a

libsec/dsaverify.c

@@ -0,0 +1,46 @@
+#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(sig->r->sign < 0 || mpcmp(sig->r, pub->q) >= 0)
+		return rv;
+	if(sig->s->sign < 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;
+}