Читать книгу Security Engineering - Ross Anderson - Страница 125

Since 1995, all cars sold in Europe were required to have a ‘cryptographically enabled immobiliser’ and by 2010, most cars had remote-controlled door unlocking too, though most also have a fallback metal key so you can still get into your car even if the key fob battery is flat. The engine immobiliser is harder to bypass using physical means and uses a two-pass challenge-response protocol to authorise engine start. As the car key is inserted into the steering lock, the engine controller sends a challenge consisting of a random -bit number to the key using short-range radio. The car key computes a response by encrypting the challenge; this is often done by a separate RFID chip that's powered by the incoming radio signal and so keeps on working even if the battery is flat. The frequency is low (125kHz) so the car can power the transponder directly, and the exchange is also relatively immune to a noisy RF environment.

Writing for the engine controller, for the transponder in the car key, for the cryptographic key shared between the transponder and the engine controller, and for the random challenge, the protocol may look something like:

This is sound in theory, but implementations of security mechanisms often fail the first two or three times people try it.

Between 2005 and 2015, all the main remote key entry and immobiliser systems were broken, whether by security researchers, car thieves or both. The attacks involved a combination of protocol errors, poor key management, weak ciphers, and short keys mandated by export control laws.

The first to fall was TI's DST transponder chip, which was used by at least two large car makers and was also the basis of the SpeedPass toll payment system. Stephen Bono and colleagues found in 2005 that it used a block cipher with a 40-bit key, which could be calculated by brute force from just two responses [298]. This was one side-effect of US cryptography export controls, which I discuss in 26.2.7.1. From 2010, Ford, Toyota and Hyundai adopted a successor product, the DST80. The DST80 was broken in turn in 2020 by Lennert Wouters and colleagues, who found that as well as side-channel attacks on the chip, there are serious implementation problems with key management: Hyundai keys have only 24 bits of entropy, while Toyota keys are derived from the device serial number that an attacker can read (Tesla was also vulnerable but unlike the older firms it could fix the problem with a software upgrade) [2050]. Next was Keeloq, which was used for garage door openers as well as by some car makers; in 2007, Eli Biham and others found that given an hour's access to a token they could collect enough data to recover the key [244]. Worse, in some types of car, there is also a protocol bug, in that the key diversification used exclusive-or: . So you can rent a car of the type you want to steal and work out the key for any other car of that type.

Also in 2007, someone published the Philips Hitag 2 cipher, which also had a 48-bit secret key. But this cipher is also weak, and as it was attacked by various cryptanalysts, the time needed to extract a key fell from days to hours to minutes. By 2016, attacks took 8 authentication attempts and a minute of computation on a laptop; they worked against cars from all the French and Italian makers, along with Nissan, Mitsubishi and Chevrolet [748].

The last to fall was the Megamos Crypto transponder, used by Volkswagen and others. Car locksmithing tools appeared on the market from 2008, which included the Megamos cipher and were reverse engineered by researchers from Birmingham and Nijmegen – Roel Verdult, Flavio Garcia and Barış Ege – who cracked it [1956]. Although it has a 96-bit secret key, the effective key length is only 49 bits, about the same as Hitag 2. Volkswagen got an injunction in the High Court in London to stop them presenting their work at Usenix 2013, claiming that their trade secrets had been violated. The researchers resisted, arguing that the locksmithing tool supplier had extracted the secrets. After two years of argument, the case settled without admission of liability on either side. Closer study then threw up a number of further problems. There's also a protocol attack as an adversary can rewrite each 16-bit word of the 96-bit key, one after another, and search for the key 16 bits at a time; this reduces the time needed for an attack from days to minutes [1957].

Key management was pervasively bad. A number of Volkswagen implementations did not diversify keys across cars and transponders, but used a fixed global master key for millions of cars at a time. Up till 2009, this used a cipher called AUT64 to generate device keys; thereafter they moved to a stronger cipher called XTEA but kept on using global master keys, which were found in 23 models from the Volkswagen-Audi group up till 2016 [748]³.

