刚学习，用整数编写了一个RSA加密算法，加深一下理解

#include<iostream>
#include<cstdlib>
#include<ctime>
#define MAX 100
using namespace std;
void Decimal_to_binary(int n,int str[],int * length)
//十进制转化为二进制，二进制存放在str数组中，length存放转化后二进制的位数
{
 int i=0;
 while(n)
 {
  str[i++]=n%2;
  n/=2;
 }
 *length=i-1;
}//Decimal_to_binary
bool WITNESS(int n)
//素数的判定
{
 int str[MAX];
 int length,a,x,d=1;
 Decimal_to_binary(n-1,str,&length);
 while(1)
 {
  a=rand()%n+1;
  if(a<n)break;
 }
 for(int i=length;i>=0;i--)
 {
  x=d;
  d=(d*d)%n;
  if(d==1&&x!=1&&x!=n-1)
   return false;
  if(str[i]==1)
   d=(d*a)%n;
 }
 if(d!=1)return false;
 return true;
}//WITNESS
bool Test_prime(int n)
//用WITNESS算法测试五次，看这个数是不是素数
{
 for(int i=0;i<5;i++)
  if(WITNESS(n)==false)return false;
 return true;
}//test_prime
bool Extended_Eucild(int f,int d,int *back)
//扩展的欧几里得算法,在d在模f的条件下的逆元
{
 int X1,X2,X3,Y1,Y2,Y3,Q,T1,T2,T3;
 //if(f<d)return Extended_Eucild(d,f,back);
 X1=1;X2=0;X3=f;Y1=0;Y2=1;Y3=d;
 while(Y3!=1&&Y3!=0)
 {
  Q=X3/Y3;
  T1=X1-Q*Y1;T2=X2-Q*Y2;T3=X3-Q*Y3;
  X1=Y1;X2=Y2;X3=Y3;
  Y1=T1;Y2=T2;Y3=T3;
 }
 if(Y3==0)
 {
  *back=X3;
  return false;
 }
 else
 {
  Y2=(Y2+f)%f;
  *back=Y2;
  return true;
 }
}//Extended_Eucild
int Fast_index(int a,int index,int n)
//快速指数算法，计算a的index次方在模n下的余数
{
 int str[MAX];
 int length;
 Decimal_to_binary(index,str,&length);
 int c=0,d=1;
 for(int i=length;i>=0;i--)
 {
  c=2*c;
  d=(d*d)%n;
  if(str[i]==1)
  {
   c=c+1;
   d=(d*a)%n;
  }
 }
 return d;
}//Fast_index
void Produce_key(int *ee,int *d,int *nn)
//产生密钥
{
 int p,q,n,fn,e,back;
 cout<<"input two primes p and q:";
 cin>>p>>q; 
 while (!(Test_prime(p)&&Test_prime(q)))
 {
  cout<<"wrong input,please make sure two number are both primes!"<<endl;
  cout<<"input two primes p and q:";
  cin>>p>>q;
 }
 n=p*q;
 fn=(p-1)*(q-1);
 cout<<"n="<<n<<",fn="<<fn<<endl;
 cout<<"please input e:";
 cin>>e;
 while(e>=fn||e<=1)
 {
  cout<<"wrong input,please make sure 1<e<"<<fn<<endl<<"Please re-enter e :";
  cin>>e;
 }
 while(Extended_Eucild(fn,e,&back)==false)
 {
  cout<<"wrong input,please make sure e and "<<fn<<" is coprime"<<endl;
  cout<<"Please re-enter e:";
  cin>>e;
 }
 cout<<"Public key is {"<<e<<","<<n<<"}"<<endl;
 cout<<"Private key is {"<<back<<","<<n<<"}"<<endl;
 *ee=e;*d=back;*nn=n;
}//Produce_key
/*
void Produce_key(int *ee,int *d,int *nn)
//产生密钥
{
 int p,q,n,fn,e,back;
 //srand((unsigned)time(NULL));
 p=rand();
 q=rand();
 while (!(Test_prime(p)&&Test_prime(q)))
 {
 p=rand();
 q=rand();
 }
 cout<<"p="<<p<<"q="<<q<<endl;
 n=p*q;
 fn=(p-1)*(q-1);
 cout<<"n="<<n<<",fn="<<fn<<endl;
 cout<<"please input e:";
 cin>>e;
 while(e>=fn||e<=1)
 {
  cout<<"wrong input,please make sure 1<e<"<<fn<<endl<<"Please re-enter e "<<endl;
  cin>>e;
 }
 while(Extended_Eucild(fn,e,&back)==false)
 {
  cout<<"wrong input,please make sure e and "<<fn<<" is coprime"<<endl;
  cout<<"Please re-enter e"<<endl;
  cin>>e;
 }
 cout<<"Public key is {"<<e<<","<<n<<"}"<<endl;
 cout<<"Private key is {"<<back<<","<<n<<"}"<<endl;
 *ee=e;*d=back;*nn=n;
}//Produce_key
*/
int Encrypt(int e,int n)
//加密
{
 int c,m;
 cout<<"please input plaintext:";
 cin>>m;
 c=Fast_index(m,e,n);
 cout<<"Ciphertext is "<<c<<endl;
 return c;
}//Encrypt
void Decrypt(int d,int n)
//解密
{
 int m,c;
 cout<<"please input ciphertext:";
 cin>>c;
 m=Fast_index(c,d,n);
 cout<<"Plaintext is "<<m<<endl;
}//Decrypt
void Print()
//打印主界面
{
 cout<<"*********************************************"<<endl;
 cout<<"***       Welcome to use RSA encoder      ***"<<endl;
 cout<<"***               p Produce key           ***"<<endl;
 cout<<"***               e Encrypt               ***"<<endl;
 cout<<"***               d Decrypt               ***"<<endl;
 cout<<"***               q Quit                  ***"<<endl;
 cout<<"*********************************************"<<endl;
 cout<<"please input your choice:"<<endl;
}//Print
int main()
{
 char ch;
 int e,d,n,c;
 Print();
 while((ch=getchar())!='q')
 {
  if(ch=='p')
   Produce_key(&e,&d,&n);
  else if(ch=='e')
   c=Encrypt(e,n);
  else if(ch=='d')
   Decrypt(d,n);
 }
 system("pause");
 return 0;
}