Encryption is the science of concealing messages. Historically, it was language-based, and involved tricks like substituting one letter for another—the way a cereal-box decoder ring does.

Today, encryption and the broader science of cryptology are rigorous, fundamentally mathematical disciplines. They can seem intellectually forbidding. Many of the basic ideas are surprisingly simple, though.

Here are some brainteasers inspired by ideas in modern cryptology. The stories in which they’re embedded are fanciful. But the concepts derive from real-world, modern cryptology.

Puzzle sequences:

• Breaking the Unbreakable Code
• The Maytruscans