A method and apparatus for sending protected media data from a data source node to a client node via an intermediate node. The data source node establishes a first hop-by-hop key to be shared with the intermediate node and an end-to-end key to be shared with the client node. A single security protocol instance is configured and used to trans-protocol form data from a media stream into transformed data using the keys. The transformed data is then sent to the intermediate node. The intermediate node uses the first hop-by-hop key to apply a security processing to the transformed data, and establishes a second hop-by-hop key with the client node. A second transformation is performed on the transformed data using the second hop-by-hop key to produce further transformed media data, which is then sent to the client node. At the client node a single security protocol instance is configured with the second hop-by-hop key and the end-to-end key, which are used to apply further security processing to the transformed media data.