diff options
Diffstat (limited to 'third_party/heimdal/lib/hcrypto/libtommath/bn_mp_montgomery_calc_normalization.c')
-rw-r--r-- | third_party/heimdal/lib/hcrypto/libtommath/bn_mp_montgomery_calc_normalization.c | 44 |
1 files changed, 44 insertions, 0 deletions
diff --git a/third_party/heimdal/lib/hcrypto/libtommath/bn_mp_montgomery_calc_normalization.c b/third_party/heimdal/lib/hcrypto/libtommath/bn_mp_montgomery_calc_normalization.c new file mode 100644 index 0000000..8379789 --- /dev/null +++ b/third_party/heimdal/lib/hcrypto/libtommath/bn_mp_montgomery_calc_normalization.c @@ -0,0 +1,44 @@ +#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 |