A data encryption apparatus operable to apply encryption to input data having at least a format identifying portion and a payload portion comprises means for discriminating between the format identifying portion of the input data and the payload portion of the input data; and means for selectively applying encryption to the input data in dependence upon an output of the discriminating means so as to encrypt at least a part of the payload portion of the input data but not to encrypt the format identifying portion. A corresponding decryption apparatus is also provided.