Running code on a machine you don't trust (was Re: Executing Encrypted Code)

[Nelson Minar <nelson@media.mit.edu>]

ph@netcom.com (Peter Hendrickson) writes:
> At the last meeting references were made to processors which only
> execute encrypted code.  Decryption occurs on chip.
> If each chip has a unique public/secret key pair, and executes
> authenticated code only, there are some interesting implications.

Yes, interesting indeed. It would also partially solve a problem I've
been thinking about: how can I safely run code on a machine that I
don't trust?

I'm working on some mobile agent / distributed computation research.
The basic model is that I send an agent to a server (say, a Java
interpreter) running somewhere. A lot has been written about security,
how to protect the server from malicious agents.

But what about protecting agents from malicious servers? Possible
threat models include servers that steal an agent's propietary code
and data or servers that deliberately misexecute the agent's code. The
latter threat model is under serious consideration with the
distributed DES cracking project that's being designed now.

The ultimate solution is trusted hardware on the server end. I think,
for a variety of reasons, this is really unlikely to be widly
deployed. But bringing the trusted hardware needed down to just a
black-box CPU that decrypts on the fly is a neat idea.

Other ideas include obfuscating code (protects against theft),
splitting up your computation across multiple machines (spread the
risk of theft), independently verify the results of remote
comptuations (protects from spoofing), or build some reputation
mechanism for servers (so bad guys are identified). None of these
solutions is very satisfying.

I suspect that really guaranteeing safety to mobile agents is
impossible, or at least very difficult, without trusted hardware. But
I'm not 100% sure. There are some interesting notes in Applied Crypto
2nd about performing computations on encrypted data (p.540). These
algorithms seem to be of very limited application. Or are they?

If anyone has any thoughts on this issue, I'd love to hear them. If
you send to cypherpunks, please also mail me privately as I'm going
offline for a few days..