWrap? other) -> bool
+DodoSSH.Contracts.VaultKeyWrap.KeyGeneration.get -> uint
+DodoSSH.Contracts.VaultKeyWrap.KeyGeneration.init -> void
+DodoSSH.Contracts.VaultKeyWrap.VaultKeyWrap(uint KeyGeneration, byte[]! WrappedKey) -> void
+DodoSSH.Contracts.VaultKeyWrap.WrappedKey.get -> byte[]!
+DodoSSH.Contracts.VaultKeyWrap.WrappedKey.init -> void
 DodoSSH.Contracts.VaultSummary
 DodoSSH.Contracts.VaultSummary.<Clone>$() -> DodoSSH.Contracts.VaultSummary!
-DodoSSH.Contracts.VaultSummary.Deconstruct(out System.Guid VaultId, out string! Name, out bool IsPersonal, out System.Guid? TeamId, out uint KeyGeneration, out int Permissions, out byte[]? WrappedVaultKey, out bool RekeyRequired) -> void
+DodoSSH.Contracts.VaultSummary.Deconstruct(out System.Guid VaultId, out string! Name, out bool IsPersonal, out System.Guid? TeamId, out uint KeyGeneration, out int Permissions, out byte[]? WrappedVaultKey, out bool RekeyRequired, out System.Collections.Generic.IReadOnlyList<DodoSSH.Contracts.VaultKeyWrap!>? PriorKeyWraps) -> void
 DodoSSH.Contracts.VaultSummary.Equals(DodoSSH.Contracts.VaultSummary? other) -> bool
 DodoSSH.Contracts.VaultSummary.IsPersonal.get -> bool
 DodoSSH.Contracts.VaultSummary.IsPersonal.init -> void
@@ -717,13 +739,15 @@ DodoSSH.Contracts.VaultSummary.Name.get -> string!
 DodoSSH.Contracts.VaultSummary.Name.init -> void
 DodoSSH.Contracts.VaultSummary.Permissions.get -> int
 DodoSSH.Contracts.VaultSummary.Permissions.init -> void
+DodoSSH.Contracts.VaultSummary.PriorKeyWraps.get -> System.Collections.Generic.IReadOnlyList<DodoSSH.Contracts.VaultKeyWrap!>?
+DodoSSH.Contracts.VaultSummary.PriorKeyWraps.init -> void
 DodoSSH.Contracts.VaultSummary.RekeyRequired.get -> bool
 DodoSSH.Contracts.VaultSummary.RekeyRequired.init -> void
 DodoSSH.Contracts.VaultSummary.TeamId.get -> System.Guid?
 DodoSSH.Contracts.VaultSummary.TeamId.init -> void
 DodoSSH.Contracts.VaultSummary.VaultId.get -> System.Guid
 DodoSSH.Contracts.VaultSummary.VaultId.init -> void
-DodoSSH.Contracts.VaultSummary.VaultSummary(System.Guid VaultId, string! Name, bool IsPersonal, System.Guid? TeamId, uint KeyGeneration, int Permissions, byte[]? WrappedVaultKey, bool RekeyRequired) -> void
+DodoSSH.Contracts.VaultSummary.VaultSummary(System.Guid VaultId, string! Name, bool IsPersonal, System.Guid? TeamId, uint KeyGeneration, int Permissions, byte[]? WrappedVaultKey, bool RekeyRequired, System.Collections.Generic.IReadOnlyList<DodoSSH.Contracts.VaultKeyWrap!>? PriorKeyWraps = null) -> void
 DodoSSH.Contracts.VaultSummary.WrappedVaultKey.get -> byte[]?
 DodoSSH.Contracts.VaultSummary.WrappedVaultKey.init -> void
 override DodoSSH.Contracts.AddTeamMemberRequest.Equals(object? obj) -> bool
@@ -789,6 +813,9 @@ override DodoSSH.Contracts.RegisterDeviceRequest.ToString() -> string!
 override DodoSSH.Contracts.RegisterDeviceResponse.Equals(object? obj) -> bool
 override DodoSSH.Contracts.RegisterDeviceResponse.GetHashCode() -> int
 override DodoSSH.Contracts.RegisterDeviceResponse.ToString() -> string!
