Perpetual motion from light

Perpetual Motion Machines are those that would produce free, endless power, thus ending all of humanity’s energy problems (and maybe some political ones at the same time). I am not going to embarrass myself showing pictures of my early perpetual motion machines on this page, since you can find some very similar ones (guess which ones they are) going to this excellent website. What this article is about is a kind of perpetual motion machine that so far I haven”t been able to prove how it doesn’t work. Maybe you will…

What is Perpetual Motion?

Perpetual motion refers to a device that produces free power forever. This does not apply to things like the motion of the earth or the moon, because those would eventually stop if we extracted power from them (in fact, the moon always shows the same face toward earth precisely because, when it was young and soft, the energy of its rotation was “extracted” into tides that eventually died down). The motion of the heavenly bodies is actually inertia, not “perpetual motion.”

Many people have tried to invent (or, more likely, pretend to have invented) devices that produce free energy. Such devices, or “perpetual motion machines,” can be classified into two broad categories:

  1. First Kind: devices that create power out of nothing.
  2. Second Kind: devices that extract power out of the environment.

The first and second laws of Thermodynamics derive from the impossibility (so far) to build these devices. The first law tells us that energy must be conserved, and thus it is impossible to make a device that will generate power without consuming some other kind of energy. The second law states that, once energy has gone into the environment, usually converted in to heat, we cannot get it back as useful power: energy is indeed conserved, but it can be degraded.

Throughout history, attempts at perpetual motion of the first kind have usually centered around imbalanced levers, where a clever mechanism shortens one of the sides of a see-saw, for instance, as it rolls around to the opposite position from where it started, or on magnets. I can claim the dubious honor of having invented (or rather, re-invented) machines of both types, before I knew any better (or even after ;-).

Putative perpetual motion of the 2nd kind is a bit more sophisticated and hard to detect. The giveaway usually is the lack of a heat sink in machines that absorb heat from the environment. The same website has a number of pretty interesting machines of this type, so I will only add here those you cannot find there. I happen to know personally the inventor of a few of them, which have obtained (fairly recent) patents from the US Patent Office, despite their long-standing policy. This is either a witness to the lack of training of the examiners or to the insistence of my friend the inventor. Maybe to both. Here are links to those patents:

4,479,354     4,663,939     5,107,682    

But I want to tell you about different types of perpetual motion. Here I will tell you about how light can be used to seemingly create perpetual motion of the 2nd kind. A new type of perpetual motion, which I would call “of the 3rd kind,” and which does not seem to be violating any physical law so far in force, is explained in this page.

Perpetual motion from light

Electromagnetic radiation is a strange beast as far as the Second Law is concerned, because it behaves either as heat or as work, depending on the circumstances. It is heat for a hamburger warming up in a microwave oven, but it would be work, capable of moving electrons in an ordered, spark-generating way, for a fork inadvertently left inside. Light is electromagnetic radiation, and because of this it sometimes behaves as heat, as in a black body glowing due to its temperature, and sometimes gets diffracted and aligned like the millimeter waves that link your cell phone to the world.

An interesting result of diffraction is holography. A hologram is a two-dimensional image, usually on film or a similar, fine-grained substrate, that actually contains a three-dimensional image. It is generated by shining a laser on the 3D object to be recorded while at the same time shining an identical laser on the film. The film will be exposed with a series of very fine lines, which look kind of like what you get when you put a semitransparent fabric on top of another, such that the image will be reconstructed by shining yet another identical laser (or a non-laser light of the same color) on it. Holograms have a lot of interesting properties in addition to looking really cool, and one of them is that you can record multiple holograms on the same film and they won’t interfere with one another so long as the laser shining directly on the film during recording (called the reference beam) changes in color or position from shot to shot.

This property is used in US patents 5,877,874 and 6,274,860 by Rosenberg (assigned to Terrasun, Inc.), to create a film that directs sunlight into a narrow beam no matter what direction it comes from. Thus, a solar panel having this film in front of it would not need to turn to track the sun, which is quite handy. This is the intended application of the film, which allegedly has been tested in a number of prototypes. The patent descriptions can be found here and here. Below is a picture from the first patent, which illustrates the concept:

All right, then. Imagine we have a piece of this film, made so that all the light (and infrared radiation) falling on one side of it will be transmitted through to the other side, where it will leave in the direction perpendicular to the surface no matter what direction it came from on the other side. This film would be the heart of the device in the picture below:

In addition to the film, the device includes a large flat black body 1, a small black body 2, and a parabolic mirror, which has black body 2 centered on its focal point. The side surface between the large black body and the film is also polished to mirror finish. All surfaces are thermally insulated on the outside.

For those whose heat transfer courses are a bit rusty, a black body is a type of surface (actually, an ideal surface, but we can get pretty close in reality) that absorbs all the radiation that falls onto it. That’s why it’s called “black,” because it absorbs all light that falls on it and thus our eyes register its location as black, but it is also “black” for thermal radiation as well, which is mostly in the infrared range and our eyes cannot see it. When hot, a black body will emit radiation at a rate given by this formula:

where A is the surface area, s is the Stefan-Boltzmann constant (5.67×10-8 Watt/m2K4), and T is the black body absolute temperature, in Kelvins (temperature in centigrade degrees + 273.15).

Now we do the following: heat up black body 1 to a (high) temperature T1, and wait until thermal equilibrium with black body 2 is established. Thermal equilibrium means that no heat flows from 1 to 2, or from 2 to 1. The 2nd law of Thermodynamics (through a corollary mistakenly called by many “the zeroth law”) commands that this shall only occur when the temperatures of 1 and 2 are the same. But let’s see if that is the case here.

Observe that, given the geometry and the presence of the holographic film, all radiation issuing from 1 will hit the film, where it will be made parallel and transmitted to the other side. Now, a parabolic mirror has the interesting property that all beams parallel to its axis are reflected toward its focus. This means that all the radiation will end up on black body 2, where it will be absorbed. No radiation will bounce back to the surface of 1 (this is important). The heat rate going from 1 to 2, therefore, is given by:


But black body 2 also emits radiation. Some of it will bounce on the mirror and then back to the surface of 2, but most of it will miss it after the reflection and will travel toward the film after that. A lot of the radiation will travel toward the film directly. This means that eventually most of it will end up on the film, where it will either be steered toward the perpendicular direction or will be somehow diffused as it travels to the other side. In either case all of that radiation will end up on surface 1, as given by:


where the factor f, representing the fraction of the heat radiated by black body 2 that ends up falling on black body 1, is less than, but close to 1. When thermal equilibrium is reached, both heat flows balance one another, so that:


and then, it follows that:


Since A2 is smaller than A1 and f is less than 1, the temperature of black body 2 is greater than the temperature of black body 1 when equilibrium is reached. This makes it a perpetual motion machine of the 2nd kind, because now we can run a heat cycle using black body 2 as a heat source and black body 1 as a heat sink, and thus generate some power.

