Rijndael & AES Combi encrytion

Public Class SimpleAes
    Implements IDisposable
    ''' <summary>
    ''' Initialization vector length in bytes.
    ''' </summary>
    Private Const IvBytes As Integer = 16

    ''' <summary>
    ''' Must be exactly 16, 24 or 32 characters long.
    ''' </summary>
    Private Shared ReadOnly Key As Byte() = Convert.FromBase64String("FILL ME WITH 16, 24 OR 32 CHARS")

    Private ReadOnly _encoder As UTF8Encoding
    Private ReadOnly _encryptor As ICryptoTransform
    Private ReadOnly _rijndael As RijndaelManaged

    Public Sub New()
        _rijndael = New RijndaelManaged() With { _
            Key .Key = Key _
        }
        _rijndael.GenerateIV()
        _encryptor = _rijndael.CreateEncryptor()
        _encoder = New UTF8Encoding()
    End Sub

    Public Function Decrypt(encrypted As String) As String
        Return _encoder.GetString(Decrypt(Convert.FromBase64String(encrypted)))
    End Function

    Public Sub Dispose()
        _rijndael.Dispose()
        _encryptor.Dispose()
    End Sub

    Public Function Encrypt(unencrypted As String) As String
        Return Convert.ToBase64String(Encrypt(_encoder.GetBytes(unencrypted)))
    End Function

    Private Function Decrypt(buffer As Byte()) As Byte()
        ' IV is prepended to cryptotext
        Dim iv As Byte() = buffer.Take(IvBytes).ToArray()
        Using decryptor As ICryptoTransform = _rijndael.CreateDecryptor(_rijndael.Key, iv)
            Return decryptor.TransformFinalBlock(buffer, IvBytes, buffer.Length - IvBytes)
        End Using
    End Function

    Private Function Encrypt(buffer As Byte()) As Byte()
        ' Prepend cryptotext with IV
        Dim inputBuffer As Byte() = _rijndael.IV.Concat(buffer).ToArray()
        Return _encryptor.TransformFinalBlock(inputBuffer, IvBytes, buffer.Length)
    End Function
End Class