diff --git a/library/bignum_internal.h b/library/bignum_internal.h
new file mode 100644
index 0000000..39909f3
--- /dev/null
+++ b/library/bignum_internal.h
@@ -0,0 +1,31 @@
+/**
+ *  Low level bignum functions
+ *
+ *  Copyright The Mbed TLS Contributors
+ *  SPDX-License-Identifier: Apache-2.0 OR GPL-2.0-or-later
+ */
+
+#ifndef MBEDTLS_BIGNUM_INTERNAL_H
+#define MBEDTLS_BIGNUM_INTERNAL_H
+
+#include "mbedtls/bignum.h"
+
+/**
+ * \brief Calculate the square of the Montgomery constant. (Needed
+ *        for conversion and operations in Montgomery form.)
+ *
+ * \param[out] X  A pointer to the result of the calculation of
+ *                the square of the Montgomery constant:
+ *                2^{2*n*biL} mod N.
+ * \param[in]  N  Little-endian presentation of the modulus, which must be odd.
+ *
+ * \return        0 if successful.
+ * \return        #MBEDTLS_ERR_MPI_ALLOC_FAILED if there is not enough space
+ *                to store the value of Montgomery constant squared.
+ * \return        #MBEDTLS_ERR_MPI_DIVISION_BY_ZERO if \p N modulus is zero.
+ * \return        #MBEDTLS_ERR_MPI_NEGATIVE_VALUE if \p N modulus is negative.
+ */
+int mbedtls_mpi_get_mont_r2_unsafe(mbedtls_mpi *X,
+                                   const mbedtls_mpi *N);
+
+#endif /* MBEDTLS_BIGNUM_INTERNAL_H */