Caesar Shift

The Caesar shift is a symmetric substitution cypher that slides every letter by $3$ places alphabetically in order to obtain the cyphertext and slides every letter back by $3$ places in order to obtain the plaintext. In other words, for every letter $l_{p}$, in order to obtain the cyphered letter $l_{c}$, you do:

\begin{eqnarray*} l_{c} &=& (l_{p}+3)\mod{26} \end{eqnarray*}

In order to obtain the plaintext letter $l_{p}$, the reverse operation can be performed:

\begin{eqnarray*} l_{p} &=& (l_{c}-3)\mod{26} \end{eqnarray*}