protected string Base64Encode(string str)
{
  return Convert.ToBase64String(
    System.Text.Encoding.GetEncoding("euc-kr").GetBytes(str)); 
}

protected string Base64Decode(string str)
{
  return System.Text.Encoding.GetEncoding("euc-kr").GetString(
    System.Convert.FromBase64String(str));
}

void Page_Load(Object src, EventArgs e )
{
  string str="인코딩할 한글 1234567890!@#$%^&*()";
  Response.Write( "인코딩 전 : " + str +"<br>" );
  string encoded = Base64Encode(str);
  Response.Write( "BASE64 인코드 : " + encoded +"<br>");
  string decoded = Base64Decode(encoded);
  Response.Write( "BASE64 디코드 : " + decoded +"<br>");
}