Schneier on Security

Clive Robinson • June 20, 2022 9:30 AM

@ John,

Re : Computers are secure?

First define what you mean by “secure”…

@ ALL,

This is another “work to time domain” side channel attack.

Not a new idea but a newish and easier way to do it.

The basic physics are simple,

1, Work requires energy over time.
2, No work process is 100% efficient.
3, The inefficiency becomes the most dangerous polution of all “heat”.

Heat or the increase in atomic vibration is highly destructive if not conducted, radiated, or convected away.

Like all movments of energy it can carry information impressed / modulated upon it.

In this case the “sensor” to the increase in heat is part of the CPU chip and as such it has a faster, finer, or both response to heat change than a sensor on the chip package or heat sink or ambiant temprature in the system case. All of which have been used before.

The problem for an attacker is that heat energy is a very ineffective communications medium. So you need a transducer to convert the change in heat energy to something that conducts or radiates much more effectively and with better bandwidth and channel charecteristics than heat and the standard channels it travels through.

The basic energy choices are,

1, Acoustic vibrations
2, Electrical charge conduction
3, The E or H fields of EM radiation.

But the choice of transducer is a Hobson’s Choice of what is built into the system.

In the past, researchers have used,

1, Fan acoustic noise.
2, Fan electrical noise.
3, Fan EM noise.
4, Xhange in system clock crystal frequency.
5, System EM noise
6, when turned on the Spread Spectrum signal of certain motherboards EMC reduction systems.
7, Software “loops” and similar.

Obviously the better transducers give an attacker more information bandwidth and less latency, but as woth fan noise may be of very limited range. Whilst the “Delta F” of the system clock gets impressed on network timing so can be detected from the other side of the world.

Mostly the bandwidth is fractions of a Hertz and the signals observed fractions of that. The change in XTAL frequencg giving only one or two bits reliably in an hour.

However, as we go for “lighter, faster, more efficient” Smart Devices, not only does the bandwidth of existing channels go up, new IO and thermal managment devices can be used as new transducers to help “libetate information”…

So expect to see more of these types of side channel attacks in the future.

Clive Robinson • June 20, 2022 10:59 AM

@ Trevor,

Would these likely be invalidated if the processor (or is) randomly added jitter and occasionally high power spikes? Or is this unlikely to add enough noise to be practical?

Back last century the Smart Card industry was the place where power spectrum side channel attacks “were happening”.

The industry tried variations on what you suggest and they all failed for various reasons (I can go into them but it would be a very long post).

In short the longer you “average” the better the “signal to noise ratio” you as an attacker get, whilst the defenders see their noise average ever lower (halves as samole average doubles).

Ross J. Anderson, decided to try “unclocked logic” which would have the effect of making the signal to difficult to average. By kind of making it Chaotic like a double or tripple armed pendulum.

As I pointed out, to Ross the trouble with “unclocked logic” is that like the effect in single arm/rod pendulums Christian Huygines had observed centuries before, you could by adding a little energy get things to synchronise in what in electronics is called “injection logic”. Back in the 1980’s I’d quite a bit of experience using EM Radiation not just to do “injection locking”, but also getting “cross modulation” to carry information out of a computer system without having to make electrical or other physical connections to it. Worse I’d worked out how to “fault inject” by amplitude and frequency modulation of the EM carrier.

A researcher in Belgium who’s name I’m ashamd to say I’ve forgoton, was kind enough at Ross’s behest to send me a preprint of a paper where they were using pulses of EM energy using “pico inductors” to actively flip memory bits from outside the chip package. It was a logical follow on from earlier research using IR laser diodes on unpackaged chips.

As our host @Bruce has observed in the past,

“Attacks always get better, they never get worse”

As I’ve noted there are three fundemental ways to get side channel attacks to work better or further,

1, Upp the energy in the signal.
2, Reduce the energy of the noise.
3, Repeate/amplify the signal via a transducer that has the effect of 1, 2, or both.

