37 lines
741 B
C
37 lines
741 B
C
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#include "os.h"
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#include <mp.h>
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#include <libsec.h>
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// find a prime p of length n and a generator alpha of Z^*_p
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// Alg 4.86 Menezes et al () Handbook, p.164
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void
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gensafeprime(mpint *p, mpint *alpha, int n, int accuracy)
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{
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mpint *q, *b;
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q = mpnew(n-1);
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while(1){
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genprime(q, n-1, accuracy);
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mpleft(q, 1, p);
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mpadd(p, mpone, p); // p = 2*q+1
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if(probably_prime(p, accuracy))
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break;
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}
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// now find a generator alpha of the multiplicative
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// group Z*_p of order p-1=2q
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b = mpnew(0);
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while(1){
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mprand(n, genrandom, alpha);
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mpmod(alpha, p, alpha);
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mpmul(alpha, alpha, b);
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mpmod(b, p, b);
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if(mpcmp(b, mpone) == 0)
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continue;
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mpexp(alpha, q, p, b);
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if(mpcmp(b, mpone) != 0)
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break;
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}
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mpfree(b);
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mpfree(q);
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}
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