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[20];
double mod[20];

int main()
{
    int n,E,x;
    while ((cin>>n>>E>>x)&&n&&E&&x)
    {
        memset(binary,0,sizeof(binary));
        memset(mod,0,sizeof(mod));
        for (int i=19;i>=0;i--)
        {
            if (1<<i <= E) 
            {
                binary[i]=1;
                E-= (1<<i);
            }
            if (E<=0.001) break;
        }
        mod[0]=x%n;
        for (int i=1;i<sizeof(binary);i++)
        {
            mod[i]=fmod(pow(mod[i-1],2),n);
        }
        double temp = 1;
        for (int i=0;i<sizeof(binary);i++)
        {
            if (binary[i]==1) 
            {
                temp = fmod(fmod(temp,n)*fmod(mod[i],n),n);
            }
        }
        cout<<temp<<endl;
    }
    return 0;
}