Hacking the Xbox

Chapter 7 - A Brief Primer on Security 103
of Xbox Live to drive hardware sales. Furthermore, Xbox Live is a
subscription service, and one year from its launch users will have to pay a
monthly fee. If Microsoft can get its subscribers hooked on Xbox Live,
then all of a sudden the Xbox business looks quite profitable, even if a
substantial amount of money is lost up-front on the hardware. The trick
is, of course, hook Xbox users on Xbox Live. Billed as the “Disneyland
of on-line gaming,” the goal of Xbox Live is to provide a well-executed
and fair gaming experience. Central to the value proposition of Xbox
Live that there are no cheaters. In order to ensure that nobody is cheating,
users must be forced to authenticate themselves against a registry
maintained by Xbox Live, and their game state must be kept secure and
unmodifiable. In addition, game software must be unpatched. Even more
crucial is the fact that you only need a few cheaters to ruin the gaming
experience of an entire user base. All of a sudden, the front-door security
protections offered by the DVD-9 format seem inadequate. The odds are
against you if you betting the success of a business on the morality and
honor of a user base of millions of twenty-something hardcore male
gamers with a reasonable amount of computer savvy distributed
throughout. The hardware must be trustable, network connections
secure, and executables signed and sealed.
The statement that the hardware must be trustable bears repeating. Given an
untrustable user base, the only way to establish a trust relationship with
clients is if a seed of trust exists in every piece of hardware. Hence,
Microsoft must include in every client a piece of tamper-proof hardware that
enables some kind of attestation. Attestation is the ability to prove that
some piece of data, such as a player’s identity or game state, is in fact
generated by untainted software and hardware. The tamper-proof
hardware does not have to implement the attestation function directly,
but it must at least ensure that the system is in a trustable state before
attestation.
There are many ways to ensure that hardware is trustable. The bruteforce
method is to make the entire piece of hardware physically secure.
Automated Teller Machines are prime examples of hardware that is
physically secure. Sealed in thick sheet metal and covered with intrusion
sensors, it is difficult to physically penetrate and modify the hardware of
an ATM. Still, hile effective, this is an impractical and expensive solution
for a video game console.
A more economical solution is to use a small piece of trusted tamper-proof
hardware that can make “measurements” on the rest of the system. These
sorts of measurements are typically accomplished through the use of a
cryptographic hash function. If all of these trust measurements conform
with the expected values, then one might be able to conclude that the entire
system is trustable.
I say might because this scheme is still vulnerable to man-in-the-middle
attacks where a hacker sends spoofed valid data in response to a measurement
query. Man-in-the-middle attacks refer to a general class of
attacks where an adversary can freely modify and control the information