diff --git a/metropolis/proto/private/private.proto b/metropolis/proto/private/private.proto
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+// Copyright 2020 The Monogon Project Authors.
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+syntax = "proto3";
+option go_package = "source.monogon.dev/metropolis/proto/private";
+package metropolis.proto.private;
+
+// Node describes a single node's state in etcd
+message Node {
+    // Node's public key..
+    bytes public_key = 1;
+    // Node's individual cluster part of the data partition encryption key. It
+    // is combined with the Node Unlock Key (NUK) kept within
+    // SealedConfiguration.
+    bytes cluster_unlock_key = 2;
+
+    // TODO(q3k): document this based on cluster lifecycle design doc once this
+    // fully stabilizes.
+    enum FSMState {
+        FSM_STATE_INVALID = 0;
+        FSM_STATE_NEW = 1;
+        FSM_STATE_STANDBY = 2;
+        FSM_STATE_UP = 3;
+        FSM_STATE_DISOWNED = 4;
+    };
+    FSMState fsm_state = 3;
+
+    message Roles {
+        message ConsensusMember {
+            // Used to be: consensus member name, can be deduced from
+            // Certificate/Pubkey/ID.
+            reserved 1;
+        }
+        ConsensusMember consensus_member = 1;
+        message KubernetesWorker {
+            // Used to be: node name, can be deduced from
+            // Certificate/Pubkey/ID.
+            reserved 1;
+        }
+        KubernetesWorker kubernetes_worker = 2;
+    }
+    Roles roles = 4;
+}
+
+// SealedConfiguration is all configuration kept in a TPM-sealed (ie.
+// confidential and tamper-resistant) blob within the node's ESP (EFI)
+// partition. It contains everything that the node needs to join an existing
+// cluster after rebooting.
+message SealedConfiguration {
+    // node_unlock_key (NUK) is the node's part of the key required to unlock
+    // the node's data partition(s). It is combined with the Cluster Unlock Key
+    // (CUK) that's stored in etcd to create a key which is then used to mount
+    // the data partition. The NUK and CUK are generated on bootstrap and
+    // registration.
+    bytes node_unlock_key = 1;
+}