Tom Weinstein wrote: > > Ian Grigg wrote: > > > Nobody doubts that it can occur, and that it *can* occur in practice. > > It is whether it *does* occur that is where the problem lies. > > This sort of statement bothers me. > > In threat analysis, you have to base your assessment on capabilities, > not intentions. If an attack is possible, then you must guard against > it. It doesn't matter if you think potential attackers don't intend to > attack you that way, because you really don't know if that's true or not > and they can always change their minds without telling you.
In threat analysis, you base your assessment on economics of what is reasonable to protect. It is perfectly valid to decline to protect against a possible threat, if the cost thereof is too high, as compared against the benefits. This is the reason that we cannot simply accept "the possible" as a basis for engineering of any form, let alone cryptography. And this is the reason why, if we can't measure it, then we are probably justified in assuming it's not a threat we need to worry about. (Of course, anecdotal evidence helps in that respect, hence there is a lot of discussion about MITMs in other forums.) iang Here's Eric Rescorla's words on this: http://www.iang.org/ssl/rescorla_1.html The first thing that we need to do is define our <i>threat model.</i> A threat model describes resources we expect the attacker to have available and what attacks the attacker can be expected to mount. Nearly every security system is vulnerable to some threat or another. To see this, imagine that you keep your papers in a completely unbreakable safe. That's all well and good, but if someone has planted a video camera in your office they can see your confidential information whenever you take it out to use it, so the safe hasn't bought you that much. Therefore, when we define a threat model, we're concerned not only with defining what attacks we are going to worry about but also those we're not going to worry about. Failure to take this important step typically leads to complete deadlock as designers try to figure out how to counter every possible threat. What's important is to figure out which threats are realistic and which ones we can hope to counter with the tools available. --------------------------------------------------------------------- The Cryptography Mailing List Unsubscribe by sending "unsubscribe cryptography" to [EMAIL PROTECTED]
