Schneier on Security

Clive Robinson • February 15, 2019 12:33 AM

@ All,

Shields only work if they’re grounded

First define what ground is…

Ground is just a refrence point that is assumed to have a low “resistance” where “current summation” produces –little or– no voltage that would be “ground noise”

However when you start talking about “ground impedence” (resistance to AC not DC) it’s a whole different can of worms. Firstly you have currents at different frequencies which by definition have different and continuously changing phases. You also get reactive components that change their impedence with frequency. Capacitors are effectively open circuit at DC but their impedence drops with increacing frequency. Inductors are effectively short circuit at DC and their impedence increases with frequency. Their reactences are opposit, and effectively cancel when the same which will happen at a “resonant frequency”. There are two types of resonance when the effective reactances are in parallel and in series. As reactive components are not perfect the two frequences are only approximately the same. There is also the “Quality factor” or Q that is related to the loaded and unloaded impedences at any given frequency. Whilst Q is most definitely not an indicator of frequency stability as a rough rule of thumb the higher the unloaded Q of a resonant circuit the more likely it is to be frequency stable, because it is often based on the physical charecteristics of the reactive components or resonators and that is important to remember when talking about sheilding components.

Shielding works by the abscence or nulling of currents. In a pure theoretical conductor the “H component” (magnetic) of the EM field induces a current in the conductor which in turn creates in antiphase it’s own field, which in effect reflects the field. As part of this the depth to which the current can penetrate the conducter is not just inversely related to frequency it is also quite shallow. Thus a high quality conductor such as silver can be plated onto a very low quality conductor like iron, or even plastic… If such a surface encapsulates an object like a tuned circuit then any radiation from the circuit will be kept inside the box. Importantly any EM radiation outside the box stays outside the box.

Things get a little more interesting when you realise that you have one set of alternating currents on the inside of the box that are unrelated to alternating currents on the outside of the box and that they can not see each other through the box unless the skin depths are insufficient.

Now the next obvious question is what happens when the box is not a box but a tube or tunnel? Well provided you are sufficient distance from the ends of the tube then it behaves in the same way as the box, what is inside stays within, what is outside stays outside.

However at the ends all sorts of things can happen, and you can look them up in an amature radio book that explains the use of coax cables as “phasing harnesses” and the like for antennas (Yes I can explain it but certain people will complain I am taking up to much thread space as it is).

However what you need to know is that in an unbalanced transmission line such as a coax line there is an outbound current on the outside of the center conductor, a return current on the inside of the outer conductor and a third current on the outside of the outer conductor. In part the third current may contain components of the other two currents depending on how you terminate the coax at either end. One way to reduce any component of the first two currents in the third current is to inductively “choke them off” this can be done in a number of ways including the use of ferrite materials, resonant circuits and lengths of transmission line that are multiples of a quaterwave or halfwave depending on if they are short or open circuit at the distant end. Knowing which is the best to use, when, why and how is a bit of a black art even for those that know the theory.

Clive Robinson • February 15, 2019 9:53 AM

@ Celos,

So in the future, we have to x-ray cables we buy before we can trust them?

The short answer is “No”.

The long answer depends on how good you are using various test instruments, from simple ohm meters upto Time Domain reflection instrumentation etc.

A USB to USB cable is generally three or four wires and or a screen. The individual wires have no reason to be connected to each other (except -DC supply to scrern in some cables).

This means the +DC supply, Data+, and Data- should be issolated from each other. That is the use of an Ohm meter using fairly high voltage (Megger or Hi-Pot tester) will indicate if as well as probably frying any “hidden electronic circuits”…

It would not be very difficult to design a little electronics box to do this (which from a Health and safety point of view would be better). In fact there has been the inverse designed some time ago the so called USB-Killer[1].

It is very simple to design a low (5V) voltage to high voltage (150V plus) generator or just rip them out of a disposable camera flash unit. The hard part is usually finding and using an appropriate trigger device (gas filed spark gap) so that both the voltage and energy dumped into the “offending” hidden electronics is not just sufficiently high but pulsed sufficiently fast, so the hidden circuit is damaged beyond repair, even if the designer has had the brains to use an AC capacitive charge circuit to power it.

Personally I’m not keen on anybody having X-Ray kit around, because at some point some joker is going to do the equivalent of “Photocopy their or somebody elses bum”. Likewise meggers and hand cranked “ring voltage” generators because some idiot will always find it funny to shock somebody for a laugh and in some cases end up killing someone. However I suspect even a carefully designed box will still get an idiot sticking some part of their anatomy in like their finger, toung, or worse. After all if a chainsaw manufacturer sees the need to warn about keeping it away from your crotch, you can almost bet that in the past some idiot has not…

[1] Now into version three atleast, it is apparently not selling well… https://en.m.wikipedia.org/wiki/USB_Killer

Clive Robinson • February 16, 2019 4:26 PM

@ Jon,

Yes, I do this sort of stuff for a living. 😉

Yes so did a lot of people a while ago…

But as “work is outsourced” abroad, it’s not just money and the very important economic churn that follows it. So does the “home knowledge” required to do the job, once that is gone, the work can not come back…

Which means the number of people with the knowledge in the general population shrinks rather rapidly to a fraction of a percent at best.

