intro

Using Modular arithmetic:

encryption: $y=ax+b \text{ mod} 26$

decryption: $x=a^{-1}(y-b) \text{ mod} 26$

$a$ has to be Coprime with $26$ (i.e. $gcd(a,26)=1$ for $a^{-1}$ to exist

Key space is small, so Brute-force attack is very easy, and also frequency analysis. So it's not secure