An encoder for encoding an MPEG transport stream of MPEG sections comprises means for processing the MPEG sections using a hash function to provide multiple hash outputs each hash output being associated with one or more MPEG sections. The hash outputs and corresponding section identifying information are signed using a private key corresponding to a public key available at receivers to produce signatures. The hash output section identifying information and signatures are then coded into one or more MPEG sections themselves which are designated as signature MPEG sections. The signature MPEG sections and MPEG transport stream are broadcast together. A receiver is able to verify using the signature MPEG sections that the MPEG transport stream has been provided by a known source and has not been tampered with.