JavaScript Solution

javascript/solution.js

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+/**
+ * Creates a 5x5 Playfair cipher matrix based on a keyword.
+ * @param {string} keyword - The keyword to base the matrix on.
+ * @returns {Array} The Playfair cipher matrix.
+ */
+const createPlayfairMatrix = (keyword) => {
+  const matrix = [];
+  const seen = new Set();
+  const alphabet = 'ABCDEFGHIKLMNOPQRSTUVWXYZ'; // 'J' is not included
+
+  //Add keyword to matrix
+  for (let char of keyword) {
+    if (!seen.has(char)) {
+      matrix.push(char);
+      seen.add(char);
+    }
+  }
+
+  //Add remaining characters to matrix
+  for (let char of alphabet) {
+    if (!seen.has(char)) {
+      matrix.push(char);
+      seen.add(char);
+    }
+  }
+  return matrix;
+};
+
+/**
+ * Gets the position of a character in the Playfair cipher matrix.
+ * @param {string} char - The character to find.
+ * @param {Array} matrix - The Playfair cipher matrix.
+ * @returns {Array} The [row, column] position of the character.
+ */
+const getCharposition = (char, matrix) => {
+  const index = matrix.indexOf(char);
+  return [Math.floor(index / 5), index % 5];
+};
+
+/**
+ * Decrypts a pair of characters using the Playfair cipher rules.
+ * @param {string} pair - The pair of characters to decrypt.
+ * @param {Array} matrix - The Playfair cipher matrix.
+ * @returns {string} The decrypted pair of characters.
+ */
+const decryptPair = (pair, matrix) => {
+  const [firstChar, secondChar] = pair;
+  const [row1, col1] = getCharposition(firstChar, matrix);
+  const [row2, col2] = getCharposition(secondChar, matrix);
+
+  let decryptedPair = '';
+
+  //if two characters are in the same row
+  if (row1 === row2) {
+    decryptedPair += matrix[row1 * 5 + ((col1 + 4) % 5)];
+    decryptedPair += matrix[row2 * 5 + ((col2 + 4) % 5)];
+  }
+  //if two characters are in the same column
+  else if (col1 === col2) {
+    decryptedPair += matrix[((row1 + 4) % 5) * 5 + col1];
+    decryptedPair += matrix[((row2 + 4) % 5) * 5 + col2];
+  }
+  //if two characters are in a rectangle position
+  else {
+    decryptedPair += matrix[row1 * 5 + col2];
+    decryptedPair += matrix[row2 * 5 + col1];
+  }
+  return decryptedPair;
+};
+
+/**
+ * Decrypts a Playfair cipher text using a given keyword.
+ * @param {string} text - The encrypted text.
+ * @param {string} keyword - The keyword used to create the Playfair cipher matrix.
+ * @returns {string} The decrypted text.
+ */
+const decryptPlayfairCipher = (text, keyword) => {
+  const matrix = createPlayfairMatrix(keyword);
+
+  let decryptedText = '';
+
+  for (let i = 0; i < text.length; i += 2) {
+    const pair = text.substring(i, i + 2);
+    decryptedText += decryptPair(pair, matrix);
+  }
+  return decryptedText.replace(/X/g, ''); //removing padding 'X'
+};
+
+/**
+ * Main function to decrypt the given ciphertext using the Playfair cipher.
+ */
+function main() {
+  const ciphertext = 'IKEWENENXLNQLPZSLERUMRHEERYBOFNEINCHCV';
+  const keyword = 'SUPERSPY';
+
+  const decryptedText = decryptPlayfairCipher(ciphertext, keyword);
+
+  console.log(decryptedText);
+}
+
+main();