A method for verifying the integrity of data in a message by a data processing device, the message comprising a plurality of packets, the method comprising: receiving, at the device from a first resource, a manifest associated with the message, the manifest comprising a plurality of group check values for the plurality of packets; receiving, at the device, from the first or a different resource, the message; generating a first progression of rolling hashes for the plurality of packets; deriving group check values from the first progression of rolling hashes for groups of the plurality of packets along one or more paths; verifying the integrity of the data in the message based on or in response to a determination that the derived group check values correspond to the plurality of group check values in the manifest.