A high-speed Galois Counter Mode-Advanced Encryption Standard (GCM-AES) block cipher apparatus and method is provided. The apparatus can operate at a low clock frequency of 125 MHz and provide a 2 Gbps link encryption function in an Optical Line Termination (OLT) and an Optical Network Unit (ONU) of an Ethernet Passive Optical Network (EPON). 11-round block cipher of 128-bit input data is implemented using an 8-round Counter-AES (CTR-AES) block cipher module and a 3-round CTR-AES block cipher module, so that it is possible to provide a 1 Gbps link security function for an input frequency of 62.5 MHz and a 2 Gbps link security function for an input frequency of 125 MHz.