diff options
Diffstat (limited to 'libtommath/bn_s_mp_invmod_slow.c')
| -rw-r--r-- | libtommath/bn_s_mp_invmod_slow.c | 119 |
1 files changed, 119 insertions, 0 deletions
diff --git a/libtommath/bn_s_mp_invmod_slow.c b/libtommath/bn_s_mp_invmod_slow.c new file mode 100644 index 0000000..4c5db33 --- /dev/null +++ b/libtommath/bn_s_mp_invmod_slow.c @@ -0,0 +1,119 @@ +#include "tommath_private.h" +#ifdef BN_S_MP_INVMOD_SLOW_C +/* LibTomMath, multiple-precision integer library -- Tom St Denis */ +/* SPDX-License-Identifier: Unlicense */ + +/* hac 14.61, pp608 */ +mp_err s_mp_invmod_slow(const mp_int *a, const mp_int *b, mp_int *c) +{ + mp_int x, y, u, v, A, B, C, D; + mp_err err; + + /* b cannot be negative */ + if ((b->sign == MP_NEG) || MP_IS_ZERO(b)) { + return MP_VAL; + } + + /* init temps */ + if ((err = mp_init_multi(&x, &y, &u, &v, + &A, &B, &C, &D, NULL)) != MP_OKAY) { + return err; + } + + /* x = a, y = b */ + if ((err = mp_mod(a, b, &x)) != MP_OKAY) goto LBL_ERR; + if ((err = mp_copy(b, &y)) != MP_OKAY) goto LBL_ERR; + + /* 2. [modified] if x,y are both even then return an error! */ + if (MP_IS_EVEN(&x) && MP_IS_EVEN(&y)) { + err = MP_VAL; + goto LBL_ERR; + } + + /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */ + if ((err = mp_copy(&x, &u)) != MP_OKAY) goto LBL_ERR; + if ((err = mp_copy(&y, &v)) != MP_OKAY) goto LBL_ERR; + mp_set(&A, 1uL); + mp_set(&D, 1uL); + +top: + /* 4. while u is even do */ + while (MP_IS_EVEN(&u)) { + /* 4.1 u = u/2 */ + if ((err = mp_div_2(&u, &u)) != MP_OKAY) goto LBL_ERR; + + /* 4.2 if A or B is odd then */ + if (MP_IS_ODD(&A) || MP_IS_ODD(&B)) { + /* A = (A+y)/2, B = (B-x)/2 */ + if ((err = mp_add(&A, &y, &A)) != MP_OKAY) goto LBL_ERR; + if ((err = mp_sub(&B, &x, &B)) != MP_OKAY) goto LBL_ERR; + } + /* A = A/2, B = B/2 */ + if ((err = mp_div_2(&A, &A)) != MP_OKAY) goto LBL_ERR; + if ((err = mp_div_2(&B, &B)) != MP_OKAY) goto LBL_ERR; + } + + /* 5. while v is even do */ + while (MP_IS_EVEN(&v)) { + /* 5.1 v = v/2 */ + if ((err = mp_div_2(&v, &v)) != MP_OKAY) goto LBL_ERR; + + /* 5.2 if C or D is odd then */ + if (MP_IS_ODD(&C) || MP_IS_ODD(&D)) { + /* C = (C+y)/2, D = (D-x)/2 */ + if ((err = mp_add(&C, &y, &C)) != MP_OKAY) goto LBL_ERR; + if ((err = mp_sub(&D, &x, &D)) != MP_OKAY) goto LBL_ERR; + } + /* C = C/2, D = D/2 */ + if ((err = mp_div_2(&C, &C)) != MP_OKAY) goto LBL_ERR; + if ((err = mp_div_2(&D, &D)) != MP_OKAY) goto LBL_ERR; + } + + /* 6. if u >= v then */ + if (mp_cmp(&u, &v) != MP_LT) { + /* u = u - v, A = A - C, B = B - D */ + if ((err = mp_sub(&u, &v, &u)) != MP_OKAY) goto LBL_ERR; + + if ((err = mp_sub(&A, &C, &A)) != MP_OKAY) goto LBL_ERR; + + if ((err = mp_sub(&B, &D, &B)) != MP_OKAY) goto LBL_ERR; + } else { + /* v - v - u, C = C - A, D = D - B */ + if ((err = mp_sub(&v, &u, &v)) != MP_OKAY) goto LBL_ERR; + + if ((err = mp_sub(&C, &A, &C)) != MP_OKAY) goto LBL_ERR; + + if ((err = mp_sub(&D, &B, &D)) != MP_OKAY) goto LBL_ERR; + } + + /* if not zero goto step 4 */ + if (!MP_IS_ZERO(&u)) { + goto top; + } + + /* now a = C, b = D, gcd == g*v */ + + /* if v != 1 then there is no inverse */ + if (mp_cmp_d(&v, 1uL) != MP_EQ) { + err = MP_VAL; + goto LBL_ERR; + } + + /* if its too low */ + while (mp_cmp_d(&C, 0uL) == MP_LT) { + if ((err = mp_add(&C, b, &C)) != MP_OKAY) goto LBL_ERR; + } + + /* too big */ + while (mp_cmp_mag(&C, b) != MP_LT) { + if ((err = mp_sub(&C, b, &C)) != MP_OKAY) goto LBL_ERR; + } + + /* C is now the inverse */ + mp_exch(&C, c); + err = MP_OKAY; +LBL_ERR: + mp_clear_multi(&x, &y, &u, &v, &A, &B, &C, &D, NULL); + return err; +} +#endif |