How did Julius Caesar write secret messages?

0:47 Welcome to Matt's Is Not Boring by kidopoly.com. I'm Mira! And I'm Finn! If you haven't already, tap follow wherever you're listening so you never miss an episode. Now, Finn, I want you to imagine you are a Roman general standing in a dusty camp about 2,000 years ago. A messenger gallops up on a tired horse and hands you a piece of parchment.

1:12 Okay, I'm picturing it. I'm unrolling the parchment to read the secret battle orders from the Emperor. But when you look at it, the letters are completely broken. It's just a jumbled mess of letters that don't make any words at all. Wait, did the messenger drop it in a puddle? Or did the person writing it just forget how to spell?

1:32 Neither! You are holding one of the most famous secrets in history: a message hiding in plain sight, using a trick so clever that it started a race that is still going on right now, hidden inside your pocket. Inside my pocket? You mean like my phone? Exactly! Today we are looking at Julius Caesar's secret messages, and how one brilliantly simple idea protects almost everything we send to each other today.

2:00 So, let's travel back to ancient Rome, about fifty years before the year one. Julius Caesar is leading massive armies across vast distances. And if you're leading an army, you have to send messages to your other generals. Right, but messengers got caught by enemies all the time. If an enemy read your plans, you could lose the whole battle.

2:24 That's really bad. So he had to hide the message, but still write it down. How do you actually do that? He used something we usually call a code. But actually, a mathematician would tell you there's a strict difference between a code and a cipher. What's the difference? They sound like the same thing.

2:44 A code is when you swap whole words. Like saying, the eagle has landed, when you really mean we arrived safely. Oh I love those. Like a secret spy language. Exactly. But a cipher is different. A cipher swaps individual letters. So we have code in our title today because people use that word all the time.

3:08 But what Caesar actually invented was a cipher. So he jumbled up the letters themselves. But how do we know he did that? We know because of a Roman writer named Suetonius. Writing over a century after Caesar lived, Suetonius tells us exactly how this famous leader locked his messages so no enemy could read them, even if they were staring right at the parchment.

3:34 Suetonius recorded that when Caesar had something secret to say, he shifted every letter three places further along the alphabet. Wait, let me picture that. So if he wanted to write the letter A, he'd count three spots forward. A goes to B, C, D. Yes! A became D, and B became B goes to C, D, E. B became E.

4:04 You've got it! That is the Caesar shift. You just slide the whole alphabet over by three. If you don't know the trick, it looks like complete nonsense. Wait, hold on. That works for the start of the alphabet. But what happens when you get to the end? What does Z turn into? You run out of letters. I love that you notice that.

4:26 You don't run out. The shift wraps right around to the start of the alphabet again. Oh, like a clock. When you go past twelve, you don't run out of hours. You just go back to one. Exactly! It's a huge, beautiful mathematical idea in disguise. Wrapping around in a circle. So Z becomes C. That is so cool.

4:49 It's like the alphabet is drawn on a spinning wheel. Yes, and for Caesar, this trick worked beautifully. To anyone without the key, his important letters just look like total gibberish. But the idea didn't stop with Caesar. His great nephew, who became the Emperor Augustus, used an even simpler version of the trick.

5:10 What did Augustus do? Did he shift it by five? Actually, he shifted it by just one letter. But Suetonius tells us Augustus didn't wrap around at the end like Caesar did. So A becomes B, and B becomes C. But wait, that seems a bit too easy for an enemy to guess. It was, and that's where the real drama begins.

5:35 Because Caesar's cipher is brilliant, but once you know the trick, it's very easy to crack. Oh no, so the enemy started figuring it out. Exactly. It started a 2,000-year race between codemakers and codebreakers. Every time someone invented a harder way to jumble the letters, someone else found a clever mathematical way to unjumble them.

5:59 Like a gigantic game of hide-and-seek that lasted forever. That's a perfect way to describe it. The mathematics got deeper and deeper. It went all the way up to the Second World War in the 1940s. What happened then? At a secret place in England called Bletchley Park, brilliant codebreakers, including a gentle mathematician named Alan Turing, worked day and night.

6:24 Did they use the Caesar Shift? The enemy ciphers they were fighting had grown vastly more complicated than Caesar's shift. They were so difficult that Turing and the others had to build huge, extraordinary machines just to crack them, and that race never actually stopped, Finn. The codebreakers and codemakers kept going, right into the world we live in today.

6:48 Wait, but I don't use secret ciphers. I just play games and watch videos. You actually use them every single day. Today, mathematics, far stronger than Caesar's, protects almost everything we do online. It's called modern encryption. Encryption? Like locking things in a vault? Exactly. Every text message, every photo you send, every video call you make from a phone is locked up tight by modern encryption.

7:21 So when I video call Grandma, my phone is jumbling up the picture. Yes. Your phone jumbles it into completely unreadable nonsense before it travels through the air. And Grandma's phone has the exact mathematical key to unjumble it perfectly on the other side. So nobody else can see it. That's incredible.

7:43 We're all sending secret messages all the time. It really is. And here is a fun fact about Roman messages. The Romans didn't even have the letter J or U in their alphabet. Julius Caesar would have spelled his own name starting with an I. And people have loved secret writing for so long that we have clay tablets from ancient Mesopotamia, over 3,000 years ago, with secret disguised writing pressed right into the clay.

8:17 Alright, Finn, it's time for the quiz. Are you ready? I'm ready. Let's see if I can crack this. First question. What was the name of the Roman writer who recorded how Caesar hid his messages? Next one. How many places along the alphabet did Julius Caesar shift his letters? Question three. Which relative of Caesar used a simpler shift of just one letter?

8:54 Here's number four. What was the name of the secret place in England where codebreakers worked during the Second World War? Last question. What do we call the powerful mathematics that locks up our phone messages and video calls today? Great job, let's see how you did. The first answer is Suetonius. He wrote it down over a century later.

9:25 For question two, three. Caesar shifted his letters three places, so A became D. Number three, the answer is Augustus, he was Caesar's great nephew. Question four, that was Bletchley Park. And the final answer is modern encryption. It keeps all our photos and messages safe. You know, I'm never going to look at my phone the same way again.

9:53 It's doing all that amazing maths, wrapping letters around millions of times a second. Just to keep your video calls safe. It's a wonderful secret world, isn't it? It really is. If you love discovering this secret with us, please leave us a five-star review. You can just scroll down on your app and tap the stars.

10:14 It really helps other codebreakers find the show. And don't forget to follow us wherever you're listening, so the next episode just appears on your device. Oh, and we have sister shows too. History, science, and philosophies not boring. Go give those a listen and a follow as well. Want your name on the show?

10:33 Send us an email at hello at kidopoly.com. We love hearing from you. Thanks for exploring with us. See you next time on Maths is not boring. Bye.