harvey/sys/src/libsec/port/dsaverify.c

#include "os.h"
#include <mp.h>
#include <libsec.h>

int
dsaverify(DSApub *pub, DSAsig *sig, mpint *m)
{
	int rv = -1;
	mpint *u1, *u2, *v, *sinv;

	if(mpcmp(sig->r, mpone) < 0 || mpcmp(sig->r, pub->q) >= 0)
		return rv;
	if(mpcmp(sig->s, mpone) < 0 || mpcmp(sig->s, pub->q) >= 0)
		return rv;
	u1 = mpnew(0);
	u2 = mpnew(0);
	v = mpnew(0);
	sinv = mpnew(0);

	// find (s**-1) mod q, make sure it exists
	mpextendedgcd(sig->s, pub->q, u1, sinv, v);
	if(mpcmp(u1, mpone) != 0)
		goto out;

	// u1 = (sinv * m) mod q, u2 = (r * sinv) mod q
	mpmul(sinv, m, u1);
	mpmod(u1, pub->q, u1);
	mpmul(sig->r, sinv, u2);
	mpmod(u2, pub->q, u2);

	// v = (((alpha**u1)*(key**u2)) mod p) mod q
	mpexp(pub->alpha, u1, pub->p, sinv);
	mpexp(pub->key, u2, pub->p, v);
	mpmul(sinv, v, v);
	mpmod(v, pub->p, v);
	mpmod(v, pub->q, v);

	if(mpcmp(v, sig->r) == 0)
		rv = 0;
out:
	mpfree(v);
	mpfree(u1);
	mpfree(u2);
	mpfree(sinv);
	return rv;
}