149 lines
3.3 KiB
C
149 lines
3.3 KiB
C
#include <tommath.h>
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#ifdef BN_FAST_MP_INVMOD_C
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/* LibTomMath, multiple-precision integer library -- Tom St Denis
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*
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* LibTomMath is a library that provides multiple-precision
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* integer arithmetic as well as number theoretic functionality.
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*
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* The library was designed directly after the MPI library by
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* Michael Fromberger but has been written from scratch with
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* additional optimizations in place.
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*
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* The library is free for all purposes without any express
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* guarantee it works.
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*
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* Tom St Denis, tstdenis82@gmail.com, http://libtom.org
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*/
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/* computes the modular inverse via binary extended euclidean algorithm,
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* that is c = 1/a mod b
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*
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* Based on slow invmod except this is optimized for the case where b is
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* odd as per HAC Note 14.64 on pp. 610
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*/
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int fast_mp_invmod (mp_int * a, mp_int * b, mp_int * c)
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{
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mp_int x, y, u, v, B, D;
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int res, neg;
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/* 2. [modified] b must be odd */
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if (mp_iseven (b) == 1) {
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return MP_VAL;
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}
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/* init all our temps */
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if ((res = mp_init_multi(&x, &y, &u, &v, &B, &D, NULL)) != MP_OKAY) {
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return res;
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}
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/* x == modulus, y == value to invert */
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if ((res = mp_copy (b, &x)) != MP_OKAY) {
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goto LBL_ERR;
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}
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/* we need y = |a| */
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if ((res = mp_mod (a, b, &y)) != MP_OKAY) {
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goto LBL_ERR;
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}
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/* 3. u=x, v=y, A=1, B=0, C=0,D=1 */
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if ((res = mp_copy (&x, &u)) != MP_OKAY) {
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goto LBL_ERR;
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}
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if ((res = mp_copy (&y, &v)) != MP_OKAY) {
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goto LBL_ERR;
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}
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mp_set (&D, 1);
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top:
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/* 4. while u is even do */
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while (mp_iseven (&u) == 1) {
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/* 4.1 u = u/2 */
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if ((res = mp_div_2 (&u, &u)) != MP_OKAY) {
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goto LBL_ERR;
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}
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/* 4.2 if B is odd then */
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if (mp_isodd (&B) == 1) {
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if ((res = mp_sub (&B, &x, &B)) != MP_OKAY) {
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goto LBL_ERR;
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}
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}
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/* B = B/2 */
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if ((res = mp_div_2 (&B, &B)) != MP_OKAY) {
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goto LBL_ERR;
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}
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}
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/* 5. while v is even do */
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while (mp_iseven (&v) == 1) {
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/* 5.1 v = v/2 */
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if ((res = mp_div_2 (&v, &v)) != MP_OKAY) {
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goto LBL_ERR;
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}
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/* 5.2 if D is odd then */
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if (mp_isodd (&D) == 1) {
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/* D = (D-x)/2 */
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if ((res = mp_sub (&D, &x, &D)) != MP_OKAY) {
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goto LBL_ERR;
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}
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}
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/* D = D/2 */
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if ((res = mp_div_2 (&D, &D)) != MP_OKAY) {
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goto LBL_ERR;
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}
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}
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/* 6. if u >= v then */
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if (mp_cmp (&u, &v) != MP_LT) {
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/* u = u - v, B = B - D */
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if ((res = mp_sub (&u, &v, &u)) != MP_OKAY) {
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goto LBL_ERR;
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}
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if ((res = mp_sub (&B, &D, &B)) != MP_OKAY) {
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goto LBL_ERR;
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}
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} else {
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/* v - v - u, D = D - B */
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if ((res = mp_sub (&v, &u, &v)) != MP_OKAY) {
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goto LBL_ERR;
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}
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if ((res = mp_sub (&D, &B, &D)) != MP_OKAY) {
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goto LBL_ERR;
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}
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}
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/* if not zero goto step 4 */
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if (mp_iszero (&u) == 0) {
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goto top;
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}
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/* now a = C, b = D, gcd == g*v */
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/* if v != 1 then there is no inverse */
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if (mp_cmp_d (&v, 1) != MP_EQ) {
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res = MP_VAL;
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goto LBL_ERR;
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}
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/* b is now the inverse */
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neg = a->sign;
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while (D.sign == MP_NEG) {
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if ((res = mp_add (&D, b, &D)) != MP_OKAY) {
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goto LBL_ERR;
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}
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}
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mp_exch (&D, c);
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c->sign = neg;
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res = MP_OKAY;
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LBL_ERR:mp_clear_multi (&x, &y, &u, &v, &B, &D, NULL);
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return res;
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}
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#endif
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/* $Source$ */
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/* $Revision$ */
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/* $Date$ */
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