Hail Ceasar

A very old encryption method is the Caesar Cipher. 

For testing purposes, usually the ASCII code is used. But you can add a little more security to your message. For example give the letters random numbers, or use some fake numbers in between to confuse the spy.

 

Rules: Shift all letters by 5

Plain Text: Attack at midnight.

Apply the rules.

Cipher text: Fyyfhp fy rnisnlmy.

 

The main problem is that same letters are the same after encryption.

An attacker (f/m/d) could find some letters due to their “letter frequency”. The letter frequency varies in different languages, books, special texts or in dictionaries. Some recent child names increase the rare letters.

A letter frequency diagram:

 

Now you could use a letter for the “space” between the words, or shift the second word by increasing with one.

 

Rules: space = R, “shift-start” with 5 and increase by 1 each word.

Plain Text: Attack at midnight.

Now with the new rules.

Cipher text: FyyfhpRgzRtpkupnoa.

 

Next step: Transfer the “key” to your colleague. In case your name is Bob, your mate is maybe called Alice. Those are the most used names in illustrations. A and B….

The main problems with a symmetric algorithm is that you decode and encode with the same key and you need to send the key together with the message.

Or a second way: The security level of your “key” depends on the safe transmission of your key to a friend. An evil dude should not take a look, in case you send the key with a “secure” postcard or letter. 

A real help gives us an asymmetric cryptography algorithm. But that’s an other story.