Ah Matryoshkas, who doesn't like these Russian nesting dolls? But why should the fun of chimeric nesting be limited to just application formats? It is possible to design PHY-layer digital modulation protocols that (1) are backward compatible with existing standards and (2) discretely contain additional information for reception by those who know the right tricks. When properly designed, these polyglot protocols look and sound much like the older protocols, causing an eavesdropping Eve to believe she has sniffed the contents of a transmission when in fact a second, hidden message is hitching a ride on the transmission. Mallory, on the other hand, may use these protocols-in-protocols to smuggle long Russian stories to all who will listen!
This fine technical lecture by two neighborly gentlemen describes techniques for designing polyglot modulation protocols, as well as concrete examples of such protocols that are fit for use in international shortwave radio communication.