module vigenere =
    let keyschedule (key:string) =
        let s = key.ToUpper().ToCharArray() |> Array.filter System.Char.IsLetter
        let l = Array.length s
        (fun n -> int s.[n % l])

    let enc k c = ((c + k - 130) % 26) + 65
    let dec k c = ((c - k + 130) % 26) + 65
    let crypt f key = Array.mapi (fun n c -> f (key n) c |> char)

    let encrypt key (plaintext:string) =
        plaintext.ToUpper().ToCharArray()
        |> Array.filter System.Char.IsLetter
        |> Array.map int
        |> crypt enc (keyschedule key)
        |> (fun a -> new string(a))

    let decrypt key (ciphertext:string) =
        ciphertext.ToUpper().ToCharArray()
        |> Array.map int
        |> crypt dec (keyschedule key)
        |> (fun a -> new string(a))

let passwd = "Vigenere Cipher"
let cipher = vigenere.encrypt passwd "Beware the Jabberwock, my son! The jaws that bite, the claws that catch!"
let plain = vigenere.decrypt passwd cipher
printfn "%s\n%s" cipher plain