diff --git a/library/base64_internal.h b/library/base64_internal.h
new file mode 100644
index 0000000..f9f56d7
--- /dev/null
+++ b/library/base64_internal.h
@@ -0,0 +1,57 @@
+/**
+ * \file base64_internal.h
+ *
+ * \brief RFC 1521 base64 encoding/decoding: interfaces for invasive 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_INTERNAL
+#define MBEDTLS_BASE64_INTERNAL
+
+#include "common.h"
+
+#if defined(MBEDTLS_TEST_HOOKS)
+
+/** Given a value in the range 0..63, return the corresponding Base64 digit.
+ *
+ * The implementation assumes that letters are consecutive (e.g. ASCII
+ * but not EBCDIC).
+ *
+ * \param value     A value in the range 0..63.
+ *
+ * \return          A base64 digit converted from \p value.
+ */
+unsigned char mbedtls_ct_base64_enc_char(unsigned char value);
+
+/** Given a Base64 digit, return its value.
+ *
+ * If c is not a Base64 digit ('A'..'Z', 'a'..'z', '0'..'9', '+' or '/'),
+ * return -1.
+ *
+ * The implementation assumes that letters are consecutive (e.g. ASCII
+ * but not EBCDIC).
+ *
+ * \param c     A base64 digit.
+ *
+ * \return      The value of the base64 digit \p c.
+ */
+signed char mbedtls_ct_base64_dec_value(unsigned char c);
+
+#endif /* MBEDTLS_TEST_HOOKS */
+
+#endif /* MBEDTLS_BASE64_INTERNAL */