Base24 encode and decode C#

Base24.cs

//
// Description: Base24 encode and decode
// Author: wangye  <pcn88 at hotmail dot com>
// Website: http://wangye.org
//
public static class Base24
{
  private static string sel = "BCDFGHJKMPQRTVWXY2346789";
 
  public static string Encode(string Text)
  {
    int i = 0;
    int Pos = 0;
    char []Buf = new char[Text.Length<<1];
 
    while ((i = Pos) < Text.Length)
    {
        Buf[i<<1] = sel[(Text[Pos]) >> 4];
        Buf[(i<<1) + 1] = sel[23 - (Text[Pos] & 0x0F)];
        Pos++;
     }
 
        return new string(Buf);
    }
 
    public static string Decode(string Text)
    {
        if (Text.Length % 2 != 0)
            return null;
 
        int [] NPos = new int[2];
        char[] N = new char[2];
        char[] Buf = new char[Text.Length >> 1];
 
        for (int i = 0; i < (Text.Length >> 1); i++)
        {
            NPos[0] = sel.IndexOf(Text[ i<<1 ]);
            NPos[1] = 23 - sel.IndexOf(Text[(i<<1) + 1]);
            if (NPos[0] < 0 || NPos[1] < 0) {
                return null;
            }
 
            Buf[i] = ((char)((NPos[0] << 4) | NPos[1]));
        }
            return new string(Buf);
    }
}