Hacking the Xbox

Chapter 13 - Onward! 201
This brief history lesson demonstrates that trusted computing does not
require the cryptographic approach that is being proposed today by
Palladium and the Trusted Computing Platform Alliance (TCPA). In fact,
cryptography on its own does not provide any security. Secure key
management is really what provides all the security in Palladium/TCPA.
Cryptographic algorithms simply transfer the security of the key into the
user’s domain.
This being said, one can draw an analogy between a capability and a
cryptographic key. Both require a trusted OS to manage their creation,
dissemination and destruction. Both are equally weak if the system
cannot protect against forged keys or capabilities. The big difference is
that if a secret key leaks, all security is lost, eternally. On the other hand,
capabilities are created and destroyed dynamically, so the leakage of a
capability might lead to a security breach, but the scope and duration of
the breach is limited. To this extent, capabilities provide a more robust
solution for computer security.
Note that relying solely on cryptographic techniques for hardware
security still leaves machines open to classic buffer-overrun style attacks
and security holes due to programming errors. “Measurements” of the
software’s state help mitigate this weakness by detecting code alterations
before executing security-critical operations, but measurements are not a
perfect solution. On the other hand, buffer-overrun attacks are impossible
in systems using hardware enforced capabilities with bounds
checking.
Memory tags can also be used to implement security features that are not
feasible using a purely cryptographic approach to trusted computing. One
example is the trustable concurrent processing of compartmentalized
secrets. 4 In this example, multiple threads with varying levels of security
clearance are operating on a single processor. The hardware enforces a policy
where all threads impress their security level upon the data that they access.
In other words, every computation simultaneously operates in two
domains: the conventional arithmetic domain, and the security domain.
Suppose an unclassified thread adds two unclassified numbers and creates
a piece of data named foo. foo’s security tag is also computed in parallel
with the add arithmetic operation. In this case, the security tag’s result is
“unclassified.” Now, suppose a top-secret thread touches foo: foo’s security
tag now changes to “top secret.” Unclassified threads can no longer read
foo, even if the unclassified thread has a valid pointer to foo; foo must be
explicitly reclassified before it can be read again by unclassified threads.
Such a strictly compartmentalized security system can be used, for
example, to ensure that no internal kernel structures are ever accessible to
4 More about security systems like this can be found in a tech
note titled “A Minimal Trusted Computing Base for Dynamically
Ensuring Secure Information Flow” by Jeremy Brown and Tom
Knight. http://www.ai.mit.edu/projects/aries/Documents/
Memos/ARIES-15.pdf