Finish revoking a device, instead of half of it

ForgetDeviceAsync stopped this machine unlocking without a passphrase and left
the server's row exactly where it was, so the account went on listing a device
nobody could account for. ADR 0007 recorded that as a deliberate gap needing an
endpoint. This is the endpoint, and the two things that turned up behind it.

DELETE /api/v1/me/devices/{id}. The device row is not the dangerous half: a
kind=device wrap is the user's identity bundle sealed to a key somebody may be
holding, and that is what has to go. It goes on the foreign key's cascade rather
than a second statement, and RevokeDevice_TakesItsWrapWithIt asserts the cascade
rather than trusting the configuration to keep saying so.

Scoped to the caller's own account, which is the only authorisation check there
is. The id is an unguessable v7 GUID, but unguessable is not a permission —
without the scope one user could withdraw another's device key by pasting an id
they saw once, and the victim's next launch would ask for a passphrase with no
explanation. 404 rather than 403 for somebody else's device, so a stranger does
not learn the id exists.

Never refused for being the last device. ADR 0001 makes an enrolled device a
recovery path, so removing the last one does cost the user something — but the
machine being revoked is most likely the one they have just lost, and a server
that argued about it would be refusing the one request that has to work
immediately. The passphrase wrap is untouched either way, which
RevokeDevice_LeavesThePassphraseWrapAlone pins.

--- Two things found on the way ---

Registering twice from one machine left two devices on the account. The server
is idempotent on the public key, but the client generates a fresh key pair every
call and the keystore holds one — so the second registration orphaned a wrap
whose private half had just been overwritten, which is precisely the leftover
this change exists to remove. Registering now withdraws the previous device.
Found by a test that asserted the property and failed.

And the fakes were lying about it. FakeAccountServer's comment claimed the real
service's idempotence while handing back a fresh Guid on every call, which is
invisible until something revokes by id — at which point a test would be
revoking an id the server never issued, and passing. Both fakes now issue one id
per public key and drop the wrap with the device, as the cascade does.

--- Reachable at all ---

ForgetDeviceAsync had exactly one caller and it was a test, so "Stop unlocking
here" now sits in the account bar where "Use Windows Hello here" was. Its own
flag rather than the negation of that one: a machine with no TPM and a machine
that is already registered are both "cannot register", and only the second has
anything to take back.

No confirmation prompt, deliberately. The cost of pressing it by accident is one
passphrase and one re-registration; the cost of a dialog is a moment's
hesitation at the point somebody has realised a machine is in the wrong hands.

Offline it does the local half and says so rather than refusing. Whether this
machine may unlock itself is decided entirely by the local cache and the local
keystore — the unlock path never asks the server — so forgetting here is what
actually revokes, and "you are offline, so this machine will go on unlocking
itself" would be the worst available answer. DeviceRevocation.LocalOnly is what
the interface reports and the status line explains what is left to do.

The local half runs first for the same reason, and the keystore call is the
first thing in the method that can yield: on Windows it raises a consent dialog,
and a dialog wants the thread it was called from. That ordering is currently
load-bearing and shakier than it looks — see the open device-unlock hang.

Four mutations, all caught: dropping the user scope from the server query
(1 test), skipping the stale-device revoke on re-registration (2), skipping the
server call in ForgetDeviceAsync (2), and the earlier version of the client that
never called it at all.

930 tests green across 16 projects, 13 of them new. Zero warnings, format clean.

tests/DodoSSH.Api.Tests/IdentityEndpointTests.cs

@@ -957,6 +957,114 @@ public sealed class IdentityEndpointTests(ApiFixture fixture)
             ProblemCodes.InvalidDeviceRegistration);
     }
 
