Base64 Encoding/Decoding (VB .Net)

'http://www.webtoolkit.info/javascript_base64.html#.YSOruI6A6Uk
'https://humanwhocodes.com/blog/2009/12/08/computer-science-in-javascript-base64-encoding/

Imports System.Text
Imports System.Text.RegularExpressions

Public Class Base64

    'Private Property
    Const _keyStr As String = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789+/="

    ' public method for encoding
    Public Function Encode(input As String, isBinaryData As Boolean) As String
        If String.IsNullOrEmpty(input) Then Return ""

        Dim output As New StringBuilder
        Dim cur, prev, byteNum As Double
        Dim i As Integer = 0

        If Not isBinaryData Then input = _utf8_encode(input)

        While i < input.Length
            cur = AscW(input(i)) 'charCodeAt
            byteNum = i Mod 3

            Select Case byteNum
                Case 0
                    output.Append(_keyStr(cur >> 2)) 'charAt
                Case 1
                    output.Append(_keyStr(((prev And 3) << 4) Or (cur >> 4)))

                Case 2
                    output.Append(_keyStr(((prev And 15) << 2) Or (cur >> 6)))
                    output.Append(_keyStr(cur And 63))
            End Select

            prev = cur
            i += 1

        End While

        If byteNum = 0 Then
            output.Append(_keyStr((prev And 3) << 4))
            output.Append("==")
        ElseIf byteNum = 1 Then
            output.Append(_keyStr((prev And 15) << 2))
            output.Append("=")
        End If

        Return output.ToString
    End Function

    ' public method for decoding
    Public Function Decode(input As String, isBinaryData As Boolean) As String

        If String.IsNullOrEmpty(input) Then Return ""

        Dim output As New StringBuilder
        Dim cur, prev, digitNum As Double
        Dim i As Integer = 0

        input = input.Replace("=", "")

        While i < input.Length
            cur = _keyStr.IndexOf(input(i))
            digitNum = i Mod 4
            Select Case digitNum

                Case 1
                    output.Append(ChrW((prev << 2) Or (cur >> 4)))
                Case 2
                    output.Append(ChrW(((prev And 15) << 4) Or (cur >> 2)))
                Case 3
                    output.Append(ChrW(((prev And 3) << 6) Or cur))
            End Select

            prev = cur
            i += 1
        End While

        If Not isBinaryData Then
            Return _utf8_decode(output.ToString)
        Else
            Return output.ToString
        End If

    End Function

    ' private method for UTF-8 encoding
    Private Function _utf8_encode(_string As String) As String

        _string = Regex.Replace(_string, "/\r\n/g", "\n")

        Dim utftext As String = ""

        For n = 0 To _string.Length - 1
            Dim c As Integer = AscW(_string(n))

            If c < 128 Then
                utftext &= ChrW(c)

            ElseIf (c > 127) AndAlso (c < 2048) Then
                utftext &= ChrW((c >> 6) Or 192)
                utftext &= ChrW((c And 63) Or 128)
            Else
                utftext &= ChrW((c >> 12) Or 224)
                utftext &= ChrW(((c >> 6) And 63) Or 128)
                utftext &= ChrW((c And 63) Or 128)
            End If
        Next

        Return utftext
    End Function

    ' private method for UTF-8 decoding
    Private Function _utf8_decode(utftext As String)
        Dim _String As String = ""
        Dim i As Integer = 0
        Dim c, c3, c2 As Integer

        While (i < utftext.Length)
            c = AscW(utftext(i))

            If (c < 128) Then
                _String &= ChrW(c)

                i += 1
            ElseIf ((c > 191) And (c < 224)) Then
                c2 = AscW(utftext(i + 1))
                _String &= ChrW(((c And 31) << 6) Or (c2 And 63))
                i += 2
            Else
                c2 = AscW(utftext(i + 1))
                c3 = AscW(utftext(i + 2))
                _String &= ChrW(((c And 15) << 12) Or ((c2 And 63) << 6) Or (c3 And 63))
                i += 3
            End If
        End While

        Return _String

    End Function

End Class