#include <iostream.h>
#include <math.h>
#include <stdio.h>

struct PublicK      // 公钥
{
    int e;
    int n;
} puk;

struct PrivateK     //私钥
{
    int d;
    int n;
} prk;

int p,q,b;//p,q分别为两大素数，b为加密指数
int fn;//fn为欧拉数
int min,mi;//min为明文，mi为密文
int flag = 0;

bool test_prime(int prime)//检测一个数是否为素数
 {
    int i;
    int limit= (int)sqrt( (double)prime );
    for(i=2; i <= limit; i++)
    {
        if((prime%i)==0)
        {
        return false;
        break;
        }
    }
    return 1;
}

 
void switch_to_bit(int b, int bin[32]) //将十进制数据转化为二进制数组
{
    int n = 0;
    while( b > 0)
    {
        bin[n] = b % 2;
        n++;
        b /= 2;
    }
}

int CountCommonData(int a, int b)//求最大公约数
{
    int c ;

    if( c= a%b )
        return CountCommonData(b,c);
    else
        return b;
}

int CountOula(int p, int q, int &fn)//生成欧拉数
{
    if( !test_prime(p) )
        return 0;
    if( !test_prime(q) )
        return 0;
    fn= (p-1)*(q-1);
        return fn;
}



void order(int &in1, int &in2) /* 将两个数排序，大的在前面*/
{
    int a = ( in1 > in2 ? in1 : in2);
    int b = ( in1 < in2 ? in1 : in2);
    in1 = a;
    in2 = b;
}



int extend_euclid(int m, int bin) // 用扩展的欧几里得算法求乘法逆元
{
    order(m,bin);
    int a[3],b[3],t[3];
    a[0] = 1, a[1] = 0, a[2] = m;
    b[0] = 0, b[1] = 1, b[2] = bin;
    if (b[2] == 0)
    {
        return a[2] = CountCommonData(m, bin);
    }
    if (b[2] ==1)
    {
        return b[2] = CountCommonData(m, bin);
    }
    while(true)
    {
        if (b[2] ==1)
        {
            return b[1];
            break;
        }
        int q = a[2] / b[2];
        for(int i=0; i<3; i++)
        {
            t[i] = a[i] - q * b[i];
            a[i] = b[i];
            b[i] = t[i];
        }
    }
}
 
int modular_multiplication(int a, int b, int n) //快速模幂算法
{
    int f = 1;
    int bin[32];
    switch_to_bit(b,bin);
    for(int i=31; i>=0; i--)
    {
        f = (f * f) % n;
        if(bin[i] == 1)
        {
            f = (f * a) % n;
        }
    }
    return f;
}

// 产生密钥
void produce_key() {
    cout<<"input two primes p and q:";
    cin>>p>>q;
    int a;
    fn=CountOula(p, q, a);
    prk.n = p * q;
    puk.n = p * q;
    cout<<"fn = "<<fn<<endl;
    cout<<"input b :";
    cin>>b;
    while((CountCommonData(fn,b)!=1))
    {
        cout<<"b is error,try again!";
        cout<<"input b :";
        cin>>b;
    }
    prk.d = (extend_euclid(fn,b) + fn) % fn;
    puk.e = b;
    flag = 1;
    cout<<"PrivateK.d: "<<prk.d<<"PrivateK.n: "<<prk.n<<endl;
    cout<<"PublicK.e: "<<puk.e<<" Publick.n: "<<prk.n<<endl;
}


 
void encrypt() //加密
{
    if(flag == 0)
    {
        cout<<"setkey first:"<<endl;
        produce_key();
    }
    cout<<"input minwen:";
    cin>>min;
    mi = modular_multiplication(min,puk.e,puk.n);
    cout<<"miwen is:"<<mi<<endl;
}

// 解密
void decrypt() {
    if(flag == 0) {
        cout<<"setkey first:"<<endl;
        produce_key();
    }
    cout<<"input miwen:";
    cin>>mi;
    min = modular_multiplication(mi,prk.d,prk.n);
    cout<<"minwen is:"<<min<<endl;
}

int mian()
{
    cout<<"…………………菜单……………………"<<endl<<endl;
    cout<<"1.encrypt加密……………………"<<endl;
    cout<<"2.decrypt解密……………………"<<endl;
    cout<<"3.setkey生成密钥…………………………"<<endl;
    int s;
    char y='y';
    while(y='y')
    {
        cout<<"请选择菜单："<<endl;
        cin>>s;
        switch(s)
        {
        case (1):{
                  encrypt();
                  
                 }break;
        case (2):{
                 decrypt();
                 
                 }break;
        case (3):{
                  produce_key();
                 }break;
        }
        cout<<endl<<"是否继续？y/n:";
        cin>>y;
        if(y=='n')  
            break;
    }
    return 0;
}


不知道怎么回事出了一个连接上的错误Linking...
LIBCD.lib(crt0.obj) : error LNK2001: unresolved external symbol _main
Debug/aaa.exe : fatal error LNK1120: 1 unresolved externals
请各位大神指点迷津，小弟不甚感激

main 没有找到入口函数 你main 写成 mian了