Earlier quoted context omitted.
> Do you trust Dropbox to not send compromised JavaScript every time? If so, why not trust them with your keys in the first place? I don't trust Dropbox to do either of these things, but at least in theory I can examine the JavaScript each time they send it to me, whereas there is no way for me to inspect Dropbox's internal operations to make sure they aren't misusing my key. The problem becomes mechanically verifyin…
Actually, checking that two pieces of code are equivalent is undecideable: http://en.wikipedia.org/wiki/Lambda_calculus#Undecidability_... So mechanically checking that Dropbox-supplied code is the decryption routine claimed is impossible.
True. However, we don't need to be able to compare two arbitrary chunks of code. As long as our algorithm provides no false positives, I would settle for something as simple as a checksum.