Function step2 

pub fn step2(
    data: Step1,
    response: ResPq,
    dc_id: i32,
) -> Result<(ReqDhParams, Step2), Error>

Process ResPQ and generate req_DH_params.

dc_id is only used if the dc-tagged constructor is requested via step2_dc_tagged. This function uses the plain p_q_inner_data.