A
substitution cipher that may consist of letters, symbols, or a mix of them. Also every character in the
cipher alphabet is associated to one character of the
plain alphabet throughout the entire
ciphertext. This is also often called a
simple substitution cipher.
The problem with messages encrypted with a monoalphabetic substitution cipher is that it is very easy to break using frequency analysis.
For example:
Plain 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
Cipher V X B G J C Q L N E F P T Z Y W U S R O M K I H D A
The Caesar shift cipher is another example.