Encrypting a Rijndael string, counter part for https://gist.github.com/Thorium/1972253

Rijndael.fsx

open System
open System.IO
open System.Security.Cryptography
open System.Text
open System.Diagnostics.Contracts

let EncryptStringWith (plain:string) (key:string) (iv:string) = 
    let enc = new ASCIIEncoding()
    use encrypted = new MemoryStream()
    use encode = new ToBase64Transform()
    let errdesc = "Failure when encrypting the string: " + plain + "\r\n"
    try
        use encryptor = Rijndael.Create().CreateEncryptor(enc.GetBytes(key), enc.GetBytes(iv))
        use tmpcrypt = new CryptoStream(encrypted, encryptor, CryptoStreamMode.Write)
        use encodestream = new CryptoStream(tmpcrypt, encode, CryptoStreamMode.Write)
            
        let encryptedbytes = enc.GetBytes(plain);
        encodestream.Write(encryptedbytes, 0, encryptedbytes.Length);
        encodestream.Close() // lazy, has to close explicitly before use. use is not enough.
    with
        | :? CryptographicException as ex -> failwith(errdesc + ex.ToString())
        | :? FormatException as ex -> failwith(errdesc + ex.ToString())
    encrypted.ToArray() |> Convert.ToBase64String