OpenSSL used fixed Values for Diffie Hellman Key g

Question

According to that piece of documentation: https://wiki.openssl.org/index.php/Diffie_Hellman#Using_the_Low_Level_APIs

Using the Low level API's for Diffie Hellman (need to perform a group Key agreement).

For simplicity I need to provide fixed values for Diffie Hellman p and g values for now I use the function DH_generate_parameters_ex but any solution using these options may add a communication overhead and there are fixed values for p and g for Diffie Hellman offering good security.

So using the approach convention over configuration, how I can set fixed values especially the ones specified in this RFC to openssl low-level api fore diffie hellman instyead of generating ones on the fly?

PS I use the OpenSSL version 1.0.2g.

Answer 1

The (outer) primes for the RFC3526 and RFC2409 groups are builtin, per this man page (should also be on your system under those names if 1.1.0+) -- they are actually in the code back to before 1.0.0 but without the BN_ prefix (though in the bn.h header) and previously undocumented. (In 1.1.0+ the old names are additionally #define'd if compatibility is set.)

AFAICS you must add the generator yourself, something like:

DH *dh = DH_new(); BIGNUM *two = BN_new(); 
if( !dh || !two ) /* error */;
BN_set_word(two,2); 

// corrected AGAIN!
DH_set0_pqg (dh, BN_dup(BN_get_rfc3526_prime_2048(NULL)), NULL, two);

// added: below 1.1.0 many API structs were not opaque, just
dh->p = BN_dup(/*not BN_*/ get_rfc3526_prime_2048(NULL));
dh->g = two; 
// leave q as unspecified

Note RFC5114 modp parameters are available prebuilt in DH* form but only in 1.1.0+.