Pays your money takes you choice…

However just the otherday we were talking about manufacturing spread, causing unique ID’s in the analogue circuitry of BlueTooth/WiFi chips analogue sections (modulator).

Such analogue circuitry as an EM signal modulator, makes an excelent “transducer” to carry out sensitive information as a “side channel”…

For reasons I’ve mentioned in the past I’d start your “stop watch” to see when the first academic paper (probably by a PhD student at the likes of Usnix) on it comes out.

Clive Robinson • June 20, 2022 11:32 AM

@ Bruce,

Perhaps it is time you wrote a piece on the fundemental problem behing these time based side channel attacks, which is,

“The need at the crypto algorithm level to do things strictly in sequence”.

The average block cipher uses Fiestel Rounds, that all have to be done in a fixed order sequence. This makes certain parts of such time based attacks completely trivial. Like glancing at a clock you know where the hands / display is going next, the only thing you have to do is synchronize your thinking to them it’s trivial.

So the question arises why are we designing algorithms to work in the strict sequence order, especially when with a little thinking it’s probably not necessary.

Take the addition or multiplication algorithms most were taught at school. Although we tend to do them in a fixed ordered sequence it takes only a moments thought to realise that much of them can be done in “any order” and it’s only the “carry” that has –sometimes– to be done in order.

If we designed cryptographic algorithms so the base logic functions could be done in any order, then we could do just that.

That is as long as they all got done they could be randomized all the time.

OK it would add a small performance hit to the algorithm but… Provided the randomization was done effectively it would increase the attackers work factor enormously because they would get not srquential signals they could relatively easily average, they would get random permutation signals they could not average.

Something algorithm designers should have a think about.

Clive Robinson • June 20, 2022 11:41 PM

@ lurker, SpaceLifeForm, ALL,

how long would it take to exfiltrate a key if the cpu is bolted down at constant speed and constant power?

Is the wrong question, based on incorrect assumptions.

If you go back to physics you know,

1, The encryption is work
2, Work is inefficient
3, Energy can not be destroyed only moved / disipated over time.
4, The movment of energy or matter in time is a signal.
5, All Signals can be measured / observed.

If you bolt down speed and power, then for actual “productive work” to be done the signal has to appear in a different domain.

All signals exist in a “Shannon Channel” it’s as fundemental to life as thermodynamics.

So you need to ask,

“Which domain will the signal appear and which Shannon Channel will it appear in?”

If you doubt this have a think about the cosmic microwave signals still leaking information about the “big bang”.

There are two basic tricks to limiting signals

1, Limit the channel bandwidth.
2, Limit the signal energy.

Both limit the energy movement in some way but it remains detectable even in the pressence of noise.

In electrical circuits it is the movment of charge in a coherent way that gives us a signal.

Charge can be stored in “cells” or “reactive” components. It becomes disipated in resistive components via heat that can be conducted, radiated or convected into the more general environment in a process ultimately called entropy. Each of these three movment types have an associated Shannon Channel…

So any information impressed / modulated on the charge movment will get out into the general environment, it can not be avoided.

It’s a little like the “air bubbles under wall paper” problem, if you push it down in one place, it simply pops up in another.

You have to work out what channel the signal appears in and how to disipate it so it becomes part of the background noise that an opponent can not monitor sufficiently well to make it usefull.

Without going into tedious detail to detect a signal you have to be able to synchronize to it in some way.

Thus the best line of defence is to prevent synchronisation, which can be difficult.

In fact “constant speed” is “constant time” which alows for very accurate synchronization even in very high noise levels. Put simply a tuned circuit bassed on reactive components in theory has no loss, so any inphase signal constructively adds to the amplitude of the resonance, so eventually a usable synchronisation signal will be obtained.

Essentially that is what you do when you “average” the signal over many many events.

Once you have synchronisation, you can then use it to make the measurment process. Again averaging over a long enough period of time will give the “key” because it is both static and in many algorithms positian invariant.

It’s why making the algorithm such that the key bits are randomly variant in order stops the averaging process from working far better than trying to hide it under added faux noise, that gets averaged out to zero over time.