diff options
Diffstat (limited to 'libtommath/bn_fast_mp_invmod.c')
| -rw-r--r-- | libtommath/bn_fast_mp_invmod.c | 223 |
1 files changed, 112 insertions, 111 deletions
diff --git a/libtommath/bn_fast_mp_invmod.c b/libtommath/bn_fast_mp_invmod.c index 12f42de..7771136 100644 --- a/libtommath/bn_fast_mp_invmod.c +++ b/libtommath/bn_fast_mp_invmod.c @@ -15,131 +15,132 @@ * Tom St Denis, tstdenis82@gmail.com, http://libtom.org */ -/* computes the modular inverse via binary extended euclidean algorithm, - * that is c = 1/a mod b +/* computes the modular inverse via binary extended euclidean algorithm, + * that is c = 1/a mod b * - * Based on slow invmod except this is optimized for the case where b is + * Based on slow invmod except this is optimized for the case where b is * odd as per HAC Note 14.64 on pp. 610 */ -int fast_mp_invmod (mp_int * a, mp_int * b, mp_int * c) +int fast_mp_invmod(mp_int *a, mp_int *b, mp_int *c) { - mp_int x, y, u, v, B, D; - int res, neg; - - /* 2. [modified] b must be odd */ - if (mp_iseven (b) == MP_YES) { - return MP_VAL; - } - - /* init all our temps */ - if ((res = mp_init_multi(&x, &y, &u, &v, &B, &D, NULL)) != MP_OKAY) { - return res; - } - - /* x == modulus, y == value to invert */ - if ((res = mp_copy (b, &x)) != MP_OKAY) { - goto LBL_ERR; - } - - /* we need y = |a| */ - if ((res = mp_mod (a, b, &y)) != MP_OKAY) { - goto LBL_ERR; - } - - /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */ - if ((res = mp_copy (&x, &u)) != MP_OKAY) { - goto LBL_ERR; - } - if ((res = mp_copy (&y, &v)) != MP_OKAY) { - goto LBL_ERR; - } - mp_set (&D, 1); + mp_int x, y, u, v, B, D; + int res, neg; -top: - /* 4. while u is even do */ - while (mp_iseven (&u) == MP_YES) { - /* 4.1 u = u/2 */ - if ((res = mp_div_2 (&u, &u)) != MP_OKAY) { - goto LBL_ERR; - } - /* 4.2 if B is odd then */ - if (mp_isodd (&B) == MP_YES) { - if ((res = mp_sub (&B, &x, &B)) != MP_OKAY) { - goto LBL_ERR; - } - } - /* B = B/2 */ - if ((res = mp_div_2 (&B, &B)) != MP_OKAY) { - goto LBL_ERR; - } - } + /* 2. [modified] b must be odd */ + if (mp_iseven(b) == MP_YES) { + return MP_VAL; + } - /* 5. while v is even do */ - while (mp_iseven (&v) == MP_YES) { - /* 5.1 v = v/2 */ - if ((res = mp_div_2 (&v, &v)) != MP_OKAY) { - goto LBL_ERR; - } - /* 5.2 if D is odd then */ - if (mp_isodd (&D) == MP_YES) { - /* D = (D-x)/2 */ - if ((res = mp_sub (&D, &x, &D)) != MP_OKAY) { - goto LBL_ERR; - } - } - /* D = D/2 */ - if ((res = mp_div_2 (&D, &D)) != MP_OKAY) { + /* init all our temps */ + if ((res = mp_init_multi(&x, &y, &u, &v, &B, &D, NULL)) != MP_OKAY) { + return res; + } + + /* x == modulus, y == value to invert */ + if ((res = mp_copy(b, &x)) != MP_OKAY) { goto LBL_ERR; - } - } + } - /* 6. if