A method for data processing, comprising providing a sequence of image frames that is encoded by identifying intra and inter frames in the sequence and applying a variable block size motion compensation (VBSMC) procedure to the inter frames, thereby generating respective parameters representing the inter frames; selectively encrypting the block sizes, using an encryptor, without encrypting all of the parameters representing the inter frames; and outputting encoded data representing the sequence of the image frames and comprising the encrypted block sizes.