*/
static int
kex_method_diffie_hellman_group_exchange_sha256_key_exchange
(LIBSSH2_SESSION * session, key_exchange_state_low_t * key_state)
{
    int ret = 0;
    int rc;

    if(key_state->state == libssh2_NB_state_idle) {
        key_state->p = _libssh2_bn_init();
        key_state->g = _libssh2_bn_init();
        /* Ask for a P and G pair */
#ifdef LIBSSH2_DH_GEX_NEW
        key_state->request[0] = SSH_MSG_KEX_DH_GEX_REQUEST;
        _libssh2_htonu32(key_state->request + 1, LIBSSH2_DH_GEX_MINGROUP);
        _libssh2_htonu32(key_state->request + 5, LIBSSH2_DH_GEX_OPTGROUP);
        _libssh2_htonu32(key_state->request + 9, LIBSSH2_DH_GEX_MAXGROUP);
        key_state->request_len = 13;
        _libssh2_debug(session, LIBSSH2_TRACE_KEX,
                       "Initiating Diffie-Hellman Group-Exchange (New Method SHA256)");
#else
        key_state->request[0] = SSH_MSG_KEX_DH_GEX_REQUEST_OLD;
        _libssh2_htonu32(key_state->request + 1, LIBSSH2_DH_GEX_OPTGROUP);
        key_state->request_len = 5;
        _libssh2_debug(session, LIBSSH2_TRACE_KEX,
                       "Initiating Diffie-Hellman Group-Exchange (Old Method SHA256)");
#endif

        key_state->state = libssh2_NB_state_created;
    }

    if(key_state->state == libssh2_NB_state_created) {
        rc = _libssh2_transport_send(session, key_state->request,
                                     key_state->request_len, NULL, 0);
        if(rc == LIBSSH2_ERROR_EAGAIN) {
            return rc;
        }
        else if(rc) {
            ret = _libssh2_error(session, rc,
                                 "Unable to send Group Exchange Request SHA256");
            goto dh_gex_clean_exit;
        }

        key_state->state = libssh2_NB_state_sent;
    }

    if(key_state->state == libssh2_NB_state_sent) {
        rc = _libssh2_packet_require(session, SSH_MSG_KEX_DH_GEX_GROUP,
                                     &key_state->data, &key_state->data_len,
                                     0, NULL, 0, &key_state->req_state);
        if(rc == LIBSSH2_ERROR_EAGAIN) {
            return rc;
        }
        else if(rc) {
            ret = _libssh2_error(session, rc,
                                 "Timeout waiting for GEX_GROUP reply SHA256");
            goto dh_gex_clean_exit;
        }

        key_state->state = libssh2_NB_state_sent1;
    }

    if(key_state->state == libssh2_NB_state_sent1) {
        unsigned char *p, *g;
        unsigned long p_len, g_len;
        struct string_buf buf = { .len = 0, .offset = 0 };
  
        if(key_state->data_len < 9) {
            ret = _libssh2_error(session, LIBSSH2_ERROR_PROTO,
                                 "Unexpected key length");
            goto dh_gex_clean_exit;
        }

        buf.data = key_state->data;
        buf.dataptr = buf.data;
        buf.len = key_state->data_len;
        
        buf.dataptr++; /* increment to big num */
        
        if((p_len = _libssh2_get_bignum_bytes(&buf, &p)) <= 0) {
            ret = _libssh2_error(session, LIBSSH2_ERROR_PROTO,
                                 "Unexpected value");
            goto dh_gex_clean_exit;
        }
        
        if((g_len = _libssh2_get_bignum_bytes(&buf, &g)) <= 0) {
            ret = _libssh2_error(session, LIBSSH2_ERROR_PROTO,
                                 "Unexpected value");
            goto dh_gex_clean_exit;
        }
        
        _libssh2_bn_from_bin(key_state->p, p_len, p);
        _libssh2_bn_from_bin(key_state->g, g_len, g);

        ret = diffie_hellman_sha256(session, key_state->g, key_state->p, p_len,
                                    SSH_MSG_KEX_DH_GEX_INIT,
                                    SSH_MSG_KEX_DH_GEX_REPLY,
                                    key_state->data + 1,
                                    key_state->data_len - 1,
                                    &key_state->exchange_state);
        if(ret == LIBSSH2_ERROR_EAGAIN) {
            return ret;
        }

        LIBSSH2_FREE(session, key_state->data);
    }

  dh_gex_clean_exit:
    key_state->state = libssh2_NB_state_idle;
    _libssh2_bn_free(key_state->g);
    key_state->g = NULL;
    _libssh2_bn_free(key_state->p);
    key_state->p = NULL;