diff --git a/compos/src/signer.rs b/compos/src/signer.rs
new file mode 100644
index 0000000..9ff1477
--- /dev/null
+++ b/compos/src/signer.rs
@@ -0,0 +1,23 @@
+/*
+ * 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.
+ */
+
+use anyhow::Result;
+
+/// Provides the ability to cryptographically sign messages.
+pub trait Signer: Send + Sync {
+    /// Sign the supplied data. The result is a raw signature over the input data.
+    fn sign(&self, data: &[u8]) -> Result<Vec<u8>>;
+}