расшифровка шифра Цезаря

(*расшифровка шифра Цезаря*)
let secret = "li bra ghflskhu wklv dqg bra nqrz lw, fods brxu kdqgv"
let keys = [ for i in 'a'..'z' -> i]
let size = keys |> List.length

let slice a =
    let (h::t) = a
    t @ [h] 
let decode (word:string) (code:char list) =
    for x = 0 to word.Length - 1 do
         let f = List.tryFindIndex (fun e -> word.[x] = e) keys
         match f with
         | Some c ->  printf "%c" (code.[c])   
         | None -> printf "%c" (word.[x]) 

let rec loop (word:string) (code:char list) count =
    match count with
    | _ when count > keys.Length -> ()
    | _ 
        -> 
        printfn "\ntry number %d" count
        let cipher = slice code
        decode word cipher
        loop word cipher (count + 1)


printfn "decoding Cezar's cipher  %s ...\n" secret

loop secret (slice keys) 0

printfn "\n" 

Запускаем:

try number 20
he xnw cdbhogdq sghr zmc xnw jmnv hs, bkzo xntq gzmcr
try number 21
if yox decipher this and yox know it, clap your hands
try number 22 

Суть шифра в сдвиге алфавита на определенное количество символов.