Now let’s discuss some things that might possibly go wrong, and why they still wouldn’t keep the machine from breaking the law.

1. It could be said that there is no such thing as a perfect black body, and neither is there a perfect mirror, or a perfect insulator, or a perfectly transparent film. True, but the 2nd law is supposed to apply even if they existed, for it is based on infinitely slow, ideal processes, which are the limit of real processes. It’s just not fair to a candidate perpetual motion machine to force it to deal with non-ideal materials. You can stop anything, including machines that don’t violate any laws, by heaping friction, losses, and leaks onto them.

2. There is a similar putative perpetual motion machine based on elliptical mirrors with black bodies in their foci. You can find it here, and here, (courtesy of Prof. L.H. Palmer, from Simon Fraser University). This is what it looks like: 

The mirror geometry supposedly conspires to put more energy on black body 2 than on the other because some of the light coming out of black body 2 will be reflected back onto itself but not so for black body 1, but this is only in appearance. In reality, since the bodies are not mathematical points some of the energy coming out of one will fail to strike the other (this happens for both bodies), so that eventually the device will be filled with a uniform radiation intensity, and therefore both bodies will end up at the same temperature. The problem here is that the same argument does not work for the device we’re dealing with, since all the radiation coming out of black body 1 is guaranteed to fall on 2, even if it is not a mathematical point (and even better if it’s not), and any radiation that, coming out of black body 2, does not strike 1 but falls back onto 2, only makes the temperature difference even greater (through the effect of < 1).

3. It could also be that the holographic film cannot steer the incoming radiation into exactly parallel beams, but rather into beams deviating from the perpendicular by up to an angle q, so that, after reflection on the parabolic mirror, they will miss black body 2, leading to a uniform radiation field as in the device mentioned in the paragraph above. If q is sufficiently small, however, the rays reflected on the parabolic mirror will concentrate within a sphere of radius e around the focal point, where e is a continuous function of q with e(0) = 0. If follows that one only has to choose a spherical black body 2 with a radius larger than e, for all the radiation from black body 1 to fall onto it, which brings us back to the original situation. For a sufficiently small qT2 will still be greater than T1.

 4. Another argument is that, since the film patents only show films working by reflection, then the machine above will not work because it works by transmission of the light through the film. This is only an apparent problem. First, every reflection hologram has a conjugate transmission hologram, the difference being whether the reference beam used in recording the hologram is placed on the same side of the film as the object to be holographed (for transmission holograms), or on the other side (for reflection holograms). It follows that, even if the film patents do not specifically speak about transmission holograms (and they should), it should be possible to make them by putting the reference beam on the same side as the object beam during recording. In addition, it is possible indeed to make a similar machine as the one above using a hologram that reflects light into a single direction, no matter what direction it originally comes from, as in the next figure:

As in the transmission case, all the light issuing from the large black body 1 will fall onto the small black body 2, and the difference in surface area will ensure that T2 > T1. But this case is even more extreme than the transmission case because, if the film reflects into a single direction all the light that strikes it, then it does it both for the radiation going from 1 to 2 and for the radiation initially going from 2 to 1, which will end up falling back onto black body 2. This means that thermal equilibrium will never be reached, since no energy flows from 2 to 1, and temperature T2 can reach arbitrarily high values.

A simpler version of the above device would have no parabolic concentrator, like this, using films that reflect light at a 45 degree angle:

Because of the geometry, light from the right side never reaches the left side, so the black body on the right will keep receiving energy by radiation while losing none. This setup acts as a Maxwell demon for photons, rather than gas molecules.

5. One could say that, in practice, there is no such thing as a holographic film that steers all incident light into a single direction. The actual prototypes made by Terrasun (the inventors of the patents mentioned above), which I have seen in action, collect light from just a couple directions and send it in a broad beam in a different direction. It is doubtful whether a perfect or near-perfect light-steering film can be made in practice, since this would involve recording many holograms into a single film, and the superimposed holograms, although theoretically able to coexist, would end up interfering with one another so they could not work. So the next question is: how well do the light-steering holograms have to work in practice so they can still be the basis of a perpetual motion machine? The last machine above violates the 2nd law by impeding backward transmission of light, without any concentration, while the first does it by concentration, without needing one-way transmission at all. If both effects are combined, as in the machine in the middle, it may be possible to still achieve a violation with imperfect holographic films.

For instance, a film that does not steer light at all, but simply prevents it from being reflected into a certain region can still form the basis of a one-way light valve, as in the picture below:

Here the film would not reflect light into a certain angle off the surface. By shaping the film into a logarithmic spiral, it is possible to ensure that none of the light that enters the device from the left (or from the right), would come out of the left port, even if the light would be otherwise dispersed. If the “blind angle” for no reflection does not come all the way to the surface, the device would be more complicated, but I don’t believe it would be impossible to come up with an example.

Likewise, the concentration effect does not need to be perfect in order to work. Even the concentration due to a change of index of refraction can do the trick if the change is strong enough, as in this device:

Here there is no holographic film at all, but partial concentration is achieved by changing the index of refraction of the transmission medium, which we assume to be non-participating in the radiation. After crossing the interface, the radiation coming from black body 1 will be partially aligned along the axis of the parabolic mirror within the total reflection angle of the interface, given by:

where n1 and n2 are the indices of refraction of the transparent media in contact with black bodies 1 and 2, respectively. This angle is measured from the perpendicular direction so that, the greater the ratio of the refraction indices, the more aligned toward the perpendicular (which also happens to be the axis of the parabolic mirror) will the light be. As we saw before, the light does not need to be fully aligned with the axis of the parabolic mirror in order to strike a surface smaller than the emission surface, leading to a temperature imbalance.

On top of all that, holograms, interfaces, and other components in the device do not need to work equally well with all the wavelengths in the spectrum, as indeed they won’t. To see this it is enough to imagine that the blackbody surfaces are coated with a filter film or paint that only allows a certain wavelength to pass through. This narrow-pass film is certainly ideal, but there are existing films that approximate this behavior quite well. In that case, all the radiation involved in the exchange will be of a single wavelength, but everything we have discussed above would still hold and there would still be a heat transfer imbalance and a temperature difference would be created.