It's easy to find out if a car is vulnerable: just try to buy a spare key. If the locksmith companies have figured out how to duplicate the key, your local garage will sell you a spare for a few bucks. We have a spare key for my wife's 2005 Lexus, bought by the previous owner. But when we lost one of the keys for my 2012 Mercedes, we had to go to a main dealer, pay over £200, show my passport and the car log book, have the mechanic photograph the vehicle identification number on the chassis, send it all off to Mercedes and wait for a week. We saw in Chapter 3 that the hard part of designing a password system was recovering from compromise without the recovery mechanism itself becoming either a vulnerability or a nuisance. Exactly the same applies here!

But the worst was still to come: passive keyless entry systems (PKES). Challenge-response seemed so good that car vendors started using it with just a push button on the dashboard to start the car, rather than with a metal key. Then they increased the radio frequency to extend the range, so that it worked not just for short-range authentication once the driver was sitting in the car, but as a keyless entry mechanism. The marketing pitch was that so long as you keep the key in your pocket or handbag you don't have to worry about it; the car will unlock when you walk up to it, lock as you walk away, and start automatically when you touch the controls. What's not to like?

Well, now you don't have to press a button to unlock your car, it's easy for thieves to use devices that amplify or relay the signals. The thief sneaks up to your front door with one relay while leaving the other next to your car. If you left your keys on the table in the hall, the car door opens and away he goes. Even if the car is immobilised he can still steal your stuff. And after many years of falling car thefts, the statistics surged in 2017 with 56% more vehicles stolen in the UK, followed by a further 9% in 2018 [824]⁴.

The takeaway message is that the attempt since about 1990 to use cryptography to make cars harder to steal had some initial success, as immobilisers made cars harder to steal and insurance premiums fell. It has since backfired, as the politicians and then the marketing people got in the way. The politicians said it would be disastrous for law enforcement if people were allowed to use cryptography they couldn't crack, even for stopping car theft. Then the immobiliser vendors' marketing people wanted proprietary algorithms to lock in the car companies, whose own marketing people wanted passive keyless entry as it seemed cool.

What can we do? Well, at least two car makers have put an accelerometer in the key fob, so it won't work unless the key is moving. One of our friends left her key on the car seat while carrying her child indoors, and got locked out. The local police advise us to use old-fashioned metal steering-wheel locks; our residents' association recommends keeping keys in a biscuit tin. As for me, we bought such a car but found that the keyless entry was simply too flaky; my wife got stranded in a supermarket car park when it just wouldn't work at all. So we took that car back, and got a second-hand one with a proper push-button remote lock. There are now chips using AES from NXP, Atmel and TI – of which the Atmel is open source with an open protocol stack.

However crypto by itself can't fix relay attacks; the proper fix is a new radio protocol based on ultrawideband (UWB) with intrinsic ranging, which measures the distance from the key fob to the car with a precision of 10cm up to a range of 150m. This is fairly complex to do properly, and the design of the new 802.15.4z Enhanced Impulse Radio is described by Srdjan Capkun and colleagues [1768]; the first chip became available in 2019, and it will ship in cars from 2020. Such chips have the potential to replace both the Bluetooth and NFC protocols, but they might not all be compatible; there's a low-rate pulse (LRP) mode that has an open design, and a high-rate pulse (HRP) variant that's partly proprietary. Were I advising a car startup, LRP would be my starting point.

Locks are not the only application of challenge-response protocols. In HTTP Digest Authentication, a web server challenges a client or proxy, with whom it shares a password, by sending it a nonce. The response consists of the hash of the nonce, the password, and the requested URI [715]. This provides a mechanism that's not vulnerable to password snooping. It's used, for example, to authenticate clients and servers in SIP, the protocol for Voice-Over-IP (VOIP) telephony. It's much better than sending a password in the clear, but like keyless entry it suffers from middleperson attacks (the beneficiaries seem to be mostly intelligence agencies).