multi-downloader-nx_mirror/modules/playready/elgamal.ts

import { ec as EC } from 'elliptic'
import { randomBytes } from 'crypto'
import BN from 'bn.js'

export interface Point {
    getY(): BN
    getX(): BN
    add(point: Point): Point
    mul(n: BN | bigint | number): Point
    neg(): Point
}

export default class ElGamal {
    curve: EC

    constructor(curve: EC) {
        this.curve = curve
    }

    static toBytes(n: BN): Uint8Array {
        const byteArray = n.toString(16).padStart(2, '0')
        if (byteArray.length % 2 !== 0) {
            return Uint8Array.from(Buffer.from('0' + byteArray, 'hex'))
        }
        return Uint8Array.from(Buffer.from(byteArray, 'hex'))
    }

    encrypt(messagePoint: Point, publicKey: Point): [Point, Point] {
        const ephemeralKey = new BN(randomBytes(32).toString('hex'), 16).mod(
            this.curve.n!
        )
        const ephemeralKeyBigInt = BigInt(ephemeralKey.toString(10))
        const point1 = this.curve.g.mul(ephemeralKeyBigInt)
        const point2 = messagePoint.add(publicKey.mul(ephemeralKeyBigInt))

        return [point1, point2]
    }

    static decrypt(encrypted: [Point, Point], privateKey: BN): Point {
        const [point1, point2] = encrypted
        const sharedSecret = point1.mul(privateKey)
        const decryptedMessage = point2.add(sharedSecret.neg())
        return decryptedMessage
    }
}