+override DodoSSH.Contracts.RekeyVaultRequest.Equals(object? obj) -> bool
+override DodoSSH.Contracts.RekeyVaultRequest.GetHashCode() -> int
+override DodoSSH.Contracts.RekeyVaultRequest.ToString() -> string!
 override DodoSSH.Contracts.RelayConfiguration.Equals(object? obj) -> bool
 override DodoSSH.Contracts.RelayConfiguration.GetHashCode() -> int
 override DodoSSH.Contracts.RelayConfiguration.ToString() -> string!
@@ -846,6 +873,9 @@ override DodoSSH.Contracts.VaultGrantsResponse.ToString() -> string!
 override DodoSSH.Contracts.VaultGrantSummary.Equals(object? obj) -> bool
 override DodoSSH.Contracts.VaultGrantSummary.GetHashCode() -> int
 override DodoSSH.Contracts.VaultGrantSummary.ToString() -> string!
+override DodoSSH.Contracts.VaultKeyWrap.Equals(object? obj) -> bool
+override DodoSSH.Contracts.VaultKeyWrap.GetHashCode() -> int
+override DodoSSH.Contracts.VaultKeyWrap.ToString() -> string!
 override DodoSSH.Contracts.VaultSummary.Equals(object? obj) -> bool
 override DodoSSH.Contracts.VaultSummary.GetHashCode() -> int
 override DodoSSH.Contracts.VaultSummary.ToString() -> string!
@@ -894,6 +924,8 @@ static DodoSSH.Contracts.RegisterDeviceRequest.operator !=(DodoSSH.Contracts.Reg
 static DodoSSH.Contracts.RegisterDeviceRequest.operator ==(DodoSSH.Contracts.RegisterDeviceRequest? left, DodoSSH.Contracts.RegisterDeviceRequest? right) -> bool
 static DodoSSH.Contracts.RegisterDeviceResponse.operator !=(DodoSSH.Contracts.RegisterDeviceResponse? left, DodoSSH.Contracts.RegisterDeviceResponse? right) -> bool
 static DodoSSH.Contracts.RegisterDeviceResponse.operator ==(DodoSSH.Contracts.RegisterDeviceResponse? left, DodoSSH.Contracts.RegisterDeviceResponse? right) -> bool
+static DodoSSH.Contracts.RekeyVaultRequest.operator !=(DodoSSH.Contracts.RekeyVaultRequest? left, DodoSSH.Contracts.RekeyVaultRequest? right) -> bool
+static DodoSSH.Contracts.RekeyVaultRequest.operator ==(DodoSSH.Contracts.RekeyVaultRequest? left, DodoSSH.Contracts.RekeyVaultRequest? right) -> bool
 static DodoSSH.Contracts.RelayConfiguration.operator !=(DodoSSH.Contracts.RelayConfiguration? left, DodoSSH.Contracts.RelayConfiguration? right) -> bool
 static DodoSSH.Contracts.RelayConfiguration.operator ==(DodoSSH.Contracts.RelayConfiguration? left, DodoSSH.Contracts.RelayConfiguration? right) -> bool
 static DodoSSH.Contracts.RelaySessionSummary.operator !=(DodoSSH.Contracts.RelaySessionSummary? left, DodoSSH.Contracts.RelaySessionSummary? right) -> bool
@@ -932,5 +964,7 @@ static DodoSSH.Contracts.VaultGrantsResponse.operator !=(DodoSSH.Contracts.Vault
 static DodoSSH.Contracts.VaultGrantsResponse.operator ==(DodoSSH.Contracts.VaultGrantsResponse? left, DodoSSH.Contracts.VaultGrantsResponse? right) -> bool
 static DodoSSH.Contracts.VaultGrantSummary.operator !=(DodoSSH.Contracts.VaultGrantSummary? left, DodoSSH.Contracts.VaultGrantSummary? right) -> bool
 static DodoSSH.Contracts.VaultGrantSummary.operator ==(DodoSSH.Contracts.VaultGrantSummary? left, DodoSSH.Contracts.VaultGrantSummary? right) -> bool
+static DodoSSH.Contracts.VaultKeyWrap.operator !=(DodoSSH.Contracts.VaultKeyWrap? left, DodoSSH.Contracts.VaultKeyWrap? right) -> bool
+static DodoSSH.Contracts.VaultKeyWrap.operator ==(DodoSSH.Contracts.VaultKeyWrap? left, DodoSSH.Contracts.VaultKeyWrap? right) -> bool
