# libtom/libtommath

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 #include #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, tomstdenis@gmail.com, http://libtom.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 /* \$Source\$ */ /* \$Revision\$ */ /* \$Date\$ */
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