diff options
Diffstat (limited to 'third_party/heimdal/lib/hcrypto/libtommath/bn_mp_dr_reduce.c')
-rw-r--r-- | third_party/heimdal/lib/hcrypto/libtommath/bn_mp_dr_reduce.c | 78 |
1 files changed, 78 insertions, 0 deletions
diff --git a/third_party/heimdal/lib/hcrypto/libtommath/bn_mp_dr_reduce.c b/third_party/heimdal/lib/hcrypto/libtommath/bn_mp_dr_reduce.c new file mode 100644 index 00000000000..ffc33a6b50e --- /dev/null +++ b/third_party/heimdal/lib/hcrypto/libtommath/bn_mp_dr_reduce.c @@ -0,0 +1,78 @@ +#include "tommath_private.h" +#ifdef BN_MP_DR_REDUCE_C +/* LibTomMath, multiple-precision integer library -- Tom St Denis */ +/* SPDX-License-Identifier: Unlicense */ + +/* reduce "x" in place modulo "n" using the Diminished Radix algorithm. + * + * Based on algorithm from the paper + * + * "Generating Efficient Primes for Discrete Log Cryptosystems" + * Chae Hoon Lim, Pil Joong Lee, + * POSTECH Information Research Laboratories + * + * The modulus must be of a special format [see manual] + * + * Has been modified to use algorithm 7.10 from the LTM book instead + * + * Input x must be in the range 0 <= x <= (n-1)**2 + */ +mp_err mp_dr_reduce(mp_int *x, const mp_int *n, mp_digit k) +{ + mp_err err; + int i, m; + mp_word r; + mp_digit mu, *tmpx1, *tmpx2; + + /* m = digits in modulus */ + m = n->used; + + /* ensure that "x" has at least 2m digits */ + if (x->alloc < (m + m)) { + if ((err = mp_grow(x, m + m)) != MP_OKAY) { + return err; + } + } + + /* top of loop, this is where the code resumes if + * another reduction pass is required. + */ +top: + /* aliases for digits */ + /* alias for lower half of x */ + tmpx1 = x->dp; + + /* alias for upper half of x, or x/B**m */ + tmpx2 = x->dp + m; + + /* set carry to zero */ + mu = 0; + + /* compute (x mod B**m) + k * [x/B**m] inline and inplace */ + for (i = 0; i < m; i++) { + r = ((mp_word)*tmpx2++ * (mp_word)k) + *tmpx1 + mu; + *tmpx1++ = (mp_digit)(r & MP_MASK); + mu = (mp_digit)(r >> ((mp_word)MP_DIGIT_BIT)); + } + + /* set final carry */ + *tmpx1++ = mu; + + /* zero words above m */ + MP_ZERO_DIGITS(tmpx1, (x->used - m) - 1); + + /* clamp, sub and return */ + mp_clamp(x); + + /* if x >= n then subtract and reduce again + * Each successive "recursion" makes the input smaller and smaller. + */ + if (mp_cmp_mag(x, n) != MP_LT) { + if ((err = s_mp_sub(x, n, x)) != MP_OKAY) { + return err; + } + goto top; + } + return MP_OKAY; +} +#endif |