add hillCipher at cryptography section (#424)

* add hillCipher.js and its test case first commit

* add README.md

* update style

Co-authored-by: Oleksii Trekhleb <trehleb@gmail.com>

src/algorithms/cryptography/hillCipher/README.md

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+# Hill Cipher
+
+* The Hill cipher is a polygraphic substitution cipher based on linear algebra.
+Each letter is represented by a number modulo 26. 
+
+* Encryption: to encrypt a message, each block of n letters (considered as an n-component vector) is multiplied by an invertible n × n matrix, against modulus 26. 
+Consider the message 'ACT', and the key below (or GYB/NQK/URP in letters):
+  | 6   24   1 |
+  | 13  16   10|
+  | 20  17   15|
+The message is the vector:
+  |  0  |
+  |  2  |
+  |  19 |
+Thus the enciphered vector is given by  
+  | 6   24   1 |  |  0  |   |  67  |    |  15 |
+  | 13  16   10|  |  2  | = |  222 | ≡  |  14 | (mod 26)
+  | 20  17   15|  |  19 |   |  319 |    |  7  |
+which corresponds to a ciphertext of 'POH'.
+
+* Decryption: to decrypt the message, each block is multiplied by the inverse of the matrix used for encryption.
+
+
+## Reference
+- [Wikipedia] https://en.wikipedia.org/wiki/Hill_cipher
+- [GeeksforGeeks]https://www.geeksforgeeks.org/hill-cipher/
+

src/algorithms/cryptography/hillCipher/_test_/hillCipher.test.js

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+import hillCipherEncrypt from '../hillCipher';
+
+describe('hillCipher', () => {
+  it('should throw an error when the length of the keyString does not equal to the power of length of the message ', () => {
+    const invalidLenghOfkeyString = () => {
+      hillCipherEncrypt('hello', 'helloworld');
+    };
+
+    expect(invalidLenghOfkeyString).toThrowError();
+  });
+  it('should throw an error when message or keyString contains none letter character', () => {
+    const invalidCharacterInMessage = () => {
+      hillCipherEncrypt('hell3', 'helloworld');
+    };
+    const invalidCharacterInKeyString = () => {
+      hillCipherEncrypt('hello', 'hel12world');
+    };
+    expect(invalidCharacterInMessage).toThrowError();
+    expect(invalidCharacterInKeyString).toThrowError();
+  });
+  it('should encrypt passed message using Hill Cipher', () => {
+    expect(hillCipherEncrypt('ACT', 'GYBNQKURP')).toBe('POH');
+    expect(hillCipherEncrypt('GFG', 'HILLMAGIC')).toBe('SWK');
+  });
+});

src/algorithms/cryptography/hillCipher/hillCipher.js

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+
+/**
+ * generate key matrix from given keyString
+ *
+ * @param {integer} length
+ * @param {string} keyString
+ * @return {Array[][]} keyMatrix
+ */
+const generateKeyMatrix = (length, keyString) => {
+  const keyMatrix = [];
+  let keyStringIndex = 0;
+  for (let i = 0; i < length; i += 1) {
+    const keyMatrixRow = [];
+    for (let j = 0; j < length; j += 1) {
+      keyMatrixRow.push((keyString.codePointAt(keyStringIndex)) % 65);
+      keyStringIndex += 1;
+    }
+    keyMatrix.push(keyMatrixRow);
+  }
+  return keyMatrix;
+};
+
+/**
+ * generate message vector from given message
+ *
+ * @param {*} message
+ * @return {Array} messageVector
+ */
+const generateMessageVector = (message) => {
+  const messageVector = [];
+  for (let i = 0; i < message.length; i += 1) {
+    messageVector.push(message.codePointAt(i) % 65);
+  }
+  return messageVector;
+};
+
+/**
+ * validate data and encrypt message from given message and keyString
+ *
+ * @param {string} message plaintext
+ * @param {string} keyString
+ * @return {string} cipherString
+ *
+ */
+
+export default function hillCipherEncrypt(message, keyString) {
+  const length = keyString.length ** (0.5);
+  // keyString.length must equal to square of message.length
+  if (!Number.isInteger(length) && length !== message.length) {
+    throw new Error('invalid key string length');
+  }
+  // keyString and messange can only contain letters
+  if (!(/^[a-zA-Z]+$/.test(message)) || !(/^[A-Za-z]+$/.test(keyString))) {
+    throw new Error('messange and key string can only contain letters');
+  }
+
+  const keyMatrix = generateKeyMatrix(length, keyString);
+  const messageVector = generateMessageVector(message);
+  let ciperString = '';
+  for (let row = 0; row < length; row += 1) {
+    let item = 0;
+    for (let column = 0; column < length; column += 1) {
+      item += keyMatrix[row][column] * messageVector[column];
+    }
+    ciperString += String.fromCharCode((item % 26) + 65);
+  }
+  return ciperString;
+}