/* Compile with:
gcc -lcrypto whatever.c -o whatever
Thanks to Phillip Lee Hellewell for bugfixes
*/

#include <stdio.h>
#include <openssl/dh.h>
#include <openssl/bn.h>

int whatever() {

BIGNUM *a, *b, *c, *d, *gen, *prime_mod;
char *c_str;
BN_CTX *ctx; /* used internally by the bignum lib */
DH *dhparms;
int csize;

ctx = BN_CTX_new();
a = BN_new();
b = BN_new();
c = BN_new();
gen = BN_new();
prime_mod = BN_new();

/* find a safe 512-bit random prime for generator 5 */
dhparms = DH_generate_parameters(512, 5, NULL, NULL);
gen = dhparms->g; /* Just a BIGNUM version of 5 */
prime_mod = dhparms->p;

BN_set_word(a, 6);
BN_set_word(b, 7);

BN_mul(c, a, b, ctx);
c_str = BN_bn2dec(c); /* there's also a BN_dec2bn() */
printf("6*7=%s\n", c_str);

csize = BN_num_bits(c);

d = BN_dup(a);

if(BN_is_bit_set(prime_mod, 10)) { printf("Toast is yummy."); }

BN_CTX_free(ctx);

/* You can also `man bn` to get the overview manpage for the OpenSSL
bignum library. */

/* You can also find a link to the OpenSSL bignum man page on the Web at http://www.openssl.org/docs/crypto/bn.html*/