a

libsec/dsagen.c

@@ -0,0 +1,61 @@
+#include "os.h"
+#include <mp.h>
+#include <libsec.h>
+
+DSApriv*
+dsagen(DSApub *opub)
+{
+	DSApub *pub;
+	DSApriv *priv;
+	mpint *exp;
+	mpint *g;
+	mpint *r;
+	int bits;
+
+	priv = dsaprivalloc();
+	pub = &priv->pub;
+
+	if(opub != nil){
+		pub->p = mpcopy(opub->p);
+		pub->q = mpcopy(opub->q);
+	} else {
+		pub->p = mpnew(0);
+		pub->q = mpnew(0);
+		DSAprimes(pub->q, pub->p, nil);
+	}
+	bits = Dbits*pub->p->top;
+
+	pub->alpha = mpnew(0);
+	pub->key = mpnew(0);
+	priv->secret = mpnew(0);
+
+	// find a generator alpha of the multiplicative
+	// group Z*p, i.e., of order n = p-1.  We use the
+	// fact that q divides p-1 to reduce the exponent.
+	//
+	// This isn't very efficient.  If anyone has a better
+	// idea, mail presotto@closedmind.org
+	exp = mpnew(0);
+	g = mpnew(0);
+	r = mpnew(0);
+	mpsub(pub->p, mpone, exp);
+	mpdiv(exp, pub->q, exp, r);
+	if(mpcmp(r, mpzero) != 0)
+		sysfatal("dsagen foul up");
+	while(1){
+		mprand(bits, genrandom, g);
+		mpmod(g, pub->p, g);
+		mpexp(g, exp, pub->p, pub->alpha);
+		if(mpcmp(pub->alpha, mpone) != 0)
+			break;
+	}
+	mpfree(g);
+	mpfree(exp);
+
+	// create the secret key
+	mprand(bits, genrandom, priv->secret);
+	mpmod(priv->secret, pub->p, priv->secret);
+	mpexp(pub->alpha, priv->secret, pub->p, pub->key);
+
+	return priv;
+}