what should I do when I want get share key in pkey DH?

[TanyaWong]

I create a new objection whice type is DH, and I got priv_key,pub_key,p, g, Now, I want to get a share key ,what show I do? thanks

[daurnimator]

Is this a request for a bn:mod_exp function so that you can efficiently compute a shared secret?

[TanyaWong]

I want user my priv_key and Other people's pub_key to calculate shared secret ，It's not clear if it has anything to do with bn：mod_exp

[daurnimator]

I'm not sure why you're trying to do this manually.
However, take a look at the general algorithm at https://en.wikipedia.org/wiki/Diffie%E2%80%93Hellman_key_exchange#Secrecy_chart
The shared secret is calculated with s = (B^a)%p --> this is a mod_exp operation (which you can do explicitly. but will be much faster with the BN_mod_exp function in PR #117)

[TanyaWong]

I search openssl C source code, and I am puzzled by how to call function DH_compute_key in luaossl? thanks

[daurnimator]

DH_compute_key is mostly just a call to bn_mod_exp. https://github.com/openssl/openssl/blob/532b11830c365fd50a6c27e4933935a8126d91ad/crypto/dh/dh_key.c#L196

[TanyaWong]

can you give me some examples about acquired shared secret which use lua? thanks

[TanyaWong]

thanks very much , problem has been solved

[daurnimator]

problem has been solved

How? What did you do?

[TanyaWong]

I used two method ：

1. Add bn_mod_exp function in bn_methods , it's same as you gave me

 3157 static int bn_mod_exp(lua_State *L) {
 3158     BIGNUM *r, *a, *b, *c;
 3159  
 3160     lua_settop(L, 3); 
 3161     bn_preptop(L, &r, &a, &b, &c);
 3162      
 3163     if (!BN_mod_exp(r, a, b, c, getctx(L))) {
 3164         return auxL_error(L, auxL_EOPENSSL, "bignum:mod_exp");
 3165     }
 3166      
 3167     return 1;
 3168 }    
 3169      
 3170 static const auxL_Reg bn_methods[] = {
 3171     { "add",       &bn__add },
 3172     { "sub",       &bn__sub },
 3173     { "mul",       &bn__mul },
 3174     { "sqr",       &bn_sqr },
 3175     { "idiv",      &bn__idiv },
 3176     { "mod",       &bn__mod },
 3177     { "nnmod",     &bn_nnmod },
 3178     { "exp",       &bn__pow },
 3179     { "mod_exp",   &bn_mod_exp },

2. Add pk_dhComputeKey function to call DH_compute_key in pk_methods, In order to I can more easy to call .

 4528 static const auxL_Reg pk_methods[] = {
 4529     { "type",          &pk_type },
 4530     { "setPublicKey",  &pk_setPublicKey },
 4531     { "setPrivateKey", &pk_setPrivateKey },
 4532 #if HAVE_EVP_PKEY_CTX_NEW
 4533     { "decrypt",       &pk_decrypt },
 4534     { "encrypt",       &pk_encrypt },
 4535 #endif                  
 4536     { "sign",          &pk_sign },
 4537     { "verify",        &pk_verify },
 4538     { "getDefaultDigestName", &pk_getDefaultDigestName },
 4539     { "toPEM",         &pk_toPEM },
 4540     { "getParameters", &pk_getParameters },
 4541     { "setParameters", &pk_setParameters },
 4542     { "dhComputeKey", &pk_dhComputeKey },
 4543     { NULL,            NULL },
 4544 };       

[daurnimator]

Did you want to send in a PR for pk_dhComputeKey?