Today’s topic is a classic - the Caesar cipher. I love this topic for many reasons, but particularly because it gives me a chance to add a history lesson into the cyber realm. It serves as another example of the interdisciplinary nature of cybersecurity.
The Caesar cipher, also called a shift cipher, dates back to…you guessed it, the days of the Roman Caesars. That means they are over 2,000 years old. With all of the intrigue among the Roman senate and nobility, they had to pass messages in some way that if the messages were intercepted they could not be understood. This is what the shift cipher did for them. Here is how it works.
Write out the letters of the alphabet in a line. Next to that line of letters, you write out another line of the alphabet but offset by some number of letters, known as the shift. Here’s an example to illustrate using a +6 character shift.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
G H I J K L M N O P Q R S T U V W X Y Z A B C D E F
Plain-text message: I LOVE MY PUPPY DOGS
Encrypted message: O RUBK SE VAVVE JUMY
As long as the recipient knows how many letters to shift the cipher, the sender and receiver can exchange messages without fear of eavesdropping. The shift can be any number of letters to the left (-) or the right (+). Shift ciphers can be used in any language and are simple to use. They have an Achilles heel, however. They are easily crackable due to the frequency distribution of letters.
Wheel of Fortune displays the frequency distribution of letters very nicely. Children in upper elementary school know the trick. Always guess letters like S, T, R, N, E, and A first because they appear the most often in words. That makes it easier to figure out the rest of the letters in each word. We also know that double letters show up in very specific ways so the appearance of double letters in shift cipher messages are also a valuable clue.
The shift cipher is probably the second cipher many of us learn. For most of us pig latin, which is a substitution cipher, was the first. Both of these are easy to learn and to solve. They still serve a great purpose. They get puzzle geeks like me hooked on codes and working in cybersecurity.
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