Passwords And Security: Upgrading Password Encryption Algorithms

Imagine you are happily programming away one day, when your boss comes over and says “Hey, remember when we sent the results of our security audit to that big potential partner, XYZ Corp?” “Of course” you say, “If we partnered with them it would be huge. Whatever happened with that?” Well, he says, “They came back to us with some security upgrades they’d like us to implement before they can integrate with us. Apparently they don’t like something called M D 5 … do you know what that is?” You knowingly nod, “Yeah, it’s the encryption algorithm we use to store passwords in our database. It was popular in 1995 when the first code for our legacy product (the fart app) was written, but it’s now known to be pretty weak.” He responds, “Well we need to upgrade that to something more secure. Can you make that happen?” “Sure”, you say, “There are some algorithms that are stronger, like SHA-512 and BCrypt. I’ll give some thought to the actual migration strategy before choosing the new algorithm.”

The Issue

You don’t store passwords in your database in plain text so you don’t actually have access to the original password. How can you re-encrypt your passwords if you don’t have the original to re-encrypt?

Some Possible Solutions

Re-Register Everybody

You could clear all the passwords and require users to re-register.
PRO: this is easiest for the developer/code/database, and you only ever has to ever worry about a single encryption algorithm at a time
CON: this is very invasive and will probably piss off a lot of people.

Re-Encrypt All Passwords At Once

You could add more protection immediately by encrypting the already-weakly-encrypted passwords using the new algorithm, so they are doubly-encrypted.
PRO: immediately have more protection for all passwords, transparent to users
CON: every login requires more hash calculation, this strategy complicates the code, the conversion cost for 100K users with BCrypt at 500ms each is 13 hours (single threaded) which may exceed your maintenance window

Re-Encrypt Passwords Incrementally

When the user logs in, you do briefly have access to the plain text password. You could test incoming passwords with multiple algorithms one at a time, once you find a match you can overwrite old encryptions with new as necessary as users log in.
PRO: transparent to users
CON: potentially need to calculate the hash multiple times, need to support multiple algorithms in perpetuity

Store Extra Encryption Information

Finally, you could store information about the encryption algorithm on a per-password basis in its own column, and use the appropriate algorithm for login. You can migrate passwords incrementally (as described above) as users log in.
PRO: shows conversion rate, can later reset just select users, only single hash is ever required
CON: more complexity in code, requires database schema update


The solution you use might depend on your specific scenario and requirements. But that said, which approach would you use? Is there another approach not listed here that you would use?


Filed under Software Engineering

2 responses to “Passwords And Security: Upgrading Password Encryption Algorithms

  1. Incrementally… for sure.. its the only option I see that doesn’t do change that basically would break everybody at the same time if something went wrong. The advantage to the incremental fix is that you can always “switch” the conversion off if something bad starts happening. I tend to implement major changes so it can have a phased approach.. we got 100k users, and its nice to be able to monitor conversion over the process of months and watch for trouble. i would have a web.config switch to control backwards compatibility if required.

    • Jason Young

      I like the incremental approach too. The last two are incremental… What do you think about maintaining the encryption type as a column vs encrypting on login until you get a match? I see it as a tradeoff of code complexity vs database complexity.

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