The Caesar cipher is a substitution cipher that shifts each letter in the message by the value of the key. For example, if the message was “send help” and the key was 3 then the encrypted message would be “vhqg khos”.
As it is a substitution cipher it is vulnerable to common decryption cracking methods such as frequency analysis meaning it provides virtually no security for communication. Even without the key, using a brute force attack on the encrypted message can be broken within 25 attempts. This is because there are only 26 characters in the alphabet meaning that there are only 25 different possible key shifts. Using a key of anything over 25 would provide an output than has already been tested.