function Decode64(S: string): string;
const
  Codes64 = '0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz+/';
var
  i: Integer; a: Integer;
  x: Integer; b: Integer;
begin
  Result := '';
  a := 0;
  b := 0;
  for i := 1 to Length(s) do
  begin
    x := Pos(s[i], codes64) - 1;
    if x >= 0 then
    begin
      b := b * 64 + x;
      a := a + 6;
      if a >= 8 then
      begin
        a := a - 8;
        x := b shr a;
        b := b mod (1 shl a);
        x := x mod 256;
        Result := Result + chr(x);
      end;
    end
    else
      Exit;
  end;
end;

procedure TForm1.Button1Click(Sender: TObject);
var
  s : String;
begin
  s := Decode64('KsXlTo14PMnmQ6a'); //  Show Delphi codificado 64
  ShowMessage(s);
end;