TheJoyOfHack

For people who like to make things

After my recent Blitz Talk at SecondConf I had some great conversations with fellow attendees in the meal lines and around the beverage table. I’m presenting them here for further discussion.


You say to hash the email. How would you send out emails, then?

If you need to send out emails when the user is logged in, and if the user’s email address is the login ‘id’, then you can save the cleartext email address in the session when the user logs in successfully. Then, when the session expires, you delete the email from the session data structure or table.

If, on the other hand, you need send out emails when the user isn’t logged in, the way sites like Amazon and eBay do, then you’re gonna have to store the email in a way in which you can read it later. You don’t have to store it cleartext; you could encrypt it and store the encrypted value. Encryption is different from hashing because with encryption you can get back to the original text (the email address). By definition, you can’t do that with a hash.

What you shouldn’t ever do is use your hash salt as an encryption key. The reason is that hash salts are well known and should not be hidden. In contrast, encyption keys are supposed to be secret. If you use a hash salt as an encryption key, you’re effectively storing your data in cleartext.

At some point you have to store some valuable information in your database - your system will always do more than provide a secure login and password. Hashes will not absolve you of the need to engage in secure data storage practices.


As a user is there anything you can do to tell if a site is secure?

Unfortunately, no. The best you can do is ask the site’s owners. You may or may not get a satisfactory answer; the people best qualified to answer that question may not be management or customer support, or even technical support. The people who can answer best may be the developers.

I do not recommend trying to determine this on your own by hunting for weaknesses in websites — that might be punishable by law.


That rehashing stuff is cool! You can just automatically increase the strength of your users’ hashes every year!

I know, right? But hold on: you can’t have your system automatically increase the strength of everyone’s password as a batch job. You can only do it when the user logs in successfully. That’s the only time you as a system know (and ought to know) the user’s password. The only opportunity you have to do the rehashing is during the login process before sending the user a successful HTTP response.

Do NOT try to be efficient here by saving the cleartext password in an on-file queue for processing so that you can return back to user as soon as possible. Saving the cleartext password on file should be forbidden for any reason.


When you said use a salt, and make sure it’s random, is that now a two step process? Generate a random system salt, and use that for the password hashes?

Oh, no no no. You use a different random salt for every hash. You really don’t need to worry about it too much - most good hash generating functions will generate a random salt for you. In fact, in most cases you have to go out of your way to use the same salt for every hash.

For example, here’s how you hash a password using PostgreSQL:

myDb=> SELECT crypt('mySuperPassword', gen_salt('bf', 10));
                            crypt                             
--------------------------------------------------------------
 $2a$10$G3dJM9XGFHuUTDOKu8bk.eCB1fl0cWiLOKkRBFQOv6tAtxxNtAjrW
(1 row)

myDb=>

In the example above, the password is mySuperPassword, the hash is bcrypt (as specified by bf and the hash strength is 10. Since I use PostgreSQL as a backend database, its native support for bcrypt makes hash generation and later strengthening very easy.

Thanks

I would like to thank SecondConf for giving me the opportunity to speak. I would also like to thank the volunteers who helped make this event such a resounding success. Finally, I would like to thank all the people who came over to me afterwards for their kind words and engaging conversation. I’m very happy to be part of this community.