to trick a browser into running code, and widely used programs with simple inputs like JPEG have had buffer overruns. A modern client OS, together with the many applications that run on it, is bound to have security bugs.

Users can’t evaluate these dangers. The only sure way to avoid the effects of dangerous inputs is to reject them. A computer that is not connected to any network rejects all inputs, and is probably secure enough for most purposes. Unfortunately, it isn’t very useful. A more plausible approach has two components:

Divide inputs into safe ones, han- ˲ dled by software that you trust to be bug-free (that is, to enforce security policy), and dangerous ones, for which you lack such confidence. Vanilla ANSI text files are probably safe and unfiltered HTML is dangerous; cases in between require judgments that balance risk against inconvenience.

Accept dangerous inputs only from ˲ sources that are accountable enough, that is, that can be punished if they misbehave. Then if the input turns out to be harmful, you can take appropriate revenge on its source.

accountability

People think that security in the real world is based on locks. In fact, real-world security depends mainly on deterrence, and hence on the possibility of punishment. The reason your house is not burgled is not that the burglar can’t get through the lock on the front door; rather, it’s that the chance of getting caught and sent to jail, while small, is large enough to make burglary uneconomic.

It is difficult to deter attacks on a computer connected to the Internet because it is difficult to find the bad guys. The way to fix this is to communicate only with parties that are accountable, that you can punish. There are many different punishments: money fines, ostracism from some community, firing, jail, and other options. Often it is enough if you can undo an action; this is the financial system’s main tool for security.

Some punishments require identifying the responsible party in the physical world, but others do not. For example, to deter spam, reject email unless it is signed by someone you know or comes with “optional postage” in the form

the most common
user model
today is “say oK
to any question
about security.”

of a link certified by a third party you trust, such as Amazon or the U.S. Postal Service; if you click the link, the sender contributes a dollar to a charity.

The choice of safe inputs and the choice of accountable sources are both made by your system, not by any centralized authority. These choices will often depend on information from third parties about identity, reputation, and so forth, but which parties to trust is also your choice. All trust is local.

To be practical, accountability needs an ecosystem that makes it easy for senders to become accountable and for receivers to demand it. If there are just two parties they can get to know each other in person and exchange signing keys. Because this doesn’t scale, we also need third parties that can certify identities or attributes, as they do today for cryptographic keys. This need not hurt anonymity unduly, since the third parties can preserve it except when there is trouble, or accept bonds posted in anonymous cash.

This scheme is a form of access control: you accept input from me only if I am accountable. There is a big practical difference, though, because accountability is for punishment or undo. Auditing is crucial, to establish a chain of evidence, but very permissive access control is OK because you can deal with misbehavior after the fact rather than preventing it up front.

freedom

The obvious problem with accountability is that you often want to communicate with parties you don’t know much about, such as unknown vendors or gambling sites. To reconcile accountability with the freedom to go anywhere on the Internet, you need two (or more) separate machines: a

green machine that demands accountability, and a red one that does not.

On the green machine you keep important things, such as personal, family and work data, backup files, and so forth. It needs automated management to handle the details of accountability for software and Web sites, but you choose the manager and decide how high to set the bar: like your house, or like a bank vault. Of course the green machine is not perfectly secure—no practical machine can be—but it is far more secure than what you have today.

On the red machine you live wild and free. You don’t put anything there that you really care about keeping secret or really don’t want to lose. If anything goes wrong, you reset the red machine to some known state.

This scheme has significant unsolved problems. Virtual machines can keep green isolated from red, though there are details to work out. However, we don’t know how to give the user some control over the flow of information between green and red without losing too much security.

conclusion

Things are so bad for usable security that we need to give up on perfection and focus on essentials. The root cause of the problem is economics: we don’t know the costs either of getting security or of not having it, so users quite rationally don’t care much about it. Therefore, vendors have no incentive to make security usable.

To fix this we need to measure the cost of security, and especially the time users spend on it. We need simple models of security that users can understand. To make systems trustworthy we need accountability, and to preserve freedom we need separate green and red machines that protect things you really care about from the wild Internet.

 

References

1. adams, a. and Sasse, a. Users are not the enemy.

Commun. ACM 42, 12 (Dec. 1999), 41–46.

2. anderson, r. economics and Security resource Page; http://www.cl.cam.ac.uk/~rja14/ econsec.html

3. Lampson, b. Practical principles for computer security. in Software System Reliability and Security, broy et al., eds., ioS Press, 2007, 151–195.

 

Butler Lampson ( [e-mail]) is a technical fellow at Microsoft research and is an aCM fellow.