' Encipher the text using the pasword.
Private Sub Cipher(ByVal password As String, ByVal _
    from_text As String, to_text As String)
Const MIN_ASC = 32  ' Space.
Const MAX_ASC = 126 ' ~.
Const NUM_ASC = MAX_ASC - MIN_ASC + 1

Dim offset As Long
Dim str_len As Integer
Dim i As Integer
Dim ch As Integer

    ' Initialize the random number generator.
    offset = NumericPassword(password)
    Rnd -1
    Randomize offset

    ' Encipher the string.
    str_len = Len(from_text)
    For i = 1 To str_len
        ch = Asc(Mid$(from_text, i, 1))
        If ch >= MIN_ASC And ch <= MAX_ASC Then
            ch = ch - MIN_ASC
            offset = Int((NUM_ASC + 1) * Rnd)
            ch = ((ch + offset) Mod NUM_ASC)
            ch = ch + MIN_ASC
            to_text = to_text & Chr$(ch)
        End If
    Next i
End Sub

' Encipher the text using the pasword.
Private Sub Decipher(ByVal password As String, ByVal _
    from_text As String, to_text As String)
Const MIN_ASC = 32  ' Space.
Const MAX_ASC = 126 ' ~.
Const NUM_ASC = MAX_ASC - MIN_ASC + 1

Dim offset As Long
Dim str_len As Integer
Dim i As Integer
Dim ch As Integer

    ' Initialize the random number generator.
    offset = NumericPassword(password)
    Rnd -1
    Randomize offset

    ' Encipher the string.
    str_len = Len(from_text)
    For i = 1 To str_len
        ch = Asc(Mid$(from_text, i, 1))
        If ch >= MIN_ASC And ch <= MAX_ASC Then
            ch = ch - MIN_ASC
            offset = Int((NUM_ASC + 1) * Rnd)
            ch = ((ch - offset) Mod NUM_ASC)
            If ch < 0 Then ch = ch + NUM_ASC
            ch = ch + MIN_ASC
            to_text = to_text & Chr$(ch)
        End If
    Next i
End Sub

' Translate a password into an offset value.
Private Function NumericPassword(ByVal password As String) _
    As Long
Dim value As Long
Dim ch As Long
Dim shift1 As Long
Dim shift2 As Long
Dim i As Integer
Dim str_len As Integer

    str_len = Len(password)
    For i = 1 To str_len
        ' Add the next letter.
        ch = Asc(Mid$(password, i, 1))
        value = value Xor (ch * 2 ^ shift1)
        value = value Xor (ch * 2 ^ shift2)

        ' Change the shift offsets.
        shift1 = (shift1 + 7) Mod 19
        shift2 = (shift2 + 13) Mod 23
    Next i
    NumericPassword = value
End Function

For lots more information on cryptography, including some very secure algorithms, see Bruce Schneier's book Applied Cryptography.