Oh and is further not helped by the level of education and intelligence required. For instance to be an accountant, MBA, lawyer etc the levels required according to University entry requirments are considerably less than for high end engineering subjects… Even a lot of medical subjects require less on the entry requirments than engineering. But… of course engineering pay in comparison is a fraction of those other subjects, which is another reason why the engineering subjects are getting studied less and less.

Thus explaining what are simple concepts to trained engineers can be almost impossible to otherwise intelligent and well educated individuals.

Try having a go your self and explain not just why the shield should be connected at one end in some cables and both in others oh and also which end it should be when it is only in one end…

Trust me it’s an uphill strugle explaining “earth bonding” issues with many electricians, especially when you are talking of having 10-50KW of HF or VHF equipment for such mundane tasks as drying of food and welding of plastics. But then with actual TX sites… you don’t need a PhD but you do need to know rather more than “that’s what the book says” for “basic safety”, otherwise people will get hurt and RF burns can take years to heal properly (I’ve got one thats fourty years old and it’s still quite visable unlike some surgery scars less than a third of it’s age).

But if you feel like pushing a very large rock up hill with your nose… so called “domestic heating engineers” ask them where they are putting “the master thermostat” for central heating and why in domestic dwellings… Trust me when I say you will find that they place them “where they have always placed them”. Which in the UK is “in the hallway” which might be valid for many semi-detached or end of terrace houses from after the 1930s but totally wrong for most other domestic dwellings especially those built since the 1980’s…

There is a story that goes around about the real reason a UK Gov initiative to make domestic property more energy efficient failed miserably. The answer they could not find enough people with the simple abilities required to meet the minimum standards to pass the exams so there were never going to be enough inspectors to work at the low level of income envisaged to make the costs of the tests viable…

Clive Robinson • February 17, 2019 3:25 AM

@ Daniel S,

It seems to me that the main problem is … that any device plugged in to the port automatically gets privileges to send input.

The problem is a bit more subtle than that which is,

“The plug in hardware and it’s firmware can not be trusted.”

It’s actually the same problem those “walled garden” “app stores” have.

It’s one of those problems that anybody who knows of it generally does not like to talk about it, because they usually have skin in the game[1] which is how they found out, and they can not think of any reliable way to fix the issue (there actually may be none).

Think of it in more human terms,

You are not a murderer untill you have killed someone.

That is simple logic but it shows that there is a before and after time for any event.

In human terms on meeting someone for the first time “what you see is what you get” because you can not “see into the mind” of that potential XXX (where you can substitute just about any descriptive noun you care to chose for XXX). It’s why we look for “refrences and referals” from others we either trust or have a reputation that can be trusted.

But even refrences and referals are based on incomplete past knowledge and no future knowledge. Therefore, sometimes you just have to “implicitly trust”. Our host @Bruce used the example of a plumber as the first example of this in one of his books.

And because humans have to trust to get things done, implicit trust can and has been abused. Which is why we have sayings about gift and Trojan horses. It is believed that various countries have as a matter of espionage planted “sleeper agents” in other countries. That is people with background stories or “legends” that as far as can be checked are solid. However at some appointed time, place, or command they will become active agents again.

The problem is there is only so much you can reasonably check about a persons background[2]. Importantly though past is usually a poor indicator of future as well, so even if a person has been trustworth untill today, that is no reason to assume the same will be true tomorrow or the day after.

Getting back to technology, the same set of rules applies in that you can only check so far. Which unfortunately combined with the limits of what you can actually check are very very limited, means that there is actually little point checking. Because it is extreamly unlikely that it will tell you anything of use. This has been the case since before the earliest PCs and is unlikely to change at any point in the near future.

So to recap, you can not check for trust, therefore there is little or no point doing so, which is the default position for by far the majoriry of systems, including USB.

[1] This is becoming less true as time goes on, people are starting to learn about the problem of trust through various people who have decided to discuss it openly. Part of this is the huge problems of App Stores and IoT, we have to pull our heads up and face reality as the consequences of not doing so are already public…

[2] It has been suggested that this might also be a reason for “collect it all” in that it builds a “virtual time machine” that allows those that want to check people the opportunity to go back far enough in time to look for anomalies etc.