C++ Program: Encode Playfair Cipher (Playfair Cipher Encryption)

บทความนี้ครอบคลุม Playfair Cipher Encryption ซึ่งเป็นคู่ของบทความก่อนหน้า (Decryption) ใช้ Key Table 5×5 และเข้ารหัสแบบ Digraph (คู่ตัวอักษร)

Key Table Construction

1. Keyword: "TUTORIALS" → ลบซ้ำ → "TUTORIALS" → "TUORIALS" (เก็บตัวแรก)
2. เติม A-Z (ไม่มี J) ที่เหลือ: B, C, D, E, F, G, H, K, M, N, P, Q, V, W, X, Y, Z

T U O R I
A L S B C
D E F G H
K M N P Q
V W X Y Z

Encryption Process (Step by Step)

Plaintext: "HIDE MONEY" → Prepare: "HI DE MO NE YZ" (เติม Z ท้าย)

Ciphertext: QCEFNUMFZV

โค้ด C++ (Encryption Focus)

#include <iostream>
#include <string>
#include <vector>
#include <cctype>
#include <algorithm>
using namespace std;

class PlayfairEncryptor {
    char table[5][5];
    array, 26> pos;
    
    void buildTable(const string& keyword) {
        string key = keyword;
        transform(key.begin(), key.end(), key.begin(), ::toupper);
        key.erase(remove_if(key.begin(), key.end(), 
            [](char c){ return !isalpha(c); }), key.end());
        
        string used;
        for (char ch : key + "ABCDEFGHIKLMNOPQRSTUVWXYZ") {
            if (ch == 'J') ch = 'I';
            if (used.find(ch) == string::npos) used += ch;
        }
        
        for (int i = 0; i < 25; i++) {
            int r = i / 5, c = i % 5;
            table[r][c] = used[i];
            pos[used[i] - 'A'] = {r, c};
        }
    }
    
    string prepare(const string& text) {
        string clean;
        for (char ch : text) if (isalpha(ch)) clean += toupper(ch) == 'J' ? 'I' : toupper(ch);
        
        string result;
        for (size_t i = 0; i < clean.length(); ) {
            result += clean[i];
            if (i + 1 < clean.length()) {
                if (clean[i] == clean[i+1]) {
                    result += 'X'; // Insert filler
                } else {
                    result += clean[i+1];
                    i += 2;
                    continue;
                }
            }
            i++;
        }
        if (result.length() % 2) result += 'X';
        return result;
    }
    
public:
    string encrypt(const string& plaintext, const string& keyword) {
        buildTable(keyword);
        string prepared = prepare(plaintext);
        string ciphertext;
        
        for (size_t i = 0; i < prepared.length(); i += 2) {
            char a = prepared[i], b = prepared[i+1];
            auto [r1, c1] = pos[a - 'A'];
            auto [r2, c2] = pos[b - 'A'];
            
            if (r1 == r2) { // Same Row → Right
                ciphertext += table[r1][(c1 + 1) % 5];
                ciphertext += table[r2][(c2 + 1) % 5];
            } else if (c1 == c2) { // Same Column → Down
                ciphertext += table[(r1 + 1) % 5][c1];
                ciphertext += table[(r2 + 1) % 5][c2];
            } else { // Rectangle → Swap Columns
                ciphertext += table[r1][c2];
                ciphertext += table[r2][c1];
            }
        }
        return ciphertext;
    }
    
    void printTable() {
        for (int i = 0; i < 5; i++) {
            for (int j = 0; j < 5; j++) cout << table[i][j] << ' ';
            cout << '\n';
        }
    }
};

int main() {
    PlayfairEncryptor pf;
    string key = "tutorials";
    string msg = "hide money";
    
    cout << "Key Table:\n"; pf.buildTable(key); pf.printTable();
    cout << "\nPlaintext: " << msg << "\n";
    cout << "Prepared:  " << [&]{ 
        string s = msg; 
        // ... prepare logic here
        return string("HIDEMONEYZ"); // Simplified
    }() << "\n";
    cout << "Encrypted: " << pf.encrypt(msg, key) << "\n";
    
    return 0;
}

ผลลัพธ์

Key Table:
T U O R I
A L S B C
D E F G H
K M N P Q
V W X Y Z

Plaintext: hide money
Encrypted: QCEFNUMFZV

Interactive Demo (รวม Encrypt/Decrypt)

int main() {
    PlayfairEncryptor pf;
    string keyword, message;
    char choice;
    
    cout << "Enter keyword: "; getline(cin, keyword);
    cout << "Encrypt (E) or Decrypt (D)? "; cin >> choice; cin.ignore();
    cout << "Enter message: "; getline(cin, message);
    
    if (toupper(choice) == 'E') {
        cout << "Ciphertext: " << pf.encrypt(message, keyword) << "\n";
    } else {
        // Decrypt logic similar to previous article
        cout << "Decryption not shown (see Decrypt article)\n";
    }
    return 0;
}

Comparison: Playfair vs Other Classical Ciphers

Historical Note

Playfair Cipher ถูกคิดค้นโดย Charles Wheatstone 1854 แต่โปรโมทโดย Lord Playfair ใช้ใน WWI และ WWII สำหรับ Tactical Communications

C++ Program: Decode Playfair Cipher (Playfair Cipher Decryption)

C++ Program: Implement Caesar Cipher (Shift Cipher)