[sgarman]

Most of the attacks described in this article are not solved by any of those though right? They protect against hacking one person but if you just do these advanced dictionary attacks you can still crack people with weak passwords.

Maybe I'm missing something?

[afreak]

No. You cannot effectively use the techniques you can use against SHA/MD5 to attack the three I mentioned.

SHA and MD5 can be calculated entirely in a CPU's registers without having to rely on RAM. Bcrypt for example requires the use of a matrix as part of its calculation, slowing down the process. A GPU has so few channels from the processor to memory that it cannot be effectively done in parallel.

All one has to do with Bcrypt is just adjust the difficulty and any advances in GPU technology or whatever can be nullified.

[sgarman]

Thanks. I thought I saw an article about a recent leak with salted bcrypted passwords where they cracked weak passwords. The conclusion was there is no way to prevent a weak password from being compromised so to prevent it require longer more complicated passwords.

[afreak]

A weak password is a weak password no matter how good the hashing is. I think that you're referring to this bit from last year:

http://www.pxdojo.net/2015/08/what-i-learned-from-cracking-4...

The author of this piece was doing about 156 hashes per second and after just over five days, he had only gone through and cracked 4,000 account passwords--we're talking 0.0001% of Ashley Madison's supposed userbase here. To run just over the 14,000,000 passwords from RockYou.txt on every single user account from AM, it would take up to at least two billion years.

[xyience]

Weak passwords are still weak, but things like bcrypt buy you time. A bcrypt configuration might have hashing take 250ms on a server's Xeon CPU (as opposed to nanoseconds with a sha-based hash). You can try the password 'password' on say 50k hashes that were dumped, but it will take about 3.5 hours. You can get this time down by parallelizing or with better hardware like GPUs / FPGAs / ASICs, but you're still looking at a base time of 3.5 hours per attempt to go through each account. Or if you're only targeting one account, 3.5 hours to try 50k words.

If the server had bcrypt configured to take a second per password, multiple everything by 4. And so on. What I think is needed as a supplement to "use bcrypt / scrypt" is a "and use it this way so you don't accidentally open your server to DoSing or give a poor experience", because a 10 seconds-to-compute-on-a-Xeon hash is great from a security standpoint if your hashes get leaked, it sucks from a user experience to have to wait at least 10 seconds to login, and if you have to service multiple logins at once your server's not going to be able to do anything else if you just use bcrypt/scrypt synchronously.

[afreak]

If the server had bcrypt configured to take a second per password, multiple everything by 4. And so on. What I think is needed as a supplement to "use bcrypt / scrypt" is a "and use it this way so you don't accidentally open your server to DoSing or give a poor experience", because a 10 seconds-to-compute-on-a-Xeon hash is great from a security standpoint if your hashes get leaked, it sucks from a user experience to have to wait at least 10 seconds to login, and if you have to service multiple logins at once your server's not going to be able to do anything else if you just use bcrypt/scrypt synchronously.

If you're in a situation where you're needing to rely on hashing a user's password for every action, your application has far worse problems than what password storage method is in play. Moving from your current password storage method that is inadequate to one that would be better also takes into account that you haven't done something completely wrong with session states.

[edit]

I misread the poster I was responding to assuming that they meant that the user was re-authenticating on each request. My thought process was that if they're storing credentials in a cookie in lieu of a session ID then that needed to be addressed first before even going down the avenue of correcting password storage.

[xyience]

I wasn't talking about verifying a password for every action, but simply having more than one user of your service at once.

[bigiain]

They're not "solved", but they're made 3 to 6 orders of magnitude more effort:

See https://gist.github.com/epixoip/a83d38f412b4737e99bbef804a27...

