关键词密码（Keyword Cipher）

关键词密码是单表替代密码，选用某一个词组或短语经过处理作为密钥。

原理：
随机选择一个词组或短语作为密钥，如果该关键词中有重复字母，则去掉除第一次出现外的所有重复字母。例如选取关键词”nextlegend”，则处理后为”nextlgd”。
将处理后的关键词依次排列在字母表的下方，并将除去这些关键词后字母表中剩余的字母依次填入剩余空间。如下图：
table - 关键词密码（Keyword Cipher）

像这样，上方的明文字母由下方的密文字母替换，即可实现加密。

解密即反向替换，通过密文得到明文。

例如，根据以上的对应关系，有：
明文：NEVER GIVE UP
密文：JLULP DBUL SM

特点：简单，脆弱。比凯撒密码略强。
破解：频率分析

加密代码实现：

#include<iostream>

#include<string>

#include<stdlib.h>

#include<windows.h>

using namespace std;

int Check(string s) { //检查输入的字符串是否为纯英文

int i = 0;

while (s[i] != '\0') {

if (!((s[i] >= 'a'&&s[i] <= 'z') || (s[i] >= 'A'&&s[i] <= 'Z'))) {

cout << "Error!Please enter again!" << endl;

Sleep(1000);

return 1;

}

i++;

}

return 0;

}

//主函数

int main() {

unsigned int i, j, k = 0, n = 0;

int flag = 0;

string st, keyword;

string alpha("abcdefghijklmnopqrstuvwxyz");

string key(20, '\0');

string rule(50, '\0');

system("color 2F");

do {

fflush(stdin);

system("cls");

cout << "-------------Keyword cipher(Encryption)------------" << endl;

cout << "Please enter the plaintext:" << endl;

cout << " Plaintext: ";

cin >> st;

flag = Check(st);

} while (flag);

do { //输入keyword

fflush(stdin);

cout << endl << "Select a keyword: ";

cin >> keyword;

flag = Check(keyword);

} while (flag);

for (i = 0; i < keyword.length(); i++) { //提取key

flag = 1;

for (j = 0; j < i; j++) {

if (key[j] == keyword[i])

flag = 0;

}

if (flag)

key[k++] = keyword[i];

}

key[k] = '\0'; //组合对应表

while (key[n] != '\0') {

rule[n] = key[n];

n++;

}

rule[n] = '\0';

for (i = 0; i < alpha.length(); i++) {

flag = 1;

for (j = 0; j < key.length(); j++) {

if (alpha[i] == key[j]) {

flag = 0;

break;

}

if (flag)

rule[n++] = alpha[i];

}

rule[n] = '\0';

cout << " Key: " << key << endl;

cout << endl << " Password Table" << endl;

cout << "---------------------------------------" << endl;

cout << "plaintext: " << alpha << endl;

cout << "ciphertext: " << rule << endl;

cout << "---------------------------------------" << endl << endl;

cout << "Ciphertext: ";

for (i = 0; i < st.length(); i++) {

if (st[i] >= 'a'&&st[i] <= 'z') {

cout << rule[(int)(st[i] - 'a')];

}

else {

cout << rule[(int)(st[i] - 'A')];

}

cout << endl << endl;

}

Encryption - 关键词密码（Keyword Cipher）

解密代码与加密代码几乎一致，只需在加密代码的基础上修改小部分内容即可实现。

解密代码实现：

#include<iostream>

#include<string>

#include<stdlib.h>

#include<windows.h>

using namespace std;

int Check(string s) { //检查输入的字符串是否为纯英文

int i = 0;

while (s[i] != '\0') {

if (!((s[i] >= 'a'&&s[i] <= 'z') || (s[i] >= 'A'&&s[i] <= 'Z'))) {

cout << "Error!Please enter again!" << endl;

Sleep(1000);

return 1;

}

i++;

}

return 0;

}

//主函数

int main() {

unsigned int i, j, k = 0, n = 0;

int flag = 0;

string st, keyword;

string alpha("abcdefghijklmnopqrstuvwxyz");

string key(20, '\0');

string rule(50, '\0');

system("color 2F");

do {

fflush(stdin);

system("cls");

cout << "-------------Keyword cipher(Decryption)------------" << endl;

cout << "Please enter the ciphertext:" << endl;

cout << " Ciphertext: ";

cin >> st;

flag = Check(st);

} while (flag);

do {

fflush(stdin);

cout << endl << "Enter the keyword: ";

cin >> keyword;

flag = Check(keyword);

} while (flag);

for (i = 0; i < keyword.length(); i++) {

flag = 1;

for (j = 0; j < i; j++) {

if (key[j] == keyword[i])

flag = 0;

}

if (flag)

key[k++] = keyword[i];

}

key[k] = '\0';

while (key[n] != '\0') {

rule[n] = key[n];

n++;

}

rule[n] = '\0';

for (i = 0; i < alpha.length(); i++) {

flag = 1;

for (j = 0; j < key.length(); j++) {

if (alpha[i] == key[j]) {

flag = 0;

break;

}

if (flag)

rule[n++] = alpha[i];

}

rule[n] = '\0';

cout << " Key: " << key << endl;

cout << endl << " Password Table" << endl;

cout << "---------------------------------------" << endl;

cout << "plaintext: " << alpha << endl;

cout << "ciphertext: " << rule << endl;

cout << "---------------------------------------" << endl << endl;

cout << "Plaintext: ";

for (i = 0; i < st.length(); i++) {

if (st[i] >= 'a'&&st[i] <= 'z') {

cout << alpha[rule.find(st[i])];

}

else {

cout << alpha[rule.find(st[i] + 32)];

}

cout << endl << endl;

}

Decryption - 关键词密码（Keyword Cipher）

（编译运行环境：Microsoft Visual Studio Community 2017）

逸·文鹤

人生如逆旅，我亦是行人。

关键词密码（Keyword Cipher）

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