* more generally could use a more complex equation to calculate the ciphertext letter for each plaintext letter

E(a,b) : i ->a.i + b mod 26

a must not divide 26 (i.e., gcd(a,26) = 1)

otherwise cipher is not reversible, e.g.,
a=2 and a=0, b=1, c=2 ... , y=24, z=25
eg E(5 7) : i ->5.i + 7 mod 26
Cryptanalysis

* use letter frequency counts to guess a couple of possible letter mappings (frequency pattern not produced just by a shift)
* use these mappings to solve 2 simultaneous equations to derive above parameters