tdsasign.c - plan9port - [fork] Plan 9 from user space
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       ---
       tdsasign.c (981B)
       ---
            1 #include "os.h"
            2 #include <mp.h>
            3 #include <libsec.h>
            4 
            5 DSAsig*
            6 dsasign(DSApriv *priv, mpint *m)
            7 {
            8         DSApub *pub = &priv->pub;
            9         DSAsig *sig;
           10         mpint *qm1, *k, *kinv, *r, *s;
           11         mpint *q = pub->q, *p = pub->p, *alpha = pub->alpha;
           12         int qlen = mpsignif(q);
           13 
           14         qm1 = mpnew(0);
           15         kinv = mpnew(0);
           16         r = mpnew(0);
           17         s = mpnew(0);
           18         k = mpnew(0);
           19         mpsub(pub->q, mpone, qm1);
           20 
           21         /* find a k that has an inverse mod q */
           22         while(1){
           23                 mprand(qlen, genrandom, k);
           24                 if((mpcmp(mpone, k) > 0) || (mpcmp(k, pub->q) >= 0))
           25                         continue;
           26                 mpextendedgcd(k, q, r, kinv, s);
           27                 if(mpcmp(r, mpone) != 0)
           28                         sysfatal("dsasign: pub->q not prime");
           29                 break;
           30         }
           31 
           32           /* make kinv positive */
           33         mpmod(kinv, pub->q, kinv);
           34 
           35         /* r = ((alpha**k) mod p) mod q */
           36         mpexp(alpha, k, p, r);
           37         mpmod(r, q, r);
           38 
           39         /* s = (kinv*(m + ar)) mod q */
           40         mpmul(r, priv->secret, s);
           41         mpadd(s, m, s);
           42         mpmul(s, kinv, s);
           43         mpmod(s, q, s);
           44 
           45         sig = dsasigalloc();
           46         sig->r = r;
           47         sig->s = s;
           48         mpfree(qm1);
           49         mpfree(k);
           50         mpfree(kinv);
           51         return sig;
           52 }