I would probably get kicked out of my job as a professor and defender of the 2nd Law of Thermodynamics if I said that these machines could work, so I won’t say that. But I will say this: they’re stumping me so far and sometimes manage to take away my sleep (well, part of it, really :-), so please let me know if you come up with a more convincing argument why they cannot work (other than saying that they seem to violate the 2nd Law of Thermodynamics).

If, on the other hand, you decide to build one and succeed (they seem pretty simple, but I’m not that handy, and forget about writing a proposal to get funding for that), I’d like to hear from you. My email is elsewhere on this website. You will be assured strict confidentiality, because I know what you’re likely to get if your machine actually works and Uncle Sam finds out about it (hint: definitely not a patent, how about a nice vacation at an exclusive resort in southern Cuba?). Should they find me, the only line that’ll cross my lips will be:

Eppure si muove!

Possible perpetual motion

What do you know? It seems there is a new kind of Perpetual Motion Machine, which would give free energy for ever, and Nature hasn’t managed to pass a law against it (just yet, at any rate). Read on if you’re strong-hearted (warning: contents under heavy math)…

Perpetual Motion Machines of the 3rd Kind

This article is highly unusual because it deals with a perpetual motion machine that the laws of Thermodynamics forgot to forbid. Perpetual motion machines, in their most common definition, are those that produce a high-grade kind of energy, such as electricity or mechanical work, for free. Naturally this cannot be or the world would go topsy-turvy: fortunes would be reduced to poverty, governments would collapse, and the earth would start warming up for good. But thankfully, Thermodynamics has laws to prevent such mayhem.

The First Law forbids anything from yielding more energy than is put in, effectively nullifying machines made out of magnets, unbalanced levers, and self-regenerating motors. The Second Law, on top of that, sets a strict limit on the efficiency of the heat to work conversion, so those who thought to solve all the world’s problems by extracting plentiful power from the environment—where it eventually returns only to be recycled back to use—were forced to put their considerable creativity to better use. A perpetual motion machine is fool’s gold, our textbooks say, meant for the doom of those who did not bother to stay awake during Thermo class. Of course, the flaws of such machines are hard to see sometimes. This one, for instance, which is based on light.

Another perpetual machine (called by some of the “third kind” to distinguish it from those of first and second kind, loosely described above), has been proposed. The trick here is to get a mass (let’s say it’s an ice-cream bar) down to absolute zero. Then one could go to the nearest hardware store, buy a perfect Carnot cycle, and stick it on top of the absolute-zero ice-cream bar as shown in the figure below

The Second Law gives, for the efficiency of such a machine, this expression, where the temperature must be expressed in Kelvins of any other absolute scale:

Where TH and TL are the high and low temperatures of the cycle, respectively. If the low temperature in the cycle is absolute zero, then the efficiency of the set is exactly one, meaning that all the heat extracted to the environment will be converted into usable power. An additional benefit is that the heat rejected to the ice-cream bar will be exactly zero, so our precious property will never see its temperature raised above absolute zero. We can keep our machine running forever (remember, it’s a perfect Carnot cycle), creating useful power out of the environment. This, of course, is forbidden by the Second Law, so this machine is actually a special type of perpetual motion machine of the second kind, and giving it its separate kind is not really warranted.

Is there, then, such a thing as a true perpetual motion machine of the third kind?

Enter exergy. This very useful concept “discounts” energy to give its potential to produce useful work. For instance, the exergy of heat of value Q is less than Q, because not all of it can be converted into work, according to the Second Law, but rather:

where T0 is the temperature of the environment, which is usually the heat sink in common thermodynamic systems. Everything that contains energy contains also exergy, including things that contain no energy at all. For instance, an evacuated tank contains no material, and therefore no energy, but it can be used to generate power by causing the environment to push a piston or putting a paddle wheel in front of the onrushing air, should the tank be punctured. The important thing is not whether the power comes from the system or not, but rather that the system is the opportunity by which the system itself or the environment will be able to produce power.

The exergy of substances can be calculated in many ways, and it is usually related to its thermodynamic state and that of the environment, as given by its temperature, T0, pressure, p0, and other properties. The particular case of interest for our perpetual motion machine of the third kind is the exergy of a substance whose constituent molecules are able to evaporate into the environment. If that substance, say, behaves like an ideal gas (and every substance will, once its vapor pressure becomes sufficiently small), its exergy is given by the following expression (assuming its specific heat Cp is constant, for simplicity):


Where y and y0 are its mole fractions within the system and in the environment, respectively, and p and p0 are the pressures. The upshot of this is that the above formula, which is derived in strict compliance with thefirst and second laws of Thermodynamics, would give an infinite exergy whenever y0 is zero, that is, whenever a substance is completely absent from the environment. Creating a substance that is completely absent in the environment is not such a far-fetched concept, though. Pharmaceutical companies are doing it all the time when they synthesize new medicines. Physicists do it routinely, at a subatomic level, when they collide particles traveling at a high speed to create new particles. It takes more or less energy to form a new substance, but it is always a finite amount. The amount of work required is, at a minimum, equal to its “chemical exergy,” which is obtained when the compound is allowed to react producing work (say, in a fuel cell), or maybe absorbing work, down to compounds that are present in the environment, and those are then allowed to diffuse into this environment, contributing more work through terms of the same form as equation (3), but which are now finite because none of the y0 concentrations in the environment is zero. Of course, equation (3) is not supposed to be applied when a substance is completely absent from the environment, but rather one must first calculate how much exergy is required to generate the substance by chemical reaction, starting from substances that are present, and then add the exergy that those substances would have before they expand into the environment, as explained above. But then the paradox remains that a substance that required no infinite exergy to synthesize, would appear to have an infinite capacity to do work if it is simply allowed to expand into the environment.

For an example of how a true perpetual motion machine of the third kind would work, look at the figure below:

The “synthesizer” is a black box system where a certain new substance (let’s call it “novium”) is being synthesized, starting from substances present in the environment. This process, as we saw above and know from experience, takes a finite amount of energy, consisting of work and heat. The novium formed in the synthesizer now travels to an expansion chamber maintained at the same temperature as the environment, where it is vaporized and meets a membrane that is permeable to all the substances present in the environment, but not to novium. The membrane, therefore, will be subject to the vapor pressure of novium on one side, and no force on the other. If it is allowed to move, the membrane will produce a work as the novium gas expands at constant temperature, absorbing thermal energy from the environment. The process can move at a vanishingly small rate, approaching equilibrium at all times. The process is also reversible, since it is always possible to push the membrane against the novium vapor pressure until it is concentrated into a small volume. Under these conditions, and having removed friction and other irreversibilities, the expansion chamber will produce a work per unit mass equal to its exergy, given in the equation above. Since y0 of novium (in the environment) is zero, then the work produced, given an infinite stroke for the membrane displacement, will also be infinite. Another way to look at it is that the novium undergoes a constant temperature process. Since it is an ideal gas at low concentrations, the work produced will be given by:


leading to a logarithmic relationship between work and volume. Eventually, for a sufficiently large volume (it does not have to be infinite, in fact), the expansion chamber will have produced enough work to generate the required novium sample, and then some. It should be noted that equations (3) and (4), far from breaking down as the expansion progresses, would be less and less of an idealization, for all substances approach ideal gas behavior as their vapor pressure tends to zero.

