diff options
Diffstat (limited to 'third_party/heimdal/lib/hcrypto/libtommath/bn_s_mp_mul_digs.c')
-rw-r--r-- | third_party/heimdal/lib/hcrypto/libtommath/bn_s_mp_mul_digs.c | 74 |
1 files changed, 74 insertions, 0 deletions
diff --git a/third_party/heimdal/lib/hcrypto/libtommath/bn_s_mp_mul_digs.c b/third_party/heimdal/lib/hcrypto/libtommath/bn_s_mp_mul_digs.c new file mode 100644 index 00000000000..64509d4cb93 --- /dev/null +++ b/third_party/heimdal/lib/hcrypto/libtommath/bn_s_mp_mul_digs.c @@ -0,0 +1,74 @@ +#include "tommath_private.h" +#ifdef BN_S_MP_MUL_DIGS_C +/* LibTomMath, multiple-precision integer library -- Tom St Denis */ +/* SPDX-License-Identifier: Unlicense */ + +/* multiplies |a| * |b| and only computes upto digs digits of result + * HAC pp. 595, Algorithm 14.12 Modified so you can control how + * many digits of output are created. + */ +mp_err s_mp_mul_digs(const mp_int *a, const mp_int *b, mp_int *c, int digs) +{ + mp_int t; + mp_err err; + int pa, pb, ix, iy; + mp_digit u; + mp_word r; + mp_digit tmpx, *tmpt, *tmpy; + + /* can we use the fast multiplier? */ + if ((digs < MP_WARRAY) && + (MP_MIN(a->used, b->used) < MP_MAXFAST)) { + return s_mp_mul_digs_fast(a, b, c, digs); + } + + if ((err = mp_init_size(&t, digs)) != MP_OKAY) { + return err; + } + t.used = digs; + + /* compute the digits of the product directly */ + pa = a->used; + for (ix = 0; ix < pa; ix++) { + /* set the carry to zero */ + u = 0; + + /* limit ourselves to making digs digits of output */ + pb = MP_MIN(b->used, digs - ix); + + /* setup some aliases */ + /* copy of the digit from a used within the nested loop */ + tmpx = a->dp[ix]; + + /* an alias for the destination shifted ix places */ + tmpt = t.dp + ix; + + /* an alias for the digits of b */ + tmpy = b->dp; + + /* compute the columns of the output and propagate the carry */ + for (iy = 0; iy < pb; iy++) { + /* compute the column as a mp_word */ + r = (mp_word)*tmpt + + ((mp_word)tmpx * (mp_word)*tmpy++) + + (mp_word)u; + + /* the new column is the lower part of the result */ + *tmpt++ = (mp_digit)(r & (mp_word)MP_MASK); + + /* get the carry word from the result */ + u = (mp_digit)(r >> (mp_word)MP_DIGIT_BIT); + } + /* set carry if it is placed below digs */ + if ((ix + iy) < digs) { + *tmpt = u; + } + } + + mp_clamp(&t); + mp_exch(&t, c); + + mp_clear(&t); + return MP_OKAY; +} +#endif |