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  1. // PLAYFAIR CIPHER
  2.  
  3. #include <iostream>
  4. #include <string>
  5. #include <vector>
  6. #include <cctype>
  7. using namespace std;
  8.  
  9. // Generate the 5x5 key table
  10. void generateKeyTable(const string &key, char table[5][5]) {
  11.     bool used[26] = {};
  12.     string processedKey = "";
  13.  
  14.     for (char c : key) {
  15.         c = tolower(c == 'j' ? 'i' : c);
  16.         if (isalpha(c) && !used[c - 'a']) {
  17.             processedKey += c;
  18.             used[c - 'a'] = true;
  19.         }
  20.     }
  21.  
  22.     for (char c = 'a'; c <= 'z'; ++c) {
  23.         if (c == 'j') continue;
  24.         if (!used[c - 'a']) {
  25.             processedKey += c;
  26.             used[c - 'a'] = true;
  27.         }
  28.     }
  29.  
  30.     int idx = 0;
  31.     for (int i = 0; i < 5; ++i)
  32.         for (int j = 0; j < 5; ++j)
  33.             table[i][j] = processedKey[idx++];
  34. }
  35.  
  36. // Find character positions in key table
  37. void findPosition(char table[5][5], char ch, int &row, int &col) {
  38.     if (ch == 'j') ch = 'i';
  39.     for (int i = 0; i < 5; ++i)
  40.         for (int j = 0; j < 5; ++j)
  41.             if (table[i][j] == ch) {
  42.                 row = i;
  43.                 col = j;
  44.                 return;
  45.             }
  46. }
  47.  
  48. // Prepare the text (insert 'x' for repeated pairs, pad with 'z' if odd length)
  49. string prepareText(string text) {
  50.     string result = "";
  51.     for (char c : text)
  52.         if (isalpha(c)) result += tolower(c == 'j' ? 'i' : c);
  53.  
  54.     string finalText = "";
  55.     for (size_t i = 0; i < result.length(); ++i) {
  56.         finalText += result[i];
  57.         if (i + 1 < result.length()) {
  58.             if (result[i] == result[i + 1])
  59.                 finalText += 'x';
  60.             else
  61.                 finalText += result[++i];
  62.         }
  63.     }
  64.     if (finalText.length() % 2) finalText += 'z';
  65.     return finalText;
  66. }
  67.  
  68. // Encrypt using Playfair cipher
  69. string encrypt(const string &text, const string &key) {
  70.     char table[5][5];
  71.     generateKeyTable(key, table);
  72.     string prepared = prepareText(text), cipher = "";
  73.  
  74.     for (size_t i = 0; i < prepared.length(); i += 2) {
  75.         int r1, c1, r2, c2;
  76.         findPosition(table, prepared[i], r1, c1);
  77.         findPosition(table, prepared[i + 1], r2, c2);
  78.  
  79.         if (r1 == r2) {
  80.             cipher += table[r1][(c1 + 1) % 5];
  81.             cipher += table[r2][(c2 + 1) % 5];
  82.         }
  83.         else if (c1 == c2) {
  84.             cipher += table[(r1 + 1) % 5][c1];
  85.             cipher += table[(r2 + 1) % 5][c2];
  86.         }
  87.         else {
  88.             cipher += table[r1][c2];
  89.             cipher += table[r2][c1];
  90.         }
  91.     }
  92.  
  93.     return cipher;
  94. }
  95.  
  96. // Decrypt using Playfair cipher
  97. string decrypt(const string &cipher, const string &key) {
  98.     char table[5][5];
  99.     generateKeyTable(key, table);
  100.     string plain = "";
  101.  
  102.     for (size_t i = 0; i < cipher.length(); i += 2) {
  103.         int r1, c1, r2, c2;
  104.         findPosition(table, cipher[i], r1, c1);
  105.         findPosition(table, cipher[i + 1], r2, c2);
  106.  
  107.         if (r1 == r2) {
  108.             plain += table[r1][(c1 + 4) % 5];
  109.             plain += table[r2][(c2 + 4) % 5];
  110.         }
  111.         else if (c1 == c2) {
  112.             plain += table[(r1 + 4) % 5][c1];
  113.             plain += table[(r2 + 4) % 5][c2];
  114.         }
  115.         else {
  116.             plain += table[r1][c2];
  117.             plain += table[r2][c1];
  118.         }
  119.     }
  120.  
  121.     return plain;
  122. }
  123.  
  124. int main() {
  125.     string key = "Monarchy";
  126.     string plaintext = "instruments";
  127.  
  128.     cout << "Key: " << key << "\nPlaintext: " << plaintext << endl;
  129.  
  130.     string ciphertext = encrypt(plaintext, key);
  131.     cout << "Ciphertext: " << ciphertext << endl;
  132.  
  133.     string decrypted = decrypt(ciphertext, key);
  134.     cout << "Decrypted: " << decrypted << endl;
  135.  
  136.     return 0;
  137. }
  138.  
Success #stdin #stdout 0.01s 5288KB
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stdin
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Standard input is empty
stdout
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Key: Monarchy
Plaintext: instruments
Ciphertext: gatlmzclrqtx
Decrypted: instrumentsz
https://ideone.com/XIh18T
language:
C++ (gcc 8.3)
created:
5 days ago
visibility:
public

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