Usually you make a rule. Let's say we have a syntax of +5 letters on the alphabet. So A becomes F. We can make this more complex by using variables, multiplying, division, and rpunding to whole numbers. Lets make a new rule, a=28+6÷(95/6) round that sh!t to the nearest whole number and account for over/underflow. I'm too lazy to do that right now, but whatever. People decode things like this, because usually there are patterns, and neural networks can teach themselves how to decode things like that.