+1, cannot happen.
I hope you guys are right. But the very words "cannot happen
" seem very dangerous--a dare.
What we mean when we say "cannot happen" is that we know precisely what it would take to make that particular attack work, and know that it is beyond anyone's capability for the foreseeable future.
The Domain Name System
(DNS) was regarded as solid and secure for almost two decades. Then three years ago somebody found a flaw
that allowed an attacker to trivially hijack an arbitrary website, e.g. a bank.
Well that's always the big risk -- that someone will find an attack you didn't think of. That has happened to bitcoin in the past (the overflow attack), and the fact that it was dealt with so well is encouraging.
In that case, fortunately, the guy that found it behaved very responsibly and it was fixed before it became public knowledge. But incidents like that make me nervous when I hear a chorus of people saying "cannot happen".
It's easy to judge the possibility of a particular proposed scheme, especially where the design was specifically made with that attack in mind. Bitcoin uses a specific cryptograph algorithm precisely the way it was designed to be used to protect against precisely this attack. We know exactly what it would take to break that scheme, and judge the scheme still reliable.
No only that, but if there were any hints that it could not be relied upon any longer, we know exactly what we would have to do to solve that problem and how to do it. It would take two years or so to do it smoothly, but it could be done even faster in an emergency. The community as a whole has an interest in protecting the value of bitcoins themselves.