1 #include "os.h"
2 #include <mp.h>
3 #include <libsec.h>
4 
5 DSApriv*
6 dsagen(DSApub *opub)
7 {
8 	DSApub *pub;
9 	DSApriv *priv;
10 	mpint *exp;
11 	mpint *g;
12 	mpint *r;
13 	int bits;
14 
15 	priv = dsaprivalloc();
16 	pub = &priv->pub;
17 
18 	if(opub != nil){
19 		pub->p = mpcopy(opub->p);
20 		pub->q = mpcopy(opub->q);
21 	} else {
22 		pub->p = mpnew(0);
23 		pub->q = mpnew(0);
24 		DSAprimes(pub->q, pub->p, nil);
25 	}
26 	bits = Dbits*pub->p->top;
27 
28 	pub->alpha = mpnew(0);
29 	pub->key = mpnew(0);
30 	priv->secret = mpnew(0);
31 
32 	/* find a generator alpha of the multiplicative */
33 	/* group Z*p, i.e., of order n = p-1.  We use the */
34 	/* fact that q divides p-1 to reduce the exponent. */
35 	exp = mpnew(0);
36 	g = mpnew(0);
37 	r = mpnew(0);
38 	mpsub(pub->p, mpone, exp);
39 	mpdiv(exp, pub->q, exp, r);
40 	if(mpcmp(r, mpzero) != 0)
41 		sysfatal("dsagen foul up");
42 	while(1){
43 		mprand(bits, genrandom, g);
44 		mpmod(g, pub->p, g);
45 		mpexp(g, exp, pub->p, pub->alpha);
46 		if(mpcmp(pub->alpha, mpone) != 0)
47 			break;
48 	}
49 	mpfree(g);
50 	mpfree(exp);
51 
52 	/* create the secret key */
53 	mprand(bits, genrandom, priv->secret);
54 	mpmod(priv->secret, pub->p, priv->secret);
55 	mpexp(pub->alpha, priv->secret, pub->p, pub->key);
56 
57 	return priv;
58 }