c++ 凯撒密码

????????凯撒密码（Caesar Cipher）是一种简单的替代加密技术，以罗马大帝凯撒·尤利乌斯·凯撒（Julius Caesar）的名字命名。它通过将每个字母按照字母表向后移动固定数量的位置来进行加密。

示例一：

/**
?* @file caesar_cipher.cpp
?* @brief Implementation of [Caesar cipher](https://en.wikipedia.org/wiki/Caesar_cipher) algorithm.
?*
?* @details
?* In cryptography, a Caesar cipher, also known as Caesar's cipher, the shift cipher,?
?* Caesar's code or Caesar shift, is one of the simplest and most widely known encryption?
?* techniques. It is a type of substitution cipher in which each letter in the plaintext?
?* is replaced by a letter some fixed number of positions down the alphabet. For example,?
?* with a left shift of 3, D would be replaced by A, E would become B, and so on.?
?* The method is named after Julius Caesar, who used it in his private correspondence.
?*
?* ### Algorithm
?* The encryption can also be represented using modular arithmetic by first transforming?
?* the letters into numbers, according to the scheme, A → 0, B → 1, ..., Z → 25.
?* Encryption of a letter x by a shift n can be described mathematically as,
?* \f[ E(x) = (x + n)\;\mbox{mod}\; 26\f]
?* while decryption can be described as,
?* \f[ D(x) = (x - n) \;\mbox{mod}\; 26\f]
?*?
?* \note This program implements caesar cipher for only uppercase English alphabet characters (i.e. A-Z).?
?*?
?* @author [Deep Raval](https://github.com/imdeep2905)
?*/
#include <iostream>
#include <string>
#include <cassert>

/** \namespace ciphers
?* \brief Algorithms for encryption and decryption
?*/
namespace ciphers {
? ? /** \namespace caesar
? ? ?* \brief Functions for [Caesar cipher](https://en.wikipedia.org/wiki/Caesar_cipher) algorithm.
? ? ?*/
? ? namespace caesar { ??
? ? ? ? namespace {
? ? ? ? ? ? /**
? ? ? ? ? ? ?* This function finds character for given value (i.e.A-Z)
? ? ? ? ? ? ?* @param x value for which we want character?
? ? ? ? ? ? ?* @returns ?corresponding character for perticular value
? ? ? ? ? ? ?*/ ? ? ? ?
? ? ? ? ? ? inline char get_char(const int x) {
? ? ? ? ? ? ? ? // By adding 65 we are scaling 0-25 to 65-90.?
? ? ? ? ? ? ? ? // Which are in fact ASCII values of A-Z.?
? ? ? ? ? ? ? ? return char(x + 65);?
? ? ? ? ? ? }
? ? ? ? ? ? /**
? ? ? ? ? ? ?* This function finds value for given character (i.e.0-25)
? ? ? ? ? ? ?* @param c character for which we want value
? ? ? ? ? ? ?* @returns returns corresponding value for perticular character
? ? ? ? ? ? ?*/ ?
? ? ? ? ? ? inline int get_value(const char c) {
? ? ? ? ? ? ? ? // A-Z have ASCII values in range 65-90.
? ? ? ? ? ? ? ? // Hence subtracting 65 will scale them to 0-25.
? ? ? ? ? ? ? ? return int(c - 65);
? ? ? ? ? ? }
? ? ? ? } // Unnamed namespace
? ? ? ? /**
? ? ? ? ?* Encrypt given text using caesar cipher.
? ? ? ? ?* @param text text to be encrypted
? ? ? ? ?* @param shift number of shifts to be applied
? ? ? ? ?* @returns new encrypted text
? ? ? ? ?*/
? ? ? ? std::string encrypt (const std::string &text, const int &shift) {
? ? ? ? ? ? std::string encrypted_text = ""; // Empty string to store encrypted text
? ? ? ? ? ? for (char c : text) { // Going through each character
? ? ? ? ? ? ? ? int place_value = get_value(c); // Getting value of character (i.e. 0-25)
? ? ? ? ? ? ? ? place_value = (place_value + shift) % 26; // Applying encryption formula
? ? ? ? ? ? ? ? char new_char = get_char(place_value); // Getting new character from new value (i.e. A-Z)
? ? ? ? ? ? ? ? encrypted_text += new_char; // Appending encrypted character
? ? ? ? ? ? }
? ? ? ? ? ? return encrypted_text; // Returning encrypted text
? ? ? ? }
? ? ? ? /**
? ? ? ? ?* Decrypt given text using caesar cipher.
? ? ? ? ?* @param text text to be decrypted
? ? ? ? ?* @param shift number of shifts to be applied
? ? ? ? ?* @returns new decrypted text
? ? ? ? ?*/ ? ? ? ?
? ? ? ? std::string decrypt (const std::string &text, const int &shift) {
? ? ? ? ? ? std::string decrypted_text = ""; // Empty string to store decrypted text
? ? ? ? ? ? for (char c : text) { // Going through each character
? ? ? ? ? ? ? ? int place_value = get_value(c); // Getting value of character (i.e. 0-25)
? ? ? ? ? ? ? ? place_value = (place_value - shift) % 26;// Applying decryption formula
? ? ? ? ? ? ? ? if(place_value < 0) { // Handling case where remainder is negative?
? ? ? ? ? ? ? ? ? ? place_value = place_value + 26;
? ? ? ? ? ? ? ? }
? ? ? ? ? ? ? ? char new_char = get_char(place_value); // Getting original character from decrypted value (i.e. A-Z)
? ? ? ? ? ? ? ? decrypted_text += new_char; // Appending decrypted character
? ? ? ? ? ? }
? ? ? ? ? ? return decrypted_text; // Returning decrypted text
? ? ? ? }
? ? } // namespace caesar
} // namespace ciphers

