diff --git a/microdroid/signature/README.md b/microdroid/signature/README.md
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+# Microdroid Signature
+
+Microdroid Signature contains the signatures of the payloads so that the payloads are
+verified inside the Guest OS.
+
+* APEX packages that are passed to microdroid should be listed in the Microroid Signature.
+
+## Format
+
+Microdroid Signature is composed of header and body.
+
+| offset | size | description                                                    |
+|--------|------|----------------------------------------------------------------|
+| 0      | 4    | Header. unsigned int32: body length(L) in big endian           |
+| 4      | L    | Body. A protobuf message. [schema](microdroid_signature.proto) |
diff --git a/microdroid/signature/microdroid_signature.proto b/microdroid/signature/microdroid_signature.proto
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+/*
+ * Copyright (C) 2021 The Android Open Source Project
+ *
+ * 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";
+
+package android.microdroid;
+
+// Microdroid Signature is the body of the signature partition.
+message MicrodroidSignature {
+  uint32 version = 1;
+
+  // Lists the signature information of the payload apexes.
+  // The payload apexes are mapped to the partitions following the signature partition.
+  repeated ApexSignature apexes = 2;
+}
+
+message ApexSignature {
+  // Required.
+  // The apex name.
+  string name = 1;
+
+  // Required.
+  // The original size of the apex file.
+  uint32 size = 2;
+
+  // Optional.
+  // When specified, the public key used to sign the apex should match with it.
+  string publicKey = 3;
+
+  // Optional.
+  // When specified, the root digest of the apex should match with it.
+  string rootDigest = 4;
+}