A digital data file encryption system and method where digital data encrypted to a high level and an encryption key are received and stored in a computer. The stored, encrypted digital data is decrypted on the basis of the encryption key and stored in a buffer of a predetermined size. Output digital data from the buffer is reencrypted to a low level of encryption and downloaded to a digital data playing device or a data storage medium. Therefore, a digital audio or video data file is prevented from being diverted in unencrypted form from a computer communication network when it is transmitted to a personal computer through the computer communication network and, in turn, downloaded to the digital data playing device. Further, the digital data playing device is prevented from rising in cost due to the use of simple encryption for data downloaded to the playing device.