Hacking the Xbox

Chapter 7 - A Brief Primer on Security 105
This section will briefly describe the kinds of cryptographic algorithms
used in the Xbox. We will focus on the practical implications and
implementation issues of these algorithms. You will need to understand
these algorithms in order to appreciate the available attacks on the Xbox
security system.
Note
I make no pretense of addressing the theoretical aspects
of cryptography; those are beyond me and beyond the
scope of the book. Instead, I refer interested readers to
Bruce Schneier’s excellent book, Applied Cryptography
(John Wiley & Sons); most of my knowledge in cryptography
comes from that book. Readers who are already familiar
with cryptography should be able to skim or skip the
remainder of this chapter.
Classes of Cryptographic Algorithms
There are a few important classes of cryptographic algorithms used in the
Xbox. These are:
• Hashes
• Symmetric ciphers
• Public key ciphers
Hashes come in several varieties. Hashes of the cryptographic variety are
used to summarize or “digest” a large amount of data. The summary is a
number of fixed length, typically around 100 to 200 bits long, while the
source data can be of almost any size. The most important property of a
hash is that it is a one-way computation. In other words, it is easy to
compute the hash, but it is very difficult (see the sidebar “Very Difficult
Problems” to understand exactly what this means) to derive sequences of
data whose hash digest are identical or to determine anything about the
original data from the hash.
The strength of a hash against finding two sequences of data that hash to
the same value is referred to as its collision resistance, and in general, a good
n-bit hash requires about 2n/2 random data sequences to be hashed and
compared in order to cause a collision. Since hashes are designed to be
very easy to compute and very collision-resistant, they are often used to
detect whether a bit has been changed in large regions of secure data. For
many applications, it is sufficient to include just an encrypted hash of a
message in lieu of expending the computational effort to encrypt the
whole message.
Symmetric ciphers are algorithms that have encryption and decryption keys
that can be easily derived from one another. Most of the time, the
encryption and decryption keys are the same. Symmetric ciphers use a
mixing function to combine a key schedule with data that has been processed
by some cryptographic function. This mixing may be repeated several times