u >= v then */ - if (mp_cmp (&u, &v) != MP_LT) { - /* u = u - v, B = B - D */ - if ((res = mp_sub (&u, &v, &u)) != MP_OKAY) { + /* we need y = |a| */ + if ((res = mp_mod(a, b, &y)) != MP_OKAY) { goto LBL_ERR; - } + } - if ((res = mp_sub (&B, &D, &B)) != MP_OKAY) { + /* 3. u=x, v=y, A=1, B=0, C=0,D=1 */ + if ((res = mp_copy(&x, &u)) != MP_OKAY) { goto LBL_ERR; - } - } else { - /* v - v - u, D = D - B */ - if ((res = mp_sub (&v, &u, &v)) != MP_OKAY) { + } + if ((res = mp_copy(&y, &v)) != MP_OKAY) { goto LBL_ERR; - } + } + mp_set(&D, 1); - if ((res = mp_sub (&D, &B, &D)) != MP_OKAY) { - goto LBL_ERR; - } - } - - /* if not zero goto step 4 */ - if (mp_iszero (&u) == MP_NO) { - goto top; - } - - /* now a = C, b = D, gcd == g*v */ - - /* if v != 1 then there is no inverse */ - if (mp_cmp_d (&v, 1) != MP_EQ) { - res = MP_VAL; - goto LBL_ERR; - } - - /* b is now the inverse */ - neg = a->sign; - while (D.sign == MP_NEG) { - if ((res = mp_add (&D, b, &D)) != MP_OKAY) { +top: + /* 4. while u is even do */ + while (mp_iseven(&u) == MP_YES) { + /* 4.1 u = u/2 */ + if ((res = mp_div_2(&u, &u)) != MP_OKAY) { + goto LBL_ERR; + } + /* 4.2 if B is odd then */ + if (mp_isodd(&B) == MP_YES) { + if ((res = mp_sub(&B, &x, &B)) != MP_OKAY) { + goto LBL_ERR; + } + } + /* B = B/2 */ + if ((res = mp_div_2(&B, &B)) != MP_OKAY) { + goto LBL_ERR; + } + } + + /* 5. while v is even do */ + while (mp_iseven(&v) == MP_YES) { + /* 5.1 v = v/2 */ + if ((res = mp_div_2(&v, &v)) != MP_OKAY) { + goto LBL_ERR; + } + /* 5.2 if D is odd then */ + if (mp_isodd(&D) == MP_YES) { + /* D = (D-x)/2 */ + if ((res = mp_sub(&D, &x, &D)) != MP_OKAY) { + goto LBL_ERR; + } + } + /* D = D/2 */ + if ((res = 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, B = B - D */ + if ((res = mp_sub(&u, &v, &u)) != MP_OKAY) { + goto LBL_ERR; + } + + if ((res = mp_sub(&B, &D, &B)) != MP_OKAY) { + goto LBL_ERR; + } + } else { + /* v - v - u, D = D - B */ + if ((res = mp_sub(&v, &u, &v)) != MP_OKAY) { + goto LBL_ERR; + } + + if ((res = mp_sub(&D, &B, &D)) != MP_OKAY) { + goto LBL_ERR; + } + } + + /* if not zero goto step 4 */ + if (mp_iszero(&u) == MP_NO) { + goto top; + } + + /* now a = C, b = D, gcd == g*v */ + + /* if v != 1 then there is no inverse */ + if (mp_cmp_d(&v, 1) != MP_EQ) { + res = MP_VAL; goto LBL_ERR; - } - } - mp_exch (&D, c); - c->sign = neg; - res = MP_OKAY; - -LBL_ERR:mp_clear_multi (&x, &y, &u, &v, &B, &D, NULL); - return res; + } + + /* b is now the inverse */ + neg = a->sign; + while (D.sign == MP_NEG) { + if ((res = mp_add(&D, b, &D)) != MP_OKAY) { + goto LBL_ERR; + } + } + mp_exch(&D, c); + c->sign = neg; + res = MP_OKAY; + +LBL_ERR: + mp_clear_multi(&x, &y, &u, &v, &B, &D, NULL); + return res; } #endif |