# AffineCypherjs

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``````/**
* @description - The affine cipher is a type of monoalphabetic substitution cipher, where each letter in an alphabet is mapped to its numeric equivalent, encrypted using a simple mathematical function, and converted back to a letter
* @see - [wiki](https://en.wikipedia.org/wiki/Affine_cipher)
*/

import { CoPrimeCheck } from '../Maths/CoPrimeCheck'
// Default key for Affine Cipher
const key = 'ABCDEFGHIJKLMNOPQRSTUVWXYZ'

/**
* Fix result for negative value in modulas equation
* @param {Number} n - Constant number
* @param {Number} m - Modulos value
* @return {Number} Return n mod m
*/
function mod (n, m) {
return ((n % m) + m) % m
}

/**
* Modular multiplicative inverse
* @param {Number} a - A coefficient
* @param {Number} m - Modulos value
* @return {Number} Return modular multiplicative inverse of coefficient a and modulos m
*/
function inverseMod (a, m) {
for (let x = 1; x < m; x++) {
if (mod(a * x, m) === 1) return x
}
}

/**
* Argument validation
* @param {String} str - String to be checked
* @param {Number} a - A coefficient to be checked
* @param {Number} b - B coefficient to be checked
* @return {Boolean} Result of the checking
*/
function isCorrectFormat (str, a, b) {
if (typeof a !== 'number' || typeof b !== 'number') {
throw new TypeError('Coefficient a, b should be number')
}

if (typeof str !== 'string') {
throw new TypeError('Argument str should be String')
}

if (!CoPrimeCheck(a, 26)) {
throw new Error(a + ' is not coprime of 26')
}

return true
}

/**
* Find character index based on ASCII order
* @param {String} char - Character index to be found
* @return {Boolean} Character index
*/
function findCharIndex (char) {
return char.toUpperCase().charCodeAt(0) - ('A').charCodeAt(0)
}

/**
* Encrypt a Affine Cipher
* @param {String} str - String to be encrypted
* @param {Number} a - A coefficient
* @param {Number} b - B coefficient
* @return {String} result - Encrypted string
*/

function encrypt (str, a, b) {
let result = ''
if (isCorrectFormat(str, a, b)) {
for (let x = 0; x < str.length; x++) {
const charIndex = findCharIndex(str[x])
if (charIndex < 0) result += '-1' + ' '
else result += key.charAt(mod(a * charIndex + b, 26)) + ' '
}
}
return result.trim()
}

/**
* Decrypt a Affine Cipher
* @param {String} str - String to be decrypted
* @param {Number} a - A coefficient
* @param {Number} b - B coefficient
* @return {String} result - Decrypted string
*/
function decrypt (str, a, b) {
let result = ''
if (isCorrectFormat(str, a, b)) {
str = str.split(' ')
for (let x = 0; x < str.length; x++) {
if (str[x] === '-1') result += ' '
else {
const charIndex = findCharIndex(str[x])
result += key[mod(inverseMod(a, 26) * (charIndex - b), 26)]
}
}
return result
}
}
export { encrypt, decrypt }``````
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