This article was published as "Privacy and Technology." Social Philosophy & Policy 17:186-212 (2000)  and in The Right to Privacy, edited by Ellen Frankel Paul, Fred D. Miller, Jr., and Jeffrey Paul, Cambridge University Press, 2000. The webbed version here is based on a late draft on my hard drive, and so differs in a few details from the published version. It is being provided on the web page with permission of the Journal.

Privacy and Technology


David Friedman



Privacy: 1. state of being apart from the company or observation of others ...

The definition above, from my 1944 Webster’s unabridged, nicely encapsulates two of the intertwined meanings of "privacy." In the first sense, physical seclusion, the level of privacy in modern developed societies is extraordinarily high by historical standards. We take it for granted that one bed in a hotel will be occupied by either one person or a couple–not by several strangers. At home, few of us expect to share either bed or bedroom with our children. In these and a variety of other ways, increased physical privacy has come as a byproduct of increased wealth.[1]

The situation with regard to informational privacy is less clear. While the ability of other people to see what we are doing with their own eyes has decreased as a result of increased physical privacy, their ability to observe us indirectly has increased–for two quite different reasons.

One is the development of increasingly sophisticated technologies for transmitting and intercepting messages. Eavesdropping requires that the eavesdropper be physically close to his victim; wiretapping does not. Current satellite observation technology may not quite make it possible to read lips from orbit, but it is getting close.

The other reason is the development of greatly improved technologies for storing and manipulating information. What matters to me is not whether information about me exists but whether other people can find it. Even if all of the information I wish to keep private–say my marital history or criminal record–exists in publicly accessible archives, it remains, for all practical purposes, private so long as the people I am interacting with do not know that it exists nor where to look for it. Modern information processing has at least the potential to drastically reduce that sort of privacy. The same search engines and collections of information that provide the ideal tools for the researcher who dives into the worldwide web in the hope of emerging with a fact in his teeth work equally well whether the fact is historical or personal. Privacy through obscurity is not, or at least soon will not be, a practical option.

The two sorts of privacy–physical and informational–are connected. Physical privacy is a means, although a decreasingly effective means, to informational privacy. And lack of informational privacy–in the limiting case, a world where anyone could know everything about you at every instant–feels like lack of physical privacy, a sort of virtual crowding.

Physical privacy can be a means to information privacy–but so can the opposite; the individual in the crowded city is more anonymous, has more informational privacy, than in the less crowded village. But the reason for that is that his privacy is protected by the difficulty of sorting through such a vast amount of data in order to find the particular facts relevant to him. That form of protection cannot survive modern information technology. Hence the connection between physical and information privacy may become stronger, not weaker, over the course of the next few decades.

A third sort of privacy is attentional privacy, the privacy violated by unsolicited email or telephone calls from people trying to sell you things that you do not want to buy. Modern technology makes sending messages less expensive, facilitating bulk email, but also makes filtering out messages without human intervention easier, thus lowering the cost of dealing with unwanted messages.

In this article I will be focussing on issues of information privacy. But, as we will see, the technology of protecting informational privacy may depend in part on the existence of physical privacy. One interesting question for the future will be whether it is possible to develop technologies that break that link, that make it practical to engage in information transactions without taking any physical actions that can be observed and understood by an outside observer.

The first section of the article explores the questions of what informational privacy is, why and whether it is a good thing, and why it is widely regarded as a good thing. The second section surveys new technologies useful for either protecting or violating an individual’s control over information about himself. The final section summarize my conclusions.

What is Informational privacy and why does it matter?

If all information about you is readily available to anyone who wants it, you have no informational privacy. If nobody else knows anything about you, you have perfect informational privacy. All of us live between those two extremes.

Informational privacy is not always desirable. Film stars and politicians pay professional public relations firms to reduce their privacy by getting (some) information about them widely distributed. Many other people, however, bear costs in order to reduce the amount other people know about them, demonstrating that, to them, privacy has positive value. And many people also bear costs learning about others, demonstrating that to them the privacy of those other people has negative value. At the same time, most people regard privacy in the abstract as a good thing. It is common to see some new product, technology, or legal rule attacked as reducing privacy, rare to see anything attacked as increasing privacy.

This raises two related questions. The first is why individuals (sometimes) value their own privacy, and so are willing to take actions to protect it. The second is why many individuals speak and act as though the cost to them of a reduction in their privacy is larger than the benefit to them of a similar reduction in other people’s privacy, making privacy in general, and not merely privacy for themselves, a good.

The answer to the first question is fairly straightforward. Information about me in the hands of other people sometimes permits them to gain at my expense. They may do so by stealing my property–if, for example, they know when I will or will not be home. They may do so by getting more favorable terms in a voluntary transaction–if, for example, they know just how much I am willing to pay for what they are selling.[2] They may do so by preventing me from stealing their property–by, for example, not hiring me as company treasurer after discovering that I am a convicted embezzler or not lending me money after discovering that I have repeatedly declared bankruptcy.

Information about me in other people’s hands may also sometimes make me better off–for example, the information that I am an honest and competent attorney. But privacy rights[3] as commonly interpreted do not prevent people from giving out information about themselves, merely from obtaining information about others without their consent. If I have control over information about myself I can release it when doing so benefits me and keep it private when releasing it would make me worse off.[4] Hence it is not surprising that people value having such control.

This does not, however, answer the second question. To the extent that my control over information about me makes me better off at the expense of other people, and their control over information about them makes them better off at my expense, it is not clear why I should regard protection of everyone’s privacy as on net a good thing. The examples I offered included one case–where my privacy protected me from burglary–in which privacy produced a net benefit, since the gain to a burglar is normally less than the loss to his victim. It included one case–where my privacy permitted me to steal from or defraud others–in which privacy produced a net loss, for similar reasons. And it included one case–bargaining–where the net effect appeared to be a wash.[5]

That third case is worth a little more attention. Suppose you have something to sell–say an apple. I am the only buyer. The apple is worth one dollar to you and two to me. We are engaged in the game known as bilateral monopoly.[6] At any price between one dollar and two, both of us benefit by the transaction, but the higher the price is within that range the more of the benefit goes to you and the less to me.

I can try to get a lower price by persuading you that the apple is worth less to me than it really is, hence that if you insist on a high price there will be no sale. You, similarly, can try to get a lower price by persuading me that the apple is worth more to you than it is, so if I don’t agree to a higher price there will be no sale. One risk with both tactics is that they may succeed too well. If you persuade me that the apple is worth more than two dollars to you, or if I persuade you that it is worth less than one dollar to me, the deal falls through.

