For several years now, it’s been standard practice among web developers who know what they’re doing to store passwords as a one-way salted SHA-1 hash. Using a salt means that they aren’t vulnerable to rainbow table attacks, for instance, so the only realistic option open to hackers is a dictionary attack, which is slower. Or so the thinking goes at any rate.
There’s just one problem. Dictionary attacks are blazingly fast these days, thanks to the massive parallelism that you can get from the GPU in your graphics card. Just how fast? Coda Hale explains:
Rainbow tables, despite their recent popularity as a subject of blog posts, have not aged gracefully. CUDA/OpenCL implementations of password crackers can leverage the massive amount of parallelism available in GPUs, peaking at billions of candidate passwords a second. You can literally test all lowercase, alphabetic passwords which are ≤7 characters in less than 2 seconds. And you can now rent the hardware which makes this possible to the tune of less than $3/hour. For about $300/hour, you could crack around 500,000,000,000 candidate passwords a second.
How fast is that? Let’s put it this way: for all but your most security-conscious users, a salted SHA-1 now offers no more protection than clear text. Commodity hardware (an off-the-shelf laptop) can test candidate passwords at a rate of up to five billion per second. Given how bad people are at choosing secure passwords, huge swathes of your user base could easily have their credentials cracked at a rate of several thousand every second.
If you are serious about protecting your users’ passwords (and you should be, because if you’re not, you’re legally on thin ice), you need to use a salted hash algorithm that is designed specifically with passwords in mind. The one that seems to be getting most mindshare at the moment is bcrypt. Its killer features are that it is (a) slow, and (b) adaptive. By tweaking its “work factor,” you can decide whether it takes milliseconds or hours, as well as making it slower as hardware gets faster.
Just how slow should you make it? As slow as you can get away with. From your point of view, your login screen will be one of the less visited pages on your website. People will only enter their password once a week or so, and when they do, they will be pretty serious about using your site and willing to put up with a short delay before they are authenticated. A delay of about 100-500 milliseconds won’t faze them. On the other hand, an attacker cares massively how fast your hash algorithm is. If he can only test a couple of candidate passwords a second, a dictionary attack will be totally impractical for all but your most idiotic users — the kind who choose “password” or “123456” as their passwords.
There are a couple of other secure password hashing algorithms available, namely PBKDF2 and scrypt. They each have their advantages and disadvantages, and as with all things programmer, they are the subject of a bit of a Religious War as to which one you should use. If you’re interested in the ins and outs of the debate, this Hacker News thread and this question on security.stackexchange.com provide some good discussion on the pros and cons of each. But whatever you do, don’t you dare use a naive MD5, or SHA-1, or SHA-256, or SHA-512 for your passwords. When it comes to hashing passwords, these general-purpose algorithms are simply not fit for purpose.