The Spartans had a system of wrapping a message around a rod so that it showed letters exposed by certain sections. The recipient would have an identical one.
It was supposed to be pretty good but it doesn’t sound very sophisticated to me.
You'd think it would take a reasonably intelligent person no more than 10 minutes to work out the possible combinations of letters and be able to read the message.
another antique method was to shave the head of a trusted slave, write the entire message on the scalp, and wait until the hair grew back before sending them on their way.
Those were not time-critical messages, of course.