RC4 stream encoding and decoding (bad performances)

RC4.fsx

open System
open System.Text
open System.IO
open System.Diagnostics

let mutable lastPercent = 0L

let computeRc4 (password:string) (content:Stream) (output:Stream) =
    let swap (arr:byte array) (i:int) (j:int) =
            let t = arr.[j]
            arr.[j] <- arr.[i]
            arr.[i] <- t

    let keySchedule (password:string) = 
        let key = password |> Encoding.ASCII.GetBytes
        let nums = [|0uy..255uy|]
        let mutable j = 0
        for i in [0..255] do
            let t = (j + (int nums.[i]) + (key.[i % key.Length] |> int)) |> int
            j <-  t % 256
            swap nums i j
        nums

    let rec compute (data:BinaryReader) (key:byte array) (i:int) (j:int) (writer:BinaryWriter) (n:int64) =
        if n % 100L = 0L then
            let percent = n * 100L / data.BaseStream.Length
            if lastPercent = percent |> not then
                printfn "progress: %d %%"  percent
                lastPercent <- percent
        if n < data.BaseStream.Length then
            let i2 = (i + 1) % 256
            let j2 = (j + (key.[i2] |> int)) % 256
            swap key i2 j2
            let t = (key.[i2] + key.[j2]) |> int
            let bk = key.[t % 256]
            let b = data.ReadByte()
            let r = bk ^^^ b
            writer.Write r
            compute data key i2 j2 writer (n + 1L)
    
    let k1 = keySchedule password
    let reader = new BinaryReader(content)
    let writer = new BinaryWriter(output)
    compute reader k1 0 0 writer 0L
    writer.Flush()

let computeRc4str (password:string) (content:string) =
    let plaintext = content |> Encoding.ASCII.GetBytes
    use stream = new MemoryStream(plaintext)
    use output = new MemoryStream()
    computeRc4 password stream output
    output.Position <- 0L
    output.ToArray()


let passwd = "github"
//let crypted = computeRc4str passwd "Appli mobile"
//let output = new MemoryStream()
//let input = new MemoryStream(crypted)
//computeRc4 passwd input output
//output.Position <- 0L
//let decrypted = Encoding.ASCII.GetString(output.ToArray())

let testCrypt () =
    use clearFile = File.OpenRead @"E:\epubs\ducobu.epub"
    use cryptedFile = File.Open(@"E:\epubs\ducobu_epub.rc4", FileMode.Create)
    computeRc4 passwd clearFile cryptedFile
    cryptedFile.Flush()

let testDecrypt () =
    use decryptedFile = File.Open(@"E:\epubs\ducobu-decrypted.epub", FileMode.Create)
    use cryptedFile2 = File.OpenRead @"E:\epubs\ducobu_epub.rc4"
    computeRc4 passwd cryptedFile2 decryptedFile
    decryptedFile.Flush()

let watch = Stopwatch.StartNew()
testCrypt()
watch.Stop()
printfn "crypting time: %A" watch.Elapsed

watch.Reset()
watch.Start()
testDecrypt()
watch.Stop()
printfn "decrypting time: %A" watch.Elapsed