The energy, of course, comes from the environment, mainly through heat interactions in the synthesizer and the expansion chamber. But the environment is at a constant temperature, so it should not be possible to produce any work by extracting heat from it, according to the Second Law. And yet, an analysis of the equations above says that this result comes directly from that law, since the logarithmic term that gives the infinite result can also be derived from:



is the entropy difference of an ideal gas (with constant specific heats) between a given state and the environmental state. Here the pressure used is the partial pressure of the gas, in case there are other gases mixed with it, which is the usual situation.

What has happened here? How did the Second Law manage to produce a result that seems to contradict the Law itself?

It should be noted that the fact that nobody has made, nor likely ever be able to make, such machine is no argument against the paradox. Likewise, nobody has been able to build something as simple as a Carnot cycle, because there are always irreversibilities such as friction and heat transfer across finite temperature gaps, and yet the science of Thermodynamics is based on it. No, the relevant fact here is that the machine proposed above, in its ideal form, seems a self-contradiction of the Second Law, which this law’s propriety cannot tolerate even in its most ideal form.

The reason why the machine is not a perpetual motion machine violating either the first or the second laws is because it does not really work in cycles. Indeed, after the first novium sample has expanded, producing as much work as we cared to collect, it is necessary to bring it back to its initial state. A valve opens on its far wall, and the membrane is allowed to move back under no pressure differential, venting the novium into the environment. But that means that, next time we try to expand a sample of novium gas, it will no longer be totally absent from the environment, and thus no infinite work will be possible.

Yes, but the same could be said about the Carnot cycle, which absorbs heat from a constant temperature thermal source without lowering its temperature, and rejects heat to a thermal sink without raising its temperature, either. The mental construct used is that those two “thermal reservoirs” are infinite for these purposes, and so a bit more or less heat does not change their temperature. It would not be fair not to give a similar capacity to absorb novium to the environment surrounding the machine, so that the concentration of novium in the environment will not be altered because a few (or a million) strokefuls are dumped in.

In addition, nothing prevents the machine operator from changing the composition of novium for the next stroke (along with that of the membrane). This is something that takes a finite amount of work, so that the next cycle works very much the same as the first, as far as the machine is concerned. There is no fear to run out of different substances to make, so the machine would keep working indefinitely. This is, however, strictly not the same process if the composition of novium has changed. We will revisit this aspect later on.

But perhaps such machine is intrinsically impossible, because no membrane will ever be able to distinguish novium from the other gases in contact with it, and thus it will stop performing as it should. The job we are asking the membrane to perform is indeed quite delicate, and not much different of the job performed by our good old friend, Maxwell’s demon.

Maxwell’s demon, pictured below, is supposed to stand guard by a little trapdoor, and allow only fast molecules to go from left to right, and slow molecules to go from right to left, with the result that a temperature difference is soon created, against the Second Law. But Maxwell’s demon cannot perform its job unless he analyzes the speed of the approaching molecules, and he creates more entropy doing so than he destroys by classifying the molecules into fast and slow. The question is, does a membrane suffer from a similar limitation? How does a membrane know novium from any other substance?

The answer is not simple, for semi-permeable membranes operate in many different ways. The membrane that is around every one of our bodies’ cells, for instance, has receptors of many kinds on its surface, and certain molecules can latch onto it by means of hydrogen bonds, provided their geometry matches that of the receptors. Scientists have been able to do a similar thing with DNA fragments on a silicon chip, using restriction enzymes that would only bind to specific sequences. If novium is DNA-based, then a substrate coated with a restriction enzyme for its particular sequence would be able to stop it as it tries to get past, while it would not stop any other molecule. The novium bound to the enzyme would eventually reach an equilibrium (controlled by the Second Law) with the free novium, so that as many molecules are released back into the expansion chamber as are captured on its surface. The result would be a barrier for novium, and the perpetual motion machine of the third kind would be able to run.

But it gets worse: making a membrane that would stop novium but would not stop anything else could be as simple as making the novium molecule larger than any other molecule present in the environment. A common wall with holes big enough for those, but not for novium, would do the trick. This is the laziest kind of Maxwell demon. A Maxwell demon who does not need to use any energy to classify the incoming molecules and therefore generates no entropy to operate. This case is different from the spring-operated Maxwell demon on the right side of the above figure, which allows through only those molecules fast enough to open the door against the spring. This may not be too far-fetched: it has become known recently that nanomaterials behave anomalously where the Second Law is concerned, probably due to their microstructure. But even the microscopic sorting door falls under the curse of the second Law. That tiny spring, indeed, would end up taking some of the energy of the incoming molecules, with the result that the door would end up shaking so badly that soon it would not be able to classify the molecules at all. But a membrane with simple holes would not take any more energy than a wall without holes. The wall material can be perfectly rigid, and still it would fulfill its mission to restrain novium safely to one side of it. Molecules too large to pass through would bounce off elastically, while those that pass need not lose any energy as they do so.

Still, our instinct tells us that there must be a reason why this machine cannot work, or the world might go upside down. Consider this: the gas in the expansion chamber does not need to be completely absent from the environment for the machine to produce power; it only needs to be rare enough so that producing it takes less power than it gives when it expands, according to eq. (4). An inventor might speculate on what gases are easy to make but are rare outside, but let us just look at carbon dioxide, for instance. Pure CO2 can be generated by a number of processes well known to freshman students, (such as dripping vinegar on marble), none of which take much energy at all. Yet, the mole fraction of CO2 in earth’s atmosphere is only 0.0003, yielding, from eq. (3), 456.6 kJ per kg of pure CO2 in the expansion chamber, if the temperature is the standard 25ºC. Once liberated, the CO2 is captured back into rocks by biotic processes, so ultimately the power produced by the machine has come from the sun. Is this, therefore, a perpetual motion machine, or isn’t it?

But perhaps this example—though possibly the basis for a power-producing method—only serves to muddy the issue, which is whether the laws of Thermodynamics can allow the infinite exergy case. Perhaps the problem is that the law of Thermodynamics that prevents this perpetual motion machine from working has not yet been promulgated, and therefore the machine would keep happily working, oblivious to any fault or misdemeanor.

