for(int 0 =1;i<str.Length-1;i++)
   str[i]=(char)(((int)str[i]+3)%128);
str=str.SubString(str.Length-2)+str.SubString(0,str.Length-1)

for(int i =0;i<str.Length-1;i++)
   str[i]=(char)(((int)str[i]+3)%128);
str=str.SubString(str.Length-2)+str.SubString(0,str.Length-1)

      private string methord(string targestr)
        {
            if (string.IsNullOrEmpty(targestr))
                return targestr;
            targestr = targestr.Trim();
            char[] list = targestr.ToCharArray ();
            char b = list[list.Length - 1];
            char a = list[0];
            char c = list[0];
            for (int i = 0; i < list.Length-1; i++)
            {                
                c=(char)( (int)a+3);
                if (list[i] > 'Z' && list[i] < 'a')
                    list[i] = 'A';
                else
                {
                  if(  list[i] > 'z')
                    list[i] = 'a';
                }
                a = list[i + 1];
                list[i + 1] = c;

            }
            list[0] = b;
            return new string(list);
        }

        private string methord(string targestr)
        {
            if (string.IsNullOrEmpty(targestr))
                return targestr;
            targestr = targestr.Trim();
            char[] list = targestr.ToCharArray ();
            char b = list[list.Length - 1];
            char a = list[0];
            char c = list[0];
            for (int i = 0; i < list.Length-1; i++)
            {                
                c=(char)( (int)a+3);
                if (c > 'Z' && c < 'a')
                    c =(char )( 'A'+c-'Z'-1);
                else
                {
                  if(  c > 'z')
                    c = (char )('a'+c-'z'-1);
                }
                a = list[i + 1];
                list[i + 1] = c;

            }
            list[0] = b;
            return new string(list);
        }

namespace ConsoleApplication1
{
    class Program
    {
        static void Main(string[] args)
        {
            string pwdLibrary = "defghijklmnopqrstuvwxyzabcDEFGHIJKLMNOPQRSTUVWXYZABC";
            string test = "sahjdshHDSGHDc";
            string pwd = test[test.Length-1].ToString();
            for (int i = 0; i < test.Length - 1; i++)
            {
                char c = test[i];
                pwd += (c >= 'a' ? pwdLibrary[c - 'a'].ToString() : pwdLibrary[26+c - 'A'].ToString());
            }
            Console.WriteLine(pwd);
        }
    }
}