Abstract: An authentication method is provided in which a first portable device generates and transmits a first random number and a first timestamp to a first USIM in the first portable device; the first USIM calculates a first sign for the first portable device; the first portable device requests authentication for authenticated communication from a second portable device through transmission of the first random number, the first timestamp, and the first sign to the second portable device; the second portable device generates a second random number and a second timestamp and transmits the information to a second USIM in the second portable device; the second USIM generates a second sign for the second portable device and a second personal key which the second portable device transmits to the first portable device; the first portable device then transmits the information to the first USIM which generates a first personal key for authenticated communication.