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May 30, 2019

Black and white

Quick 'philosophy of programming' entry this time. General solutions to common programming probelms are often called 'design patterns' (e.g. the wiki articleor the book which started the name). The idea of these is to have language independent (as far as possible) 'patterns', like clothing patterns, which can be tweaked to fit a specific situation. A lot of these patterns seem obvious, which is good, and since they're developed and tested by many people they can be very valuable in showing you questions you hadn't even thought of.

This weeks topic is perhaps too simple to really call a pattern, but it is a very useful thing to keep in mind when doing anything that deals with restricting function which exists, but should not be allowed. For example, forms which take user information often disallow anything except numbers in a 'telephone' field. A code I work on has a lot of user-specifiable options, but as the programmer I know that some are incompatible where it might not be obvious to the user - and I want to either warn or abort if these are used together.

There are two general approaches to things like this, and which you choose depends on many things. You have to maintain some kind of list to check against, but you can choose to use either the "blacklist" or the "whitelist". The former, the "blacklist" is a list of the things which aren't allowed, and anything not in the list is OK. The "whitelist" approach means keeping a list of the things which are allowed, and anything not in the list is excluded.

Sometimes the choice is fairly easy, because one method is a much, much simpler list. For instance, in the phone number example, it is fair easier to use the whitelist, allowing only '1234567890', but nothing else. If you try the other way, you might think to exclude letters, but what about Greek or Cyrillic characters? On the other hand, this is a source of deep annoyance if you forget any needed character - in the example I just gave, one could not put any spaces in, which is annoying, nor brackets or the '+' symbol.

A classic example of the poorly-thought out whitelist is in name fields which often exclude characters like the apostrophe, annoying the Scots and the Dutch for eternity. And what about accented letters, or the German ess-tsett. With a whitelist, you need to be sure you've caught everything, or people will be, rightly, upset. For a user-name on a website, and for a password, it is probably fine to allow any ASCII or Unicode character and set up your systems to handle them, leaving far less upset without any real cost to you.

On the other hand, with a blacklist, anything not forbidden is permitted. These are generally used in cases where certain characters have a function and so must be excluded, even if this annoys. So, for instance, in most programming languages variable names may not start with a number, nor contain a comment character.

Apart from the length of the lists, the two methods trade off this annoyance to your user, who must wait until you fix the omission (with a whitelist) against potential unknown failures and security risk (with a blacklist). Imagine the 'incompatible features' problem with both methods. If I use a whitelist, and forget to allow some pairing, my worst case is that I will likely be asked (somewhat irately) why X and Y can't be used together. I realise they can be, I update the code and I make a new release version to fix the omission and everybody is happy. If I use a blacklist and forget that some X and Y don't work together, my worst case is that one day I have to tell somebody that their last n years of research is all invalid, because the simulation they ran didn't work as expected, and since nothing actually went wrong they didn't know. Worse still, would be having to tell them that their fascinating effect is just a code error, and it's my fault.

In some cases, only one or other list type is really viable. For instance, virus scanners keep a list of 'tells' for malicious code, because even though they let things slip through until their lists update, they could never describe all of the 'allowed' code. App permissions (on better, more granular systems) are a whitelist - you give an app the permissions you choose, and only those.

So as a general rules of thumb:

  • If only one method is viable, obviously use that
  • If one or other list is going to be much much shorter, you have better chances of getting it right, so use that method
  • If it is really important not to let things slip through, use a carefully managed, kept up to date, whitelist. If possible, put it into a file or something, so that updates just require sending out new definitions, not modifying the entire code
  • If it's really important not to get accidental exclusions (false positives) use a, similarly carefully managed, kept up to date etc, blacklist
  • In some cases, combine the two. Programming languages generally have a set of allowed characters (a whitelist) and small blacklists for specific contexts such as the first character of a name.

As well as the literal 'blacklist' and 'whitelist' there is a more general principle here - do I selectively forbid, or selectively allow? Do I stop somebody doing this thing here, here and perhaps here, or do I only permit them to do it there and there. If you find the 'here's' or 'there's' proliferating, re-examine whether you're doing it the right way around. In safety or security critical situations, you almost always must allow only what is permitted. If you find yourself trying to plug up security holes with ever growing blacklists, you should probably change tack and think about what should be allowed instead.


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