Solitaire Encryption Algorithm implementation in F#

I first read about the Solitaire algorithm in the novel Cryptonomicon, by Neal Stephenson. Being a geeky kid fascinated by computers I thought that it was a brilliant idea, but my curiosity didn’t go so far as to make me write an implementation of it (you can find a perl version in the book, which by the way I consider to be another great way to encrypt information).

Solitaire is a Stream Cipher created by Bruce Schneier and designed to be used with only a deck of cards. No electronic devices are needed and its security is intended to be of military-strength; that means that even if the way to generate/decipher messages is manual and might look a bit rudimentary, ideally not even the most powerful government could ever decipher a message generated with Solitaire (although some potential flaws have been [reported]

So after playing with F# lately, I decided to write some non-trivial code in order to see how it suits my coding style and habits, and yet another naive implementation of this curious crypto-algorithm was born.

Notes about the implementation

After the initial struggle learning the particularities of F# (which are basically the same as ocaml) I quickly felt comfortable with it. It’s a very versatile language, given that it allows the programmer to use functional and imperative programming at his own will, so the learning curve is not very steep. And although the functional approach should be used most of the time, sometimes imperative code is more convenient and can make some programmers with a C or Java background feel more in control. For an example, consider this snippet of code:

member s.KeyStream (c: char list) f =
    match c with
    |x :: xs  -> let mutable o = s.getOutput
                 while o = jokerA or o = jokerB do o <- s.getOutput;
                 (Convert.ToChar (((f x o) % 26) + 65) :: s.KeyStream xs f)
    |[] ->  [];

Here I am using a functional approach to go through a list of characters and apply a given function and some calculations to each one of them. The whole transformation is based on the value of the variable o, but I only want to use o if it is not equal
to a joker value (you can read about the algorithm internals on the wikipedia article), so I use a very convenient while block that does the job. Sure, it can be done with recursivity as well, but F# lets you choose
what suits you better. In this snippet I am also using one of the most powerful ideas of the
functional world: pattern matching. As the name
says, a pattern match takes an expression between the match … with block and compares it against a set of rules which test the expression given and returns a result. The concept is close to the good old switch/case statement, but much more powerful, because it allows the programmer to use conditions, name binding and object decomposing in the rules. The net is full of examples and articles about it where you can learn how to use them effectively.

The code also makes a basic use of some of the different paradigms that F# allows. For example, it uses OO for the Solitaire class to take advantage of mutable variables (a clear deviation from functional pureness and as I said, one of the strong points of the language) so the
same variable can be used when keying the deck and by the encryption/decryption methods, but I use global functions for card manipulation operations. Sure, there could be a Deck class that includes card manipulation methods, but as I said before, this code is just for my own experimentation purposes.

I borrowed some utility functions from Haskell Prelude (span, splitAt) to help me with string manipulation, although I am sure that there are dozens of Ocaml libraries that already have a version of them. I am lazy and I just implemented the ones I needed and knew from Haskell.

This implementation is not intended to be efficient and of course there might be better ways to write it, this code was only written for fun and to teach myself some F# discipline, so it is not written in the most optimal way, but in the clearest one. I would gladly accept suggestions
of how to improve it. I feel like I just started scratching the surface of what can be done with F# and I can’t wait to unleash its real potential.

The code below is the working algorithm, and you can find the complete source code including tests here.

#r "FSharp.PowerPack.dll"

open System

(* Utility functions *)

let getCharValue c =
    let alphabet = Seq.to_list "ABCDEFGHIJKLMNOPQRSTUVWXYZ"
    (List.find_index (fun x -> c = x) alphabet)

(* Adds 'X' characters until the list has a length multiple of 5, as stated in
   Schneier's page *)
let rec mod5 (msg: char list) =
    if (List.length msg) % 5 <> 0 then mod5 (msg @ ['X']) else msg

let sanitize _string =
    |> Seq.to_list
    |> List.filter (fun c -> Char.IsLetter c)
    |> Char.ToUpper

(* Functions borrowed from Haskell *)
let span p xs = (Seq.to_list (Seq.take_while p xs), Seq.to_list (Seq.skip_while p xs))

(* Splits a list into two lists at the position corresponding to the given
   integer *)
let rec splitAt n xs =
    match (n, xs) with
    |(0, xs) -> ([], xs)
    |(_, []) -> ([], [])
    |(n, x :: xs) ->  let t = splitAt (n-1) xs;
                      (x :: fst t, snd t)