The Third Law exists already, so the new law (if God decides to pass it) would have to be called the “Fourth Law” at best. It might read like this:

“It is impossible for the exergy of any system to be infinite.”

Or, more specifically:

“It is impossible for the concentration of any substance to be zero.”

In its second form, the Fourth Law smells suspiciously like Schrodinger’s cat, which is both dead and alive at the same time. In our case, the undead “novium” has acquired the ability to tunnel, ghost catlike, through any wall, so that it is on both sides of it at the same time. There was always a certain amount of tunneling, controlled by Heisenberg’s principle, but now we are imposing a minimum value: there must be at least enough tunneling so that this cat’s exergy drops below the power it takes to make it.

But maybe God will be happy with this loophole: the “novium” in our machine won’t the same in every stroke, and therefore the machine isn’t strictly running in cycles. In that case, he might let us keep running it to produce infinite power as long as we don’t fill this universe with our creations (such as the different kinds of novium). And then, we’ll finally be able to say (in a rather subdued voice, just in case):

Eppure si muove!

PassLok in the UK

david-cameronIt is already illegal for a Briton to refuse to surrender his/her/its password to law-enforcement authorities, and Prime Minister Cameron is now trying to make all non-backdoored encryption illegal as well. What can you do if you are affected by this situation?

My answer, as you have already guessed, is: use PassLok.

Actually, it’s not only UK citizens who have this problem. This article explains how you may be in the same boat if you happen to live in France, India, Australia, and other places. But let’s say you are a Briton.

There are five reasons why you should use PassLok rather than other solutions to secure your email (besides being much simpler, and prettier too):

1. No servers or storage. Since nobody stores your data or keys, nobody can get an order to surrender them. In PassLok, your secret Key is never stored anywhere, no even on your trusted devices, which you may be forced to produce and unlock at law enforcement’s more or less kindly request. You lock your data with PassLok, and it is in this form that they get transmitted through your separate email program. The email provider will produce your locked data if pressured to do so, but not the Key that unlocks it because they don’t have it. The only place where that Key resides is in your head.

2. Decoy mode. But let’s say it’s illegal for you to refuse to surrender this precious Key. Then go ahead and give it to them, so long as you use PassLok’s Decoy mode. In this mode, locked items actually contain two plaintext messages, one of them completely undetectable. This way, people can be carrying a conversation through the main messages, and an entirely different one through the hidden messages, locked under a different set of Keys. Should they be forced to surrender their Keys, the hidden messages remain secret and just as undetectable. It would be unreasonable for any authority to demand the surrender of a second Key that, most likely, doesn’t even exist.

3. Perfect Forward Secrecy (PFS). PassLok is probably the only application meant for asynchronous communications such as email (this means that both parties are not necessarily online at the same time) that implements PFS. When PFS mode is used, locked messages become unlockable when the next message is exchanged, because the Keys used for locking are overwritten on the device and are not stored anywhere else. This way, if one is forced to surrender his/her/its secret Key, past messages cannot be unlocked no matter what. If this is not enough, PassLok has Read-once mode, where Key deletion happens as soon as a message is read, resulting in the closest thing to self-destruction this side of the Great Firewall of China.

4. Steganography. This is the Greek name for the science of hiding. If you live in China (or the UK in the near future), you might get in trouble just for emailing random-looking encrypted messages. Unlike pretty much all other programs, PassLok offer you the opportunity to transform its random-looking output into apparently normal text taken from a user-selectable cover text, which the recipient can return to its original state before unlocking it. PassLok also includes two ways to hide its output into images, which can then be sent as attachments to email messages. Images can hide a lot of data.

5. PassLok is human-readable. Britain and other countries with democratic leanings have laws protecting free speech (which, rather inconsistently, doesn’t always extend to cryptography). Well, the possession and distribution of at least the html version of PassLok should be protected under those laws because PassLok is human-readable code, not machine code. This is precisely how PGP survived the onslaught of the US Government back in the 1980’s, when its developer began to distribute it as a book containing the source code, which people could then type or scan and compile to executable code. PassLok is used directly in this form, simply by loading it into a browser.

Modified interlock protocol for authentication

Of the many difficult problems dealing with public key cryptography, there are few so hard to crack as public key authentication. Public keys are easy to obtain (that’s why they are “public”), and because of this, it is hard to be sure that a certain key belongs to a certain person, what is known as authentication. Usually it is recommended that the key be handed out in person or that it be identified (directly or through a one-way hash) by a rich communication medium such as voice or video.

But sometimes it might be possible to do it strictly by email, even though you suspect someone might be watching. Read on for details.

If you cannot make contact with the other person through a rich channel such as voice or video, you’re going to have to start using somebody’s public key without knowing for sure if that public key is genuine. Trust will build up gradually, as the messages sent back and forth serve to confirm the identity of the participants. But there are ways to discover if a public key is fake with only a few messages traveling back and forth.

The easiest thing to do is to send a message to the public key owner, including a question whose answer only the two of you know, and asking him/her to send you his/her public key, encrypted with a symmetric key that is the answer to that question. If the answer is correct, you’ll be able to decrypt the message, and thus retrieve the genuine public key (which should match the one you have). An interloper who is watching and perhaps modifying your traffic won’t be able to decrypt the message in order to change it, and thus he/she must be content with preventing you from getting that message, in which case you’ll know the public key in your possession have is fake.

But the easy way has the problem that the other person’s answer must be exactly the answer I know, down to the smallest spelling detail, or the message won’t decrypt. There is another way to authenticate a public key using a variation on the “interlock protocol,” which admits answers that don’t have to be exact. It is enough if the persons asking the questions can recognize the answers as valid in a more general sense.

This protocol is based on the “interlock protocol,” first described by Rivest and Shamir in this paper. In essence, people encrypt messages but only send half, and wait to get the other person’s first half of a reply before sending out the second half. This causes a “man in the middle” a big headache because, since he cannot decrypt half an encrypted message, he needs to make up messages to keep the exchange going, or his cover is blown. Unfortunately, it has been shown that this protocol can be defeated by a particularly clever interloper, using this method. Does that mean that the interlock protocol is no good? Far from it. What’s needed is extra cleverness in the way it is set up. As a clever user says in this posting, the key lies in asking questions that require an answer.

So here ‘s the way you can set up a slightly modified interlock protocol in order to authenticate a public key. Since the following is extracted from the manual for PassLok, which uses non-standard nomenclature for the sake of novice users, let’s get that out of the way first. In PassLok, a public key is called a Lock, and a private key a Key. A digital signature is a Stamp, and a one-way hash is an ID. Encrypting is referred to as “locking” and decrypting as “unlocking”.