Suppose I get accurate information on the value of the apple to you. One result is that your persuasion no longer works, making it more likely that I will get the apple at a low price. That is merely a transfer from you to me, with no net benefit. A second result is to make bargaining breakdown less likely. I will still try to persuade you that the apple is worth less than two dollars to me, but I will not try to persuade you that it is worth less than one dollar, because I now know that doing so is against my interest. This second result represents a net benefit, since it increases the chance that the apple will sell (net gain one dollar) instead of not selling (net gain zero).

Generalizing the argument, it looks as though privacy produces, on average, a net loss in the bargaining case.[7] In the other cases, it produces a gain if it is being used to protect other rights (assuming that those rights have been defined in a way that makes their protection efficient) and a net loss if it is being used to violate other rights (with the same assumption). There is no obvious reason why the former situation should be more common than the latter. So it remains puzzling why people in general support privacy rights–why they think it is, on the whole, a good thing for people to be able to control information about themselves.

Privacy Rights and Rent Seeking

One possible approach to this puzzle starts by viewing privacy rights as a mechanism for reducing costs associated with rent seeking–expenditures by one person designed to benefit himself at the cost of another. Consider again our bilateral monopoly bargaining game. Assume this time that each player can, at some cost, obtain information about the value of the apple to the other player. I can plant listening devices or miniature video cameras about your home in the hope of seeing or hearing something that will tell me just how much you value the apple. You can take similar actions with regard to me. Such activities may produce some net gain, by reducing the risk of bargaining breakdown, but they also produce a net cost–the cost of the spying.

The argument becomes clearer if we include not only your efforts to learn things about me but my efforts to prevent you from doing so. Suppose, for example, that I have a taste for watching pornographic videos and my boss is a puritan who does not wish to employ people who enjoy pornography. We consider two possible situations–one in which my boss is, and one in which he is not, able to keep track of what I am renting from the local video store. We assume that I know which is the case.

If I know the boss is monitoring my rentals from that store, I respond by renting videos from a more distant and less convenient outlet. My boss is no better off as a result of the reduction in my privacy; I am still viewing pornography and he is still ignorant of the fact. I am worse off by the additional driving time required to visit the more distant store.

Generalizing the argument, we consider a situation where I have information about myself and can, at some cost, prevent other people from having that information. Under one legal (or technological) regime, the cost of doing so is low, under another it is high. Under both regimes, however, the cost is low enough so that I am willing to pay it. The former regime is then superior, not because I end up with more privacy but because I end up getting it at a lower cost. Hence laws, norms, or technologies that lower the cost of protecting privacy may produce net benefits.[8]

I say “may” because the conclusion depends on assuming that it will, in either case, be worth the cost to me to protect my privacy. If we assume instead that under the second regime protecting my privacy is prohibitively expensive, and if we are considering situations where the loss of privacy produces a transfer from me to someone else but no net cost (or, a fortiori, if it produces a net benefit), we get the opposite result.[9] If privacy is cheap I buy it and, even though it is cheap, it still costs something. If privacy is expensive, I don’t buy it and, while I am then worse off for not having it, someone else is better off at my expense, so on net we are better off by the elimination of what I would have spent for privacy.

Privacy as a way of reducing rent seeking provides a possible explanation for why circumstances that make privacy easier to obtain might be desirable, but an explanation very much dependant on assumptions about the technology of getting and concealing information. In a world where concealing information is costly but not too costly to be worth doing, making it less costly produces a net benefit. In a world where concealing information is so costly that nobody bothers to do it, making it less costly increases the amount spent protecting privacy, which is a net loss. More generally and precisely lowering the cost of privacy reduces expenditures on privacy if the demand is inelastic, increases them if it is elastic.[10]

This explanation also depends on another assumption–that the information about me starts in my control, so that facilitating privacy means making it easier for me to protect what I already possess. But much information about me comes into existence in other people’s possession. Consider, for example, court records which record my conviction on a criminal charge, or a magazine’s mailing list with my name on it. Protecting my privacy with regard to such information requires some way of removing that information from the control of those people who initially possess it and transferring control to me. That is, in most cases, a costly process. There are lots of reasons, unconnected with privacy issues, why we want people in general to have access to court records, and there is no obvious non-legal mechanism by which I can control such access,[11] so if we do nothing to give people rights over such information about them, the information will remain public and nothing will have to be spent to restrict access to it.

Privacy as Property

An alternative argument in favor of making privacy easier to obtain starts with a point that I made earlier: if I have control over information about me but transferring that information to someone else produces net benefits, then I can give or sell that information to him. Hence, one might argue, by protecting my property rights in information about me we establish a market in information. Each piece of information moves to the person who values it most, maximizing net benefit.

So far this is an argument not for privacy but for private property in information.[12] To get to an argument for privacy requires two further steps. The first is to observe that most information about me starts out in my possession, although not necessarily my exclusive possession. Hence giving anyone else exclusive rights to it requires somehow depriving me of it–which, given the absence of technologies to produce selective amnesia, is difficult. It would be possible to deprive me of control over information by making it illegal for me to make use of it or transmit it to others, but enforcing such a restriction would be costly, perhaps prohibitively costly.

The second step, following a general line of argument originated by Coase,[13] is to note that, to the extent that our legal rules assign control over information to the person to whom it is most valuable, they save us the transaction costs of moving it to that person. My earlier arguments suggest that information about me is sometimes most valuable to me (where it protects me from a burglar), sometimes to someone else. There are, however, a lot of different someone else’s. So giving each person control over information about himself, especially information that starts in his possession, is a legal rule that should minimize the transaction cost of getting information to its highest valued user.

Stated in the abstract, this sounds like a reasonable argument–and it would be, if we were talking about other forms of property. There are two problems with applying a property solution to personal information. The first is that transacting over information is often difficult, because it is hard to tell the customer what you are selling without, in the process, giving it to him. The second is that a given piece of information can be duplicated a cost close to zero, so that while the efficient allocation of a car is to the single person who has the highest value for it, the efficient allocation of a piece of information is to everyone to whom it has positive value. That implies that legal rules that treat information as a commons, free for everyone to make copies, lead to the efficient allocation.

That conclusion must be qualified in two ways. First, as we have already seen, legal protection of information may be a cheaper substitute for private protection, in which case if the information is going to be protected because it is in someone’s interest to do so, we might as well have it protected as inexpensively as possible. Second, you cannot copy information unless it exists. Thus we get the familiar argument from the economics of intellectual property, which holds that patent and copyright result in a suboptimal use of existing intellectual property, since marginal cost on the margin of number of users is zero while price is positive, but that in exchange we get a more nearly optimal production of intellectual property.

