What's the purpose of $LoginKey from the Ashley Madison leak?

Question

According to this site Ashley Madison was keeping an insecure hash stored in their database using MD5, in addition to the far more secure bcrypt hash. This group used the MD5 hashes to quickly crack the password database rather than use the far more difficult to crack bcrypt hashes.

The article only gives this brief clue:

The $loginkey variable seemed to be used for automatic login, but we didn’t spend much time investigating further. It was generated upon user account creation and was re-generated when the user modified their account details including username, password and email address.

The question is, what's the purpose of this $loginkey variable? It's obviously pointless to use bcrypt which is purposefully slow when you're using a far, far faster hash like md5 elsewhere that's an easier target. Obviously this is a large blunder on the part of the Ashley Madison developers, but I'm curious as to why they would have put this in in the first place, and why it was never fixed since (someone) knew to use bcrypt for the main password hash.

Answer 1

Probably as a token for users to login (i.e. "Remember my login").

If you want users to be able to use remembered login functionality, then you need to use a crytographically secure pseudo random number generator (CSPRNG) to generate the token. Then calculate an expiration date for that token to be used, and store both the token and the expiration date in your database. Once the user logs in (and indicated at login that they wanted to remember their login) you would store the plain text of the token on their machine in the form of a cookie or local storage. Then next time they tried to access a page which required a login, you would search for that token in their browser's cookie or local storage. If the token matches with a token that you stored in your database, and if the expiration date for that token is still in the future, then consider the user to be logged in; delete the token from their browser as well as the database, and repeat the process of generating and storing a new token all over again.

For added security, in your database you probably want to store the hash of the token rather than the token itself. As long as you used a CSPRNG to generate the token, you should be able to use even a fast hashing algorithm to hash the token.

I think the Ashley Madison devs simply didn't use a CSPRNG to create the $loginkey and instead used a non-random sequence of characters (username plus the MD5 hash of the password), which currently is what made it so easy for crackers to determine the unhashed portion of the $loginkey, which in this case was the password.

Answer 2

The coverage at Ars Technica has a good discussion in the comments. http://arstechnica.com/security/2015/09/once-seen-as-bulletproof-11-million-ashley-madison-passwords-already-cracked/

Some speculation:

• A legacy hash. At some point the devs switched to bcrypt but kept the old hash around "just in case".
• $loginkey was used to login faster (bcrypt is slow by design) or keep a user logged in.
• Used to keep a user logged in across other applications. Like on a forum platform that couldn't do bcrypt.