Создать программное средство криптографической защиты данных

Option Explicit
Dim dPRNG#
Private Function PRNGFor7Bit(Optional ByVal seed As Double, Optional _
    da As Double = 1664525, Optional dc As Double = 1013904223 _
    , Optional dm As Double = 4294967296#) As Long
    Dim d#
    If seed = 0 Then seed = dPRNG
    d = da * seed + dc
    dPRNG = d - Fix(d / dm) * dm
    PRNGFor7Bit = Fix(dPRNG / (dm - 1) * 127)
End Function
Sub Encrypt(sSourcefile As String)
    On Error GoTo OnErr
    Dim sEncryptedFile$
    Dim iInputFile&, iOutputFile&, i&
    Dim bSource() As Byte
    dPRNG = 10000
    If Dir(sSourcefile) = "" Then MsgBox sSourcefile & " not exists": Exit Sub
    i = InStrRev(sSourcefile, ".")
    sEncryptedFile = Left(sSourcefile, i - 1) & "_encrypted" & Right(sSourcefile, Len(sSourcefile) - i + 1)
    If Dir(sEncryptedFile) <> "" Then Kill sEncryptedFile
    iOutputFile = FreeFile
    Open sEncryptedFile For Binary As #iOutputFile
        iInputFile = FreeFile
        Open sSourcefile For Binary As #iInputFile
            ReDim bSource(1 To LOF(iInputFile))
            Get #iInputFile, , bSource
            For i = 1 To UBound(bSource)
                Select Case bSource(i)
                Case 97 To 122
                    bSource(i) = Atbash(bSource(i), 219)
                Case 65 To 90
                    bSource(i) = Atbash(bSource(i), 155)
                End Select
                bSource(i) = GammaXOR(bSource(i), PRNGFor7Bit)
            Next i
            Put #iOutputFile, , bSource
        Close #iInputFile
    Close #iOutputFile
    Exit Sub
OnErr:
    Close
    MsgBox "Error"
End Sub
Private Function Atbash(Char As Byte, Number As Byte) As Byte
    Atbash = Number - Char
End Function
Private Function GammaXOR(Char As Byte, Gamma As Long) As Byte
    GammaXOR = Char Xor Gamma
End Function
Sub Decrypt(sSourcefile As String)
    On Error GoTo OnErr
    Dim sDecryptedFile$
    Dim iInputFile&, iOutputFile&, i&
    Dim bSource() As Byte
    dPRNG = 10000
    If Dir(sSourcefile) = "" Then MsgBox sSourcefile & " not exists": Exit Sub
    i = InStrRev(sSourcefile, ".")
    sDecryptedFile = Left(sSourcefile, i - 1) & "_decrypted" & Right(sSourcefile, Len(sSourcefile) - i + 1)
    If Dir(sDecryptedFile) <> "" Then Kill sDecryptedFile
    iOutputFile = FreeFile
    Open sDecryptedFile For Binary As #iOutputFile
        iInputFile = FreeFile
        Open sSourcefile For Binary As #iInputFile
            ReDim bSource(1 To LOF(iInputFile))
            Get #iInputFile, , bSource
            For i = 1 To UBound(bSource)
                bSource(i) = GammaXOR(bSource(i), PRNGFor7Bit)
                Select Case bSource(i)
                Case 97 To 122
                    bSource(i) = Atbash(bSource(i), 219)
                Case 65 To 90
                    bSource(i) = Atbash(bSource(i), 155)
                End Select
            Next i
            Put #iOutputFile, , bSource
        Close #iInputFile
    Close #iOutputFile
    Exit Sub
OnErr:
    Close
    MsgBox "Error"
End Sub