Schneier-Ranum Face-Off on Whitelisting and Blacklisting
This essay appeared as the second half of a point/counterpoint with Marcus Ranum.
The whitelist/blacklist debate is far older than computers, and it's instructive to recall what works where. Physical security works generally on a whitelist model: if you have a key, you can open the door; if you know the combination, you can open the lock. We do it this way not because it's easier -- although it is generally much easier to make a list of people who should be allowed through your office door than a list of people who shouldn't--but because it's a security system that can be implemented automatically, without people.
To find blacklists in the real world, you have to start looking at environments where almost everyone is allowed. Casinos are a good example: everyone can come in and gamble except those few specifically listed in the casino's black book or the more general Griffin book. Some retail stores have the same model -- a Google search on "banned from Wal-Mart" results in 1.5 million hits, including Megan Fox -- although you have to wonder about enforcement. Does Wal-Mart have the same sort of security manpower as casinos?
National borders certainly have that kind of manpower, and Marcus is correct to point to passport control as a system with both a whitelist and a blacklist. There are people who are allowed in with minimal fuss, people who are summarily arrested with as minimal a fuss as possible, and people in the middle who receive some amount of fussing. Airport security works the same way: the no-fly list is a blacklist, and people with redress numbers are on the whitelist.
Computer networks share characteristics with your office and Wal-Mart: sometimes you only want a few people to have access, and sometimes you want almost everybody to have access. And you see whitelists and blacklists at work in computer networks. Access control is whitelisting: if you know the password, or have the token or biometric, you get access. Antivirus is blacklisting: everything coming into your computer from the Internet is assumed to be safe unless it appears on a list of bad stuff. On computers, unlike the real world, it takes no extra manpower to implement a blacklist -- the software can do it largely for free.
Traditionally, execution control has been based on a blacklist. Computers are so complicated and applications so varied that it just doesn't make sense to limit users to a specific set of applications. The exception is constrained environments, such as computers in hotel lobbies and airline club lounges. On those, you're often limited to an Internet browser and a few common business applications.
Lately, we're seeing more whitelisting on closed computing platforms. The iPhone works on a whitelist: if you want a program to run on the phone, you need to get it approved by Apple and put in the iPhone store. Your Wii game machine works the same way. This is done primary because the manufacturers want to control the economic environment, but it's being sold partly as a security measure. But in this case, more security equals less liberty; do you really want your computing options limited by Apple, Microsoft, Google, Facebook, or whoever controls the particular system you're using?
Turns out that many people do. Apple's control over its apps hasn't seemed to hurt iPhone sales, and Facebook's control over its apps hasn't seemed to affect Facebook's user numbers. And honestly, quite a few of us would have had an easier time over the Christmas holidays if we could have implemented a whitelist on the computers of our less-technical relatives.
For these two reasons, I think the whitelist model will continue to make inroads into our general purpose computers. And those of us who want control over our own environments will fight back -- perhaps with a whitelist we maintain personally, but more probably with a blacklist.
Categories: Computer and Information Security