diff --git a/lib/services/hub/hub_client.dart b/lib/services/hub/hub_client.dart
index 58e8d4d3f..789fbced2 100644
--- a/lib/services/hub/hub_client.dart
+++ b/lib/services/hub/hub_client.dart
@@ -18,6 +18,7 @@ import 'sessions_api.dart';
 import 'system_api.dart';
 import 'tasks_api.dart';
 import 'templates_api.dart';
+import 'vault_api.dart';
 
 // HubConfig, HubApiError, and HubTransport now live in hub_transport.dart;
 // re-exported here so the many `import '.../hub_client.dart'` call sites
@@ -106,6 +107,11 @@ class HubClient {
   /// Projects + their channels, channel events, principals, project docs.
   late final ProjectsApi projects = ProjectsApi(_t);
 
+  /// Zero-knowledge SSH key vault (ADR-052 D-4) — blind cross-device sync of
+  /// the client-encrypted connection+key bundle, its recovery envelope, and
+  /// per-device wrapped keys.
+  late final VaultApi vault = VaultApi(_t);
+
   /// Optional read-through cache for list/get responses. Set by the
   /// provider after construction; forwarded to the transport so the
   /// sub-clients and the legacy methods share one cache. When null, the
diff --git a/lib/services/hub/vault_api.dart b/lib/services/hub/vault_api.dart
new file mode 100644
index 000000000..82c921196
--- /dev/null
+++ b/lib/services/hub/vault_api.dart
@@ -0,0 +1,113 @@
+import 'hub_transport.dart';
+
+/// Zero-knowledge SSH key-vault sync (ADR-052 D-4). Thin transport over the
+/// hub's blind blob store: everything here is opaque, client-encrypted base64
+/// (sealed by `services/vault/vault_crypto.dart`) — the hub never sees plaintext.
+///
+/// Endpoints under `/v1/teams/{team}/vault`:
+///   GET  /                    pull the sealed bundle
+///   PUT  /                    push it (optimistic concurrency via base_version)
+///   GET  /recovery            fetch the recovery envelope
+///   PUT  /recovery            set/replace it
+///   DELETE /recovery          clear it
+///   GET  /devices             list enrolled devices
+///   PUT  /devices/{device}    enroll a device / distribute its wrapped key
+///   DELETE /devices/{device}  revoke a device
+class VaultApi {
+  final HubTransport _t;
+  VaultApi(this._t);
+
+  String get _base => '/v1/teams/${_t.cfg.teamId}/vault';
+
+  /// Pull the sealed vault bundle `{ciphertext, version, updated_at}`.
+  /// Returns null when the principal has none yet (404) so a fresh client
+  /// knows to create one.
+  Future<Map<String, dynamic>?> pullVault() async {
+    try {
+      final out = await _t.get(_base);
+      return (out as Map).cast<String, dynamic>();
+    } on HubApiError catch (e) {
+      if (e.status == 404) return null;
+      rethrow;
+    }
+  }
+
+  /// Push the sealed bundle with optimistic concurrency. Pass the version you
+  /// last pulled as [baseVersion] (0 to create). Throws [HubApiError] with
+  /// status 409 on a stale/duplicate version — pull, re-seal, and retry.
+  /// Returns `{version, updated_at}`.
+  Future<Map<String, dynamic>> pushVault(
+    String ciphertext, {
+    required int baseVersion,
+  }) async {
+    final out = await _t.put(_base, {
+      'ciphertext': ciphertext,
+      'base_version': baseVersion,
+    });
+    return (out as Map).cast<String, dynamic>();
+  }
+
+  /// Fetch the recovery envelope `{recovery_envelope, recovery_hint?, updated_at?}`.
+  /// Returns null when the vault or the envelope is absent (404).
+  Future<Map<String, dynamic>?> getRecovery() async {
+    try {
+      final out = await _t.get('$_base/recovery');
+      return (out as Map).cast<String, dynamic>();
+    } on HubApiError catch (e) {
+      if (e.status == 404) return null;
+      rethrow;
+    }
+  }
+
+  /// Set/replace the recovery envelope. The vault must already exist
+  /// (else [HubApiError] 404). Returns `{updated_at}`.
+  Future<Map<String, dynamic>> setRecovery(
+    String envelope, {
+    String? hint,
+  }) async {
+    final body = <String, dynamic>{'recovery_envelope': envelope};
+    if (hint != null && hint.isNotEmpty) body['recovery_hint'] = hint;
+    final out = await _t.put('$_base/recovery', body);
+    return (out as Map).cast<String, dynamic>();
+  }
+
+  /// Clear the recovery envelope.
+  Future<void> deleteRecovery() => _t.delete('$_base/recovery');
+
+  /// List enrolled devices — each
+  /// `{device_id, device_name?, public_key, wrapped_key?, created_at, updated_at}`.
+  Future<List<Map<String, dynamic>>> listDevices() async {
+    final out = await _t.get('$_base/devices');
+    if (out == null) return const [];
+    final devices = (out as Map)['devices'];
+    if (devices == null) return const [];
+    return (devices as List).cast<Map<String, dynamic>>();
+  }
+
+  /// Enroll or update a device. A new device sends its [publicKey]
+  /// (`wrappedKey` empty); an already-enrolled device later sends [wrappedKey]
+  /// for that `deviceId` to distribute the vault key to it. Returns
+  /// `{device_id, updated_at}`.
+  Future<Map<String, dynamic>> putDevice(
+    String deviceId, {
+    String? deviceName,
+    String? publicKey,
+    String? wrappedKey,
+  }) async {
+    final body = <String, dynamic>{};
+    if (deviceName != null && deviceName.isNotEmpty) {
+      body['device_name'] = deviceName;
+    }
+    if (publicKey != null && publicKey.isNotEmpty) body['public_key'] = publicKey;
+    if (wrappedKey != null && wrappedKey.isNotEmpty) {
+      body['wrapped_key'] = wrappedKey;
+    }
+    final out = await _t.put('$_base/devices/$deviceId', body);
+    return (out as Map).cast<String, dynamic>();
+  }
+
+  /// Revoke a device (remove its envelope). The client should re-key the vault
+  /// afterward (ADR-052 D-4).
+  Future<void> deleteDevice(String deviceId) =>
+      _t.delete('$_base/devices/$deviceId');
+}