With that clarified, let’s look at this exchange between Alice and Bob:

1.     Alice obtains her friend Bob’s Lock, but she fears that it might be counterfeit and someone else might be unlocking the messages she sends to Bob, reading them, perhaps changing them, and then re-locking them with Bob’s actual Lock for him to read. So Alice sends Bob this email:

“Dear Bob. I just got your Lock for the app called PassLok from your email signature. I fear that I’m under surveillance, so it’s very important that I make sure that this Lock actually belongs to you. Here’s what I want you to do:

a.      Write a question whose answer only the two of us know, and lock it with my Lock, which is included at the bottom of this email. Then split it in two parts and send me the first part. I’ll be waiting for it.

b.     When I get it, I’ll send you the first half of a similar question, which has been locked with your Lock. When you get it, send me the second half of your locked question.

c.      When I get that, I’ll send you the second half of my question, and also the answer to yours, which then I’ll be able to read. I’ll send the answer locked with your Lock.

d.     If I answered your question correctly, put together the two halves of my question and unlock it. Then write the answer, lock it with my Lock, and send it back to me. Then I’ll know that your Lock is authentic. Your friend, Alice.”

2.     When Bob gets this, he decides it’s going to be fun to do all his, and writes a question whose answer only Alice knows, locks it with her Lock, which was appended to her email, splits the locked message into two parts, and sends Alice the first half.

3.     Alice gets the first half, and writes her question to Bob. Then she locks it with Bob’s presumed Lock, but only sends him the first half. Because nobody can unlock half a message, she must wait to get the second half of Bob’s message in order to answer his question.

4.     Bob gets the first half of Alice’s locked question, and he sends her the second half of his locked question.

5.     Alice gets the second half of Bob’s question. Now she can put the two halves together and unlock them with her Key. She writes a message answering Bob’s question, locks it with Bob’s presumed Lock (no need to split it now), and sends it back to him along with the second half of her question to Bob.

6.     Bob gets Alice’s email and can now unlock both messages from her. He sees the correct answer to his question in the first one, so he unlocks the one containing Alice’s question. He writes the answer, locks it with Alice’s Lock, and sends it to her. Had he been unable to unlock Alice’s question, he would have told her so. If her answer to his question was wrong, he would have told her, too.

7.     Alice gets Bob’s message, unlocks it and, seeing the correct answer to her question, is satisfied that Bob’s Lock is authentic. Otherwise she gets a message from Bob telling her that things didn’t work out, or something other than a message answering her question, or nothing at all, and she decides that “Bob’s Lock” was bad.

An alternative to splitting locked messages is to make the ID of the locked or unlocked message and send it ahead of the message itself, or apply a Stamp to the message and send the Stamp ahead of the message. The recipient will then check the ID or Stamp after the message is received, and will know that something’s wrong if it is not the same. Another option is not to lock the answers to the questions, in steps 5 and 6, since authentication also works if those messages are not locked; locking just preserves those answers, which might be sensitive, from a less-than-powerful eavesdropper who might see the exchange.

If Alice does not know Bob well enough to be able to ask him a question whose answer is known only to the two of them, or maybe Bob doesn’t know Alice well enough to ask a similar question, there is still something they can do, so long as Alice can recognize Bob in some way, and Bob can recognize Alice. Instead of a personal question, the asker can direct the other person to simply repeat something contained in the “question” message, but to do so in a video or audio recording, which is then put somewhere in the cloud, and the URL is sent back as an answer. The asker will then see or hear the other person, whom he/she recognizes, saying something that she/he would not be saying unless she/he has read the question message.

Let’s see how this protocol foils Mallory, who is able to intercept and modify their communications without them knowing anything. He poses as Alice before Bob, and as Bob before Alice. In this case, Alice does not have Bob’s genuine Lock, but one that Mallory made in order to impersonate Bob. Likewise, Bob does not get the Lock that Alice sent in step 1, but one for which Mallory has the Key.

Things begin to go wrong for Mallory in step 3. Since Mallory cannot yet read Alice’s question but nevertheless has to send something to Bob to keep the exchange going, he must send him a question from “Alice” that likely has nothing to do with the question the real Alice has asked. That, or pretend in step 2 that Bob is refusing to go along with the game, which is not going to do much to reassure Alice.

Mallory will then get the whole question from Bob, so he will be able to unlock it, re-lock it, and pass it along to Alice in step 4, and then get from her a reply that will satisfy Bob in step 6. But the damage has been done. Mallory is committed to sending Bob the second half of a question from “Alice” that is most likely not the question the real Alice asked, or otherwise Bob won’t be able to unlock the message, and Mallory’s cover will be blown. Bob might not discover the ruse at this point, but it is highly unlikely that his answer, or whatever else Mallory can come up with to replace it, will satisfy the real Alice’s question. Then she’ll know someone’s in-between and Bob’s Lock is not authentic.

If now they repeat steps 2 to 6 all over with one new question from either side, but this time with Alice asking the first question, Bob will also notice that something is wrong. But what if there is no Mallory, and “Bob’s Lock” was not being used to listen in but was simply corrupted or mistaken for another Bob’s Lock? Then Bob will simply be unable to unlock Alice’s question in step 6, and he will alert her of that fact. It is possible that a Mallory could still be watching without attempting to modify the messages passing through him unless he really has to, but it is unlikely that he could replace Bob’s announcement that the protocol failed with something that would satisfy Alice, because at that stage Alice won’t accept anything but a correct answer to her question, or she will decide that “Bob’s Lock” is bad.

It took some homework and three emails from each side, after which they still don’t know each other’s authentic Lock (which would be impossible with Mallory changing everything, anyway), but Alice has avoided being duped by an enemy.

Lessons from the VIC cipher

abelIn the mid-1950’s, the Soviet spy Reino Hayhanen, codenamed VIC, and his handler Rudolph Abel (in the picture) pulled off an incredible feat: they utilized a paper-and-pencil cipher that not even the FBI (the NSA wasn’t operating within US borders back then) was able to crack until Hayhanen defected and explained its inner workings. Computers already existed, and they were used primarily to crack ciphers like VIC. In this article, I go over some of its features, and how they can be used to enhance other simple ciphers. Read More

Remember strong passwords with this keyboard trick

keyboardEveryone knows that real people suck at coming up with strong passwords. They are either easy to remember and laughably weak, or decently strong and impossible
to recall. On top of that, it is highly recommended to use different passwords for different sites, so that compromising one won’t compromise the others. In this article, I follow Nobel laureate Manuel Blum’s recommendation of using not a password, but an easy to remember algorithm to come up with a way to generate strong, specific passwords for each site, and be able to remember them all.

