forked from ibphoenix/tomsfastmath
99 lines
1.9 KiB
C
99 lines
1.9 KiB
C
/* TomsFastMath, a fast ISO C bignum library.
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*
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* This project is meant to fill in where LibTomMath
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* falls short. That is speed ;-)
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*
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* This project is public domain and free for all purposes.
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*
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* Tom St Denis, tomstdenis@iahu.ca
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*/
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#include <tfm.h>
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/* c = 1/a (mod b) for odd b only */
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int fp_invmod(fp_int *a, fp_int *b, fp_int *c)
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{
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fp_int x, y, u, v, B, D;
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int neg;
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/* 2. [modified] b must be odd */
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if (fp_iseven (b) == FP_YES) {
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return FP_VAL;
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}
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/* init all our temps */
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fp_init(&x); fp_init(&y);
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fp_init(&u); fp_init(&v);
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fp_init(&B); fp_init(&D);
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/* x == modulus, y == value to invert */
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fp_copy(b, &x);
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/* we need y = |a| */
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fp_abs(a, &y);
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/* 3. u=x, v=y, A=1, B=0, C=0,D=1 */
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fp_copy(&x, &u);
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fp_copy(&y, &v);
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fp_set (&D, 1);
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top:
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/* 4. while u is even do */
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while (fp_iseven (&u) == FP_YES) {
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/* 4.1 u = u/2 */
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fp_div_2 (&u, &u);
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/* 4.2 if B is odd then */
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if (fp_isodd (&B) == FP_YES) {
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fp_sub (&B, &x, &B);
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}
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/* B = B/2 */
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fp_div_2 (&B, &B);
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}
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/* 5. while v is even do */
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while (fp_iseven (&v) == FP_YES) {
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/* 5.1 v = v/2 */
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fp_div_2 (&v, &v);
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/* 5.2 if D is odd then */
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if (fp_isodd (&D) == FP_YES) {
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/* D = (D-x)/2 */
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fp_sub (&D, &x, &D);
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}
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/* D = D/2 */
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fp_div_2 (&D, &D);
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}
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/* 6. if u >= v then */
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if (fp_cmp (&u, &v) != FP_LT) {
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/* u = u - v, B = B - D */
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fp_sub (&u, &v, &u);
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fp_sub (&B, &D, &B);
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} else {
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/* v - v - u, D = D - B */
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fp_sub (&v, &u, &v);
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fp_sub (&D, &B, &D);
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}
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/* if not zero goto step 4 */
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if (fp_iszero (&u) == FP_NO) {
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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 (fp_cmp_d (&v, 1) != FP_EQ) {
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return FP_VAL;
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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 == FP_NEG) {
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fp_add (&D, b, &D);
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}
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fp_copy (&D, c);
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c->sign = neg;
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return FP_OKAY;
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}
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