"8x Nvidia GTX 1080 Hashcat Benchmarks"

TL;DR:

Hashtype: MD5 Speed.Dev.#.: 200.3 GH/s

Hashtype: SHA1 Speed.Dev.#.: 68771.0 MH/s

Hashtype: bcrypt, Blowfish(OpenBSD) Speed.Dev.#.: 105.7 kH/s

Hashtype: scrypt Speed.Dev.#.: 3493.6 kH/s

Hashtype: PBKDF2-HMAC-SHA512 Speed.Dev.#.: 3450.1 kH/s

Hashtype: PBKDF2-HMAC-MD5 Speed.Dev.#.: 59296.5 kH/s

You can't protect against people using "password123" or "dadada" as their passwords, but for those of us using long randomly generated passwords bcrypt makes cracking it well outside the realms of possibility for anyone short of nation-state attackers. (I bet the NSA can get quite a lot more than 100kH/s for bcrypt if they're determined enough, but I wonder if even _they_ can throw 6 orders of magnitude more compute at the task?)

[OrpheanBeholder]

Some of these don't make sense. The point of bcrypt, scrypt, and pbkdf2 is the difficulty of them is configurable.

Digging into the comments on that gist, it says that the bcrypt benchmark used a workfactor of 5 (= 32 rounds). The lowest possible bcrypt workfactor is 4 (= 16 rounds).

For comparison, the default for the login hashes on OpenBSD is 8 (= 256 rounds) for standard accounts and 9 (= 512 rounds) for the root account, and OpenBSD's bcrypt_pbkdf(3) function uses a workfactor of 6 (= 64 rounds) and that's intended to be called with multiple rounds itself eg. in signify(1) it uses 42 rounds (a bit over the equivalent of bcrypt with a workfactor of 11) and ssh-keygen(1) defaults to 16 (roughly equivalent to bcrypt with a workfactor of 10 (= 1024 rounds).

The point I'm trying to make here is that bcrypt benchmark uses a ridiculously low workfactor, and it looks like the scrypt and pbkdf2 ones did to.

[bigiain]

Yep - and even in it's "ridiculously low work factor" configuration, it's around 6 orders of magnitude slower than SHA. In context, the author of that piece was trying to show off the hashrate, so would be expected to err on the side of making his monster 8 gpu rig look better rather than worse.

Any of bcrypt, scrypt, or PBKDF2 can easily (as in, by design in the hash function parameters) be made however much slower is needed for you (so long as your normal login process is then still "fast enough", I had a WordPress site a while back where I couldn't wind the bcrypt plugin up much past 11 before the inexpensive webhosting would time out before login succeeded... (And yeah, feel free to mock me for "securing" WP with bcrypt - it was mostly because I wanted to be confident if/when the site got exploited, the hashes in the DB weren't going to be too easily attackable for anyone who'd used a decent password))

[mikeokner]

bcrypt uses unique salts per-password. The hash outputted by the bcrypt function is actually several delimited fields that have been joined into a single string.

This means that if multiple users use a common password like 'pass123', the hashes stored in the database will still each be unique. Any attacker trying to reverse all of the password hashes will have to reverse every single one individually in a targeted manner rather than using pre-computed rainbow tables or generating hashes from a wordlist and scanning the database for matching hashes.

[ralala]

What are the advantages of bcrypt compared to SHA based hashes with unique salts?

[niftich]

General-purpose cryptographic hash functions like the (now-broken) MD5, SHA1, SHA256, etc. are designed to be computationally easy, ie. fast.

Salting protects against rainbow tables [1], but it doesn't change the fact that computing a SHA256 hash is fast.

Password hash functions like PBKDF2, bcrypt, scrypt, Argon2 are designed to be computationally expensive, to make a password-cracking endeavor take even longer.

Argon2, the winner of the Password Hashing Competition and the current state-of-the-art, for example, has two ready-made variants: Argon2d is more resistant to GPU cracking, while Argon2i is more resistant to time-memory tradeoff attacks [2].

[1] https://en.wikipedia.org/wiki/Rainbow_table

[2] https://github.com/p-h-c/phc-winner-argon2

[bigiain]

bcrypt is significantly slower to compute. Something like 5 or 6 orders of magnitude slower. (se my other comment in here with numbers for cracking various hash types on an 8gpu rig...)

[ralala]

Can I achieve the same by applying SHA x times?

[Ambroos]

If X is millions (or even billions), maybe, but you shouldn't. Just use one of the real password algorithms. Never ever roll your own hashing system assuming it's secure enough. It won't be.

[ralala]

Of course. I just wanted to get an idea of the reasons without going too much into mathematical details. For projects I would just use argon 2 or bcrypt.