I remember a meme years ago that described how the type, color, number, and arrangement of flowers in a bouquet could be used to convey complex messages. I’ll have to scroll through my hoard, but I’ll post it if I find it.

But, honestly, as long as there exists a code that is shared between dissenters, and ways to represent discrete elements of that code, communication itself is easy. You could encode anything in a medium that can represent a binary state, like lining up shiny pebbles on your windowsill: two close to each other for a binary 1, or a single one for binary 0. Or you could represent messages by how you hang your laundry to dry. Or embed it into how you play a musical instrument. You could hang Christmas ornaments in your window and use any obvious property to represent a message. You could use a rudimentary radio transmitter to send messages through radio noise. The options are limitless.

The difficult part is maintaining operational security. The limitations presented by human mental capabilities means that a very simple pre-shared secret must be used, which may be leaked or deduced. You have to know where to look, how to decode or decrypt the message, how to respond, and how to do all of that covertly. Prisoners have successfully used hand motions while cleaning their cell window to convey messages outside, apparently for years before authorities caught on.