diff --git a/library/bignum_core.c b/library/bignum_core.c
index 00837298b0..457405d277 100644
--- a/library/bignum_core.c
+++ b/library/bignum_core.c
@@ -511,4 +511,20 @@ void mbedtls_mpi_core_montmul( mbedtls_mpi_uint *X,
     mbedtls_ct_mpi_uint_cond_assign( AN_limbs, X, T, (unsigned char) ( carry ^ borrow ) );
 }
 
+int mbedtls_mpi_get_montgomery_constant_unsafe( mbedtls_mpi *X,
+                                                mbedtls_mpi const *N )
+{
+    int ret = MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED;
+
+    if ( X == NULL || N == NULL ) goto cleanup;
+
+    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 );
+}
+
 #endif /* MBEDTLS_BIGNUM_C */
diff --git a/library/bignum_core.h b/library/bignum_core.h
index 56a3bf874f..3100bd5ff9 100644
--- a/library/bignum_core.h
+++ b/library/bignum_core.h
@@ -412,4 +412,23 @@ void mbedtls_mpi_core_montmul( mbedtls_mpi_uint *X,
                                const mbedtls_mpi_uint *N, size_t AN_limbs,
                                mbedtls_mpi_uint mm, mbedtls_mpi_uint *T );
 
+/**
+ * \brief Calculate initialisation value for fast Montgomery modular
+ *        multiplication
+ *
+ * \param[out] X  A pointer to the result of the calculation of
+ *                Montgomery const 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.
+ * \return        #MBEDTLS_ERR_ERROR_CORRUPTION_DETECTED \p N, \p X are NULL
+ *                or other operations fail.
+ */
+int mbedtls_mpi_get_montgomery_constant_unsafe( mbedtls_mpi *X,
+                                                mbedtls_mpi const *N );
+
 #endif /* MBEDTLS_BIGNUM_CORE_H */