利用kasiski测试法对维吉尼亚密码进行解密的代码

下面是利用 Kasiski 测试法对维吉尼亚密码进行解密的 C++ 代码示例：

#include <iostream>
#include <string>
#include <vector>
#include <map>
#include <algorithm>

using namespace std;

// 计算两个字符串的最大公因子长度
int gcd(const string& a, const string& b) {
    int len_a = a.length();
    int len_b = b.length();

    for (int i = min(len_a, len_b); i > 0; i--) {
        if (len_a % i == 0 && len_b % i == 0) {
            string sub_a = a.substr(0, i);
            string sub_b = b.substr(0, i);

            if (sub_a == sub_b) {
                return i;
            }
        }
    }

    return 1;
}

// 计算所有可能的密钥长度
vector<int> get_possible_key_length(const string& ciphertext) {
    vector<int> possible_key_length;

    for (int i = 2; i <= 20; i++) {
        map<string, vector<int>> substrings;

        for (int j = 0; j < ciphertext.length() - i; j++) {
            string substring = ciphertext.substr(j, i);

            if (substrings.find(substring) != substrings.end()) {
                substrings[substring].push_back(j);
            } else {
                substrings[substring] = vector<int> {j};
            }
        }

        for (auto it = substrings.begin(); it != substrings.end(); it++) {
            if (it->second.size() > 1) {
                vector<int> distances;

                for (int j = 0; j < it->second.size() - 1; j++) {
                    int distance = it->second[j + 1] - it->second[j];
                    distances.push_back(distance);
                }

                int gcd_result = distances[0];

                for (int j = 1; j < distances.size(); j++) {
                    gcd_result = gcd(gcd_result, distances[j]);
                }

                if (gcd_result >= 3) {
                    possible_key_length.push_back(gcd_result);
                }
            }
        }
    }

    return possible_key_length;
}

// 对密文进行解密
string decrypt(const string& ciphertext, int key_length) {
    string plaintext;

    for (int i = 0; i < key_length; i++) {
        string sub_ciphertext;

        for (int j = i; j < ciphertext.length(); j += key_length) {
            sub_ciphertext += ciphertext[j];
        }

        int max_count = 0;
        char max_char = 'A';

        for (char c = 'A'; c <= 'Z'; c++) {
            int count = 0;
            string sub_plaintext;

            for (int j = 0; j < sub_ciphertext.length(); j++) {
                char sub_char = sub_ciphertext[j] - c;

                if (sub_char < 0) {
                    sub_char += 26;
                }

                sub_plaintext += sub_char + 'A';
            }

            for (int j = 0; j < sub_plaintext.length(); j++) {
                if (sub_plaintext[j] == 'E' || sub_plaintext[j] == 'T') {
                    count++;
                }
            }

            if (count > max_count) {
                max_count = count;
                max_char = c;
            }
        }

        string sub_plaintext;

        for (int j = 0; j < sub_ciphertext.length(); j++) {
            char sub_char = sub_ciphertext[j] - max_char;

            if (sub_char < 0) {
                sub_char += 26;
            }

            sub_plaintext += sub_char + 'A';
        }

        plaintext += sub_plaintext;
    }

    return plaintext;
}

int main() {
    string ciphertext = "LXQZCLYXQZCLYXQZCLYXQZCLYXQZCLYXQZCLYXQZCLY";

    vector<int> possible_key_length = get_possible_key_length(ciphertext);

    cout << "Possible key length: ";

    for (int i = 0; i < possible_key_length.size(); i++) {
        cout << possible_key_length[i] << " ";
    }

    cout << endl;

    for (int i = 0; i < possible_key_length.size(); i++) {
        int key_length = possible_key_length[i];
        string plaintext = decrypt(ciphertext, key_length);

        cout << "Key length: " << key_length << endl;
        cout << "Plaintext: " << plaintext << endl;
    }

    return 0;
}

该代码首先通过 Kasiski 测试法计算出所有可能的密钥长度，然后对每个密钥长度使用单个字母频率分析法进行解密，最后输出解密结果。