Option _Explicit

Dim As String udata, decoded
udata = "VG8gZXJyIGlzIGh1bWFuLCBidXQgdG8gcmVhbGx5IGZvdWwgdGhpbmdzIHVwIHlvdSBuZWVkIGEgY29tcHV0ZXIuCiAgICAtLVBhdWwgUi5FaHJsaWNo"

decoded = decode(udata)

Print udata
Print decoded

Function findIndex& (value As _Unsigned _Byte)
    If Asc("A") <= value And value <= Asc("Z") Then
        findIndex = value - Asc("A")
        Exit Function
    End If
    If Asc("a") <= value And value <= Asc("z") Then
        findIndex = value - Asc("a") + 26
        Exit Function
    End If
    If Asc("0") <= value And value <= Asc("9") Then
        findIndex = value - Asc("0") + 52
        Exit Function
    End If
    If value = Asc("+") Then
        findIndex = 62
        Exit Function
    End If
    If value = Asc("/") Then
        findIndex = 63
        Exit Function
    End If
    findIndex = -1
End Function

Function encode$ (source As String)
    Dim As String Base64: Base64 = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/"
    Dim As _Unsigned _Integer64 length: length = Len(source)
    Dim As _Unsigned _Integer64 it, strend
    Dim As Long acc
    Dim As String sink
    strend = length
    While it <> strend
        Dim As _Unsigned _Byte b1, b2, b3, b4
        it = it + 1
        b1 = Asc(Mid$(source, it, 1))
        sink = sink + Mid$(Base64, _SHR(b1, 2), 1)
        acc = _SHL(b1 And &H3, 4)
        If it <> strend Then
            it = it + 1
            b2 = Asc(Mid$(source, it, 1))
            acc = acc Or _SHR(b2, 4)
            sink = sink + Mid$(Base64, acc, 1)
            acc = _SHL(b2 And &HF, 2)
            If it <> strend Then
                it = it + 1
                b3 = Asc(Mid$(source, it, 1))
                acc = acc Or _SHR(b3, 6)
                sink = sink + Mid$(Base64, acc, 1)
                sink = sink + Mid$(Base64, b3 And &H3F, 1)
            Else
                sink = sink + Mid$(Base64, acc, 1)
                sink = sink + "="
            End If
        Else
            sink = sink + Mid$(Base64, acc, 1)
            sink = sink + "="
            sink = sink + "="
        End If
    Wend
    encode = sink
End Function

Function decode$ (source As String)
    Dim As _Unsigned _Integer64 length: length = Len(source)
    Dim As _Unsigned _Integer64 it, strend
    Dim As Long acc
    Dim As String sink
    strend = length
    While it <> strend
        Dim As _Unsigned _Byte b1, b2, b3, b4
        it = it + 1
        b1 = Asc(Mid$(source, it, 1))
        it = it + 1
        b2 = Asc(Mid$(source, it, 1))
        it = it + 1
        b3 = Asc(Mid$(source, it, 1))
        it = it + 1
        b4 = Asc(Mid$(source, it, 1))
        Dim As Long i1, i2
        i1 = findIndex(b1)
        i2 = findIndex(b2)
        acc = _SHL(i1, 2)
        acc = acc Or _SHR(i2, 4)
        sink = sink + Chr$(acc)
        If b3 <> Asc("=") Then
            Dim As Long i3
            i3 = findIndex(b3)
            acc = _SHL(i2 And &HF, 4)
            acc = acc Or _SHR(i3, 2)
            sink = sink + Chr$(acc)
            If b4 <> Asc("=") Then
                Dim As Long i4
                i4 = findIndex(b4)
                acc = _SHL(i3 And &H3, 6)
                acc = acc Or i4
                sink = sink + Chr$(acc)
            End If
        End If
    Wend
    decode = sink
End Function