Speed up encryption code

Function XorC(ByVal sData As String, ByVal sKey As String) As String
    Dim l As Long, i As Long, byIn() As Byte, byOut() As Byte, byKey() As Byte
    Dim bEncOrDec As Boolean
    'confirm valid string and key input:
    If Len(sData) = 0 Or Len(sKey) = 0 Then XorC = "Invalid argument(s) used": Exit Function
    'check whether running encryption or decryption (flagged by presence of "xxx" at start of sData):
    If Left$(sData, 3) = "xxx" Then
        bEncOrDec = False   'decryption
        sData = Mid$(sData, 4)
    Else
        bEncOrDec = True   'encryption
    End If
    'assign strings to byte arrays (unicode)
    byIn = sData
    byOut = sData
    byKey = sKey
    l = LBound(byKey)
    For i = LBound(byIn) To UBound(byIn) - 1 Step 2
        byOut(i) = ((byIn(i) + Not bEncOrDec) Xor byKey(l)) - bEncOrDec 'avoid Chr$(0) by using bEncOrDec flag
        l = l + 2
        If l > UBound(byKey) Then l = LBound(byKey)  'ensure stay within bounds of Key
    Next i
    XorC = byOut
    If bEncOrDec Then XorC = "xxx" & XorC  'add "xxx" onto encrypted text
End Function

  A         B          C         
1 abc       xxx!      abc       
2 123       xxxq]K     123       
3 Your text xxx"U Your text 
Prod
[Table-It] version 09 by Erik Van Geit

RANGE FORMULA (1st cell)
B1:C3 =XorC(A1,"AnyString")
[Table-It] version 09 by Erik Van Geit