This is a legitimate argument for property rules in contexts such as copyright or patent. It is less convincing in the context of privacy, since information about me is either produced by me as a byproduct of other activities or produced by other people about me–in which case giving me property rights over it will not give them an incentive to produce it. It does provide an argument for privacy in some contexts, usually commercial, where privacy is used to protect produced information.

Privacy as an Inefficient Norm

Robert Ellickson, in Order Without Law, argues that close knit communities tend to produce efficient norms. One of his examples is the set of norms developed by 19th century whalers to deal with situations in which one ship harpooned a whale and another ship eventually brought it in. He offers evidence that those norms changed over time in a way that efficiently adapted them to the characteristics of the changing species of whales being hunted.

This story raises a puzzle. The reason the whalers had to change the species they were hunting, and the associated norms, was that they were hunting one species after another into near extinction. That suggests that a norm against overwhaling would have produced sizable benefits. Yet no such norm developed.

My explanation starts with a different puzzle: what is the mechanism that produces efficient norms? My answer starts by distinguishing between two different sorts of efficient norms. A locally efficient norm is a norm which it is in the interest of a small group of individuals to follow among themselves–an example would be a norm of fair dealing. A globally efficient norm is one that it is in the interest of everyone to have everyone follow.

Locally efficient norms can be adopted by small groups. Since the groups benefit by the norm, adoption spreads. Eventually everyone follows the norm. That mechanism does not work for a norm that is globally but not locally efficient, such as a norm against over whaling. If some whalers follow it, it is in the interest of other whalers to take advantage of the opportunity by increasing their efforts. Hence we would expect systems of private norms to be locally but not globally efficient–which corresponds to what Ellickson found for whaling.[14]

This brief sketch of norms provides a possible explanation for the widespread existence of norms of privacy–norms holding that individuals are entitled to conceal personal information about themselves, and that other individuals ought not to seek to discover such information. Such norms may well be locally efficient even if globally inefficient.

Why would such norms be locally efficient? Consider some piece of information about me–say my value for the apple in the earlier discussion of bilateral monopoly. If only I possess that piece of information, I can either withhold it, to my benefit, or offer to sell it to my trading partner, supposing that there is some way in which I can prove to him that the information I am selling is truthful. If only you possess the information, you can offer to sell it to either me or my trading partner, whichever bids more. But if several people possess the information, no one of them can sell it for a significant price since if he tries he will be underbid by one of the others–the cost of reproduction of information being near zero. The logic is exactly the same as if we wished to maximize the revenue from a patent and were comparing the alternatives of having one owner of the patent or several, where in the latter case each owner could freely license to third parties.

It follows that if we are members of a closeknit group containing all of the people who can readily discover personal information about each other, and if we are also engaged in dealings with non-members of the group such that possession by them of personal information about one of us would make him better off at our expense, a norm of privacy is likely to be in our interest. Its effect is to give each of us monopoly ownership of information about himself, permitting him to maximize the return from that information, whether by keeping it secret or by selling it. To the extent that the return comes at the expense of the non-members we are dealing with, the norm may be globally inefficient. But it is locally efficient, which provides a possible explanation of why it exists.

Blackmail and Privacy

"If blackmail were legal, blackmailers and their customers (today called "victims") would enter into legally enforceable contracts whereby the blackmailer would agree for a price never to disclose the information in question; the information would become the legally protected trade secret of the customer." (Posner 1993)

Laws against blackmail provide an interesting puzzle. Suppose you know something about me that I would prefer not to be public. I agree to pay for your silence. At first glance, the transaction seems obviously beneficial. I value your silence more than the money, which is why I made the offer; you value the money more than publishing my secret, which is why you accepted. We are both better off, so why should anyone object?[15]

One answer is that we have started too late in the process–after you obtained the information. The possibility of blackmail gives people an incentive to spend resources acquiring information about other people and gives potential targets an incentive spend resources concealing such information. If blackmail is legal I spend a thousand dollars trying to conceal the information, you spend a thousand dollars trying to discover it. If you succeed I then pay you three thousand dollars to keep your mouth shut, leaving us, on net, two thousand dollars worse off than when we started (you are two thousand dollars better off, I am four thousand worse off). If you fail, we are each out a thousand dollars, so again we are, on net, two thousand dollars worse off than when we started. So a law that made it impractical for you to profit by discovering such information provides a net benefit of two thousand dollars. We are back with the rent seeking explanation of privacy.

An alternative answer is that we ought to include more people in our calculations–in particular, we ought to include the people you are threatening to tell my secret to. The reason I am willing to pay for your silence is that doing so makes me better off–possibly at their expense. Perhaps the secret is my record for fraud or malpractice–and, having moved from where my misdeeds were first unveiled, I am looking for new, and poorly informed, customers. Perhaps the secret is what happened to my first wife–and I am now seeking to obtain a replacement. In these and many other circumstances, when the blackmailer accepts a payment for silence he imposes an external cost on those who would otherwise have learned what he knows. So perhaps legal rules that permit me to buy his silence make the society as a whole worse off, by keeping him silent and others ignorant.

As should be clear, these two arguments for banning blackmail are not only different, they are in an important sense inconsistent. If we assume that the same amount of information will be produced whether or not blackmail is legal–if we are imagining that the typical blackmailer obtained his information by accident not effort–then the rent seeking argument vanishes but the public good argument replaces it. The potential blackmailer has the information; if he cannot sell it he might as well give it away. If, on the other hand, we assume that the information on which blackmail is based is primarily obtained for that purpose, the rent seeking argument is revived but the other argument vanishes. If blackmail is illegal, the information is never generated so the public is never warned.

So far we seem to have arguments against permitting blackmail both in the case where blackmailers discover information by accident and in the case where they deliberately search for it. The conclusion becomes less clear if we assume that the information a blackmailer discovers is not merely useful to other people in dealing with the victim but discreditable to the victim–as we usually do assume in discussing blackmail. If we suppose that the blackmailer discovered the information by accident and will publish it if he cannot sell it to the victim–perhaps in the hope of a financial or reputational reward–then laws against blackmail make sense, since they result in the potential victim being convicted and punished for his crimes. If we permit blackmail he is still punished, with the punishment taking the form of a payment to the blackmailer, but the reason he makes the payment is that it costs him less than having his crime revealed, so the result is a lower punishment.

