Project progress v2

/*Code by Surya a.k.a Sunny*/
/* by https://www.codechef.com/users/spsc */
#include <bits/stdc++.h>
#include<string.h>
//#include <boost/multiprecision/cpp_int.hpp>
#define lli long long
#define pi 3.14159265358979323846
#define MOD 1000000007
#define foi(n)  for(lli i=0;i<n;i++)
#define foj(n)  for(lli j=0;j<n;j++)
#define test(T) lli T;cin>>T;while(T--)
#define loop(i, a, b) for(int i = (a); i<= (b); i++)
#define unbuffer cin.clear(); cin.sync();
using namespace std;
//using namespace boost::multiprecision;
void decrypt(string &backup, lli &len)
{
  lli temp,i=0,x;
    while(i<len)
      {
        temp=backup[i];
        temp-=i;
        x=(char)temp;
        backup[i]=x;
        i++;
      }
    backup[len]='\0';
    cout<<backup;
  }
int main()
{
  //ios_base::sync_with_stdio(false);
   //cin.tie(NULL);
   //cout.tie(NULL);
   lli a,i=0,temp,len;
   string str,backup;
   char x;

   cout<<"DATA ENCRYPTION SOFTWARE v1.00\n\n\n";
   //gotoxy(24,30);
   cout<<"Choose from one of the options below : \n\n\n";
   cout<<" 1. Encryption of a text \n";
   cout<<" 2. Decryption of a text \n\n\n \t";
   cin>>a;
   system("cls");
   if(a==1)
   {
   cout<<" 1.  Enter a text \n";
   cout<<" 2. Choose a .txt or .dat (binary) file by giving its name and extension \n\n\n \t";

   cin>>a;

   /*---------X------------X-----------X-----------X----------X----------X--------X---------*/

   system("cls");
   if(a==1)
    {
      cout<<"Enter your text below : \n";
      unbuffer
      getline(cin,str);
      len=str.length();
      str[len]='\0';
      char enc[len],dec[len];
      while(i<len)
        {
          /*switch(str[i])
            {
              case 's':
                enc[i]='m';
                break;
              case 'u':
                enc[i]='n';
                break;
              case 'r':
                enc[i]='g';
                break;
              case 'y':
                enc[i]='l';
                break;
              case 'a':
                enc[i]='p';
                break;
            }
          */
          temp=str[i];
          temp+=i;
          x=(char)temp;
          //cout<<x;
          enc[i]=x; //*********************************
          i++;
        }
      //cout<<str<<'\n';
      cout<<"\n_________________________________________________________________\n\n"<<enc<<"\n\n";
    i=0;
    backup=enc;
cout<<"THIS IS YOUR ENCRYPTED TEXT \n Choose an option from below : \n \t1. Manually copy this text. \n \t2. Save this text to a file. \n\n";

/*---------X------------X-----------X-----------X----------X----------X--------X---------*/

}
    /*while(i<len)
      {
        temp=backup[i];
        temp-=i;
        x=(char)temp;
        backup[i]=x;
        i++;
      }
    backup[len]='\0';
    cout<<backup;*/

    decrypt(backup,len);
 }
}