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One of the simplest ciphers ever invented is the Substitution Cipher. It is probably also the easiest to crack.
A simplest substitution cipher is a system of encoding where one letter is replaced with another throughout a piece of text. As an example, "MISSISSIPPI" could become "MIXXIXXIPPI", replacing "S" with "X."
The most famous use of this type of cipher was during Roman times, when the commander of the Roman army, Julius Caesar had a problem. He needed to figure out how to order his troops from a distance, but make it so that if the enemy intercepted his messengers, it couldn’t understand the message. To do this, Caesar came up with a variation on what is now called the substitution cipher. Simply, he shifted the position of letters by three, substituting the resulting letter for the original.
Here is a guide. To use it, find the letter you want to encrypt on the top row. Then, trace down to the bottom row and substitute the letter you find.
For example, the message "ET TU, BRUTE", would become "HW WX, EUXWH".
However, by today’s standards (even last century’s standards), Julius Caesar’s messages were not safe at all. Using modern computer technology, messages encrypted with the Caesar Cipher can be cracked in matter of seconds.
In attempts to make their encryption schemes harder to break, mathematicians came up with several ways to make the simple substitution cipher harder break.
First, one can expand the cipher from simply shifting the placement of letters to random substitution. For example,
Second, you can create additional substitutions, based on multiple letters. Examples:
Original: CH SH ER
Cipher: J j p
So now, by combining the shift-three Caesar Cipher with multiple letter substitutions, the message "CHECK THIS MESSENGER TWICE" becomes
"J HFN WKLV PHVVHQJp WZLFH"
Of course, any creative person could come up with countless more variations of the substitution cipher, however these are the major ones.
You can visit our page specifically devoted to Julius Caesar here, or you can try out an applet we made, which lets you encrypt and decrypt text using the substitution cipher.