Public Function EncryptFile(ByVal sSource As String, ByVal sTarget As _
      String, ByVal sPassword As String, Optional ByVal BufferSize As Long = _
      4096) As Boolean
        Try
            Dim sKey As Byte() = (New UnicodeEncoding).GetBytes( _
              sPassword.PadRight(8, "*"))
            Dim OldFileStream As New FileStream(sSource, FileMode.Open)
            Dim NewFileStream As New FileStream(sTarget, FileMode.Create)
            Dim RijndaelManaged As New RijndaelManaged
            Dim CryptoStream As New CryptoStream(NewFileStream, _
              RijndaelManaged.CreateEncryptor(sKey, sKey), _
              CryptoStreamMode.Write)
 
            Dim Data(BufferSize) As Byte
            Dim Buffer As Long
            Dim BufferCount As Long
            Dim FileLen As Long = OldFileStream.Length
 
            Do
                If BufferCount >= FileLen - BufferSize Then
                    ReDim Data(FileLen - BufferCount)
                    Buffer = OldFileStream.Read(Data, 0, FileLen - BufferCount)
                    CryptoStream.Write(Data, 0, Buffer)
                    Exit Do
                End If
 
                Buffer = OldFileStream.Read(Data, 0, BufferSize)
                CryptoStream.Write(Data, 0, BufferSize)
                BufferCount += Buffer
            Loop
 
            OldFileStream.Close()
            CryptoStream.Close()
            NewFileStream.Close()
        Catch
            Return False
        End Try
        Return True
    End Function

Imports System.Security.Cryptography
Imports System.IO
Imports System.Text
 
Public Function DecryptFile(ByVal sSource As String, ByVal sTarget As String, _
  ByVal sPassword As String, Optional ByVal BufferSize As Long = 4096) As _
  Boolean
    Try
        Dim sKey As Byte() = (New UnicodeEncoding).GetBytes(sPassword.PadRight( _
          8, "*"))
        Dim OldFileStream As New FileStream(sSource, FileMode.Open)
        Dim NewFileStream As New FileStream(sTarget, FileMode.Create)
        Dim RijndaelManaged As New RijndaelManaged
        Dim CryptoStream As New CryptoStream(OldFileStream, _
          RijndaelManaged.CreateDecryptor(sKey, sKey), CryptoStreamMode.Read)
 
        Dim Data(BufferSize) As Byte
        Dim Buffer As Long
        Dim BufferCount As Long
        Dim FileLen As Long = OldFileStream.Length
 
        Do
            If BufferCount >= FileLen - BufferSize Then
                ReDim Data(FileLen - BufferCount)
                Buffer = CryptoStream.Read(Data, 0, FileLen - BufferCount)
                NewFileStream.Write(Data, 0, Buffer)
                Exit Do
            End If
 
            Buffer = CryptoStream.Read(Data, 0, BufferSize)
            NewFileStream.Write(Data, 0, BufferSize)
            BufferCount += Buffer
        Loop
 
        NewFileStream.Close()
        CryptoStream.Close()
        OldFileStream.Close()
    Catch
        Return False
    End Try
    Return True
End Function