[adrianN]

At least for some problems, you need very little trust. For example, you don't have to trust software for sorting your stuff, because checking that something is correctly sorted is very easy. There is an entire field of algorithms that produce answer that are easy to check, it's called Certifying Algorithms.

Unfortunately that doesn't really work if you have to trust your software _not_ to do tasks that you don't want done, like sending your personal stuff to a third party.

[SilasX]

Even there, you have to verify that it doesn't have other side effects beyond sorting (like phoning home your data) and that the mechanisms that enforce any sandboxing meant to stop this also work, etc.