1 #include "os.h"
2 #include <mp.h>
3 #include <libsec.h>
4 
5 /* find a prime p of length n and a generator alpha of Z^*_p */
6 /* Alg 4.86 Menezes et al () Handbook, p.164 */
7 void
8 gensafeprime(mpint *p, mpint *alpha, int n, int accuracy)
9 {
10 	mpint *q, *b;
11 
12 	q = mpnew(n-1);
13 	while(1){
14 		genprime(q, n-1, accuracy);
15 		mpleft(q, 1, p);
16 		mpadd(p, mpone, p); /* p = 2*q+1 */
17 		if(probably_prime(p, accuracy))
18 			break;
19 	}
20 	/* now find a generator alpha of the multiplicative */
21 	/* group Z*_p of order p-1=2q */
22 	b = mpnew(0);
23 	while(1){
24 		mprand(n, genrandom, alpha);
25 		mpmod(alpha, p, alpha);
26 		mpmul(alpha, alpha, b);
27 		mpmod(b, p, b);
28 		if(mpcmp(b, mpone) == 0)
29 			continue;
30 		mpexp(alpha, q, p, b);
31 		if(mpcmp(b, mpone) != 0)
32 			break;
33 	}
34 	mpfree(b);
35 	mpfree(q);
36 }