This paper discusses the hardware foundations of the cryptosystem employed by the Xbox TM video game console from Microsoft. A secret boot block overlay is buried within a system ASIC. This secret boot block decrypts and verifies portions of an external FLASH-type ROM. The presence of the secret boot block is camouflaged by a decoy boot block in the external ROM. The code contained within the secret boot block is transferred to the CPU in the clear over a set of high-speed busses where it can be extracted using simple custom hardware. The paper concludes with recommendations for improving the Xbox security system. One lesson of this study is that the use of a high-performance bus alone is not a sufficient security measure, given the advent of inexpensive, fast rapid prototyping services and high-performance FPGAs.

Every cryptosystem is based on some kind of secret, such as a key. Regardless of the cipher, the security of a cryptosystem is only as strong as the secrecy of the key. Thus, some of the most startlingly effective attacks on a cryptosystem involve no ciphertext analysis, but instead find flaws in the protocols that manage the keys. Cryptosystems based on symmetric ciphers are particularly vulnerable to protocol attacks, since both the sender and the receiver must be trusted to have a copy of the same secret key. Despite the difficulty of key management in symmetric ciphers, they remain attractive because of their algorithmic simplicity and high throughput when compared to public key ciphers.

Symmetric cipher key management becomes especially problematic when the receiving party is not trusted or is in a position that can be easily compromised. This is where tamper-resistant hardware comes into play; a summary of tamper-resistance guidelines can be found in [6]. Many systems employ tamper-resistant hardware techniques in varying degrees, including the Sandia National Labs’ “Stronglink” micromechanical 24-bit lock [2], the Clipper chip [1], IBM’s 4758 PCI Cryptographic Coprocessor [3], Cryptographic Smartcards [5] [4], Automatic Teller Machines (ATMs), and now, video game consoles. However, trusting inadequate physical security measures to protect important secrets is risky. [14] and [15] present examples of how some of the aforementioned tamper-resistant systems can be defeated with surprisingly simple and direct methods.

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