Have you ever sent a hidden message to someone? You probably have, in one form or another. Maybe you created a secret language as a kid or passed notes in class written in invisible ink. Today, you might use a common secure messaging service like iMessage.
If any of these sound familiar to you, congratulations! You’ve used cryptography.
What Is Cryptography?
At its most basic, cryptography is the practice of concealing messages from anyone besides the intended recipient. This includes everything from hiding messages in plain sight to scrambling the contents of a message so that no one without the secret key can understand the message.
While today’s applications are fairly high-tech, cryptography has a surprisingly long history that stretches back to ancient times.
One aspect of cryptography is steganography, the practice of hiding a message within an object or another message. The earliest record of steganography can be traced back to 440 BC when the ancient Greeks used it to send secret messages.
In his famous book Histories, Herodotus mentions two methods the ancient Greeks used to send and receive messages in secret. According to one account, the general Histiaeus shaved the head of a servant, wrote a message on his bare scalp, then waited for the hair to regrow before sending him to Aristagoras with instructions to shave the servant’s head and look at it.
In a second account, Demaratus sent a message warning the Greeks that the Persians were planning to attack Sparta. To prevent the message from being intercepted, Demaratus inscribed it into the wooden backing of a wax tablet before applying the tablet’s beeswax surface. The message slipped right past the Persian sentries, and the Greeks were warned in time.
Besides hiding messages in everyday objects, the ancient Greeks also developed a tool for writing and sending secret messages. The scytale is a small cylinder with a narrow strip of parchment or leather wrapped around it. The sender writes their message vertically down the strip, and the message is illegible—that is until it is wound around the corresponding decryption tool. All the recipient has to do is wrap the inscribed strip around their scytale to decode the message. The ancient Greeks commonly used this method to communicate during military campaigns.
One of the most famous examples of cryptography is Caesar’s cipher. Named after the famous Julius Caesar, Caesar’s cipher is an encryption technique in which each letter of a message is shifted to a certain number of places in the alphabet to correspond to another. For example, with a left shift of 7, the letter “H” becomes “A”.
The method is simple but effective. The word “zljyla” looks like gibberish, and without the secret key, we might never know what it says. Only after decrypting it using Caesar’s cipher can we read the real message: “secret”.
Let’s give it a try! Here’s a message encrypted with a left shift of 7. See if you can decode it:
“Lujyfwapvu pz jvvs!”
Did you figure it out? The answer should be pretty obvious based on the subject of this post, but if you didn’t get it—don’t worry! You can use this tool to automatically decrypt it.
Next Up for Cryptography: Encryption Fundamentals
This is just a brief introduction to a fascinating topic, and we’ll cover all this and more in my new Encryption Fundamentals course. If you’re new to AWS in general and interested in security, you can also check out our AWS Security For Beginners tips.
If you have any questions about the course, cryptography, or security in general, feel free to get in touch on the Linux Academy Community Slack!
I hope you enjoy the course. Happy learning!