According to an embodiment, the invention provides a method for decrypting content, the comprising: receiving the content without a source encryption key from a source device connected to the electric reproducing device, the content having been encrypted with the source encryption key in the source device; performing a first addition operation by using a first device internal key and an ID, the first device internal key being associated with the electric reproducing device; generating a device encryption key based on an output of the first addition operation and a second device internal key by using a predetermined encryption algorithm, wherein the second device internal key is associated with the electric reproducing device; decrypting the content using the device encryption key; decoding the decrypted content; and outputting the decoded content.