diff --git a/library/bignum.c b/library/bignum.c
index 3e1c48c..551fd81 100644
--- a/library/bignum.c
+++ b/library/bignum.c
@@ -2039,6 +2039,20 @@
     return ret;
 }
 
+int mbedtls_mpi_get_mont_r2_unsafe(mbedtls_mpi *X,
+                                   const mbedtls_mpi *N)
+{
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+
+    MBEDTLS_MPI_CHK(mbedtls_mpi_lset(X, 1));
+    MBEDTLS_MPI_CHK(mbedtls_mpi_shift_l(X, N->n * 2 * biL));
+    MBEDTLS_MPI_CHK(mbedtls_mpi_mod_mpi(X, X, N));
+    MBEDTLS_MPI_CHK(mbedtls_mpi_shrink(X, N->n));
+
+cleanup:
+    return ret;
+}
+
 /*
  * Sliding-window exponentiation: X = A^E mod N  (HAC 14.85)
  */
@@ -2153,9 +2167,7 @@
      * If 1st call, pre-compute R^2 mod N
      */
     if (prec_RR == NULL || prec_RR->p == NULL) {
-        MBEDTLS_MPI_CHK(mbedtls_mpi_lset(&RR, 1));
-        MBEDTLS_MPI_CHK(mbedtls_mpi_shift_l(&RR, N->n * 2 * biL));
-        MBEDTLS_MPI_CHK(mbedtls_mpi_mod_mpi(&RR, &RR, N));
+        mbedtls_mpi_get_mont_r2_unsafe(&RR, N);
 
         if (prec_RR != NULL) {
             memcpy(prec_RR, &RR, sizeof(mbedtls_mpi));
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 */