This is not true if we assume that the incentive provided by the ability to blackmail people plays a major role in the production of the information used to do so. In that case blackmail becomes private enforcement of law.[16] If blackmail is legal, people have an incentive to look for evidence of other people’s crimes and use it to blackmail them, thus imposing a punishment on criminals who would otherwise go free. The same argument applies if the information concerns violations of norms rather than laws, assuming that we believe the norms are efficient ones and punishment for their violation is appropriate.

Earlier I pointed out that one argument for intellectual property law is that it provides an incentive to generate valuable information. Similarly here, the form of transferable property right that exists if blackmail is legal also creates an incentive to generate valuable information. The information, once generated, is suppressed–but there is still a benefit, since the process generates a penalty for the behavior the information is about, and blackmail is particularly likely with regard to behavior that we would like to penalize.

Privacy and Government

“It would have been impossible to proportion with tolerable exactness the tax upon a shop to the extent of the trade carried on in it, without such an inquisition as would have been altogether insupportable in a free country.”

(Adam Smith’s explanation of why a sales tax is impossible; Wealth of Nations Bk V, Ch II, Pt II, Art. II)

“The state of a man’s fortune varies from day to day, and without an inquisition more intolerable than any tax, and renewed at least once every year, can only be guessed at.” (Smith’s explanation of why an income tax is impossible, Bk V Article IV)

So far I have ignored an issue that is central to much of the concern over privacy: privacy from government. The logic of the situation is the same as in the situations we have been discussing. If the government knows things about me–for example my income–that permits the government to benefit itself at my expense. In some cases it also permits government to do things that benefit me–pay me money because my income is low, for example–but in such situations privacy rights leave me free to reveal the information if I wish.

The case of privacy from government differs from the case of privacy from private parties in two important respects. The first is that although private parties occasionally engage in involuntary transactions such as burglary, most of their interactions with each other are voluntary ones, which makes it less likely that someone else having information about me will result in an inefficient transaction. Governments engage in involuntary transactions on an enormously larger scale. The second difference is that governments almost always have an overwhelming superiority of physical force over the individual citizen. It follows that while I can protect myself from my fellow citizens, to a considerable degree, by locks and burglar alarms, I can protect myself from government actors only by keeping from them the information they need to benefit themselves at my expense.[17]

The implications of these differences for the value of privacy depends very much on one’s view of government. If, at one extreme, one regards government as the modern equivalent of the philosopher king, then individual privacy simply makes harder for government actors to do good. If, at the other extreme, one regards government as a particularly large and well organized criminal gang supporting itself at the expense of the taxpayers, individual privacy against government becomes an unambiguously good thing. Most Americans appear, judging by expressed views on privacy, to be close enough to the latter position to consider privacy against government as on the whole desirable, although with an exception for cases where they believe that privacy might be used primarily to protect private criminals.

The Weak Case for Privacy: A Summary

Explaining why individuals wish control over information about themselves is easy. Explaining why it is in my interest that both I and the people I deal with have such control, or why people believe that it is in their interest and act accordingly, is more difficult.

We have considered three reasons why privacy might be in the general interest—might be efficient in the economic sense. One is that people want to control information about themselves, so the easier it is to do the less they will have to spend to do it. People want information about other people, and the harder it is to get, the less they will spend getting it. As the odd asymmetry of the two sides of the argument—in one case lowering a price reduces expenditure, in one case raising a price reduces expenditure—suggests, the argument depends on specific assumptions about relevant demand and supply functions. It goes through rigorously if the demand for privacy is inelastic and the demand for information about others is elastic—which might, but need not, be true.[18]

Put in that form the argument sounds abstract, but the concrete version should be obvious to anyone who has ever closed a door behind him, loosened his tie, taken off his shoes, and put his feet up on his desk. Privacy has permitted him to maintain his reputation as someone who behaves properly without having to bear the cost of actually behaving properly—which is why there is no window between his office and the adjacent hallway.

The second reason privacy might be efficient is that property rights permit goods to be allocated to their highest valued use. If protection of privacy is easy, then individuals have reasonably secure property rights over information about themselves. It makes sense to give such rights to the individual the information is about because he is more likely to be the highest valued user than any single other person—and if he is not, he can always sell the information to someone else. The problem with this argument is that information, unlike other goods, can be reproduced at near zero cost, making it likely that the highest valued user is everybody—and there are significant transaction costs preventing the transfer of information from its subject to everybody even if that transfer produces net benefits.

The third reason that privacy might be efficient is that it provides a way in which individuals may protect themselves against government. The strength of that argument depends very much on one’s view of the nature of government.

We also saw one argument against privacy—that it permits people to act badly while evading the consequence of having people know that they acted badly—an argument worked out in the context of arguments for and against legalizing blackmail.

The conclusion so far is that the case for privacy—for the claim that it is desirable to lower the cost to individuals of controlling information about themselves—is a weak one. Under some circumstances privacy produces a net gain but under others a net loss.

Other Privacies

So far we have been talking only about informational privacy. The link to physical privacy is fairly obvious; if there is someone else in the room, he will probably notice when you loosen your tie and take off your shoes. Physical privacy is, among other things, a means to maintain informational privacy.

The link to attentional privacy is also obvious but the implications less clear. When someone sends me a message such as a phone call or email it costs me something to examine the message and determine whether it is of interest. In a world of uncertainty, some messages are of interest to me, some are not, and neither I nor the sender knows for certain until I have examined the message.

Both I and the sender would prefer that the sender send me messages that are of interest to me; there is no point to calling someone up in order to sell him something he has no interest in buying. Where we differ is in just where we draw the line between messages that are or are not worth their cost. The sender wants to send messages if and only if the chance I will be interested, and respond in a way which benefits him, is sufficient to justify the cost to him of sending the message.[19] I want him to send messages if and only if the chance is sufficient to justify the cost to me of examining and evaluating the message. The result in a world where sending messages is expensive and evaluating them inexpensive is that I receive inefficiently few messages—so I buy additional messages by (for example) subscribing to magazines. The result in a world where sending messages is cheap and evaluating them expensive is that I receive more messages than I want. Resolving that problem requires a negative subscription price, a mechanism by which I can charge people for sending me messages.

The connection to informational privacy comes because the sender needs information about me in order to decide whether it is worth the cost to him of sending a message. The implication is ambiguous because increasing the amount of such information available to him may make the outcome better or worse for me. In the limiting case of complete information, potential senders know for certain whether I want to buy what they are selling, so I receive all the offers I would want to receive and do not have to waste time examining any that I do not want to receive.[20] In the limiting case of no information, and in a world where the cost of sending messages is significant, it is never worth sending a message; this cannot be an improvement on other alternatives, since the other alternatives always permit the option of ignoring all messages—cutting the bottom out of my mailbox and putting a waste basket underneath it.

