How to implement hmac sha256 in VBA from scratch?

Question

For some reasons, I have to implement hmac sha256 in VBA from Scratch.

I've written the following code but I'm not getting the correct results (for verification, I'm using the following tables : https://www.rfc-editor.org/rfc/rfc4231#section-4.2)

Here is the code implemented :

In my code, the function Hash which corresponds to SHA256 is used from an external library and works fine (I could check the results). I assume then it's working properly but I'm quite sure the problem is coming from the conversion : Decimal, Hex, Byte... It's not something I master well.

I got the formulae from : https://en.wikipedia.org/wiki/Hash-based_message_authentication_code

Can anyone help please with the following code. What's wrong with this HMAC implementation ?

Thank you in advance.

    Option Explicit
     
    Public Sub Main()
        'Key declaration
        Dim Key As String
        Dim KeyBytes() As Byte
        
        'Message declaration
        Dim Message As String
        Dim MessageBytes() As Byte
        
        'ipad, opad
        Dim iPad_() As Byte
        Dim oPad_() As Byte
        
        'Size
        Dim n As Long

        'Output
        Dim Result() As Byte
        Dim A() As Byte, B() As Byte
        
        n = 64
           
        'Define the key
        Key = "123"
        KeyBytes = Key  'Convert string to byte
        KeyBytes = CompleteKey(KeyBytes, n) 'At this step there are 64 Bytes in the key
        
        'Define the message
        Message = "TestMessage"
        MessageBytes = Message  'Convert string to byte
        
        'Define ipad, opad
        iPad_ = ipad(n)
        oPad_ = opad(n)
            
        'Get the XOR operation of the key and ipad
        B = XOR_(KeyBytes, iPad_)
        
        'Concatenate the result with the message
        B = Concatenate(B, MessageBytes)
        
        'Compute the final hash
        A = XOR_(KeyBytes, oPad_)
        Result = Hash(Concatenate(A, Hash(B)))
    End Sub

    Public Function CompleteKey(KeyBytes() As Byte, n As Long) As Byte()
        Dim O() As Byte
        
        If UBound(KeyBytes) < n Then
            O = KeyBytes
            ReDim Preserve O(n - 1)
        End If
               
        CompleteKey = O
    End Function
     
    Public Function XOR_(Item1() As Byte, Item2() As Byte) As Byte()
        Dim i As Long, j As Long
        Dim A As String, B As String
        Dim Output()  As Byte
        
        ReDim Output(UBound(Item2) - 1)
        
        For i = 0 To UBound(Item1) - 1
            A = WorksheetFunction.Dec2Bin(Item1(i), 8)
            B = WorksheetFunction.Dec2Bin(Item2(i), 8)
            Output(i) = WorksheetFunction.Bin2Dec(XorBinary(A, B))
        Next i
                                       
        For j = i To UBound(Item2) - 1
            Output(i) = Item2(j)
        Next j
     
        XOR_ = Output
    End Function
     
    Public Function XorBinary(x As String, y As String) As String
        Dim i As Long
        Dim n As Long
        Dim A As Long
        Dim B As Long
        Dim C As String
        Dim O As String
       
        n = Len(x)
       
        For i = 1 To n
            A = CLng(Right(Left(x, i), 1))
            B = CLng(Right(Left(y, i), 1))
           
            If A * B = 0 And A + B = 1 Then
                C = "1"
            Else
                C = "0"
            End If
           
            O = O & C
        Next i
    End Function
     
     
    Public Function ipad(n As Long) As Byte()
        Dim S() As Byte
        Dim i As Long
          
        ReDim S(n)
            
        For i = 1 To n
            S(i) = 36
        Next i
       
        ipad = S
    End Function
     
    Public Function opad(n As Long) As Byte()
        Dim S() As Byte
        Dim i As Long
          
        ReDim S(n)
            
        For i = 1 To n
            S(i) = 92   '5C
        Next i
       
        opad = S
    End Function

    Public Function Concatenate(A() As Byte, B() As Byte) As Byte()
        Dim C() As Byte
        Dim i As Long, j As Long
        
        ReDim C(UBound(A) + UBound(B) + 1)
        
        For i = 0 To UBound(A)
            C(i) = A(i)
        Next i
        
        For j = i To i + UBound(B) - 1
            C(j) = B(j - UBound(A) - 1)
        Next j
        
        Concatenate = C
    End Function