ROT13 cipher - Encrypt & Decript & lussen door de getallen 1 t/m 26

Public Function CaesarCipher(ByVal TextToEncrypt As String, ByVal CaesarShift As Long) As String

    Dim OutputText As String
    TextToEncrypt = UCase(TextToEncrypt)

    If CaesarShift > 26 Then
        CaesarShift = CaesarShift Mod 26
    End If

    If CaesarShift = 0 Then
        OutputText = TextToEncrypt
    ElseIf CaesarShift > 0 Then
        OutputText = ShiftRight(TextToEncrypt, CaesarShift)
    Else
        CaesarShift = Abs(CaesarShift)
        OutputText = ShiftLeft(TextToEncrypt, CaesarShift)
    End If

    CaesarCipher = OutputText
End Function

Private Function ShiftRight(ByVal ShiftString As String, ByVal ShiftQuantity As Long) As String

    Dim TextLength As Long
    TextLength = Len(ShiftString)

    Dim CipherText As String
    Dim CharacterCode As Long
    Dim AsciiIndex As Long
    Dim AsciiIdentifier() As Long
    ReDim AsciiIdentifier(1 To TextLength)

    For AsciiIndex = 1 To TextLength
        CharacterCode = Asc(Mid(ShiftString, AsciiIndex, 1))
        If CharacterCode + ShiftQuantity > 90 Then
            CharacterCode = CharacterCode - 26 + ShiftQuantity
        ElseIf CharacterCode = 32 Then GoTo Spaces
        Else:  CharacterCode = CharacterCode + ShiftQuantity
        End If
Spaces:
        AsciiIdentifier(AsciiIndex) = CharacterCode
    Next

        For AsciiIndex = 1 To TextLength
            CipherText = CipherText & Chr(AsciiIdentifier(AsciiIndex))
        Next
    ShiftRight = CipherText
End Function


Private Function ShiftLeft(ByVal ShiftString As String, ByVal ShiftQuantity As Long) As String

    Dim TextLength As Long
    TextLength = Len(ShiftString)

    Dim CipherText As String
    Dim CharacterCode As Long
    Dim AsciiIndex As Long
    Dim AsciiIdentifier() As Long
    ReDim AsciiIdentifier(1 To TextLength)

    For AsciiIndex = 1 To TextLength
        CharacterCode = Asc(Mid(ShiftString, AsciiIndex, 1))
        If CharacterCode = 32 Then GoTo Spaces
        If CharacterCode - ShiftQuantity < 65 Then
            CharacterCode = CharacterCode + 26 - ShiftQuantity
        Else: CharacterCode = CharacterCode - ShiftQuantity
        End If
Spaces:
        AsciiIdentifier(AsciiIndex) = CharacterCode
    Next

        For AsciiIndex = 1 To TextLength
            CipherText = CipherText & Chr(AsciiIdentifier(AsciiIndex))
        Next
    ShiftLeft = CipherText
End Function

=REST(getal-minimum+term;maximum-minimum+1)+minimum

Option Explicit

Public Function ModA(lngNumber As Long, lngMinimum As Long, lngMaximum As Long)
'https://stackoverflow.com/questions/3057640/math-looping-between-min-and-max-using-mod
'bij negatieve getallen geeft de VBA mod-functie een andere uitkomst dan Excel REST-functie
    Dim lngRange As Long
        lngRange = lngMaximum - lngMinimum + 1
        ModA = (lngNumber - lngMinimum) Mod lngRange
        If (ModA < 0) Then
            ModA = ModA + lngRange
        End If
        ModA = ModA + lngMinimum
End Function