diff --git a/library/common.h b/library/common.h
new file mode 100644
index 0000000..ba2c52e
--- /dev/null
+++ b/library/common.h
@@ -0,0 +1,55 @@
+/**
+ * \file common.h
+ *
+ * \brief Utility macros for internal use in the library
+ */
+/*
+ *  Copyright (C) 2019, ARM Limited, All Rights Reserved
+ *  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.
+ *
+ *  This file is part of mbed TLS (https://tls.mbed.org)
+ */
+
+#ifndef MBEDTLS_LIBRARY_COMMON_H
+#define MBEDTLS_LIBRARY_COMMON_H
+
+#if defined(MBEDTLS_CONFIG_FILE)
+#include MBEDTLS_CONFIG_FILE
+#else
+#include "mbedtls/config.h"
+#endif
+
+/** Helper to define a function as static except when building invasive tests.
+ *
+ * If a function is only used inside its own source file and should be
+ * declared `static` to allow the compiler to optimize for code size,
+ * but that function has unit tests, define it with
+ * ```
+ * MBEDTLS_STATIC_TESTABLE int mbedtls_foo(...) { ... }
+ * ```
+ * and declare it in a header in the `library/` directory with
+ * ```
+ * #if defined(MBEDTLS_TEST_HOOKS)
+ * int mbedtls_foo(...);
+ * #endif
+ * ```
+ */
+#if defined(MBEDTLS_TEST_HOOKS)
+#define MBEDTLS_STATIC_TESTABLE
+#else
+#define MBEDTLS_STATIC_TESTABLE static
+#endif
+
+#endif /* MBEDTLS_LIBRARY_COMMON_H */