This is a simple [[Substitution Cipher]] where we replace the letter corresponding to the number $n$ in the *clear text* with the letter corresponding to the number $m=n+k \pmod{26}$ for some $k$, called the **key**, if we number each letter in a natural way: | A | B | C | D | E | ... | Z | | --- | --- | --- | --- | --- | --- | --- | | 0 | 1 | 2 | 3 | 4 | ... | 25 | The recipient of the message then simply does the reverse substitution $n=m-k \pmod{26}$ See [[Substitution Cipher]].