--- /dev/null
+#include "tommath_private.h"
+#ifdef BN_MP_MONTGOMERY_CALC_NORMALIZATION_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis */
+/* SPDX-License-Identifier: Unlicense */
+
+/*
+ * shifts with subtractions when the result is greater than b.
+ *
+ * The method is slightly modified to shift B unconditionally upto just under
+ * the leading bit of b. This saves alot of multiple precision shifting.
+ */
+mp_err mp_montgomery_calc_normalization(mp_int *a, const mp_int *b)
+{
+ int x, bits;
+ mp_err err;
+
+ /* how many bits of last digit does b use */
+ bits = mp_count_bits(b) % MP_DIGIT_BIT;
+
+ if (b->used > 1) {
+ if ((err = mp_2expt(a, ((b->used - 1) * MP_DIGIT_BIT) + bits - 1)) != MP_OKAY) {
+ return err;
+ }
+ } else {
+ mp_set(a, 1uL);
+ bits = 1;
+ }
+
+
+ /* now compute C = A * B mod b */
+ for (x = bits - 1; x < (int)MP_DIGIT_BIT; x++) {
+ if ((err = mp_mul_2(a, a)) != MP_OKAY) {
+ return err;
+ }
+ if (mp_cmp_mag(a, b) != MP_LT) {
+ if ((err = s_mp_sub(a, b, a)) != MP_OKAY) {
+ return err;
+ }
+ }
+ }
+
+ return MP_OKAY;
+}
+#endif