Data encryption/decryption method and monitoring system

A data encryption/decryption method performed in a monitoring system including a distribution apparatus which encrypts continuous data and distributes the encrypted continuous data via a network, a reproduction apparatus which decrypts the encrypted data distributed via the network to reproduce the continuous data, and a key management apparatus which has a key management database,

• wherein the distribution apparatus obtains a key number correlated with the distribution apparatus and key information correlated with the key number from the key management apparatus, encrypts data with using the obtained key information, and distributes the encrypted data with the obtained key number, and
• the reproduction apparatus transmits the key number appended to the encrypted data to the key management apparatus, obtains key information correlated with the transmitted key number, and decrypts the encrypted data with using the obtained key information.