More generally, increasing the information other people have about you can benefit you by making it easier for those who have offers you are interested in to find you and easier for those whose offers you are not interested in to discover the fact and save themselves the cost of making the offers. If only all the world knew that I didn’t have a mortgage on my house, I would no longer be annoyed by phone calls from people offering to refinance it.

As this example suggests, one way of getting the best of both worlds is to have control over information about yourself and use that control to make some information public while keeping other information private. We will return to that possibility later, after discussing technologies that facilitate that approach.

Finally, it is worth noting that different societies have had different norms with regard to privacy, some of which surely reflect the differing value to individuals of having information about themselves widely known. Consider the English upper class at the beginning of the nineteenth century, as depicted by Jane Austen. Every gentleman’s income appears to have been a matter of public knowledge. One reason may have been that the information was crucial to families with daughters on the marriage market. A gentleman who went to some trouble to conceal his financial situation would be signaling not a taste for privacy but an income below his pretended status.[21]

We are now finished with our theoretical discussion of privacy. One thing that discussion has made clear is that whether it is desirable for individuals to be able to control information about them depends on a variety of technologies—in the economist’s sense, in which a technology is simply a way of transforming inputs to outputs. In particular, it depends on technologies for obtaining, concealing, and transmitting information—which will be the subject of the next part of this article.

The technology of privacy

Over the course of the past fifty years, a variety of technologies have developed which substantially affect the cost of obtaining information about other people, concealing information about oneself and transacting in information. For our purposes, they may be grouped into three broad categories: information processing, encryption, and surveillance.

Information Processing

The earliest and best known of these technologies is information processing. Fifty years ago, a firm or government bureau possessing information on millions of individuals faced daunting problems in making use of it. Today, the average citizen can afford, and may well own, computer hardware and software capable of dealing with a database of that size.

One implication is that organizations that already have large-scale data collections are more able to use them, hence that privacy rights, in the sense in which I have been using the term, are weaker. A second implication is that dispersed information which nobody found worth collecting in the past may be routinely collected in the future.

It is possible to hinder that development by legal rules restricting the collection and sale of data, and such rules—for example, the Fair Credit Reporting Act—exist. But doing so is costly, and it is far from clear that it is useful. For the most part, the information is used by private parties to facilitate voluntary transactions with others—an activity that typically produces net benefits.[22] Given that information is collected for that purpose, it is hard to design legal rules that prevent its occasional use for other purposes, such as locating potential targets for criminal activity. And as the growth of the internet moves more and more of the commercial activity relevant to U.S. citizens outside of the jurisdiction of U.S. courts, regulation over the collection and use of such information will become even more difficult.

An alternative approach is control over information about individuals by those individuals, through a combination of physical privacy and contract. Such information is frequently produced by voluntary transactions, such as purchases of goods and services, and thus starts out in the possession of both parties to the transaction. If one party wishes the information to be kept confidential, it can so specify in the terms of the initial transaction—as is, of course, often done in a variety of settings. The same information processing technology that makes it relatively inexpensive to keep track of large numbers of facts about large numbers of people also makes it inexpensive to keep track of which of them have provided you information with conditions, perhaps detailed conditions, on its disclosure.

A more exotic and potentially more secure approach that may become increasingly practical as a result of technologies to be discussed in the next section is to engage in transactions anonymously, thus never putting the relevant information in the control of anyone else, not even the other party to the transaction. More generally, one possibility implicit in the combination of technologies for information processing and encryption is a shift to something more like a private property/freedom of contract model for personal information.


Many forms of modern communication, including email and cellular telephony, are physically insecure—intercepting messages is relatively easy. In order to protect the privacy of such communications, it is necessary to make them unreadable by those who might intercept them. This is done by encryption—scrambling a message in such a way that only someone with the proper information, the key, can unscramble it.

The most important modern development in this field is public key encryption.[23] An individual generates a pair of keys—two long numbers having a particular mathematical relation to each other. If one key is used to scramble a message the other is required to unscramble it. In order to make sure that messages sent to me remain confidential, all I have to do is to make sure that one of my keys (my “public key”) is widely available, so that anyone who wants to send me a message can find it. The other (my “private key”) is my secret, never revealed to anyone. Anyone who has my public key can use it to encrypt a message to me. If someone else obtains the public key, he can send me secret messages too. But only someone with my private key, which I need never make available to any other person, can read the messages.

The same technology solves a related problem—how to prove to the recipient of my message that it is really from me. In order to digitally sign a message, I encrypt it with my private key.[24] The recipient decrypts it with my public key. The fact that what he gets is a message rather than gibberish demonstrates that it was encrypted with the matching private key, which only I have.

A digital signature not only demonstrates, more securely than an ordinary signature, that I really sent the message, it also demonstrates it in a way that I cannot later deny. You now possess a digitally signed message—the original, before decryption—which you could not have created yourself. So you can prove to interested third parties that I actually sent the message, whether or not I am willing to admit it. And since there is no way of changing the digitally signed message without making the signature invalid, a digital signature, unlike a physical signature, demonstrates that the message has not been altered since it was signed.

The same technology also has two other privacy enhancing applications. One is an anonymous remailer. If I wish to communicate with someone without the fact of our communication being known, I send the message through a third party in the business of relaying messages. In order to preserve my privacy from both the remailer and potential snoops, I encrypt my message with the recipient’s public key, add to it the recipient’s email address, encrypt the whole package with the remailer’s public key, and sent it to the remailer. The remailer uses his private key to strip off the top layer of encryption, permitting him to read the email address and forward the message. If I am concerned that the remailer himself might want to keep track of who I am communicating with I can bounce the message through multiple remailers, providing each with the address of the next. Unless all of them are jointly spying on me, my secret is safe.

The other important application is anonymous digital cash. Using encryption, it is possible for a money issuer to create the digital equivalent of currency, permitting one person, by sending a message to another, to transfer claims against the issuer without either person having to know the other’s identity and without the issuer having to know the identity of either.

Consider a world in which all of these technologies exist and are in general use. In such a world, it will be possible to do business anonymously but with reputation. Your cyberspace identity is defined by your public key. Anyone who can read messages encrypted with that public key must have the matching private key–which is to say, must be you. The same is true for anyone who can sign messages with the private key that matches that public key.

