凯撒密码加密解密

加密方法：

凯撒密码需要事先设置一个偏移量值（如：3,4,5等）；

然后将自己明文中每个字母加上该偏移量转变为另一字母（如偏移量为3，a字母转变为d字母）。

加密程序:

string encrypt(string mingwen,int k)
{
    string miwen;
    int i;
    for(i=0; i<mingwen.length(); i++)
    {
        if(mingwen[i]>='a'&&mingwen[i]<='z')
        {
            miwen += (mingwen[i]-'a'+k)%26+'a';
        }
        else if(mingwen[i]>='A'&&mingwen[i]<='Z')
        {
            miwen += (mingwen[i]-'A'+k)%26+'A';
        }
        else
        {
            miwen += mingwen[i];
        }
    }
    return miwen;
}

解密时刚好与加密相反，将密文字母减去偏移量即可。

解密程序：

string decrypt(string miwen,int k)
{
    string mingwen;
    int i;
    for(i=0; i<miwen.length(); i++)
    {
        if(miwen[i]>='a'&&miwen[i]<='z')
        {
            mingwen += (miwen[i]-'a'+26-k)%26+'a';
        }
        else if(miwen[i]>='A'&&miwen[i]<='Z')
        {
            mingwen += (miwen[i]-'A'+26-k)%26+'A';
        }
        else
        {
            mingwen+=miwen[i];
        }
    }
    return mingwen;
}

完整程序为：

#include<iostream>
#include<stdio.h>
#include<string>
#include<cstdio>
#include<fstream>
using namespace std;

//加密操作
string encrypt(string mingwen,int k)
{
    string miwen;
    int i;
    for(i=0; i<mingwen.length(); i++)
    {
        if(mingwen[i]>='a'&&mingwen[i]<='z')
        {
            miwen += (mingwen[i]-'a'+k)%26+'a';
        }
        else if(mingwen[i]>='A'&&mingwen[i]<='Z')
        {
            miwen += (mingwen[i]-'A'+k)%26+'A';
        }
        else
        {
            miwen += mingwen[i];
        }
    }
    return miwen;
}

//解密操作
string decrypt(string miwen,int k)
{
    string mingwen;
    int i;
    for(i=0; i<miwen.length(); i++)
    {
        if(miwen[i]>='a'&&miwen[i]<='z')
        {
            mingwen += (miwen[i]-'a'+26-k)%26+'a';
        }
        else if(miwen[i]>='A'&&miwen[i]<='Z')
        {
            mingwen += (miwen[i]-'A'+26-k)%26+'A';
        }
        else
        {
            mingwen+=miwen[i];
        }
    }
    return mingwen;
}

int main()
{
    string s;
    int i,k;
    ifstream fin("in.txt");
    ofstream fout("out.txt");

    cout<<"选择你需要的操作(1:加密 2:解密 3:**):";
    cin>>i;

    switch(i)
    {
    case 1:
        cout<<"输入偏移量(小于26):";
        cin>>k;
        while(getline(fin,s))
        {
            fout<<encrypt(s,k)<<endl;
        }
        fin.close();
        fout.close();
        break;
    case 2:
        cout<<"输入偏移量(小于26):";
        cin>>k;
        while(getline(fin,s))
        {
            fout<<decrypt(s,k)<<endl;
        }
        fin.close();
        fout.close();
        break;
    case 3:
        for(int j=1; j<26; j++)
        {
            ifstream fin1("in.txt");
            ofstream fbr("break.txt",ios::app);
            while(getline(fin1,s))
            {
                fbr<<decrypt(s,j)<<endl;
            }
            fin1.close();
            fbr.close();
        }
        break;
    default :
        cout<<"输入不符合规则"<<endl;
    }

}