[CWuestefeld]

Relevant parts:

Paradoxically, the best terrorist strategy (as long as they have enough volunteers) under unpredictable screening may be to prepare a cadre of suicide bombers for the least rigorous screening to which they might be subjected, and not, as the strategy assumes, for the most rigorous. Sent on their way, each will either succeed at destroying a plane or be caught, but either outcome serves the terrorists' objective. ...

We might reflexively assume that any passenger screening system needs to be 100% effective at detecting all possible weapons and dangerous objects, an obviously difficult task. But, fortunately, that's not the requirement. Instead, the mechanisms need only be highly effective at detecting objects that can create actual terror under the conditions they will be subjected to in an actual flight. That is, in order to have meaningful security screening, we first must understand what it realistically takes to bring down an airplane. The security system can then be designed specifically to eliminate the preconditions for successful terrorism.

The TSA's much maligned "three ounce" liquid rule is, in fact, a nice example of good security engineering of this kind. ...

[tptacek]

The idea that Matt Blaze thinks the three-ounce rule is sensible was surprising to me; I hit it, jumped back to the top, and re-read the whole article. What's the flaw in his reasoning? The three-ounce rule always seemed like one of the more ridiculous TSA measures.

[ghshephard]

What is the alternatives to preventing liquid bombs from bringing down an Airplane? What Matt Blaze is saying, is that instead of eliminating _all_ liquids (which would have also prevented aircraft from catastrophic explosions) - the TSA simply did the engineering to determine what the smallest amount of liquid was required to do significant damage, and then limited bottled liquids to something smaller than that. People can still bring on toothpaste, shampoo, and other useful liquids/medicines, but at the same time can't bring them on in large enough volumes to destroy an airplane.

[jz]

I was also surprised by this. The three-ounce rule applies to a single passenger. All it takes is a bit of team work to get around it. A 747 for example can carry between 400 and 500 passengers depending on class layout. A team of 3 terrorists would represent less than 1% of the passengers.

[ghshephard]

Kip Hawley, former head of the TSA has answered this question a number of times, including conversations with Schneier. Most of the liquids that can do the type of damage the TSA are concerned about are likely highly oxygen reactive, or otherwise have significant obstacles to being combined outside of a lab environment.

If that weren't the case, the TSA would have simply banned all liquids.

[blasdel]

Why would you have to combine the liquids into one reservoir? Couldn't the N passengers just make N bombs from their 3x 3oz, each independently capable of going off, but intended to be detonated together for maximum effect?

[tptacek]

Because then you have to get two suicide bombers on the plane at the same time.

By all indications, the 9/11 bombers were very high on the AQ food chain. The underpants bomber didn't get tens or hundreds of thousands of dollars wired to him before the op; he was a shmuck, in a strategy designed to weaponize schmucks.

If a TSA measure doubles the manpower required to carry off an AQ op, it is almost prima facie "effective". Is, I think, the logic you'd deploy against the "combine the 3oz bottles" argument.

[ghshephard]

If the TSA is acting rationally, and I have to believe they are, I would have to believe that the ingredients can't be combined outside of a lab, or trivially in a washroom. Otherwise you are right - you don't even need to have more than one person on the plane carry the dangerous liquid, in fact, you wouldn't even need the person about to blow up the plane carry the liquid - just have three or four people taking planes to random areas get together, reformulate the bomb, and then hand it off to person number five who is clean.

Of course, we're now talking about a _conspiracy_ - and a conspiracy of four average people is 100x more likely to be discovered by a intelligence team than a single attacker.

So perhaps it's the forcing of attacks to be _conspiracies_ is the goal of allowing only small amounts of liquids.

Regardless - I think we can all agree that it makes liquid bomb attacks substantially more difficult than simply allowing people to carry on as much liquid as they chose, and therefore opening the door wide open to a lone attacker just blowing up airplanes at will without possibility of detection.

[almost]

If you think about it there are things you can do with a not too large an amount of liquid chemicals that would cause serious problems on plane. I don't think making explosives is one of them.

[NikkiA]

His reasoning appears to be 'no-one has blown a plane up with liquid explosives, therefore it must be working', at least that was the impression I got from the article.

[rogersm]

After all, the latest bomber was carrying 80 grams, less than three ounces.

[ghshephard]

Note that it was 80 grams of _powder_, not liquid.