http://www.ruanyifeng.com/blog/2013/06/rsa_algorithm_part_one.html
RSA算法原理（一）
http://www.ruanyifeng.com/blog/2013/07/rsa_algorithm_part_two.html
RSA算法原理（二）

#include <stdio.h>
int candp(int a,int b,int c)//men  m^e = k*n + r
{ 
    int r=1;
    b=b+1;
    while(b!=1)
    {
        r=r*a;
        r=r%c;
        b--;
    }
    printf("%d\n",r);
    return r;
}

void main()
{
    int p,q,e,d,m,n,t,c,r;
    char s;
    printf("please input the p,q: ");
    scanf("%d%d",&p,&q);
    n=p*q;
    printf("the n is %3d\n",n);
    t=(p-1)*(q-1);
    printf("the t is %3d\n",t);
    printf("please input the e: ");
    scanf("%d",&e);
    if(e<1||e>t)
    {
        printf("e is error,please input again: ");
        scanf("%d",&e);
    }
    d=1;
    while(((e*d)%t)!=1)   d++;
    printf("then caculate out that the d is %d\n",d);

    while(1)
    {
        printf("the cipher please input 1\n");
        printf("the plain please input 2\n");
        scanf("%d",&r);
        switch(r)
        {
        case 1: printf("input the m: "); /*输入要加密的明文数字*/
            scanf("%d",&m);
            c=candp(m,e,n);
            printf("the cipher is %d\n",c);break;
        case 2: printf("input the c: "); /*输入要解密的密文数字*/
            scanf("%d",&c);
            m=candp(c,d,n);
            printf("the cipher is %d\n",m);break;
        }
    }
    getch();
}