234.cpp

凯撒密码的过程,再用配对字母取代讯息里的原始字母位移加密法(shift cipher)：模数计算。Ek(x)=(x+k)mod 26

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

void main() 
{ 
char M[100]; 
char C[100]; 
int k,i; 
cout<<"\n***加密***\n"<<endl;


cout<<"请输入明文M(注意不要输入空白串)"<<endl; 

gets(M);

cout<<"请输入您的密钥(必需是整数):"<<endl;

cin>>k;

for(i=0;M[i]!='\0';i++) 
C[i]=(M[i]-'a'+k)%26+'a'; 
C[i]='\0'; 

cout<<"所得的凯撒密码为:"<<endl;

cout<<C<<endl;

cout<<"\n***解密***\n"<<endl;

cout<<"您的凯撒密码:"<<C<<endl;

cout<<"你的密钥是："<<k<<endl;

for(i=0;C[i]!='\0';i++) 
 M[i]=(C[i]-'a'-k+26*(k/26+1))%26+'a'; 
  M[i]='\0'; 

cout<<"你的明文为:"<<M<<endl;
}