# code: drawterm

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#include "os.h"
#include <mp.h>
#include <libsec.h>

// Gordon's algorithm for generating a strong prime
//	Menezes et al () Handbook, p.150
void
genstrongprime(mpint *p, int n, int accuracy)
{
mpint *s, *t, *r, *i;

if(n < 64)
n = 64;

s = mpnew(n/2);
genprime(s, (n/2)-16, accuracy);
t = mpnew(n/2);
genprime(t, n-mpsignif(s)-32, accuracy);

// first r = 2it + 1 that's prime
i = mpnew(16);
r = mpnew(0);
itomp(0x8000, i);
mpleft(t, 1, t);	// 2t
mpmul(i, t, r);		// 2it
mpadd(r, mpone, r);	// 2it + 1
for(;;){
if(probably_prime(r, 18))
break;
mpadd(r, t, r);	// r += 2t
}

// p0 = 2(s**(r-2) mod r)s - 1
itomp(2, p);
mpsub(r, p, p);
mpexp(s, p, r, p);
mpmul(s, p, p);
mpleft(p, 1, p);
mpsub(p, mpone, p);

// first p = p0 + 2irs that's prime
itomp(0x8000, i);
mpleft(r, 1, r);	// 2r
mpmul(r, s, r);		// 2rs
mpmul(r, i, i);		// 2irs
mpadd(p, i, p);		// p0 + 2irs
for(;;){
if(probably_prime(p, accuracy))
break;
mpadd(p, r, p); // p += 2rs
}

mpfree(i);
mpfree(s);
mpfree(r);
mpfree(t);
}