+    /// <remarks>
+    /// The wrap is the half that matters. A device row nobody can use is untidy; a <c>kind=device</c> wrap
+    /// left behind is the user's identity bundle still sealed to a key somebody may be holding. This asserts
+    /// the foreign key's cascade rather than trusting the configuration to go on saying so.
+    /// </remarks>
+    [Fact]
+    public async Task RevokeDevice_TakesItsWrapWithIt()
+    {
+        using var enrollment = NewEnrollment();
+        var client = enrollment.CreateClient(fixture);
+        await EnrollAsync(client, enrollment.Build());
+
+        var registered = await RegisterDeviceAsync(
+            client, new RegisterDeviceRequest("a laptop", DeviceKey(), Wrap()));
+
+        var response = await client.DeleteAsync(
+            new Uri($"{DevicesUrl}/{registered.DeviceId}", UriKind.Relative));
+
+        response.StatusCode.ShouldBe(HttpStatusCode.NoContent);
+
+        await using var scope = fixture.CreateScope();
+        var database = scope.ServiceProvider.GetRequiredService<DodoDbContext>();
+
+        (await database.Devices.CountAsync(d => d.Id == registered.DeviceId)).ShouldBe(0);
+        (await database.UserKeyWraps.CountAsync(w => w.DeviceId == registered.DeviceId)).ShouldBe(0);
+    }
+
+    /// <remarks>
+    /// The passphrase wrap is what makes revocation safe to offer at all: withdrawing every device must
+    /// never be able to lock somebody out of their own vault, so this checks the one row that guarantees it
+    /// is still there afterwards.
+    /// </remarks>
+    [Fact]
+    public async Task RevokeDevice_LeavesThePassphraseWrapAlone()
+    {
+        using var enrollment = NewEnrollment();
+        var client = enrollment.CreateClient(fixture);
+        await EnrollAsync(client, enrollment.Build());
+
+        var registered = await RegisterDeviceAsync(
+            client, new RegisterDeviceRequest("a laptop", DeviceKey(), Wrap()));
+
+        await client.DeleteAsync(new Uri($"{DevicesUrl}/{registered.DeviceId}", UriKind.Relative));
+
+        var me = await ReadMeAsync(client);
+
+        me.EnrollmentRequired.ShouldBeFalse();
+        me.WrappedPrivateKey.ShouldNotBeNull();
+    }
+
+    [Fact]
+    public async Task RevokeDevice_ThatIsNotThere_Is404()
+    {
+        // Rather than a bland 204. "Revoked" is what the user reads, and reading it about the wrong id is
+        // worse than being told to look again — a client driving towards "this machine cannot unlock" can
+        // treat 404 as having arrived, and the desktop one does.
+        using var enrollment = NewEnrollment();
+        var client = enrollment.CreateClient(fixture);
+        await EnrollAsync(client, enrollment.Build());
+
+        var response = await client.DeleteAsync(
+            new Uri($"{DevicesUrl}/{Guid.CreateVersion7()}", UriKind.Relative));
+
+        response.StatusCode.ShouldBe(HttpStatusCode.NotFound);
+    }
+
+    /// <remarks>
+    /// The authorisation check, and the only one there is: the id is an unguessable v7 GUID, but unguessable
+    /// is not a permission. Without the user scope on the query, one account could withdraw another's device
+    /// key by pasting an id it saw once — and the victim's next launch would ask for a passphrase with no
+    /// explanation.
+    /// </remarks>
+    [Fact]
+    public async Task RevokeDevice_BelongingToSomebodyElse_Is404AndChangesNothing()
+    {
+        using var mine = NewEnrollment();
+        var myClient = mine.CreateClient(fixture);
+        await EnrollAsync(myClient, mine.Build());
+
+        var registered = await RegisterDeviceAsync(
+            myClient, new RegisterDeviceRequest("my laptop", DeviceKey(), Wrap()));
+
+        using var theirs = NewEnrollment();
+        var theirClient = theirs.CreateClient(fixture);
+        await EnrollAsync(theirClient, theirs.Build());
+
+        var response = await theirClient.DeleteAsync(
+            new Uri($"{DevicesUrl}/{registered.DeviceId}", UriKind.Relative));
+
+        // 404 rather than 403, because a stranger must not learn that the id exists.
+        response.StatusCode.ShouldBe(HttpStatusCode.NotFound);
+
+        await using var scope = fixture.CreateScope();
+        var database = scope.ServiceProvider.GetRequiredService<DodoDbContext>();
+
+        (await database.Devices.CountAsync(d => d.Id == registered.DeviceId)).ShouldBe(1);
+        (await database.UserKeyWraps.CountAsync(w => w.DeviceId == registered.DeviceId)).ShouldBe(1);
+    }
+
+    [Fact]
+    public async Task RevokeDevice_WithoutAToken_Is401()
+    {
+        var response = await fixture.CreateClient().DeleteAsync(
+            new Uri($"{DevicesUrl}/{Guid.CreateVersion7()}", UriKind.Relative));
+
+        response.StatusCode.ShouldBe(HttpStatusCode.Unauthorized);
+    }
+
     [Fact]
     public async Task RegisterDevice_WithABlankName_IsRejected()
     {

tests/DodoSSH.Client.App.Tests/FakeVaultServer.cs

@@ -46,6 +46,9 @@ internal sealed class FakeVaultServer : IVaultServer, IAccountApi, ISyncApi, IKe
     /// <summary>Device wraps registered after enrollment, keyed on the device public key.</summary>
     internal Dictionary<string, byte[]> RegisteredDevices { get; } = new(StringComparer.Ordinal);
 
