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The Affine cipher is a type of monoalphabetic substitution cipher that employs modular arithmetic to encrypt the letters of a message. The encryption formula is c $=($a $*$ p $+$ b$)$ mod n$,$ where $\text{'}$p$\text{'}$ is the plaintext

letter, $\text{'}$c$\text{'}$ is the ciphertext letter, $\text{'}$n$\text{'}$ is the modulus $($usually the size of the alphabet, which is $26$ for English letters$),\text{'}$a$\text{'}$ and $\text{'}$b$\text{'}$ are keys, and $\text{'}$a$\text{'}$ must be chosen such that $\text{'}$a$\text{'}$ and $\text{'}$n$\text{'}$ are coprime $($relatively prime$).$

In this cipher, each letter of the plaintext is first converted into an integer: A$=0,$ B$=1,$ C$=2,$ and so on up to Z$=25.$