A key management apparatus receives a key request including a first device identification information and a second device identification information, encrypts a common key using the first device identification information to generate a first encrypted common key, encrypts the common key using the second device identification information to generate a second encrypted common key, and transmits a key response including the first encrypted common key and the second encrypted common key. A first device receives the key response, decrypts the first encrypted common key using the first device identification information to obtain the common key, and transmits the second encrypted common key. A second device receives the second encrypted common key and decrypts the second encrypted common key using the second device identification information to obtain the common key.