+    /// <summary>The id issued for each registered public key, so revocation has something to name.</summary>
+    private readonly Dictionary<string, Guid> deviceIds = new(StringComparer.Ordinal);
+
     /// <summary>When set, the next sign-in throws — how an unreachable server is exercised.</summary>
     internal Exception? SignInFailure { get; set; }
 
@@ -150,11 +153,37 @@ internal sealed class FakeVaultServer : IVaultServer, IAccountApi, ISyncApi, IKe
                 "This account has no identity key yet.");
         }
 
-        RegisteredDevices[Convert.ToHexString(request.PublicKey)] = request.WrappedPrivateKey;
+        var key = Convert.ToHexString(request.PublicKey);
 
-        return Task.FromResult(new RegisterDeviceResponse(
-            DeviceId: Guid.CreateVersion7(),
-            EnrolledAt: DateTimeOffset.UnixEpoch));
+        RegisteredDevices[key] = request.WrappedPrivateKey;
+
+        // One id per public key, as the real service issues, so a revocation can name the device that was
+        // actually registered rather than one this fake invented on the way past.
+        if (!deviceIds.TryGetValue(key, out var deviceId))
+        {
+            deviceId = Guid.CreateVersion7();
+            deviceIds[key] = deviceId;
+        }
+
+        return Task.FromResult(new RegisterDeviceResponse(deviceId, DateTimeOffset.UnixEpoch));
+    }
+
+    /// <inheritdoc />
+    public Task<bool> RevokeDeviceAsync(Guid deviceId, CancellationToken cancellationToken)
+    {
+        var key = deviceIds.FirstOrDefault(entry => entry.Value == deviceId).Key;
+
+        if (key is null)
+        {
+            return Task.FromResult(false);
+        }
+
+        deviceIds.Remove(key);
+
+        // With its wrap, as the foreign key's cascade does on the real server.
+        RegisteredDevices.Remove(key);
+
+        return Task.FromResult(true);
     }
 
     // ---- Sync ----

tests/DodoSSH.Client.App.Tests/ShellFlowTests.cs

@@ -1809,6 +1809,93 @@ public sealed class ShellFlowTests : IAsyncLifetime
         shell.Passphrase.ShouldBeEmpty("nothing was typed");
     }
 
+    [Fact]
+    public async Task WithdrawingTheDevice_SendsThisMachineBackToThePassphraseAndClearsTheAccount()
+    {
+        // The button that makes revocation reachable at all. Until it existed, ForgetDeviceAsync had one
+        // caller and that caller was a test.
+        await UnlockedAsync();
+        await shell.RegisterDeviceCommand.ExecuteAsync(null);
+
+        shell.CanForgetDevice.ShouldBeTrue("there is a device key here now");
+
+        await shell.ForgetDeviceCommand.ExecuteAsync(null);
+
+        shell.CanForgetDevice.ShouldBeFalse("the offer is spent");
+        server.RegisteredDevices.ShouldBeEmpty("the account must not go on listing it");
+
+        await shell.LockCommand.ExecuteAsync(null);
+        await shell.StartAsync(Token);
+
+        shell.CanUnlockWithDevice.ShouldBeFalse("there is nothing left to unlock with");
+
+        // And the passphrase still opens it, which is what makes withdrawing safe to offer without a
+        // confirmation prompt.
+        shell.Passphrase = Passphrase;
+        await shell.UnlockCommand.ExecuteAsync(null);
+
+        shell.State.ShouldBe(ShellState.Unlocked, shell.StatusMessage);
+    }
+
+    /// <remarks>
+    /// The offer and the withdrawal are two flags rather than one and its negation, and this is why: on a
+    /// machine with no keystore both are false, and a single flag would have made "cannot register" mean
+    /// "has something to withdraw".
+    /// </remarks>
+    [Fact]
+    public async Task OnAMachineWithNoKeystore_ThereIsNothingToWithdrawEither()
+    {
+        deviceKeys.IsAvailable = false;
+
+        await UnlockedAsync();
+
+        shell.CanRegisterDevice.ShouldBeFalse();
+        shell.CanForgetDevice.ShouldBeFalse();
+    }
+
+    [Fact]
+    public async Task WithdrawingTheDeviceOffline_StopsThisMachineAndSaysTheAccountStillListsIt()
+    {
+        // Somebody who has just realised a machine is in the wrong hands may well be on a train. Refusing
+        // until they are online would leave the device unlocking itself for the whole journey.
+        await UnlockedAsync();
+        await shell.RegisterDeviceCommand.ExecuteAsync(null);
+        await shell.LockCommand.ExecuteAsync(null);
+
+        // The same keystore, so this machine still holds its device key — only the network is gone.
+        var offline = new MainWindowViewModel(
+            paths,
+            caches,
+            workspace,
+            new VaultKnownHostStore(),
+            deviceKeys,
+            (_, _) => throw new InvalidOperationException("The shell went to the network."),
+            TimeProvider.System,
+            CheapProfile);
+
+        await using var _ = offline.ConfigureAwait(false);
+
+        await offline.StartAsync(Token);
+        offline.IsOnline.ShouldBeFalse();
+
+        offline.Passphrase = Passphrase;
+        await offline.UnlockCommand.ExecuteAsync(null);
+        offline.State.ShouldBe(ShellState.Unlocked, offline.StatusMessage);
+        offline.CanForgetDevice.ShouldBeTrue();
+
+        await offline.ForgetDeviceCommand.ExecuteAsync(null);
+
+        offline.StatusMessage.ShouldContain("still lists it");
+        offline.CanForgetDevice.ShouldBeFalse();
+        server.RegisteredDevices.Count.ShouldBe(1, "nothing reached the server, and it must not pretend");
+
+        // The half that decides whether this machine may let itself in happened anyway.
+        await offline.LockCommand.ExecuteAsync(null);
+        await offline.StartAsync(Token);
+
+        offline.CanUnlockWithDevice.ShouldBeFalse();
+    }
+
     [Fact]
     public async Task OnAMachineWithNoKeystore_NeitherAffordanceAppears()
     {

