Abstract Algebra chapter 7

7.7:Encrypt each of the following RSA messages x so that x is divided into blocks of integers of length 2; that is, if x = 142528,encode 14,25,and 28 separately.

RSA加密方法：y=x^E mod n

计算时可采用重复乘方法（repeated squares）

 #include<iostream>
 #include<algorithm>
 #include<cstring>
 #include<string>
 #include<cmath>
 #include<cstdio>
 using namespace std;

 int binary[];
 double mod[];

 int main()
 {
     int n,E,x;
     while ((cin>>n>>E>>x)&&n&&E&&x)
     {
         memset(binary,,sizeof(binary));
         memset(mod,,sizeof(mod));
         for (int i=;i>=;i--)
         {
             if (<<i <= E)
             {
                 binary[i]=;
                 E-= (<<i);
             }
             if (E<=0.001) break;
         }
         mod[]=x%n;
         for (int i=;i<sizeof(binary);i++)
         {
             mod[i]=fmod(pow(mod[i-],),n);
         }
         double temp = ;
         for (int i=;i<sizeof(binary);i++)
         {
             if (binary[i]==)
             {
                 temp = fmod(fmod(temp,n)*fmod(mod[i],n),n);
             }
         }
         cout<<temp<<endl;
     }
     return ;
 }