简单实现的rsa

#include<iostream>
#include<stdio.h>
#include<cmath>
#include<time.h>
#include<stdlib.h>
using std::cin;
using std::cout;
using std::endl;
class Pair{
	public:
	Pair(){};
	int e,d,n;
};
bool is_prime(int n,int *p,int c){
	if(c>sqrt((double)n))
	c=sqrt((double)n);
	for(int i=1;i<=c;i++)
	if(n%p[i]==0)
	return false;
	return true;
}
Pair rsa(){
	while(true){
	cout<<"finding......."<<endl;
	bool find=true;
	long begain,end;
	begain=clock();
	srand((int)time(0));
	int n=100+rand()%100;
	int *prime = new int[n];
	prime[1] = 2;
	int current=1;
	for(int i=3;i<=n;i++){
		if(is_prime(i,prime,current)){
			prime[++current]=i;
		}else continue;
	}
	int p = current-rand()%10;
	int q = current-rand()%10;
	while(p==q){
		q = current-rand()%1;
	}
	p = prime[p];
	q = prime[q];
	n = p*q;
	int f_n = (p-1)*(q-1);
	int e = (int)rand()%20;
	int i=0;
	e=prime[e];
	while((e==p)||(e==q)||(e>f_n)||(e==0)){
		e = rand()%10;
		i-=1;
		e=prime[e];
	}
	int d = 1;
	while(((e*d)%f_n)!=1){
		d++;
		end=clock();
		if(end-begain>5000000){
		find = false;
		break;
		}
	}
	Pair pair;
	pair.e=e;
	pair.d=d;
	pair.n=n;
	if(find)
	return pair;
	}
}
int Mont(int b,int e,int n){
	int rt=1;
	int x=b;
	while(e>0){
		if(e%2==1)
		rt=rt*x%n;
		x=x*x%n;
		e/=2;
	}
	return rt;
}
int main(){
	cout<<"Welcome to use rsa , because of the aplication don't support big number , your key will not security enough"<<endl;
	Pair pair = rsa();
	cout<<"YOUR PU("<<pair.e<<","<<pair.n<<")"<<endl;
	cout<<"YOUR PR("<<pair.d<<","<<pair.n<<")"<<endl;
	cout<<"please input an string :";
	char plain[300];
	int cipher[300];
	fgets(plain,300,stdin);
	int last = 0;
	for(int i=0;i<300;i++)
	if((plain[i]=='n')||(plain[i]=='