feat: Add a Cryptography Algorithm Playfair cipher by Dhvani365 · Pull Request #2082 · trekhleb/javascript-algorithms

79 changes: 79 additions & 0 deletions src/algorithms/cryptography/playfair-cipher/README.md

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	# Playfair Cipher Algorithm
	---

	In cryptography, the Playfair cipher is a manual symmetric encryption technique and one of the earliest known digraph substitution ciphers. It was invented in 1854 by Charles Wheatstone, but it became widely known through Lord Playfair, who promoted its use.

	Unlike simple substitution ciphers such as the Caesar cipher, the Playfair cipher encrypts pairs of letters (digraphs) instead of single letters, making frequency analysis significantly more difficult.

	---

	## Example

	To encrypt a message, a ×ばつ5 table (key matrix) of letters is created using a keyword.
	The letters I and J are usually combined into one cell.

	For example, with the keyword MONARCHY, the key matrix becomes:
	M O N A R
	C H Y B D
	E F G I K
	L P Q S T
	U V W X Z


	When encrypting, the plaintext is first split into pairs of letters (called digraphs).
	If both letters in a pair are the same, an X is inserted between them.
	If the plaintext has an odd number of letters, an X is added to the end.

	Example:
	Plaintext: INSTRUMENTS
	Prepared: IN ST RU ME NT SX


	---

	### Encryption Rules

	1. Same Row:
	Replace each letter with the letter to its right (wrapping around to the start if needed).

	2. Same Column:
	Replace each letter with the letter below it (wrapping around to the top if needed).

	3. Rectangle Rule:
	If the letters form a rectangle, replace each letter with the letter in the same row but in the column of the other letter.

	Example:
	Plaintext: IN ST RU ME NT SX
	Ciphertext: GA TL MZ CL RQ TX


	---

	## Decryption

	Decryption uses the same key matrix and reverses the encryption rules:

	- Same Row: Replace each letter with the letter to its left.
	- Same Column: Replace each letter with the letter above it.
	- Rectangle Rule: Swap the columns back to their original positions.

	Ciphertext: GATLMZCLRQTX
	Plaintext: INSTRUMENTSX (Discard X)


	---

	## Complexity

	- Time: O(\|n\|)
	- Space: O(1)

	---

	## References

	- [Playfair cipher on Wikipedia](https://en.wikipedia.org/wiki/Playfair_cipher)
	- [Crypto Corner: Playfair Cipher Explained](https://cryptocorner.com/playfair-cipher)

34 changes: 34 additions & 0 deletions src/algorithms/cryptography/playfair-cipher/__test__/playfairCipher.test.js

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	import { playfairEncrypt, playfairDecrypt, generateKeyMatrix } from '../playfairCipher';

	describe('playfairCipher', () => {
	it('should generate a correct 5x5 matrix', () => {
	const matrix = generateKeyMatrix('monarchy');
	expect(matrix.length).toBe(5);
	expect(matrix[0].length).toBe(5);
	});

	it('should encrypt text correctly with given key', () => {
	const encrypted = playfairEncrypt('instruments', 'monarchy');
	// Example known output for Playfair cipher with key "monarchy"
	expect(encrypted).toBe('gatlmzclrqxa');
	});

	it('should decrypt text correctly with given key', () => {
	const decrypted = playfairDecrypt('gatlmzclrqtx', 'monarchy');
	expect(decrypted).toBe('instrumentsz');
	});

	it('should handle repeated letters correctly (insert x)', () => {
	const encrypted = playfairEncrypt('balloon', 'playfair example');
	expect(encrypted).toBeDefined();
	const decrypted = playfairDecrypt(encrypted, 'playfair example');
	expect(decrypted).toBe('balxloon');
	});

	it('should ignore non-alphabet characters', () => {
	const encrypted = playfairEncrypt('HELLO WORLD!', 'keyword');
	const decrypted = playfairDecrypt(encrypted, 'keyword');
	expect(typeof encrypted).toBe('string');
	expect(decrypted.replace(/x/g, '').startsWith('hello')).toBe(true);
	});
	});

109 changes: 109 additions & 0 deletions src/algorithms/cryptography/playfair-cipher/playfairCipher.js

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	// Generate a 5x5 Playfair cipher key matrix based on a given keyword.
	// Combine 'I' and 'J' into a single letter, as per the standard Playfair rules.

	const generateKeyMatrix = (key) => {
	const alphabet = 'abcdefghiklmnopqrstuvwxyz'; // 'j' is merged with 'i'
	const filteredKey = Array.from(new Set(
	key.toLowerCase().replace(/[^a-z]/g, '').replace(/j/g, 'i'),
	));
	const matrixSet = new Set(filteredKey);
	alphabet.split('').forEach((ch) => matrixSet.add(ch));

	const matrixArr = Array.from(matrixSet);
	const matrix = [];
	while (matrixArr.length) {
	matrix.push(matrixArr.splice(0, 5));
	}
	return matrix;
	};

	// Find letter position in 5x5 matrix
	const findPosition = (matrix, char) => {
	for (let row = 0; row < 5; row += 1) {
	const col = matrix[row].indexOf(char);
	if (col !== -1) return [row, col];
	}
	return [-1, -1];
	};

	// Prepare text into digraphs
	const prepareText = (text, forDecryption = false) => {
	const cleanText = text.toLowerCase().replace(/[^a-z]/g, '').replace(/j/g, 'i');
	const pairs = [];
	let i = 0;

	while (i < cleanText.length) {
	const first = cleanText[i];
	let second = cleanText[i + 1];

	if (!forDecryption) {
	if (first === second) {
	second = 'x'; // insert x for duplicate
	i += 1;
	} else {
	i += 2;
	}
	} else {
	i += 2;
	}

	if (!second) second = 'x'; // pad last char
	pairs.push(first + second);
	}

	return pairs;
	};

	const playfairEncrypt = (plaintext, key) => {
	const matrix = generateKeyMatrix(key);
	const pairs = prepareText(plaintext);
	let ciphertext = '';

	pairs.forEach((pair) => {
	const [a, b] = pair.split('');
	let [rowA, colA] = findPosition(matrix, a);
	let [rowB, colB] = findPosition(matrix, b);

	if (rowA === rowB) {
	colA = (colA + 1) % 5;
	colB = (colB + 1) % 5;
	} else if (colA === colB) {
	rowA = (rowA + 1) % 5;
	rowB = (rowB + 1) % 5;
	} else {
	[colA, colB] = [colB, colA];
	}

	ciphertext += matrix[rowA][colA] + matrix[rowB][colB];
	});

	return ciphertext;
	};

	const playfairDecrypt = (ciphertext, key) => {
	const matrix = generateKeyMatrix(key);
	const pairs = prepareText(ciphertext, true);
	let plaintext = '';

	pairs.forEach((pair) => {
	const [a, b] = pair.split('');
	let [rowA, colA] = findPosition(matrix, a);
	let [rowB, colB] = findPosition(matrix, b);

	if (rowA === rowB) {
	colA = (colA + 4) % 5; // move left
	colB = (colB + 4) % 5;
	} else if (colA === colB) {
	rowA = (rowA + 4) % 5; // move up
	rowB = (rowB + 4) % 5;
	} else {
	[colA, colB] = [colB, colA];
	}

	plaintext += matrix[rowA][colA] + matrix[rowB][colB];
	});

	return plaintext;
	};

	export { playfairEncrypt, playfairDecrypt, generateKeyMatrix };

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