One disturbing implication, which I have discussed elsewhere, is the possibility of criminal firms operating anonymously but with brand name reputation. A more attractive implication is that, in such a world, the private property model of personal information becomes a practical possibility. If, when I buy something from you, neither of us knows the identity of the other, then neither of us can obtain the relevant transactional information–the fact that a certain other person bought or sold a particular good–without the cooperation of the other. Hence transactional information starts as the sole property of the person it is information about; that person is then free to either suppress it, publish it, or sell it, whichever best serves his interests.

A second feature of this world relevant to the issues we have been discussing comes from a different use of the technology of encryption: technological protection of intellectual property.[25] It may some become practical to distribute intellectual property in a cryptographic container–as part of a computer program which controls access to its contents, what IBM refers to as a “cryptolope.” Use of the contents then requires a payment, perhaps in digital cash, with the container regulating the form of use. Combining such technologies with the use of intelligent software agents for negotiating online contracts, we have the possibility of a world where it will be practical to treat information as something close to ordinary property. One could, for example, sell or give away transactional data about oneself in a form that could only be used for specified purposes, or only in association with specified payments.

This set of possibilities represent one part of a more general pattern. The combination of online communications, encryption, and information processing permits a much more detailed control over information flows, at least information flows online or flows of information that originates online, than was possible in the past. Thus, to take an entirely different example, there is no technical barrier to prevent the creation of an email program designed to permit someone who wished to protect his attentional privacy from charging a price for receiving email–and simply trashing, without human intervention, any messages that came without an associated payment. Nor is there any barrier to making such software distinguish among senders, receiving messages for free if they are digitally signed by people the owner of the software wants to receive messages from.

For a less exotic example, consider the marketing of mailing lists. With current transactional technology, the fact of a transaction is known to both parties, so I cannot directly control the fact that I am a subscriber–as I could if the transaction were taking place online between anonymous parties. But a magazine may, and some do, restrict its use of that information by contract, by promising not to make its mailing list available to others or by giving the customer the choice of having or not having his name and address sold to other merchants. Such contractual arrangements will become easier as more and more transactions shift to digital forms, where individualized contract terms are considerably less expensive than with conventional contracting technology.

One option is to keep your name and address private, another is to permit it to be freely sold. A third, which many might find more attractive than either of the others, is for the magazine to sell access to its customers but not their identity. This could be done easily enough by having the magazine operate its own remailer. Information about each customer would be provided to merchants interested in communicating with him–information about what the customer had purchased, and any other information the magazine had that was relevant to what the consumer would want to buy but could not be used to identify him. The merchant then sends a message directed at that particular (identified but unnamed) customer, which the magazine forwards to him.

This is, in fact, how the selling of mailing lists is commonly conducted at present, although for a different reason and with considerably less information available to the sender. A magazine does not normally sell its mailing list, it rents it, for a fixed number of uses. The purchaser gets, not the list, but the opportunity to have its message sent to the names on the list, with the remailer replaced by a third firm in the business of arranging such transactions. Modern technology makes possible a more sophisticated version of such a transaction. Ultimately we could have third party remailers holding large amounts of information on unidentified individuals in ways that permit their customers to search for individuals possessing combinations of characteristics that make them attractive targets for specfic offers but do not permit any outsider to link information with identity. Alternatively, the same result could be produced even more securely–without having to trust the remailer–by having individuals interact via anonymous online personas, making the facts of transactions public, in order to attract desirable offers, but keeping the identity of the realspace person corresponding to the cyberspace persona private.[26] One thus abandons privacy for purposes of voluntary transactions, which can take the form of an offer to an unknown identity, but retains it for protection against involuntary transactions. It is hard to burgle the house of someone identified only by his public key.

With the exception of anonymous ecash, which we know how to do but which nobody has so far done,[27] and cryptolopes, which are still mostly in the development stage, all of the fundamental technologies I have described already exist. Public key encryption has been implemented in a variety of forms, including a widely distributed free program.[28] Anonymous remailers currently exist. Digital signatures are widely used. But for the most part these technologies have been applied only to text and so have affected only that part of private and commercial life that is embodied in text messages.

As computers become more powerful and the bandwidth of digital networks increases, that situation will change. Using wide bandwidth networks and virtual reality programs, it will be possible to create the illusion of any transaction that involves only the senses of sight and sound. Further in the future we may succeed in cracking the dreaming problem, figuring out how our nervous system encodes the information that reaches us as sensory experience. At that point the limitation to two senses will disappear. We will be able to create, by the transmission of information in digital form, the illusion of any interaction that could take place in realspace.

As more and more of our activity shifts into cyberspace encryption and related technologies make possible a degree of control over both the creation and the transfer of information very much greater than we now have. At that point the property justification for privacy, rejected in the first part of this article, comes back into the argument.

What about the argument against privacy–that one reason I may wish to conceal information about myself is in order to defraud my trading partner, whether in the context of a mortgage or a marriage? That becomes a less serious problem online, where the technology restricts parties to voluntary transactions. You can, of course, conceal information about yourself if that information is under your control. But I can refuse to transact with you unless you agree to reveal the relevant information–and if you decline, that fact itself signals something about the information you are keeping private.

Surveillance devices—Towards a transparent society,

While technological developments in online communication are moving us towards a high level of privacy in cyberspace, developments in surveillance technology may be moving realspace in precisely the opposite direct, for two reasons. One is that surveillance devices provide a relatively inexpensive and effective way of reducing crime, one that is becoming increasingly popular. The other is that, as the relevant electronic devices become smaller and cheaper, it becomes more difficult to prevent surveillance. We may be moving towards a world in which video cameras with the size and aerodynamic characteristics of a mosquito will be widely available.

David Brin, on whose book The Transparent Society this section is largely based, argues that general privacy will no longer be an option. We will be limited to two choices: a world in which those in power know everything they want to know about everyone, and a world in which everyone knows everything he wants to know about everyone. Brin, not surprisingly, prefers the latter. He envisages a future with video cameras everywhere–including every police station–all generating images readily accessible to anyone interested, via some future equivalent of the web.

If he is correct, physical privacy in realspace will vanish. One implication is that individuals will protect their informational privacy in the same ways in which people in primitive societies without physical privacy protect it, by adopting patterns of speech and behavior that reveal as little as possible of what they actually believe and intend. That will represent a substantial rent seeking cost, to be added to the rent seeking cost of individuals processing the public information in order to learn things about all those with whom they expect to interact.

Two qualifications are worth making to Brin’s picture. The first is that he is assuming that the technology of surveillance is going to outrun the technology of physical privacy, that the bugs will beat the scanners, that the video mosquitos will not fall victim to automated dragonflies. While he may be correct, it is hard to predict in advance how the balance will turn out. We might end up in a world where legal surveillance is cheap and easy, illegal surveillance difficult, giving us the choice of how much privacy we will have.

