decrypt.js

// public key: [23, 55]
// private key: [7, 55]

class Decrypter {

    constructor (priv, pub, mod) {
        this.private = priv
        this.public = pub
        this.mod = mod
    }

    decryptMessage (arr) {
        return arr
            .map((num) => Decrypter.powerMod(num, this.private, this.mod))
            .map(Decrypter.mapToChar)
            .join('')
    }

    encryptMessage (message) {
        return message
            .split('')
            .map(Decrypter.mapFromChar)
            .map((num) => Decrypter.powerMod(num, this.public, this.mod))
    }

    static mapFromChar (char) {
        let code = char.toUpperCase().charCodeAt(0)
        if (65 <= code && code <= 90) {
            return code - 64
        }
        switch (code) {
            case 46: return 27      // 27 is for period
            case 32: return 28      // 28 is for space
            case 33: return 29      // 29 is for exclamation point!
            default: throw new Error('Out of bounds exception!')
        }
    }

    static mapToChar (num) {
        if (0 < num && num < 27) {
            return String.fromCharCode(num + 64)
        }
        switch (num) {
            case 27: return '.'
            case 28: return ' '
            case 29: return '!'
            default: throw new Error('Out of bounds exception!')
        }
    }

    static powerMod (n, pow, mod) {
        if (pow <= 1) return n

        let r = Decrypter.powerMod(n, pow >> 1, mod)

        if (pow % 2 === 0) {
            return (r * r) % mod
        } else {
            return (n * r * r) % mod
        }
    }
}

let message = [
    25, 17, 1, 49, 11, 39, 7, 51, 20, 2, 7, 23, 15, 1, 2, 49, 14, 49, 13, 7, 1,
    8, 20, 21, 25, 7, 1, 8, 39, 25, 2, 1, 27, 25, 7, 52, 1, 25, 17, 15, 52, 1,
    25, 14, 27, 39, 48, 17, 1, 33, 15, 7, 1, 7, 13, 2, 15, 1, 25, 7, 12, 14, 49,
    25, 15, 2, 7, 8, 2, 15, 1, 11, 24
]

let RSA = new Decrypter(7, 23, 55)

let decryptedMessage = RSA.decryptMessage(message)
let reEncryptedMessage = RSA.encryptMessage(decryptedMessage).join(' ')

console.log(`DECRYPTED MESSAGE:\t${decryptedMessage}`)
console.log('------------')
console.log(`ORIGINAL MESSAGE:\t${reEncryptedMessage}`)