As kid’s, we used to write a dummy message on the faceside of the letter and on the backside we wrote the real message in secret ink (lemon juice).
The recipient, then used heat (mom, can I borrow a candle) to de-cypher the real message on the backside.
To be extra secure we encrypted the real message using our encoder/decoder rings (free from a well known cereal manufacturer) and a key which the recipient had to find by trial and error before the message would make sense.
Double secure. You bet. Fun. You bet.
Sorry, no Internet or TV...4th graders had to do something besides build model planes and play Monopoly back then. Maybe Jimmah is having a flashback or something.
BTW, when I was a 5th grader and began my SWL (short wave listener) hobby, I sent a reception report and a request to Radio Bulgaria for a QSL card and a program schedule.
One day there was a knock at the door, mom answered (we were told to never answer the door if a parent was present), and there was a tall stranger dressed in a dark business suit and looking like your typical Jehovah Witness or LDS visitor.
The stranger asked if I was home and he needed to speak with me. Mom was quite perplexed about this. Turned out I was a kid and he was from the FBI (Holy sh*t Batman!). To make a long story short, I showed him my shortwave radio and QSL card collection and then he left. Mom wanted to know what I was doing on my radio (No porn back then) to warrant a visit from the FBI. I was able to convince her I was not doing anything wrong and I got to keep my radio (parents used to take things away back then).
Years later while undergoing a security clearance check for an aerospace contractor, I was questioned about the “spying” incident. Seems I had an FBI file and had been under surveillance for spying for an iron curtain country.
Spying by the government on the general public is nothing new.