 static DodoSSH.Contracts.VaultSummary.operator !=(DodoSSH.Contracts.VaultSummary? left, DodoSSH.Contracts.VaultSummary? right) -> bool
 static DodoSSH.Contracts.VaultSummary.operator ==(DodoSSH.Contracts.VaultSummary? left, DodoSSH.Contracts.VaultSummary? right) -> bool

src/DodoSSH.Contracts/Teams.cs

@@ -404,6 +404,37 @@ public sealed record IssueVaultGrantRequest(
     byte[] GrantSignature,
     DateTimeOffset GrantedAt);
 
+/// <summary>
+/// Moves a vault to a fresh key, wrapped to the caller.
+/// </summary>
+/// <remarks>
+/// <para>
+/// The new key is generated by a client that already holds the current one, and arrives sealed to that
+/// same client — the server can neither produce it nor tell that it differs from the old one. What the
+/// server does is decide the moment it takes effect: the generation advances in one transaction, so
+/// there is no instant at which two clients disagree about which generation is current.
+/// </para>
+/// <para>
+/// <b>Grants for earlier generations are kept, not revoked.</b> Every item still carries the generation
+/// it was sealed under, so withdrawing them would make the vault's whole history unreadable to the
+/// people who are still in it. The departed member's grants are revoked — that is what
+/// <c>RevokeGrantAsync</c> and removal from the team already do — and this is what stops them reading
+/// anything written from here on. It does not reach back; see ADR 0001.
+/// </para>
+/// </remarks>
+/// <param name="KeyGeneration">
+/// The generation being created. Must be exactly one past the vault's current one, so two clients
+/// rotating at once cannot both believe they succeeded.
+/// </param>
+/// <param name="WrappedVaultKey">The new vault key, sealed to the caller's own encryption key.</param>
+/// <param name="GrantSignature">Ed25519 signature over the canonical grant tuple.</param>
+/// <param name="GrantedAt">Signing timestamp, part of the signed tuple.</param>
+public sealed record RekeyVaultRequest(
+    uint KeyGeneration,
+    byte[] WrappedVaultKey,
+    byte[] GrantSignature,
+    DateTimeOffset GrantedAt);
+
 /// <summary>One vault key grant, as the sharing interface sees it.</summary>
 /// <remarks>
 /// The wrapped key itself is deliberately not here. A member reads their own through
@@ -436,7 +467,12 @@ public sealed record VaultGrantSummary(
 /// compares against rather than inferring from <see cref="VaultGrantSummary.State"/> alone.
 /// </param>
 /// <param name="RekeyRequired">Whether a membership change has left this vault needing a rekey.</param>
-/// <param name="Grants">Every grant, including revoked ones.</param>
+/// <param name="Grants">
+/// One row per holder, including those whose access has been withdrawn. Not one per grant: a rotated
+/// vault leaves a member holding one grant per generation, and the row carries the best of them — so
+/// <see cref="VaultGrantSummary.KeyGeneration"/> below <paramref name="KeyGeneration"/> means they have
+/// not been wrapped the current key yet, rather than that one of their grants is old.
+/// </param>
 public sealed record VaultGrantsResponse(
     Guid VaultId,
     uint KeyGeneration,