(* Deck Manipulation functions *)
let jokerA = 53
let jokerB = 54

(* Moves down a card in the deck *)
let md (c:int) (d) =
    match span (fun x -> x <> c) d with
    | (y :: ys, [x])     -> y :: x :: ys
    | (ys, x :: z :: zs) -> ys @ [z;x] @ zs
    | (_,_) -> []

let md2 c d = md c (md c d)

let tripleCut deck =
    let tripleCut2 c acc =
        match acc with
        | []      -> [c] :: acc
        | h :: t  -> if c = jokerA or c = jokerB then [] :: [c] :: acc
                     else (c :: h) :: t
    let s = List.fold_right tripleCut2 deck [ [] ]
    [s.[4]; s.[1]; s.[2]; s.[3]; s.[0]] |> List.concat

let countCut index deck =
    match splitAt index deck with
    | (xs, [x]) ->  deck
    | (xs, ys)  ->  let split = splitAt (ys.Length-1) ys;
                    fst split @ xs @ snd split

type Solitaire() =
    let mutable Cards = [1 .. jokerB]
    member s.Deck with get() = Cards

    member s.getOutput =
        Cards <- Cards |> md jokerA |> md2 jokerB |> tripleCut
        Cards <- countCut (if Cards.[jokerA] = jokerB
                           then jokerA
                           else Cards.[jokerA]) Cards

        if Cards.Head = jokerB then Cards.[jokerA] else Cards.[Cards.Head]

    member s.keyDeck keys =
        Cards <- [1 .. jokerB]
        let keyArray = keys |> sanitize |> Seq.to_list
        List.iter (fun (c:char) -> s.getOutput |> ignore;
                                   Cards <- countCut (Convert.ToInt32 c - 64) Cards) keyArray

    member s.KeyStream (c: char list) f =
        match c with
        |x :: xs  -> let mutable o = s.getOutput
                     while o = jokerA or o = jokerB do o <- s.getOutput;
                     (Convert.ToChar (((f x o) % 26) + 65) :: s.KeyStream xs f)
        |[] ->  [];

    member s.Encrypt msg = s.KeyStream (msg |> sanitize |> mod5)
                                       (fun x o -> (getCharValue x) + (o % 26))

    member s.Decrypt msg = s.KeyStream (msg |> sanitize |> mod5)
                                       (fun x o -> let k = (getCharValue x) - (o % 26)
                                                   if k < 65 then k + 26 else k)
(* Unit testing *)
let vectors =

let rec test (arr:string list list) =
    match arr with
    |x::xs    ->    let key = x.Tail.Head
                    let correct_enc = List.hd ( x.Tail)
                    print_endline("Testing vector: " ^ x.Head ^
                                      "\nwith key: " ^ key ^
                                      "\nshould output:\t" ^ correct_enc)

                    let cipher = new Solitaire()
                    cipher.keyDeck key
                    let enc = new String (List.to_array (cipher.Encrypt x.Head))
                    cipher.keyDeck key
                    let dec = String.sub (new String (List.to_array (cipher.Decrypt enc))) 0 x.Head.Length
                        ("and it outputs:\t" ^ enc ^ "\ndecryption:\t" ^ dec ^
                        "\n[Encryption test: " ^
                            if enc = correct_enc then "CORRECT!]" else "FAIL!]")
                        ("[Decryption test: " ^
                            if dec = x.Head then "CORRECT!]\n\n" else "FAIL!]\n\n\n")
                    test xs
    |[]       ->    print_endline "All tests Finished."

(* Command line parser *)

let _ = match Sys.argv with
        |[|_;"-test"|]      ->  test vectors
        |[|_;"-enc";_;_|]   ->  let cipher = new Solitaire()
                                cipher.keyDeck (Sys.argv.[3])
                                    (new String (List.to_array
                                        (cipher.Encrypt (Sys.argv.[2]))))
        |[|_;"-dec";_;_|]   ->  let cipher = new Solitaire()
                                cipher.keyDeck (Sys.argv.[3])
                                    (new String (List.to_array
                                        (cipher.Decrypt (Sys.argv.[2]))))
        |_  ->  print_endline("Usage:\tSolitaire -test\n\t" ^
                                  "Solitaire -enc text key\n\t" ^
                                  "Solitaire -dec ciphertext key\n\n")
                print_endline("Example: Solitaire -enc SECRETMESSAGE foo")

The great thing about F# is that is a member of the .NET family of languages, so it can interact with the .NET framework just as C# would. In the code above I am not using much of explicit .NET related code, but I definitely want to explore it…just imagine the potential uses of combining ideas like pattern matching with existing .NET objects. Cool.

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