diff --git a/build/binary_tarball/spec/spec.proto b/build/binary_tarball/spec/spec.proto
new file mode 100644
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+++ b/build/binary_tarball/spec/spec.proto
@@ -0,0 +1,32 @@
+// 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";
+
+package build.binary_tarball.spec;
+
+// Spec is a spec of what goes into a binary_tarball
+message Spec {
+  repeated File file = 1;
+}
+
+// File is a single file in the tarball
+message File {
+  // src contains the path of the file on the build host
+  string src = 1;
+  // path contains the path in the tarball
+  string path = 2;
+}