In this talk, Manuel Blum asks four volunteers from the audience, who we presume not to have been prepared before the lecture, and explains to them a method which, when given a name to apply it to, leads them all to the same, apparently random result. The video does not reveal the method used, but some articles by Blum speak about mapping the alphabet (plus numbers) into a secret scrambled grid, and applying a secret walk to the successive letters of the name (presumably a website name) to be converted into something else. Thus, the user only needs to memorize the scrambled alphabet and the steps taken in the walk.

I don’t think I could do that, though, so here’s my counter-proposal: use the computer keyboard as my grid, and just memorize the method, plus maybe a simple code that I can change from time to time. Let’s say I want to come up with a strong password for I start, therefore, from the word “amazon”, which I am going to turn into something else. In order to increase security, I memorize the secret code “1234” (maybe I can’t memorize a scrambled alphabet, but this I can memorize). Now I do the following on an American qwerty keyboard like this:


  1. Starting with the first letter in the original word, move down (and slightly to the right) on the keyboard as many keys as the first digit of the code. If this causes me to fall off the lower edge, I continue on the top row, on the key directly above the last one I touched. Since the first letter is “a” and the first digit is “1”, I move one key down from “a”, which is “z”.
  2. Repeat with the second letter of the name and the second digit, then do the same until all the letters have been transformed. If I run out of numbers, I take the first number again, and so on. Therefore the other letters are:
    1. “m” + 2 = “i” (wrap to “8” on the first step, and then down to “i”)
    2. “a” + 3 = “w” (wrap from “z” to “2” on the second step, and then down to “w”)
    3. “z” + 4 = “x” (wrap from “z” to “2” on the first step, and then down to “x”)
    4. “o” + 1 = “l” (observe that we go back to “1”, since we ran out of digits on the key)
    5. “n” + 2 = “u” (wrap from “n” to “7” on the first step, and then down to “u”)

Final result: “ziwxlu”, which likely does not appear in any dictionary and is therefore as hard to crack as a group of random letters. If the website demands that you add numbers, go ahead and add a few that you can easily remember (except “1234”, which would compromise the key). This time I will add “1111”, making the final password  “ziwxlu1111”. Never mind that the numerical part is weak; the strength is in the first part, which is one out of 366 = 2,176,782,336 possibilities (numbers are also part of the set).

What we have done is essentially to apply the Vigenère encryption algorithm to the word “amazon”, using “1234” as key and the qwerty keyboard read column by column as starting alphabet. Not secure by today’s standards, but again, we are using it to generate a password, which itself has a small probability of being revealed. Additionally, anyone having access to the password will still have to figure out the algorithm, since what I’ve presented above is just a sample. There are many other ways you can apply a key to a website name. For instance:

  • Do as above but instead moving up, or left, or right on the keyboard. Or maybe alternating directions, or even switching directions in a more complex pattern that is still easy to remember: a cross, a circle, etc.
  • Use an alphanumerical key, and then find the result key by doing a “Playfair” combination. In the classic Playfair cipher, two letters at a time combine into one, this way: if there are in the same row or column, take the letter following the first, right or down respectively, depending on whether they are on the same row (or is the same letter) or column. If they are on different row and column, trace a rectangle with the two letters as opposite corners, and then choose the upper new corner (or lower, or right, or left) as result. For instance: “”q” + “t” = “w” (same row), “r” + “v” = “f” (same column), “i” + “c” = “e” (neither). This method also can be subject to direction changes, if so desired.
  • Use no key at all, and just rely on direction changes to get the result. For instance, Blum used a one-step north-east-south-west repeating walk, which would turn “amazon” into “q,z/9m” on the qwerty keyboard, excluding non-character keys and the space bar.

There are several reasons why this method, even when not using a key at all, is easier and more secure than others.

  1. It is certainly easier than having to remember Blum’s secret square containing a scrambled alphabet. The keyboard is there, so why not use it? Otherwise, I might feel tempted to write down the square on a piece of paper because it is hard to do the translations all in the head. It is still hard to extract the key from a compromised password, since the details of the algorithm are unknown and the text sample very short.
  2. It is more secure than memorizing a single high-security password because, if that is compromised, then all logins are compromised. The result of the algorithm is very random-looking, which makes it hard to crack using a dictionary. Cracking by trying to guess the algorithm is hopeless, since you can use so many different possibilities for that too.
  3. Even if one password is compromised, that does not necessarily reveal the master key (“1234” in the above example) because the attacker does not know the exact series of steps in your algorithm. If he does, of course, he’ll get your key in no time at all, so don’t reveal your method. This is probably why Blum did not reveal his in the video.

Is self-destruct email possible?

Earlier this week, my new app SeeOnce was rejected (hopefully only temporarily) by the iOS app store for allegedly misleading users into thinking that it could make self-destructing messages. Leaving aside what exactly “self-destruct” means for a digital message and whether or not SeeOnce actually achieves this, a number of current and past apps have claimed precisely this. In this article, companion to the one on Privnote vs. SeeOnce, I go over these apps, how they work, and how they can be used most profitably. Read More

Privnote vs. SeeOnce

In this post, I review Privnote and compare it withSeeOnce. Both apps claim to generate self-destructing messages. Which one will be the winner?

Ladies and gentlemen, on this corner is Privnote, a really, really simple app to send self-destructing messages to someone. It works like this:

  1. Navigate to The interface contains a single box where you write your message and a big red button to create the note. Nobody asks you for any password or private info of any kind. There is an additional button for some options, such as encrypting it with a password (using AES), changing the default time for self-destruction, or getting an emailed confirmation that the note has been read and destroyed.
  2. Write the note and click the big red button. The box is replaced by a link like this: already selected for you to copy.
  3. Copy the link and paste it into an email or whatever.
  4. When the recipient clicks on the link, Privnote loads on a secure page and displays the original message, plus assurances that the message has been destroyed at the source. Sure enough, reloading the page displays a warning that the message no longer exists.

Privnote is beautifully simple and it seems to work. Can anyone beat it?

