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.

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