HOWTO securely hash passwords

In the wake of a series of very high-profile password leaks, Brian Krebs talks to security researcher Thomas H. Ptacek about the best practices for securing passwords. The trick isn't to merely hash with a good salt -- you must use a slow password hash that takes a lot of work, so that making rainbow tables is impractical.

Ptacek: The difference between a cryptographic hash and a password storage hash is that a cryptographic hash is designed to be very, very fast. And it has to be because it’s designed to be used in things like IP-sec. On a packet-by-packet basis, every time a packet hits an Ethernet card, these are things that have to run fast enough to add no discernible latencies to traffic going through Internet routers and things like that. And so the core design goal for cryptographic hashes is to make them lightning fast.

Well, that’s the opposite of what you want with a password hash. You want a password hash to be very slow. The reason for that is a normal user logs in once or twice a day if that — maybe they mistype their password, and have to log in twice or whatever. But in most cases, there are very few interactions the normal user has with a web site with a password hash. Very little of the overhead in running a Web application comes from your password hashing. But if you think about what an attacker has to do, they have a file full of hashes, and they have to try zillions of password combinations against every one of those hashes. For them, if you make a password hash take longer, that’s murder on them.

So, if you use a modern password hash — even if you are hardware accelerated, even if you designed your own circuits to do password hashing, there are modern, secure password hashes that would take hundreds or thousands of years to test passwords on.

The problem is that you really need to make this design decision from the start -- it's hard to retrofit once you've got millions of users.

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