凯撒密码

文件结构如上图

/*    作者：朱鑫    语言: C++    说明：从in.txt读取明文或者密文，经过加密或者解密后输出到          out.txt中，如果out.txt与test.txt一样则加密或者解密          成功，否则，不成功。    日期：2011/11/21    邮箱：[e-mail]*/#include <iostream>#include <fstream>using namespace std;#define N 26#define CRYPT_OK 1#define CRYPT_ERROR -1char loweralpha[26]={                'a','b','c','d','e','f','g',                'h','i','j','k','l','m','n',                'o','p','q','r','s','t',                'u','v','w','x','y','z'             };char upperalpha[26]={                'A','B','C','D','E','F','G',                'H','I','J','K','L','M','N',                'O','P','Q','R','S','T',                'U','V','W','X','Y','Z'             };int k;int encrypt();int decrypt();int main (void){    char choice;    //密钥    cout << "please enter k:" << endl;    cin>>k;    //必须输入E(e)或者D(d)    do    {        cout << "do you want to Encryption or Decryption:"             << "(please enter E(e) or D(d)):" << endl;        cin >> choice;    }while (choice != 'E' && choice != 'e' && choice != 'D' && choice != 'd');    //加密    if (choice =='E' || choice == 'e')    {        cout << encrypt() << endl;    }    //解密    else if (choice == 'D' || choice == 'd')    {        cout << decrypt() << endl;    }    return 0;}int encrypt(){    fstream in("in.txt",ios::in);    fstream test("test.txt",ios::in);    fstream out("out.txt",ios::out);    string str;    if(!in)    {        cerr << "can't open file!" <<endl;    }    while(!in.eof())    {        getline(in, str);        for (size_t i=0;i<str.length();i++)        {            if (isalpha(str[i]))            {                if(str[i]>='a'&&str[i]<='z')                {                    if(str[i]-'a'+k > 25)                    {                        out << loweralpha[str[i]-'a'+k-26];                    }                    else                    {                        out << loweralpha[str[i]-'a'+k];                    }                }                else                {                    if(str[i]-'A'+k > 25)                    {                        out << upperalpha[str[i]-'A'+k-26];                    }                    else                    {                        out << upperalpha[str[i]-'A'+k];                    }                }            }            else            {                out << str[i];            }        }        out << endl;    }    out.close();    out.open("out.txt", ios::in);    while( !out.eof() && !test.eof() )    {        string line1, line2;        getline(out, line1);        getline(test, line2);        if(line1 != line2)        {            return CRYPT_ERROR;        }    }    in.close();    test.close();    return CRYPT_OK;}int decrypt(){    fstream in("in.txt", ios::in);    fstream test("test.txt", ios::in);    fstream out("out.txt", ios::out);    string str;    if(!in)    {        cerr << "can't open file!" << endl;    }    while(!in.eof())    {        getline(in, str);        for (size_t i=0; i<str.length(); i++)        {            if (isalpha(str[i]))            {                if (str[i] >= 'a' && str[i] <= 'z')                {                    if(str[i]-'a'-k < 0)                    {                        out << loweralpha[str[i]-'a'-k+26];                    }                    else                    {                        out << loweralpha[str[i]-'a'-k];                    }                }                else                {                    if(str[i]-'A'-k < 0)                    {                        out << upperalpha[str[i]-'A'-k+26];                    }                    else                    {                        out << upperalpha[str[i]-'A'-k];                    }                }            }            else            {                out << str[i];            }        }        out << endl;    }    out.close();    out.open("out.txt", ios::in);    while( !out.eof() && !test.eof() )    {        string line1, line2;        getline(out, line1);        getline(test, line2);        if(line1 != line2)        {            return CRYPT_ERROR;        }    }    in.close();    test.close();    return CRYPT_OK;}