Feat: Add playfair cipher

ciphers/playfair_cipher.cpp

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+/**
+ * @file playfair_cipher.cpp
+ * @brief Implementation of [Playfair cipher](https://en.wikipedia.org/wiki/Playfair_cipher) algorithm.
+ *
+ * @details
+ * The Playfair cipher is a digraph substitution cipher, meaning that it encrypts pairs of letters (digraphs)
+ * instead of individual letters. It uses a 5x5 grid filled with the letters of the alphabet (combining 'I' and 'J')
+ * and a keyword to create the grid. Each pair of letters in the plaintext is encrypted by locating the letters in the grid.
+ * 
+ * @algorithm
+ * The encryption process works by first finding the letters of the digraph in the 5x5 grid:
+ *  - If the two letters are in the same row, each letter is replaced by the letter to its immediate right (wrapping to the leftmost letter if needed).
+ *  - If the two letters are in the same column, each letter is replaced by the letter immediately below it (wrapping to the topmost letter if needed).
+ *  - If the letters form a rectangle, each letter is replaced by the letter in its row at the other corner of the rectangle.
+ * 
+ * Decryption reverses these steps using the same key.
+ * 
+ * @example
+ * For Example:
+ * If the key is "PLAYFAIR", the 5x5 grid will be:
+ * ```
+ *  P L A Y F
+ *  I R B C D
+ *  E G H K M
+ *  N O Q S T
+ *  U V W X Z
+ * ```
+ * To encrypt the plaintext "HELLO", it would be first split into digraphs "HE", "LX", and "LO" (since "LL" forms a duplicate digraph, an 'X' is inserted).
+ * The encrypted pairs would then be calculated using the rules above.
+ *
+ * \note This implementation assumes that all text is in lowercase and only handles alphabetic characters ('a'-'z'). The letter 'j' is replaced with 'i'.
+ * 
+ * @author [Jayditya Dhaka]
+ */
+
+
+#include <iostream>     // For input/output stream operations
+#include <vector>       // For using vectors to create the 5x5 grid
+#include <string>       // For handling strings
+#include <cassert>      // For assertions in case of unexpected input
+
+/** @namespace ciphers
+ *  @brief Algorithms for encryption and decryption
+ */
+namespace ciphers {
+    /** @namespace playfair
+     *  @brief Functions for [Playfair cipher](https://en.wikipedia.org/wiki/Playfair_cipher) algorithm.
+     */
+    namespace playfair {
+
+        namespace {
+            /**
+             * This function cleans the input string: converts to lowercase, replaces 'j' with 'i', and removes non-alphabet characters.
+             * @param input the string to clean
+             * @return cleaned string with only lowercase alphabetic characters
+             */
+            std::string cleanString(const std::string &input) {
+                std::string cleaned;
+                for (char c : input) {
+                    if (std::isalpha(c)) {
+                        cleaned += std::tolower(c == 'j' ? 'i' : c); // Replace 'j' with 'i' and ensure lowercase
+                    }
+                }
+                return cleaned;
+            }
+
+            /**
+             * This function removes duplicate characters from the string while preserving order.
+             * @param input the string from which duplicates will be removed
+             * @return string without duplicates
+             */
+            std::string removeDuplicates(const std::string &input) {
+                std::string result;
+                for (char c : input) {
+                    if (result.find(c) == std::string::npos) {
+                        result += c;
+                    }
+                }
+                return result;
+            }
+
+            /**
+             * This function formats the plain text by handling duplicate characters and appending 'z' if needed.
+             * @param text the plain text to format
+             * @return formatted text with no duplicate letters in pairs
+             */
+            std::string formatPlainText(const std::string &text) {
+                std::string cleanedText = cleanString(text);
+                std::string formattedText;
+
+                for (size_t i = 0; i < cleanedText.length(); ++i) {
+                    formattedText += cleanedText[i];
+                    if (i < cleanedText.length() - 1 && cleanedText[i] == cleanedText[i + 1]) {
+                        formattedText += 'x'; // Insert 'x' between repeating characters
+                    }
+                }
+
+                if (formattedText.length() % 2 != 0) {
+                    formattedText += 'z'; // Add 'z' if text length is odd
+                }
+
+                return formattedText;
+            }
+
+            /**
+             * This function creates a Playfair cipher grid based on the cleaned key phrase.
+             * @param key the key phrase for the Playfair cipher
+             * @return 5x5 grid of characters for Playfair cipher
+             */
+            std::vector<std::vector<char>> createPlayfairGrid(const std::string &key) {
+                std::vector<std::vector<char>> grid(5, std::vector<char>(5));
+                size_t index = 0;
+                for (int row = 0; row < 5; ++row) {
+                    for (int col = 0; col < 5; ++col) {
+                        grid[row][col] = key[index++];
+                    }
+                }
+                return grid;
+            }
+
+            /**
+             * This function returns the coordinates of a character in the Playfair grid.
