98 lines
1.6 KiB
C
98 lines
1.6 KiB
C
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#include "os.h"
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#include <mp.h>
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#define iseven(a) (((a)->p[0] & 1) == 0)
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// extended binary gcd
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//
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// For a anv b it solves, v = gcd(a,b) and finds x and y s.t.
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// ax + by = v
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//
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// Handbook of Applied Cryptography, Menezes et al, 1997, pg 608.
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void
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mpextendedgcd(mpint *a, mpint *b, mpint *v, mpint *x, mpint *y)
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{
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mpint *u, *A, *B, *C, *D;
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int g;
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if(a->top == 0){
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mpassign(b, v);
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mpassign(mpone, y);
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mpassign(mpzero, x);
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return;
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}
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if(b->top == 0){
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mpassign(a, v);
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mpassign(mpone, x);
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mpassign(mpzero, y);
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return;
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}
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g = 0;
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a = mpcopy(a);
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b = mpcopy(b);
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while(iseven(a) && iseven(b)){
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mpright(a, 1, a);
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mpright(b, 1, b);
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g++;
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}
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u = mpcopy(a);
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mpassign(b, v);
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A = mpcopy(mpone);
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B = mpcopy(mpzero);
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C = mpcopy(mpzero);
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D = mpcopy(mpone);
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for(;;) {
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// print("%B %B %B %B %B %B\n", u, v, A, B, C, D);
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while(iseven(u)){
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mpright(u, 1, u);
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if(!iseven(A) || !iseven(B)) {
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mpadd(A, b, A);
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mpsub(B, a, B);
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}
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mpright(A, 1, A);
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mpright(B, 1, B);
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}
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// print("%B %B %B %B %B %B\n", u, v, A, B, C, D);
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while(iseven(v)){
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mpright(v, 1, v);
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if(!iseven(C) || !iseven(D)) {
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mpadd(C, b, C);
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mpsub(D, a, D);
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}
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mpright(C, 1, C);
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mpright(D, 1, D);
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}
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// print("%B %B %B %B %B %B\n", u, v, A, B, C, D);
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if(mpcmp(u, v) >= 0){
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mpsub(u, v, u);
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mpsub(A, C, A);
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mpsub(B, D, B);
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} else {
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mpsub(v, u, v);
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mpsub(C, A, C);
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mpsub(D, B, D);
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}
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if(u->top == 0)
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break;
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}
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mpassign(C, x);
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mpassign(D, y);
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mpleft(v, g, v);
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mpfree(A);
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mpfree(B);
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mpfree(C);
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mpfree(D);
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mpfree(u);
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mpfree(a);
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mpfree(b);
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}
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