I think there are two issues here. One is the intractability of defeating encryption protocols such as RSA, digital signatures, blinded signatures, etc. These form the basis for digital cash and they appear to be quite secure. The other issue, which I know less about, is the possibility of cryptograph- ically strong obfuscated code. Mike Duvos first mentioned this. You could have an algorithm running on your own computer and have it be impossible to determine what it is doing, or (presumably) to effectively alter the internals of the algorithm. This seems a lot more difficult to achieve, since all the information needed to tell what the program is doing is in principle in your hands. Yet the ability to actually determine this is computationally out of reach. It's not just a matter of the kinds of complexity and obscurity we have been discussing here (self-decrypting code and such tricks), but rather some mathematically strong transformation has been done on the structure of the code to hide it in a cryptographically strong way. I vaguely recall hearing about such technologies, but I can't remember where now. Can anyone provide some references, or (better) a summary of how this works and what can actually be accomplished along these lines? Thanks - Hal