tests/DodoSSH.Client.Session.Tests/DeviceUnlockTests.cs

@@ -175,7 +175,44 @@ public sealed class DeviceUnlockTests : IAsyncLifetime
     }
 
     [Fact]
-    public async Task ForgettingTheDevice_SendsThisMachineBackToThePassphrase()
+    public async Task ForgettingTheDevice_SendsThisMachineBackToThePassphraseAndClearsTheAccount()
+    {
+        await using (var first = await UnlockAsync())
+        {
+            await first.RegisterDeviceAsync(server, deviceKeys, "this laptop", Token);
+        }
+
+        server.RegisteredDevices.Count.ShouldBe(1);
+
+        await using (var second = (await Opener().UnlockWithDeviceAsync(deviceKeys, Token)).Session!)
+        {
+            var revocation = await second.ForgetDeviceAsync(server, deviceKeys, Token);
+
+            revocation.ShouldBe(DeviceRevocation.Complete);
+        }
+
+        var outcome = await Opener().UnlockWithDeviceAsync(deviceKeys, Token);
+
+        outcome.Status.ShouldBe(UnlockStatus.NoDeviceKey);
+
+        // The half that used to be left behind. A kind=device wrap is the identity bundle sealed to a key
+        // somebody may still hold, so a revocation that only cleared this machine left the dangerous part
+        // exactly where it was.
+        server.RegisteredDevices.ShouldBeEmpty();
+
+        // And the passphrase still works, which is the property that makes forgetting safe to offer.
+        await using var byPassphrase = await UnlockAsync();
+        byPassphrase.ActiveVaultId.ShouldNotBe(Guid.Empty);
+    }
+
+    /// <remarks>
+    /// Revoking is most wanted at the moment a machine is lost, and being offline is no reason to leave it
+    /// able to let itself in. The local half is the half that decides whether this machine may unlock — the
+    /// unlock path never asks the server — so doing it anyway is strictly better than refusing, provided the
+    /// caller is told the account has not been told.
+    /// </remarks>
+    [Fact]
+    public async Task ForgettingTheDeviceOffline_StillStopsThisMachineAndSaysWhatWasNotDone()
     {
         await using (var first = await UnlockAsync())
         {
@@ -184,16 +221,47 @@ public sealed class DeviceUnlockTests : IAsyncLifetime
 
         await using (var second = (await Opener().UnlockWithDeviceAsync(deviceKeys, Token)).Session!)
         {
-            await second.ForgetDeviceAsync(deviceKeys, Token);
+            var revocation = await second.ForgetDeviceAsync(api: null, deviceKeys, Token);
+
+            revocation.ShouldBe(DeviceRevocation.LocalOnly);
         }
 
