It's 46 BC. Caesar is powerful. He does however have severe issues to tackle. How will he send orders to his General to the other side of his empire? He may write down a message and ask a trustful man to deliver it... but is he really trustworthy or will he reveal the orders to the enemies? What if the enemies killed his man and stole the message? 

Caesar comes up with a cunning solution (probably it's a courtier's idea, but that’s another story ...). He encryptes the message!

Each letter is replaced by another letter which is 3 places after in the alphabet. So, instead of the letter A, Caesar writes the letter D, instead of B he writes E etc.

https://en.wikipedia.org/wiki/Caesar_cipher

Let's say, for example, that Caesar wants to write the word DAY:

• D shifts 3 places forward in the alphabet and becomes G
• A becomes D,
• Y becomes B

and instead of DAY Caesar writes GDB to the message to his General!

When his General receives the message, he knows what to do to read it, exactly the opposite procedure: Replaces every letter with the one 3​​​​​​ places before in the alphabet: 

• G shifts 3 places backwards and becomes D,
• D becomes A
• B becomes Y

https://en.wikipedia.org/wiki/Caesar_cipher

Instead of GDB that was written on the message, the General reads correctly the word DAY! But if someone stole the message and did not know the trick, GDB means nothing.

Decode a message by replacing each red letter with the corresponding green:

---

If you find Caesar cipher interresting, you may watch this video: