5 Real Codes Used by Real Spies (And How to Try Them)

Real spies use codes. Not just the simple kind — secret codes have been used for thousands of years to hide messages that only the right person could read. Here are five that actually worked.

1. The Caesar Cipher

Julius Caesar — yes, the Roman general — used this one over 2,000 years ago. You shift every letter in the alphabet by a set number. If you shift by 3, A becomes D, B becomes E, and so on. “HELLO” becomes “KHOOR.” Simple but effective when nobody else knew the shift number.

🔑 Try it: Can you decode this? DJHQW ILYH (shift of 3, read backwards)

2. Invisible Ink

During World War One, spies used lemon juice to write messages. The writing is invisible when dry but turns brown when you hold the paper near heat. You can try this at home with lemon juice and a cotton bud. Write your message, let it dry completely, then hold it near a lamp.

3. The Pigpen Cipher

Used by the Freemasons and later by soldiers during the American Civil War, this code replaces each letter with a shape — a section of a grid or a V shape. You draw the grid, put letters in the sections, and your code looks like a strange series of angles and lines that only someone with the key can read.

4. The Book Cipher

Pick any book. Instead of writing the actual words of your message, you write a series of numbers. Each number refers to a page, a line, and a word in the book. Unless someone has the exact same edition of the same book, the message is unreadable. This one has been used by real spies as recently as the 20th century.

5. Morse Code

Not exactly a secret code — Morse translates letters into dots and dashes — but it has been used to send hidden messages in ways people could not intercept. POW prisoners in Vietnam tapped Morse code on walls to communicate without guards understanding. A US Navy officer blinked it in a message broadcast on live television.

Agent 5 uses codes in every mission. Which of these would YOU use to send a secret message? Let us know in the comments.