/**
?* Function to test above algorithm
?*/
void test() {
? ? // Test 1
? ? std::string text1 = "ALANTURING";
? ? std::string encrypted1 = ciphers::caesar::encrypt(text1, 17);
? ? std::string decrypted1 = ciphers::caesar::decrypt(encrypted1, 17);
? ? assert(text1 == decrypted1);
? ? std::cout << "Original text : " << text1;
? ? std::cout << " , Encrypted text (with shift = 21) : " << encrypted1;
? ? std::cout << " , Decrypted text : "<< decrypted1 << std::endl;
? ? // Test 2
? ? std::string text2 = "HELLOWORLD";
? ? std::string encrypted2 = ciphers::caesar::encrypt(text2, 1729);
? ? std::string decrypted2 = ciphers::caesar::decrypt(encrypted2, 1729);
? ? assert(text2 == decrypted2);
? ? std::cout << "Original text : " << text2;
? ? std::cout << " , Encrypted text (with shift = 1729) : " << encrypted2;
? ? std::cout << " , Decrypted text : "<< decrypted2 << std::endl;
}

/** Driver Code */
int main() {
? ? // Testing
? ? test();
? ? return 0;
}

示例二：

使用C++实现凯撒密码的示例：

#include <iostream>
#include <string>

using namespace std;

// 凯撒密码加密函数
string encryptCaesarCipher(string plaintext, int shift) {
? ? string ciphertext = "";
? ??
? ? for (int i = 0; i < plaintext.length(); i++) {
? ? ? ? if (isalpha(plaintext[i])) {
? ? ? ? ? ? // 获取字符的ASCII码
? ? ? ? ? ? int ascii = int(plaintext[i]);
? ? ? ? ? ??
? ? ? ? ? ? // 根据移动位数进行替代
? ? ? ? ? ? if (islower(plaintext[i])) {
? ? ? ? ? ? ? ? ascii = (ascii - 97 + shift) % 26 + 97;
? ? ? ? ? ? } else {
? ? ? ? ? ? ? ? ascii = (ascii - 65 + shift) % 26 + 65;
? ? ? ? ? ? }
? ? ? ? ? ??
? ? ? ? ? ? // 将替代后的字符添加到密文中
? ? ? ? ? ? ciphertext += char(ascii);
? ? ? ? } else {
? ? ? ? ? ? // 非字母字符直接添加到密文中
? ? ? ? ? ? ciphertext += plaintext[i];
? ? ? ? }
? ? }
? ??
? ? return ciphertext;
}

// 凯撒密码解密函数
string decryptCaesarCipher(string ciphertext, int shift) {
? ? string plaintext = "";
? ??
? ? // 将移位数转换为负数，以实现向前移动的效果
? ? shift = -shift;
? ??
? ? // 调用加密函数进行解密
? ? plaintext = encryptCaesarCipher(ciphertext, shift);
? ??
? ? return plaintext;
}

int main() {
? ? string plaintext = "Hello, World!";
? ? int shift = 3;
? ??
? ? // 加密并输出密文
? ? string ciphertext = encryptCaesarCipher(plaintext, shift);
? ? cout << "Ciphertext: " << ciphertext << endl;
? ??
? ? // 解密并输出明文
? ? string decryptedText = decryptCaesarCipher(ciphertext, shift);
? ? cout << "Decrypted Text: " << decryptedText << endl;
? ??
? ? return 0;
}
????????在上述示例代码中，`encryptCaesarCipher`函数实现了凯撒密码的加密过程，`decryptCaesarCipher`函数实现了凯撒密码的解密过程。使用`main`函数来测试加密和解密功能，并输出结果。

????????注意，该凯撒密码实现只对字母字符进行加密和解密，非字母字符将保持原样。同时，移位数应为正整数，表示向后移动的位数。解密过程是通过将移位数转换为负数，然后调用加密函数实现的。