On this other corner is SeeOnce, which also claims to generate self-destructing messages in a pretty simple matter. SeeOnce works like this:

  1. Navigate to You are immediately asked to come up with a Password, though you are assured that you are not making an account anywhere. After supplying this, the interface contains a single box with way more buttons than Privnote, though many of them are things like “Select”, “Clear”, “Help”, etc. (Privnote doesn’t have any help, perhaps because it doesn’t need any?). There are no options to set in SeeOnce.
  2.  Write the note and click the highlighted Lock button. Now another dialog pops up, asking you to identify the recipient on a list, or send it as an (insecure) invitation to a still unknown user. This dialog doesn’t appear if you loaded SeeOnce from a link sent by someone else.
  3. After you do this, a piece of text containing a longish random-looking link fills the box. The link may be something like this: and it is already selected as in Privnote.
  4. You can copy and paste the contents (the link alone is enough) into email or texting, or simply click the Email button, which will put the whole thing into your default email. Alternatively, you can click a Hide button, which will convert the stuff into apparently normal text taken from a cover text (a popup asks you for the cover text, if you haven’t loaded one yet), before emailing it.
  5. When the recipient clicks on the link, SeeOnce loads on a secure page and asks for a Password. After supplying this, the original message is revealed. Reloading the link and re-typing the Password leads to a message stating that “unlocking has failed” (SeeOnce needs to exchange two messages between the same parties before this happens right away, otherwise the link does not fail immediately but rather after writing a reply).

A little more complicated than Privnote, but still quite manageable. Now, the devil is in the details, as they say. We need to look at what’s inside as well as the features and the overall simplicity of the process. Price is not much of an issue since both apps are free, but availability on different platforms might be.

Price. winner: SeeOnce

Both apps are free, but Privnote has ads. This is not only uncool, but poses a security risk since the ads could potentially inject malicious code into the page, compromising everything. SeeOnce, on the contrary, stays true to the open source ethos and contains no ads. SeeOnce can do this because it doesn’t rely on servers for its operation and therefore expenses are insignificant.

Simplicity. winner: Privnote

It’s hard to be simpler than Privnote: you click a link, enter a message, copy a link; on the receiving side, just click a link and you’re done. SeeOnce is almost there, but it does ask you to come up with a Password, which is extra work and requires the user to exercise his/her memory, never a good thing (we’ll see later that this isn’t as bad as it looks). On the other hand, emailing can be done without cut and paste by just clicking an Email button. Still, Privnote wins this one.

Features. winner: SeeOnce

Privnote does have a few extra settings, such as the ability to encrypt the message with a chosen password rather than the default 54-bit key (but then, how do you send the password to the recipient in a secure manner?), whereas SeeOnce encryption is always under user control (and this is why it asks you for a Password before it does anything). Privnote also has the ability to send a read receipt, which SeeOnce lacks (we see why below). Still, SeeOnce wins this one because it has a comprehensive Help system (to its credit, Privnote hardly needs one) and the ability to hide its output as innocent text, which can be life-saving in places where encryption is frowned upon. SeeOnce also has the ability to switch to secure real-time chat if the correspondents find themselves emailing one another every few minutes.

Availability. winner: tie

Both apps are available from secure links on a regular browser, though SeeOnce can run offline and Privnote cannot. SeeOnce is also available as a Chrome extension and in the Android store. So SeeOnce has an edge here, but I’m going to call it a tie since sending emails requires Internet and most likely a browser.

Security. winner: SeeOnce

Ah, here’s the biggie. Both apps stem from radically different approaches to achieve the same goal. Privnote is fundamentally server-based (except its encryption option, which appears to be client-based), whereas SeeOnce is strictly client-based (after the web server delivers the code, that is). Let’s see what’s underneath each one:

  • In Privnote, the message (encrypted with a symmetric key, which is sent in plaintext with the link but the server does not see) is sent to a server, where it is stored. Clicking on the sender-generated link first instructs the server to send the encrypted message the recipient’s machine, where it is decrypted with the key contained in the link. The Privnote server will deliver the data if this is the first time this particular link has been clicked by anyone, and the other optional conditions, such as expiration date, have been met. Then the server deletes the stored data, or so we are told, so that a repeated request using the same link cannot be met. Still, Privnote can tell the difference between an expired link and one that was never issued, which leads me to think that some memory of having stored the message remains for a while.
  • In SeeOnce, the message is locally encrypted with the result of combining a public key, which was received in a previous message from the same correspondent, and a random private key that is stored locally and is overwritten when a new message for this correspondent is encrypted. The underlying protocol is fairly complex but transparent to the user. SeeOnce never communicates with servers, so the reason why a message “self-destructs” (actually, no longer can be decrypted) is that at least one of the keys has been overwritten and cannot be obtained anywhere else, even if someone has been copying every message exchanged. This is also why SeeOnce cannot produce a read receipt: it was a different program that actually sent the message; the SeeOnce server never knew about the sender or any of his/her data.

There are three reasons why the approach followed by SeeOnce is much more secure:

  1. The first one is that Privnote displays the decrypting key in plaintext (or an equivalent, given that the client-side code can be viewed at any time) as part of the link. It needs to do this because it does not ask for any information about the recipient before preparing the link, so anyone should be able to follow the link. If the link is sent by email, for instance (and remember we are encrypting the message because we believe email to be insecure), the link can be nabbed by someone else, who then can read the message, instead of the intended recipient. Getting some control over who can actually read the message would require some sort of recipient authentication, a password at the minimum, which is what SeeOnce does.
  2. Whenever data is stored in a server, the user loses control over it. Privnote can say they have destroyed the message until they are blue in the face, but they cannot prove it. If a government agency serves them a request to make a copy, they might be doing it without the users’ knowledge. A hacker can break in and look at the data. The server itself may be saving the data as part of a scheduled backup. Now, Privnote states that this data is encrypted with a key that is not sent to the server, but since that key is included at the end of the link sent by email (otherwise the recipient would never be able to decrypt the message), if the link is compromised as we saw above, then the agency or hacker can decrypt the message. The only protection against complete loss of security is user-controlled symmetric local encryption with a separate key, which Privnote offers as an option, but then the user has the problem of transmitting the key. SeeOnce stores data only locally, and so this is much less of a problem. Stored data is encrypted by the user Password (is it beginning to look like this wasn’t such a hassle after all?) and can optionally be wiped or moved to a different machine. Anything transmitted is encrypted with a public-key algorithm, so that key transmission is never an issue.
  3. Code executing on a server is invisible to the user. Therefore, a Privnote user has no way of making sure that the code is honest and free from errors. In Privnote, this means the code that supposedly is keeping track of how many times a particular link has been followed, and which deletes the data on the server. On the other hand, the complete SeeOnce code is visible to the user by just typing ctrl-U on the browser. It is long and complex, to be sure, but it hides nothing. The program itself has a mechanism to ensure that the code has not been tampered with by a third party, fully documented in the Help pages.

Both programs have features to recommend them but in the end it comes down to a personal choice: do you value ease of use above anything else, or is it security what you value the most in a security product? Perhaps the only way to tell is to take both for a spin and decide for yourself.