Hacking the Xbox

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Hacking the Xbox: An Introduction to Reverse Engineering
being passed between two parties. Because of the man-in-the-middle
weakness, it does not make sense to use an extremely sophisticated tamperproof
module to make the system measurements. A single packaged
silicon chip is probably good enough, as it is typically easier to intercept
and spoof the measurement data going past on a printed circuit board
than it is to penetrate the epoxy package of a chip and modify the chip’s
circuitry.
The trust measurement system can be implemented using a measure-once
approach. Starting with the processor cold-boot sequence, every piece of
code is measured for trust before execution. If the processor never executes
untrusted code, then what is there not to trust? This scheme requires a very
simple tamper-proof hardware module — a tamper-proof ROM that
stores the cold-boot code, a “seed” of trust. The type of cryptography
used for the measurement and verification process is typically a combination
of hashes and public-key cryptography. Public-key cryptography is
preferred for this application because the private key required to generate
a valid code segment is a secret kept by only the hardware vendor. Again,
this scheme is vulnerable to many kinds of man-in-the-middle attacks, as
well as pure cryptographic attacks and attacks on the implementation of
the system.
A Brief Primer on Cryptography
ci·pher (n): 1 a: ZERO b: one that has no weight, worth, or influence
: NONENTITY. 2 a: a method of transforming text in order to
conceal its meaning — compare to CODE 2
Ciphers provide no security on their own. More specifically, ciphers only
provide security if the key is secure, if the algorithm is strong, and if there
are no back doors into the system. If someone hands you a CD-ROM
encrypted with a strong cipher and locks you in a padded room with a
supercomputer, the sun will probably go supernova before you can decrypt
the CD-ROM. On the other hand, if you could observe and probe the
machine as it was working to encrypt the CD-ROM, the encryption is moot.
You could get the enciphering key by eavesdropping the keyboard. Or, you
could dump the contents of the computer’s memory and obtain the
plaintext without knowing the key.
The situation with the Xbox is similar to the latter. Ultimately, the Xbox
must access and run the programs presented to it on valid disks. Furthermore,
the Pentium CPU used in the Xbox cannot tell the difference
between an authorized instruction and an unauthorized instruction.
Finally, the user has full access to probe and modify the Xbox hardware.
Thus, even if the Xbox uses strong ciphers, the security of keys is
questionable, and there may be back doors into the system.
2 Merriam-Webster OnLine Dictionary (www.webster.com).