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