The embodiments of the present disclosure disclose a method and apparatus for reducing the parameter transmission bandwidth. The parameter sender reduces the values of the parameters before sending the parameters to the parameter receiver. This scheme reduces the bandwidth consumed during parameter transmission, thus makes the transmission more efficient. The embodiment of the present disclosure also discloses a method for key exchange. This method reduces the values of the transmission parameters before sending the transmission parameters. This saves the bandwidth compared with the protocol in the prior art. Besides, the embodiment of the present disclosure discloses a system for key exchange. The parameter sender sends the transmission parameters to the bandwidth processing unit. The bandwidth processing unit performs a modulo operation on the received transmission parameters and then sends the processed transmission parameters to the parameter receiver, thus reducing the bandwidth consumed in the transmission of transmission parameters.