-        var outcome = await Opener().UnlockWithDeviceAsync(deviceKeys, Token);
+        (await Opener().UnlockWithDeviceAsync(deviceKeys, Token)).Status
+            .ShouldBe(UnlockStatus.NoDeviceKey);
 
-        outcome.Status.ShouldBe(UnlockStatus.NoDeviceKey);
+        server.RegisteredDevices.Count.ShouldBe(1, "nothing reached the server, and it must not pretend");
+    }
 
-        // And the passphrase still works, which is the property that makes forgetting safe to offer.
-        await using var byPassphrase = await UnlockAsync();
-        byPassphrase.ActiveVaultId.ShouldNotBe(Guid.Empty);
+    [Fact]
+    public async Task ForgettingADeviceThatWasNeverRegistered_SaysSoAndAsksTheServerNothing()
+    {
+        await using var session = await UnlockAsync();
+
+        var revocation = await session.ForgetDeviceAsync(server, deviceKeys, Token);
+
+        revocation.ShouldBe(DeviceRevocation.NothingRegistered);
+    }
+
+    /// <remarks>
+    /// Registering twice from one machine must not leave two revocable devices behind, which is a property of
+    /// the id the server issues rather than of the client: the real service is idempotent on the public key
+    /// and returns the id it already has. Pinned here because a fake that invented a fresh id per call would
+    /// make every revocation test pass while revoking something that was never registered.
+    /// </remarks>
+    [Fact]
+    public async Task RegisteringTwice_KeepsOneDeviceAndOneIdToRevoke()
+    {
+        await using var session = await UnlockAsync();
+
+        await session.RegisterDeviceAsync(server, deviceKeys, "this laptop", Token);
+        await session.RegisterDeviceAsync(server, deviceKeys, "this laptop", Token);
+
+        server.RegisteredDevices.Count.ShouldBe(1);
+
+        var revocation = await session.ForgetDeviceAsync(server, deviceKeys, Token);
+
+        revocation.ShouldBe(DeviceRevocation.Complete);
+        server.RegisteredDevices.ShouldBeEmpty();
     }
 
     [Fact]

tests/DodoSSH.Client.Session.Tests/FakeAccountServer.cs

@@ -50,6 +50,9 @@ internal sealed class FakeAccountServer : IAccountApi
     /// </remarks>
     internal Dictionary<string, byte[]> RegisteredDevices { get; } = new(StringComparer.Ordinal);
 
+    /// <summary>The id issued for each registered public key, so revocation has something to name.</summary>
+    private readonly Dictionary<string, Guid> deviceIds = new(StringComparer.Ordinal);
+
     /// <inheritdoc />
     public Task<MeResponse> GetMeAsync(CancellationToken cancellationToken)
     {
@@ -130,14 +133,37 @@ internal sealed class FakeAccountServer : IAccountApi
 
         var key = Convert.ToHexString(request.PublicKey);
 
-        if (!RegisteredDevices.TryAdd(key, request.WrappedPrivateKey))
+        RegisteredDevices[key] = request.WrappedPrivateKey;
+
+        // The same id for the same public key, which the real service does and this fake used to claim in a
+        // comment while handing back a fresh Guid every call. That difference is invisible until something
+        // revokes by id, at which point a test would be revoking an id the server never issued.
+        if (!deviceIds.TryGetValue(key, out var deviceId))
         {
-            RegisteredDevices[key] = request.WrappedPrivateKey;
+            deviceId = Guid.CreateVersion7();
+            deviceIds[key] = deviceId;
         }
 
-        return Task.FromResult(new RegisterDeviceResponse(
-            DeviceId: Guid.CreateVersion7(),
-            EnrolledAt: DateTimeOffset.UnixEpoch));
+        return Task.FromResult(new RegisterDeviceResponse(deviceId, DateTimeOffset.UnixEpoch));
+    }
+
+    /// <inheritdoc />
+    public Task<bool> RevokeDeviceAsync(Guid deviceId, CancellationToken cancellationToken)
+    {
+        var key = deviceIds.FirstOrDefault(entry => entry.Value == deviceId).Key;
+
+        if (key is null)
+        {
+            return Task.FromResult(false);
+        }
+
+        deviceIds.Remove(key);
+
+        // The wrap goes with it, as the foreign key's cascade does on the real server. A fake that kept the
+        // wrap would let a test claiming to prove revocation pass while the dangerous half survived.
+        RegisteredDevices.Remove(key);
+
+        return Task.FromResult(true);
     }
 
     /// <summary>Drops the vault grant, as a rekey does until it is re-issued.</summary>