tdsagen.c - plan9port - [fork] Plan 9 from user space
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       ---
       tdsagen.c (1151B)
       ---
            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 }