You don't get to go just anywhere in a city, and the same is true of cyberspace. In both domains, barriers and thresholds play crucial roles.

In the built fabric of a city, the enclosing surfaces of the constituent spaces-walls, floors, ceilings, and roofs-provide not only shelter, but also privacy. Breaches in these surfaces-gates, doors, and windows-incorporate mechanisms to control access and maintain privacy; you can lock your doors or leave them open, lower the window shades or raise them. Spatial divisions and access-control devices are carefully deployed to organize places into hierarchies grading from completely public to utterly private. Sometimes you have to flip your ID to a bouncer, take off your shoes, pay admission, dress to a doorman's taste, slip a bribe, submit to a search, speak into a microphone and wait for the buzzer, smile at a receptionist, placate a watchdog, or act out some other ritual to cross a threshold into a more private space. Traditions and laws recognize these hierarchies and generally take a dim view of illicit boundary crossing by trespassers, intruders, and Peeping Toms.

Different societies have distinguished between public and private domains (and the activities appropriate to them) in differing ways, and urban form has reflected those distinctions. According to Lewis Mumford, domestic privacy was "a luxury of the well-to-do" up until the seventeenth century in the West. ²³ The rich were the people who could do pretty much what they wanted, as long as they didn't do it in the street and frighten the horses. As privacy rights trickled down to the less advantaged classes, the modern "private house" emerged, acquired increasingly rigorous protections of constitutional law and public policy, and eventually became the cellular unit of suburban tissue. ²⁴ Within the modern Western house itself-in contrast to some of its ancient and medieval predecessors -there is a staged gradation from relatively public verandahs, entry halls, living rooms, and parlors to more private, enclosed bedrooms and bathrooms, where you can shut and lock the doors and draw down the shades against the outside world.

It doesn't rain in cyberspace, so shelter is not an architectural issue. But privacy certainly is. So the construction technology for virtual cities-just like that of bricks-and-mortar onesmust provide for putting up boundaries and erecting access controls, and it must allow cyberspace architects and urban designers to organize virtual places into public-to-private hierarchies.

Fortunately, some of the necessary technology does exist. Most obviously, in cyberspace construction the rough equivalent of a locked gate or door is an authentication system. ²⁵ This controls access to virtual places (such as your e-mail inbox) by asking for identification and a password from those who request entry. If you give the correct password, you're in. ²⁶ The trouble, of course, is that passwords, like keys, can be stolen and copied. And they can sometimes be guessed, systematically enumerated until one that works is found, or somehow extorted from the system manager who knows them all. So password protection -like putting a lock on a door-discourages illicit entry but does not block the most determined break-in artists.

Just as you can put the valuables that you really want to protect in a sturdy vault or crypt, though, you can build the strongest of enclosures around digital information by encrypting it-scrambling it in a complex way so that it can be decoded only by someone with the correct secret numerical key. The trick is not only to have a code that is difficult to crack, but also to manage keys so that they don't fall into the wrong hands. The cleverest known way to do this is to use a technique called RSA public-key encryption. In this system, which derives its power from the fundamental properties of large prime numbers, each user has both a secret "private" key and a "public" key that can be distributed freely. If you want to send a secure message, you obtain the intended recipient's public key and use it to encode the information. Then the recipient decodes the message using the private key.

Under pressure from cops and cold warriors, who anticipate being thwarted by impregnable fortresses in cyberspace, the US federal government has doggedly tried to restrict the availability ofstrong encryption software. But in June 1991, hacker folk hero Philip Zimmerman released his soon-to-be-famous, RSA-based Pretty Good Privacy (PGP) encryption program. By May 1994 commercial versions had been licensed to over four millionusers, and MIT had released a free, noncommercial version that anybody could legally download from the Internet. ²⁷ From that moment, you could securely fence off your private turf in cyberspace.

Meanwhile, the Clinton administration pushed its plans for the Clipper Chip, a device that would accomplish much the same thing as RSA but would provide a built-in "trapdoor" for law-enforcement wiretapping and file decoding. ²⁸ The effect is a lot like that of leaving a spare set of your front door keys in a safe at FBI headquarters. Opinion about this divided along predictable lines. A spokesman for the Electronic Frontier Foundation protested, "The idea that the Government holds the keys to all our locks, before anyone has even been accused of committing a crime, doesn't parse with the public." ²⁹ But an FBI agent, interviewed by The New York Times, disagreed: "O.K., someone kidnaps one of your kids and they are holding this kid in this fortress up in the Bronx. Now, we have probable cause that your child is inside this fortress. We have a search warrant. But for some reason, we cannot get in there. They made it out of some new metal, or something, right? Nothing'll cut it, right? And there are guys in there laughing at us. That's what the basis of this issue really is-we've got a situation now where a technology has become so sophisticated that the whole notion of a legal process is at stake here. ... If we don't want that, then we have to look at Clipper." ³⁰

So the technological means to create private places in cyberspace are available, but the right to create these places remains a fiercely contested issue. Can you always keep your bits to yourself? Is your home page your castle? ³¹ These are still open questions.

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