HWid Decryption Functions

hwid.js

    private _parseHandshake (handshake: Buffer) {
        const publicKey = handshake.slice(25, 58);
        const salt = handshake.slice(132, 136);
        this._theirPublicKey = publicKey;
        this._theirSalt = salt;
    }


    private _sendHandshake () {
        const ecdh = crypto.createECDH("prime256v1");
        ecdh.generateKeys();
        // @ts-ignore
        const publicKey = Buffer.from(ecdh.getPublicKey("binary", "compressed"), "binary");
        const sharedSecret = ecdh.computeSecret(this._theirPublicKey as Buffer);
        const salt = Buffer.allocUnsafe(4); // TODO NK: Randomise this later
        this._mySalt = salt;
        this._secretKey = this._deriveSecretKey(sharedSecret);
        this._bleDevice.writeToCharacteristic(Consts.BLECharacteristic.SESSION, Buffer.concat([publicKey, salt]));
    }


    private _deriveSecretKey (sharedSecret: Buffer) {
        const iv = Buffer.concat([Buffer.alloc(9), Buffer.from("mattel"), Buffer.alloc(1)]);
        let secretKey = sharedSecret.slice(0, 16);
        const cipher = crypto.createCipheriv("aes-128-ctr", secretKey, iv);
        while (iv[7] < 99) {
            secretKey = cipher.update(secretKey);
            iv[7]++;
        }
        return secretKey;
    }


    private _decryptMessage(data: Buffer) {
        const len = data.readUInt16BE(5);
        const encrypted = data.slice(8, len);
        // @ts-ignore
        const iv = Buffer.concat([data.slice(1, 5), this._theirSalt, this._mySalt, Buffer.alloc(4)]);
        // @ts-ignore
        const cipher = crypto.createDecipheriv("aes-128-ctr", this._secretKey.slice(0, 16), iv);
        return cipher.update(encrypted);
    }