If what you are communicating needs to be guaranteed to never be crackable, you only need to use the oldest and most secure way of communicating before the days of internet and computers: short wave transmitted one time pad codes.

The reason that number stations continue to operate is that it really is the best way to communicate/order under surveillance which almost certainly is happening in every embassy in non-friendly countries.

One time pads are truly unbreakable and it's techniques have been refined for the past 80 years since Cold War began.

Luckily you don't need a shortwave radio anymore and the CIA I believe utilizes emoticons. Even if you intercept a bunch of smiley faces followed by a wink, how do you figure out the contents of the message when the medium to decode is completely unknowable?

It was only when they captured a few Cuban spies were they able to break the number stations operating out of Havana but this is perhaps the only case in history.