Codes & Ciphers
📋 What it is
A cipher hides a message by swapping letters using a rule; knowing the rule lets you decode it.
🗣️ Coach says
A cipher is a secret rule for scrambling letters. The Caesar cipher shifts each letter a fixed number (shift 3: A→D, B→E). Crack it and you can read any message written with that rule — the rule IS the key.
🧠 Memory hook
Find the rule (the shift) and you hold the key to every message.
😲 Whoa!
Julius Caesar really used a shift cipher 2,000 years ago to send military orders — and the same idea, made far stronger, protects your messages and passwords online today.
✅ Quick check: With a Caesar shift of 1 (A→B), decode “IFMMP”.
Say your answer out loud first — then reveal.
“HELLO” — shift each letter back by 1: I→H, F→E, M→L, M→L, P→O.
Reversing the known shift rule decodes the message exactly.
🪄 Trick question: If you don’t know the shift number, is the message unreadable forever?
Careful — think it through, then reveal.
No — there are only 25 possible shifts, so you can just try them all (a “brute-force” crack) until it reads.
A small key space means every option can be tested; that’s why real ciphers use huge keys.
🧪 Try it! (2 minutes)
Write your name with a shift-of-2 cipher, then have a friend decode it using the rule you tell them.