As I understood the paper, blocks MA and MB can have arbitrary contents as long as possessing the following block (MA' or MB' respectively) is required for deco

As I understood the paper, blocks MA and MB can have arbitrary contents as long as possessing the following block (MA' or MB' respectively) is required for decoding. An eavesdropper cannot then substitute the contents of either MA or MB without either breaking the decoding process or dropping portions of the conversation. Either way, the eavesdropper cannot fully conceal their presence.