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Comment by aw3c2 | Hacker News Reader

A basic salt could be just a fixed string being used. That would not prevent this. The salt would need to vary per hash.

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This is a common misconception, but a "global salt" isn't actually a salt. Not sure what a "basic salt" is though.

> A new salt is randomly generated for each password. In a typical setting, the salt and the password are concatenated and processed with a cryptographic hash function, and the resulting output (but not the original password) is stored with the salt in a database.

http://en.wikipedia.org/wiki/Salt_%28cryptography%29

Ah, I did not know that. And I wrote some password hashing with a global salt myself. So I stand by my comment with "one laughing and one crying eye". I thought salts were mostly against precomputed rainbow tables for specific hash functions, but of course if my hashes are valuable enough, a global salt makes cracking them easier than if I used a random/unique one per hash.

This is why programmers should stop trying to write their own password systems.

To be exact, programmers should stop using their own password systems (unless - well, let's not go there :).

Writing them is actually an excellent exercise, and humbling one too. Especially if you show your code to some skilled hacker / security expert.

From what I understand rainbow tables are not really used anymore -- they were useful when it took longer to calculate hashes, but nowadays you just need a decent graphics card to calculate a few billion hashes (MD5 or SHA1) PER SECOND.

Per-hash salts don't really slow that down at all, unfortunately.

Short-version: don't use the hash algorithms that are designed to be fast for protecting sensitive data; use something solid with a work factor instead.

Pepper is the more common term I've seen for a global salt - although I've never seen it implemented independently of a salt.

Isn't there value in having a long global salt to make bruteforcing harder in case the database but not the server code is compromised, though?

Think about it like this: Using a SHA256 + global salt is nothing different from using SHA256 with a different set of constants/initial values.

True, but if the global salt is secret, it'll at least make it slightly harder for the attacker to figure out the password.

For example, if the password is "a" and an unique salt is available in the database, it'll be trivial to brute force. If the password function is something like this, however: sha256("longsalt" + password + salt) the attacker will first have to figure out longsalt which can be slow if it's long.

Or am I completely mistaken?

But if hackers can break in and steal your database, they'll probably also be able to steal the global salt.

The recent MongoHQ breach is a counter example.

If you can steal the password hash database, stealing the shared "salt" is usually going to be trivial. Second, lots of hash functions are vulnerable to length extension attacks, so even if extracting the shared secret directly is hard, you can probably exploit the system to see what an empty password hashes to and then use that as a basis for brute-forcing the rest.

In other words: there is a reason why security experts and cryptographers recommend the password storage mechanisms they do.

I see, that makes sense. Thanks for explaining!

The obvious solution is to salt it with something that is unique to that account: The internal account id, a string randomized at account creation or something like that.

An identical password for two different accounts should never encrypt to the same string.

The salt should always be random. If it's based on some account information it's possible that the salt can be regenerated if that account information would become available, in a DB dump like this for example.

If they had used different salts for every account, and that salt was published in this dump it would have been much more difficult to find identical passwords compared to the current situation.

I'm not sure how you can keep the salt random and secret, and still use it when users log in?

Salts aren't secret. The point of salting a password is to prevent the password (like "alskdjfalsdf") from always hashing to the same result (like "j9p+HwtWWT86aMjgZFLzYg=="). That prevents rainbow table attacks (among others) and it doesn't require the salt be secret.

Right. But the parent comment was replying to a comment suggesting that a user id (or other account information) shouldn't be used as it's not random.

I can't see why a user id could not be used as a salt though.

"I can't see why a user id could not be used as a salt though" thats exactly how people break crypto implementations, it seems ok but you dont really know. Don't just invent your own way, use the standard one.

If lots of sites all use user ids as a salt and all count up from 1 then a much weaker version of this attack applies: you can identify all the people who have both the same password and the same user id.

If this were common, someone constructing a lookup table for password hashes could include ones salted with the first N user ids. A N-times bigger table isn't something you'd be excited about as a cracker, but might still be practical.

If your user ids are randomly assigned this should be pretty much the same as generating a random salt. Still, the extra storage of a salt is going to be small compared to whatever else you're storing per-user, so better to do the safe thing and go with what's standard.

The salt is typically stored in the password db, alongside the salted hash.

If Adobe has taken that approach, the intruders would have all the salts, but they would not be able to do this attack of seeing how many people use the same password.

However, Snowden's silly password hint would still have given the game away. If that is in fact him.

Yes, it should be random but the salt is public information.

Exactly because you need it to calculate the hash and compare it.

If it's random, you need to store it, and it becomes just as available on a leak. The salt still works though, because it makes all hashes different. You need to attack each password individually rather than tackling them all with a rainbow table or something. The approach from this article also stops working. What you use as a salt is fairly irrelevant as far as I can tell.

If you want to get more security on top of that out of your salt, the only thing I can think is to add some security by obscurity on top: If it doesn't matter which field in the DB you use, just pick an unlikely one and keep secret which one it is. Might be id, email, favourite pet, or something else. Just don't forget to update the hash when the user changes that field.

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