Suppose we want to encrypt the plaintext message (with
spaces removed)
fourscoreandsevenyearsago
In a simple substitution cipher we would
replace each letter by another letter of the alphabet,
according to a fixed key. For example, we could use the
following key.
| 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 | d | e | f | g | h | i | j | k | l | m | n | o | p | q | r | s | t | u | v | w | x | y | z | a | b | c |
Using this key, the message above
becomes the ciphertext is
irxuvfruhdqgvhyhqbhduvdjr
Anyone who knows the key in the table above can easily recover the plaintext. This version of the simple substitution is known as the Caesar's cipher, since Julius Caesar used it to send encrypted messages.