+             * @param c the character to find in the grid
+             * @param grid the 5x5 Playfair grid
+             * @return coordinates of the character in the grid as a pair (row, column)
+             */
+            std::pair<int, int> getCharacterCoordinates(char c, const std::vector<std::vector<char>> &grid) {
+                for (int row = 0; row < 5; ++row) {
+                    for (int col = 0; col < 5; ++col) {
+                        if (grid[row][col] == c) {
+                            return {row, col};
+                        }
+                    }
+                }
+                return {-1, -1}; // Should never reach here with valid input
+            }
+        } // unnamed namespace
+
+        /**
+         * Encrypts the plain text using the Playfair cipher.
+         * @param plainText the text to encrypt
+         * @param keyPhrase the key used for encryption
+         * @return encrypted text
+         */
+        std::string encrypt(const std::string &plainText, const std::string &keyPhrase) {
+            std::string formattedText = formatPlainText(plainText);
+            std::string cleanedKey = cleanString(keyPhrase) + "abcdefghiklmnopqrstuvwxyz";
+            cleanedKey = removeDuplicates(cleanedKey);
+            auto grid = createPlayfairGrid(cleanedKey);
+
+            std::string encryptedText;
+            for (size_t i = 0; i < formattedText.length(); i += 2) {
+                char char1 = formattedText[i];
+                char char2 = formattedText[i + 1];
+                std::pair<int, int> coord1 = getCharacterCoordinates(char1, grid);
+                std::pair<int, int> coord2 = getCharacterCoordinates(char2, grid);
+
+                int row1 = coord1.first, col1 = coord1.second;
+                int row2 = coord2.first, col2 = coord2.second;
+
+                if (row1 == row2) {
+                    col1 = (col1 + 1) % 5;
+                    col2 = (col2 + 1) % 5;
+                } else if (col1 == col2) {
+                    row1 = (row1 + 1) % 5;
+                    row2 = (row2 + 1) % 5;
+                } else {
+                    std::swap(col1, col2);
+                }
+
+                encryptedText += grid[row1][col1];
+                encryptedText += grid[row2][col2];
+            }
+
+            return encryptedText;
+        }
+
+        /**
+         * Decrypts the encrypted text using the Playfair cipher.
+         * @param cipherText the text to decrypt
+         * @param keyPhrase the key used for decryption
+         * @return decrypted text
+         */
+        std::string decrypt(const std::string &cipherText, const std::string &keyPhrase) {
+            std::string cleanedKey = cleanString(keyPhrase) + "abcdefghiklmnopqrstuvwxyz";
+            cleanedKey = removeDuplicates(cleanedKey);
+            auto grid = createPlayfairGrid(cleanedKey);
+
+            std::string decryptedText;
+            for (size_t i = 0; i < cipherText.length(); i += 2) {
+                char char1 = cipherText[i];
+                char char2 = cipherText[i + 1];
+                std::pair<int, int> coord1 = getCharacterCoordinates(char1, grid);
+                std::pair<int, int> coord2 = getCharacterCoordinates(char2, grid);
+
+                int row1 = coord1.first, col1 = coord1.second;
+                int row2 = coord2.first, col2 = coord2.second;
+
+                if (row1 == row2) {
+                    col1 = (col1 - 1 + 5) % 5;
+                    col2 = (col2 - 1 + 5) % 5;
+                } else if (col1 == col2) {
+                    row1 = (row1 - 1 + 5) % 5;
+                    row2 = (row2 - 1 + 5) % 5;
+                } else {
+                    std::swap(col1, col2);
+                }
+
+                decryptedText += grid[row1][col1];
+                decryptedText += grid[row2][col2];
+            }
+
+            return decryptedText;
+        }
+
+    } // namespace playfair
+} // namespace ciphers
+
+void test() {
+     // Test 1
+    std::string text1 = "heyo";
+    std::string encrypted1 = ciphers::playfair::encrypt(text1, "oldtavern");
+    std::string decrypted1 = ciphers::playfair::decrypt(encrypted1, "oldtavern");
+    assert(text1 == decrypted1);
+
+    // Test 2
+    std::string text2 = "maid";
+    std::string encrypted2 = ciphers::playfair::encrypt(text2, "oldtavern");
+    std::string decrypted2 = ciphers::playfair::decrypt(encrypted2, "oldtavern");
+    assert(text2 == decrypted2);
+}
+
+/** Driver Code */
+int main() {
+    test();
+    return 0;
+}