An encryption method and apparatus thereof is disclosed. In one embodiment the method includes (a) receiving the content data via the network, (b) changing a sequence of the received content data according to a predetermined algorithm or randomly to store in a form of a cache file, (c) generating a decryption key including information of the changed sequence of the data, and (d) encrypting the decryption key according to a predetermined encryption system, wherein if a playback or transmission of the stored content is requested, the encrypted decryption key is decrypted and the corresponding data is extracted from the cache file according to the sequence information of the decryption key to be played back or transmitted. According to one embodiment of the invention, a user is unable to illegally use content data stored in his local client and content data stored in a local client can be quickly decrypted so as not to affect playback of the content data.