One obvious compromise is privacy in private spaces but not in public spaces. That would represent a further development along the same lines as computerized databases. What you do in public spaces, like the public records produced by your life,[29] has always been public in a legal sense. A video surveillance network coupled to computers running pattern recognition software and sorting and saving the resulting data would simply put that public information in a form permitting other people to find and use it.

A second qualification is that although Brin’s technology produces information, it may not always be reliable or verifiable information. Suppose I am conducting an adulterous affair. My suspicious wife can obtain video footage of me in flagrante delicto with my paramour, via a suitably programmed video camera. But that footage may be of very limited use in court, since it could have been produced just as easily if I was not conducting an affair–using video editing software instead of a camera. To the extent that modern technology makes it easy to forge evidence, the evidence itself, without a provable pedigree, becomes worthless. It may be easy to get a mosquito camera into my bedroom but it is not so easy to get a witness in as well, to prove that that camera really took that film.[30]

Encryption technology provides one approach to solving that problem. Conceivably, a manufacturer could build a sealed, tamperproof camera, complete with its own private key. The camera would then digitally sign and timestamp[31] its films as it produced them, making it possible at a later date to prove that those particular films were created by that camera at that time and have not since been edited.

One difficulty with this approach is that a camera records, not facts about the outside world, but facts about the pattern of light coming in its lens. To defeat such a camera, I build a lens cap capable of generating computer synthasized holographic images. I then put the lens cap on the camera and play whatever I want the camera to see; it sees and signs it. As this example suggests, figuring out the implications of technologies that do not yet exist, or exist only in primitive forms, is a nontrivial problem.


In the first part of this article I sketched out an economic analysis of privacy. The conclusion was that increasing the ability of individuals to control information about themselves had both desirable and undesirable effects, making it unclear whether, on net, we were better off with more or less privacy. One argument that I considered and rejected was that increased privacy rights, at least over information that originates with the person it is about, were efficient because they made it possible to convert such information into private property and then allocate it efficiently through market transactions.

That argument is harder to reject when applied to the information technology of a few decades hence. It may become possible to create transactional information in such a way that each piece of information originates in the possession of a single person. And it may be possible, given the much lower transactions costs of online transactions, to then use private transactions to allocate information to its highest valued users. If so, we end up with a world in which information generated by cyberspace events, online transactions, is characterized by both a high degree of control by those it is information about and an efficient market for its creation and allocation.

There is no reason to expect the same to be true in realspace. If anything, the combination of improved surveillance technology and improved information processing technology is likely to make increasingly large amounts of realspace information about everyone inexpensively available to everyone else. We then have both the advantages of a low privacy environment–individuals cannot hide unattractive facts about their doings in realspace from those they transact with, making many forms of fraud, commercial and social, impractical–and the disadvantages. The cost of privacy becomes the cost of behaving in a way that reveals as little as possible about oneself.

If realspace is public and cyberspace private, the amount of privacy individuals have depends critically on the importance of each, and on the links between the two. It does me no good to protect my messages with strong encryption if a mosquito camera is watching me type the unencrypted original. In extreme versions of this scenario, versions where both Brin’s vision of realspace and my vision of cyberspace are realized in full, privacy depends critically on mechanisms for inputting to a computer that cannot be observed from the outside. The low tech version is touch typing under a very secure hood; the high tech a link directly from mind to machine. If some such method makes it possible to protect cyberspace privacy from realspace prying, the balance between public and private then depends on how much of what we do is done in cyberspace and how much in realspace. It is going to be an interesting century.


Friedman, David, "In Defense of Private Orderings: Comment on Julie Cohen's 'Copyright and the Jurisprudence of Self-Help.'"Berkeley Technology Law Journal (1999,1).

........................ Why Is Law?: An Economist's View of the Elephant,”forthcoming from Princeton University Press c. 12/99.(1999,2)

........................ Hidden Order: The Economics of Everyday Life. Harper-Collins, 1996. (1996,1)

........................ “A World of Strong Privacy: Promises and Perils of Encryption,” Social Philosophy and Policy , 1996 (1996,2)

........................ "Standards As Intellectual Property: An Economic Approach," University of Dayton Law Review, Vol. 19, No. 3, (Spring 1994) pp. 1109-1129. (1994,1)

........................ “A Positive Account of Property Rights,” Social Philosophy and Policy 11 No. 2 (Summer 1994) pp. 1-16. (1994,2)

........................ “Less Law than Meets the Eye,” a review of Order Without Law, by Robert Ellickson, The Michigan Law Review vol. 90 no. 6, (May 1992) pp. 1444-1452.

........................ “Some Economics of Trade Secret Law,” with William Landes and Richard Posner, Journal of Economic Prospectives, vol. 5, Number 1 (Winter 1991) pp. 61-72.

Lindgren, James, “Blackmail: On Waste, Morals, and Ronald Coase, 36 UCLA L. REV. 597 (1989);

........................ “Kept in the Dark: Owens's View of Blackmail,” 21 CONN. L. REV. 749 (1989);

........................ “Secret Rights: A Comment on Campbell's Theory of Blackmail,” 21 CONN. L. REV. 407 (1989);

........................ “In Defense of Keeping Blackmail a Crime: Responding to Block and Gordon,” 20 LOY. L.A. L. REV. 35 (1986);

........................ “More Blackmail Ink: A Critique of Blackmail, Inc., Epstein's Theory of Blackmail,” 16 CONN. L. REV. 909 (1984);

........................ “Unraveling the Paradox of Blackmail,” 84 COLUM. L. REV. 670.

Posner, Richard, “The Right of Privacy,” 12 Georgia Law Review 393 (1978).

........................ “An Economic Theory of Privacy,” Regulation, May/June 1978, at 19.

........................ The Economics of Justice, Chs 9-10 (1981)

........................ Overcoming Law, ch 25 (1995)

“Blackmail, Privacy, and Freedom of Contract,” 141 University of Pennsylvania Law Review 1817 (1993).

Murphy, Richard S., “Property Rights in Personal Information: An Economic Defense of Privacy, 84 Geo L.J. 2381 (1996)

[1] References in Posner (1978) to anthropological literature on lack of privacy in other societies.

[2] One example occurs in the context of a takeover bid. In order for the market for corporate control to discipline corporate managers, it must be in the interest of someone to identify badly managed corporations and take them over. That depends on a takeover bid remaining secret long enough for the person responsible to accumulate substantial ownership at the pre-takeover price. In a very public world, that is hard to do. Currently it is also hard to do because of legal rules deliberately designed to prevent it.

