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diff --git a/library/constant_time_invasive.h b/library/constant_time_invasive.h
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+/**
+ * \file constant_time_invasive.h
+ *
+ * \brief Constant-time module: interfaces for invasive testing only.
+ *
+ * The interfaces in this file are intended for testing purposes only.
+ * They SHOULD NOT be made available in library integrations except when
+ * building the library for testing.
+ */
+/*
+ *  Copyright The Mbed TLS Contributors
+ *  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.
+ */
+
+#ifndef MBEDTLS_CONSTANT_TIME_INVASIVE_H
+#define MBEDTLS_CONSTANT_TIME_INVASIVE_H
+
+#include "common.h"
+
+#if defined(MBEDTLS_TEST_HOOKS)
+
+/** Turn a value into a mask:
+ * - if \p low <= \p c <= \p high,
+ *   return the all-bits 1 mask, aka (unsigned) -1
+ * - otherwise, return the all-bits 0 mask, aka 0
+ *
+ * \param low   The value to analyze.
+ * \param high  The value to analyze.
+ * \param c     The value to analyze.
+ *
+ * \return      All-bits-one if \p low <= \p c <= \p high, otherwise zero.
+ */
+unsigned char mbedtls_ct_uchar_mask_of_range( unsigned char low,
+                                              unsigned char high,
+                                              unsigned char c );
+
+#endif /* MBEDTLS_TEST_HOOKS */
+
+#endif /* MBEDTLS_CONSTANT_TIME_INVASIVE_H */