Hacking the Xbox

Chapter 8 - Reverse Engineering Xbox Security 135
alternate ROM image is valuable for running production test programs
using the native Xbox CPU. The physical structure of the Xbox LPC
interface implementation allows users, as well as Microsoft’s contract
manufacturer, to install a properly designed FLASH ROM override
device without any soldering.
Clearly, the ability to override the trust mechanism used in the Xbox has
sticky legal implications. While my intent was mostly to satisfy my curiosity
and secondly to run my own code on the Xbox under my fair-use rights,
other people have a desire to copy games and to modify and redistribute
Microsoft’s copyrighted kernel code. Because a cipher is blind to its application,
the extraction of the RC-4 key enables all applications equally. As a
result, I contacted the Electronic Frontier Foundation (EFF) to help me sort
through the legal issues. The legal process is a slow and ponderous one. I
had extracted the key in February, 2002, and it took until almost June before
I was allowed to publish the results of my study in the appropriate
academic forum.
Never had I experienced so much ado over 128 bits. The Digital Millennium
Copyright Act (DMCA) of 1998 has eternally changed the landscape
of hardware hacking. Reverse engineering used to be a protected
act, deemed part of what makes a marketplace healthy and competitive.
Now, tinkering with and bypassing a cryptographic security system to
exercise your fair-use rights in the privacy of your own home could serve
you thousands of dollars of fines and lawsuits. I strongly recommend that
you read Chapter 12, “Caveat Hacker,” so that you understand your legal
rights and responsibilities.
Security Through Obscurity
The technique used by Microsoft in the first version of the
Xbox security is an excellent example of security through
obscurity. A strong cipher, RC-4/128, was used to encrypt
the ROM image in order to prevent people from analyzing
the ROM contents or from creating their own ROMs. However,
RC-4/128 is a symmetric cipher, which means that the
Xbox must contain a decryption key also usable as an
encryption key. This decryption/encryption key is the important
piece of information buried inside the secret boot
ROM. Hiding this key is security through obscurity: once the
key is found, the cipher is moot and all security is lost.
True security would require that the user have access to
every single piece of the Xbox and still be unable to encrypt
their own valid FLASH ROM image. This implies that
some secret must be kept outside of the Xbox. Public-key
cryptography was invented for precisely this scenario. If
Microsoft had used a public-key cipher to encrypt or sign
the Xbox boot code, then knowing the entire contents of
the secure boot ROM would be useless, since the main
secret, Microsoft’s private key, remains safely out of our reach
in a vault somewhere in Redmond, Washington.