codebreaking

the code is binary.

that's kinda a low-key wild question to try to answer.

sudoku is a code.

acrostics were commonly used. you'd write a pamphlet and distribute it and the person receiving would know what kind of pamphlet to look for and whether it was the first letter of each word or line or whatever. You break that code, you get all the messages.

Harder codes to break are the fully private, never-written-down codes. Say within a couple. The only weak point is one of that couple being broken.

Codes can be super easy to create, based on cultural knowledge. If I say 'that guy did a Hellmuth' that's not something that a non-gambler would know, but a gambler has the cultural context to place that. That sort of code can work on multiple levels. As a fascinating example, members of the Umbrella revolution movement in Hong Kong have created a multi-layered hybrid code language that draws on cantonese and english and mixes sayings in such a way as to sound like they're making total sense in one context but they mean something else. It's like a private slang, but taken to the point of being a code used to communicate. The context is what tells you the score.