Abstract: Disclosed are a security processing system and method for HLS transmissions. An aspect of the invention provides a content key conversion device connected over a network to a content proxy device configured to provide encryption key information to a content operating device for a content received from a content provider device of an external network. The content key conversion device includes: a reception part that receives a double encryption key of a content from the content proxy device; an interface part that receives key decryption information corresponding to the double encryption key from an encryption key provider device of an external network; a decryption part that decrypts the double encryption key of the content using the key decryption information and thereby converts the double encryption key to an encryption key; and a transmission part that transmits the encryption key converted by the decryption part to the content proxy device.