[3] “Rights,” as I use the term here, are defined not by legal rules but by control. If I have a legal right not to have you tap my phone but it is impractical to enforce that right–the situation at present for those using cordless phones without encryption–then in my sense I have no right to that particular form of privacy. On the other hand, I have substantial rights to privacy with regard to my own thoughts, even though it is perfectly legal for other people to use the available technologies–listening to my voice and watching my facial expressions–to try to figure out what I am thinking. I have those rights because those technologies are not adequate to read my mind. For a more general discussion of rights from a related perspective see Friedman (1994)

[4] An exception is the case where the relevant information is negative–the fact that I have not declared bankruptcy, say. If individuals have control over such information, then the absence of evidence that I have declared bankruptcy provides no evidence that I have not. One solution, if the control is based on legal rules, is to permit theinformation to be released with the permission of the person it is information about.

[5] Many of the points made in this section of the article can be found, in somewhat different form, in Posner (1978). The author also finds the general desirability of privacy to be dubious.

[6] Discussions can be found in Friedman (1996) chapter 11 and in Friedman (1999,2) Chapter 8; the latter is forthcoming c. 12/99 and currently webbed at

[7] One exception is the situation in which my gains from the bargain provide the incentive for me to generate valuable information. There is little point to spending time and money predicting a rise in wheat prices if everything you discover is revealed to potential sellers before you have a chance to buy from them. This is a somewhat odd case because, as Hirshleifer pointed out in a classic article [find the reference], while successful speculation is both socially useful and profitable, there is no particular connection between the two facts. Hence the opportunity for speculative gain may produce either too much or too little incentive to obtain the necessary information.

[8] This argument is proposed as a possible justification for trade secret law in Friedman, Landes and Posner (1991).

[9] One reason that the assumption may be correct is the difficulty of propertizing information. Suppose that keeping secret some particular fact about me benefits me at the expense of people I deal with; for simplicity assume that the benefit is a simple transfer, with no net gain or loss. If you discover the fact, you have no incentive to keep it hidden, so you tell other people. You end up getting only a small fraction of the benefit, while I bear all of the cost, so I am willing to spend much more to conceal the fact than you to discover it. This would not be the case if you could sell the information to other people who deal with me—as credit agencies, of course, do. But in many contexts such sales are impractical, due to the problems of transacting over information briefly discussed below.

[10] A demand is elastic (aka more than unit elastic) if a one percent increase in price results in more than a one percent decrease in quantity demanded. A demand is inelastic (aka less than unit elastic) if a one percent increase in price results in less than a one percent decrease in quantity demanded.

[11] There may be very costly ways of doing so. During some of the litigation involving the Church of Scientology, information which the CoS wished to keep private became part of the court record. The CoS responded by having members continually checking out the relevant records, thus keeping anyone else from getting access to them. And I might preserve my privacy in a world where court records were public by changing my name.

[12] For a discussion of why it makes sense to treat some things as property and some as commons, see Friedman (1994,1) and (1999, 2,chapter 10).

[13] See Friedman (1999, 2, chapter 4).

[14] A longer version of this argument can be found in Friedman (1992).

[15] These issues are explored in Murphy (1996), Posner (1978 1,2), (1981, 1993, 1995), Lindgren (1989 1,2) (1986, 1984, ??) and by Ronald Coase in ... .

[16] Posner has argued that laws against blackmail are desirable in circumstances where private law enforcement is for some reason inefficient.

[17] Or, of course, by political activity–lobbying Congress or making contributions to the police benevolent fund. For most individuals such tactics are of very limited usefulness.

[18] Privacy might also be on net efficient if one of the two functions met the required condition and produced gains that more than outweighed the loss from the function that did not.

[19] Throughout this discussion I am assuming that the purpose of messages is to propose voluntary transactions. I am thus ignoring cases such as harassment, where the benefit to the sender does not depend on the buyer deciding that the message is of value to him, and emailbombing, flooding someone’s mailbox in order to prevent him from using it, where the purpose of the message is to create the cost to the recipient.

[20] This result is not quite as rigorous as it sounds, since the cost of evaluating an offer is already sunk at the point when you decide whether to accept it. Consider an offer that costs fifteen cents to evaluate and proposes a transaction that would produce a gain of ten cents for the receiver of the offer. The receiver, having already paid the examination cost, accepts the offer and so produces a gain for the sender sufficient to more than cover the cost of sending. I will ignore such complications since I doubt they are of much real world importance.

[21] In modern day Israel, judging by my observations, asking someone his salary is considered perfectly normal, whereas in the U.S. it is a violation of norms of privacy. I have no good explanation for the difference.

[22] Although it might under some circumstances produce net costs associated with attentional privacy.

[23] For a much longer discussion see Friedman (1996,2).

[24] The process used for digital signatures in the real world is somewhat more elaborate than this, but the difference are not important for the purposes of this article. A digital signature is produced by using a hash function to generate a message digest—a string of numbers much shorter than the message it is derived from—and then encrypting the message digest with the sender’s private key. The process is much faster than encrypting the entire message and almost as secure.

[25] See Friedman (1999,1).

[26] For an early and still interesting fictional exposition of the idea of separating realspace and cyberspace identities, see Verner Vinge, “True Names,” included in (among other books) True Names and Other Dangers. A more recent fictional effort, and one picturing something much closer to what we are actually likely to see in a few decades, is the forthcoming Earthweb by Marc Stiegler.

[27] There have been experiments with ecash, most notably by the Mark Twain Bank of St. Louis working with David Chaum, the cryptographer responsible for many of the fundamental ideas in the field. The notes were semi-anonymous, meaning that the issuing bank could identify one party to the transaction if it had the cooperation of the other.

[28] PGP.

[29] With a few exceptions created by the law.

[30] A human solution to the problem of forged data was proposed by Robert Heinlein in Stranger in a Strange Land–a body of specially trained “fair witnesses,” whose job it was to accurately observe and honestly report.

[31] One way of timestamping a digital document is to calculate a hash of that document, a much shorter string of digits derived from it in a fashion difficult to reverse, and post the hash in some publicly observable place. The document is still secret, since it cannot be derived from the hash. But the existence of the hash at a given date can later be used to prove that the document from which it was derived–in our case, a digital video–existed at that time. That fact that the hash function cannot easily be reversed means that you cannot post a random hash, then later create a suitable document which that particular hash is a hash of.

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