diff --git a/library/base64_invasive.h b/library/base64_invasive.h
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+++ b/library/base64_invasive.h
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+/**
+ * \file base_invasive.h
+ *
+ * \brief Base64 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_BASE64_INVASIVE_H
+#define MBEDTLS_BASE64_INVASIVE_H
+
+#include "common.h"
+
+#if defined(MBEDTLS_TEST_HOOKS)
+/* Return 0xff if low <= c <= high, 0 otherwise.
+ *
+ * Constant flow with respect to c.
+ */
+unsigned char mbedtls_base64_mask_of_range( unsigned char low,
+                                            unsigned char high,
+                                            unsigned char c );
+
+/* Given a value in the range 0..63, return the corresponding Base64 digit.
+ *
+ * Operates in constant time (no branches or memory access depending on val).
+ */
+unsigned char mbedtls_base64_enc_char( unsigned char val );
+
+/* Given a Base64 digit, return its value.
+ * If c is not a Base64 digit ('A'..'Z', 'a'..'z', '0'..'9', '+' or '/'),
+ * return -1.
+ *
+ * Operates in constant time (no branches or memory access depending on c).
+ */
+signed char mbedtls_base64_dec_value( unsigned char c );
+#endif /* MBEDTLS_TEST_HOOKS */
+
+#endif /* MBEDTLS_SSL_INVASIVE_H */