[thomasptacek]

Salt length and hash speed are orthogonal. Increasing the length of your nonces doesn't make your passwords less crackable. If this is news to you, don't design your own password scheme; use someone else's good one, so you aren't accidentally exposing people's bank accounts.

[aston]

Increasing the length of the nonce definitely makes the password less crackable. Since brute force methods search the entire space, the more space, the better. It's still possible to crack in by finding a hash collision, but the longer your non-password trash, the harder it is to recover the password via brute force.

[thomasptacek]

All due respect, but you shouldn't be designing password schemes. Modern password schemes are cracked using incremental crackers. This "rainbow table" stuff has totally confused the developer community. John the Ripper doesn't make a time/space tradeoff; it uses the (public) salt and the hash and iteratively recomputes hashes, and it is terrifyingly fast even on good password schemes.

[aston]

That's one attack vector. Assuming someone does attempt to use a brute force method, longer salts do matter. I think that qualifies as non-orthogonal to "crackability," even if orthogonal in some cases.

Truthfully, this isn't even worth talking about. If your user's passwords are compromised, you've already lost the security battle. Hopefully you weren't actually storing something important.

[thomasptacek]

That's horribly irresponsible. Using your logic, you might as well just store the passwords in plaintext. Most people don't use different passwords for different applications, and your web application is inevitably going to expose all your users passwords, like every other web app that has been SQL-injectable (ie, almost all of them).

[aston]

As a person interested in security, you're well-familiar with the fact that nothing is 100% secure. Choosing a less secure option over a more secure one is not tantamount to choosing no security at all.

That said, I personally don't mind plaintext passwords if there's a good usability story that goes along with it and if the security tradeoff is negligible. I put the odds of my user database being exposed at approximately zero, so generally it's a fine design decision. When was the last time you heard of passwords being stolen en masse from a major site that didn't also include a hard drive being stolen?

[thomasptacek]

As a person who spends 3-4 days a week assessing other people's web apps, I'm well-familiar with the fact that it's a very good bet that everyone's web app is SQL-injectable, whether they've tried to stop that attack or not. For example, do you know how internationalization works?

I have no idea why you think the public is told every time a a password table is dumped. In fact, change that "every" to "any".

Here, let me make this easier for you: if you ever plan to monetize your application, you will fail PCI audits for doing a crappy job with password storage. But I'll do you one better and give you a tip from the trenches: if some lame PCI auditor sees that you don't know what you are doing, his company is going to roll you for 8 billable weeks, laughing at you the whole time, before they give you the meaningless stamp of approval that lets your process credit cards.

[michaelneale]

reddit was - although it was a hard drive stolen.

[jsjenkins168]

Whether the attack is brute force or a time/space trade-off, longer salts increase addressable space which increases time or space required to achieve a collision. It doesn't matter how you're doing it, this is a simple fact.

A larger key is always more secure.

[thomasptacek]

The salt is public. If you have the password table, you have the salt. The attacker doesn't have to guess the salt with an incremental password cracker (read: almost any password cracker).

The mistake you're making is your misuse of the word "key". Larger keys are more secure. A salt is a nonce, not a key.

Can I say again that people shouldn't be rolling their own password scheme? This is a problem that has been well-addressed for decades, but the majority of new applications still ship with code that is inferior to public domain code from 1976.

[brlewis]

Are you saying that nobody uses hash dictionaries anymore and one need only worry about brute-forcing of individual passwords?

I'm not ready to stop thinking about hash dictionaries, and the 4-character salt I prepend to passwords before hashing is too short. A 4-character salt plus a 7-character password is a value that could be vulnerable to dictionaries roughly the size of the Rainbow tables.

But if you can cite a paper demonstrating that brute-force cracks of individual passwords are the only thing one need be concerned about, I'll look at it and see if it changes my mind.

[thomasptacek]

Nobody is saying you don't need to use salted passwords. What we're saying is, if your password scheme is (user, nonce, SHA1(nonce, password)), don't bother; just store your passwords in plaintext. Your users passwords are so weak (dictionary word + number) that a 200 line incremental cracker is going to blaze through it in hours.

The answer to this is to use someone else's (good) password scheme. If you're shipping on Unix, your system undoubtedly comes with one. Use it.

[brlewis]

If you mean to say that users should be forced to choose strong passwords, that's a different thing from saying longer salts don't make a set of passwords less crackable.

[thomasptacek]

I don't understand why people are hyper-focusing on this one attack. Attackers have been cracking passwords, quite effectively, long before tools like Ophcrack were available. Any decent salt scheme beats Ophcrack; it doesn't take much to make hash chain storage infeasible. But you can make your salts longer than JSESSIONID's and they still won't be a speed bump to a "conventional" password cracker.

I'm not arguing that users should be forced to choose secure passwords. They should, of course, but we're talking about how you store them. If you store them using a single-iteration SHA1 hash, then no matter how you structure your nonces, your scheme is insecure and you should be embarassed.

[brlewis]

When you say, "Increasing the length of your nonces doesn't make your passwords less crackable," you can expect that people will talk about the type of attack that a longer nonce helps deter. That isn't hyper-focusing; it's polite conversation. The impolite alternative would be to say you should be embarrassed for making a statement like that.

[thomasptacek]

I'm not saying you should be embarassed for conversing about this subject. I'm saying that if you store real people's passwords in a publically accessible web application using a single-iteration SHA1 hash scheme, you're insecure, and should be embarassed that you were outdone by password schemes that came with your operating system and were largely refined during the 1970s.

I ask, what type of attack do you think lengthening the nonce deters? The attacker who is going to construct 4 billion rainbow tables?