dropbear: bn_mp_montgomery_calc_normalization.c comparison

Import of libtommath 0.35 From ltm-0.35.tar.bz2 SHA1 of 3f193dbae9351e92d02530994fa18236f7fde01c

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+:91fbc376f010
+#include <tommath.h>
+#ifdef BN_MP_MONTGOMERY_CALC_NORMALIZATION_C
+/* LibTomMath, multiple-precision integer library -- Tom St Denis
+*
+* LibTomMath is a library that provides multiple-precision
+* integer arithmetic as well as number theoretic functionality.
+*
+* The library was designed directly after the MPI library by
+* Michael Fromberger but has been written from scratch with
+* additional optimizations in place.
+*
+* The library is free for all purposes without any express
+* guarantee it works.
+*
+* Tom St Denis, [email protected], http://math.libtomcrypt.org
+*/
+/*
+* 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.
+*/
+int mp_montgomery_calc_normalization (mp_int * a, mp_int * b)
+{
+int     x, bits, res;
+/* how many bits of last digit does b use */
+bits = mp_count_bits (b) % DIGIT_BIT;
+if (b->used > 1) {
+if ((res = mp_2expt (a, (b->used - 1) * DIGIT_BIT + bits - 1)) != MP_OKAY) {
+return res;
+}
+} else {
+mp_set(a, 1);
+bits = 1;
+}
+/* now compute C = A * B mod b */
+for (x = bits - 1; x < (int)DIGIT_BIT; x++) {
+if ((res = mp_mul_2 (a, a)) != MP_OKAY) {
+return res;
+}
+if (mp_cmp_mag (a, b) != MP_LT) {
+if ((res = s_mp_sub (a, b, a)) != MP_OKAY) {
+return res;
+}
+}
+}
+return MP_OKAY;
+}
+#endif

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