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Comment by gibybo | Hacker News Reader

>All major internet sites, anybody with more than 50.000 passwords, should design or configure a unique algorithm for their site

He probably means to use some combination of well known/tested algorithms rather than inventing your own crypto, but I think his wording is ambiguous enough to be dangerous. While there is some benefit to using a unique algorithm for your site, it's almost certainly more risky than using a secure algorithm (i.e. bcrypt/scrypt/etc) even if every other site was using it too.

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One, often over-looked reason, that website designers chose simple MD5 or SHA1 hashing (and why many still do) is for portability. Years ago this was more of an issue than it is today. They may wish to change web frameworks, programming languages, service providers, etc. and when doing so they want a simple, easy way to use the hashes they have.

Also, many big service providers still use MD5 or SHA1. I saw a company migrate to Google Mail from an old legacy mail system and part of that was integrating some authentication services too. This was several years ago, but at that time, Google accepted hashes in two formats only... MD5 or SHA1.

It's pretty easy to migrate users from one hashing algorithm to another when they next log in:

  if( user has an old style hash ){
    if( password verifies against old style hash ){
      add a new style hash
      delete the old style hash
      log them in
    }
  } else {
    if( password verifies against new style hash ){
       log them in
    }
  }

But if you have code to run the old style hash you don't need the new one. (Although it's useful to upgrade the work factor.)

The issue is you don't always have the ability to do the old style, that's why people prefer a hash that they can be assured will be found everywhere.

You mean you're sending unhashed passwords from the client?

Yes. That is the standard behaviour that nearly all sites use, and is perfectly fine.

It's not great, though - it means that someone who hacks your server can get users' passwords a lot easier. I read a couple of weeks ago that Blizzard's games don't send the passwords to the server, they send a hash. Things are necessarily crappier on the web, of course.

Edit: there's more detail at the link below. It looks clear that in at least some of their schemes they deliberately do not send the client password to the server, which sounds like a decent idea.

http://www.skullsecurity.org/blog/2012/battle-net-authentica...

If the server is hacked, then all bets are off. They can just modify the webpage if they want the un-hashed password.

Exactly - "Things are necessarily crappier on the web, of course."

I wonder, though. Could there be a "code has changed" warning from the client? I mean, authentication should be pretty damn stable, and maybe even universal. If someone does modify the page, it'd be nice to know if that change was reflected on other sites, and it'd be nice to know that someone I trust had signed off on it (cryptographically).

A simple alternative is to build it into browsers. A password field could generate a salt per-domain and automatically encrypt any queries to password-fields. The server doesn't even need to know about it. You'd have to be more than a little careful building it, obviously, and you'd have to find a way to deal with passwords used on more than one site, but it could work.

Sending a hash is no different than sending a plain text password. Because the attacker has complete control at their end and can just hack a client that sends that same hash even if they don't know the original password.

Would it not be an aid to users in cases where they are reusing the same password on multiple sites?

Note that getting the client to send only the hashed password is incredibly silly. If there's a leak, the hackers do not even have to crack those passwords.

How many web login forms (as an example) are doing client side hashing? The only ones I know are of the recent 'test your password in the linkedin leak' variety.

How do you degrade for NoScript users?

And where's the harm in that anyway (assuming TLS)?

Graceful degradation is pretty easy. Override the form submit handler to hash the password, set a flag that the password is hashed, empty the original password, and submit the form. Users with JS blocked will just submit the unhashed password as usual.

Of course, if you're salting the hash uniquely for each user, then this approach isn't very helpful.

Finally, if your server will accept hashed passwords, then getting the hash is as good as getting the password for access to your site. The only benefit is that the password is hidden so you may avoid compromising security on other sites.

> Of course, if you're salting the hash uniquely for each user, then this approach isn't very helpful.

I've seen this advised a lot. However, where are you going to put the per-user salt? I presume in your users table, or somewhere in your db. Does this not mean that the salts are just as likely to be hacked as the encrypted passwords?

Are you not better off with either a single salt that's stored somewhere outside your db or with some other scheme for algorithmically picking a salt based on the user id?

Salts aren't meant to be secret (or rather, no more secret than the password hashes themselves). The goal is to prevent a precomputed brute force attack.

If you store straight unsalted hashes, you can precompute the hash for every likely password, store the precomputed hash -> password mappings in a nice efficient data structure (http://en.wikipedia.org/wiki/Rainbow_table) and use the same precomputed tables to reverse every hash in the system with a very fast lookup. And every other system using the same unsalted hash scheme.

If you have a salt, this doesn't work -- you'd need a set of precomputed tables for each salt value, which rather defeats the object of precomputation.

If you take it a step further, and use a storage scheme like bcrypt or PBKDF2, not only are you protected from the precomputation attacks, but testing each password candidate takes much longer than a straight cryptographic hash -- so brute force attacks becomes much slower, too.

To put it differently, hashing without salting lets an attacker crack the entire database at once. Try a password, see if the hash is in the database, repeat.

With salts, the attacker can only attack a single user at a time. Try a password with one user's salt, see if the hash is in the database, try the next user's salt, see if that hash is in the database, repeat.

Salts don't need to be secret to work.

My point was that sending the hash the server doesn't work if the hash is salted per user since you don't know what the salt is. (I suppose you asynchronously fetch the salt value after the user puts in a username, but at the solution as a whole starts to fall apart.)

There's no real point to doing this. If you want to stop them being sniffed, send them over SSL.

What would be the point of hashing on the client? Let's say you're using md5, and you hash the password before you send it. Chances are, that un-salted hash can be easily decrypted by any number of reverse-lookup tables around online. You can't salt the hash client side, because anyone can look at your JS and find your salting tactic.

The best approach to securing information going from client to server is SSL.

It doesn't matter that they know your salting tactic, it still stops reverse-lookup tables with common passwords online.

However any salting tactic that can be pushed and used on the client side would have a tough time using a salt that is on a per-user basis. This means that if you could salt it client side, you would need to have a static salt, which is significantly less secure than a unique salt per user.

> However any salting tactic that can be pushed and used on the client side would have a tough time using a salt that is on a per-user basis.

How so? Surely the client would know their username too?

Eh, why not just bcrypt the SHA1/MD5 hashes? Your auth check will just become bcrypt(SHA1(pass)) rather than bcrypt(pass). You can convert all passwords right away and I don't see any significant downside to it.

Will there not be a time lapse whilst you run bcrypt on all the existing hashes to update your users table? Given that bcrypt is intentionally 'slow', this might be a problem for applications with large numbers of users (albeit a one-off cost).

I agree that for small sites, changing the auth check and updating all the existing rows (and deleting/overwriting the old hashes) is probably the best solution though.

> Will there not be a time lapse whilst you run bcrypt on all the existing hashes to update your users table?

No, just SHA1 the password and check, and then bcrypt(SHA1) and check, until the encryption process finishes. Or just check the length, or store the hash type along with the hash, a la Django. These problems are trivial, really.

If you assume 6 million users and one second to compute a password hash, that's a couple of months on a single CPU. Takes a while, but no big deal, and it can proceed while your site stays live.

That would be fine if you wanted to retain the legacy hashing function. I was describing a migration from one hashing algorithm to another. You're describing modifying the existing hashing algorithm, which is something different.

It depends if your goal is to have security for everyone right away or if it's to only use one hashing function. I don't see anything wrong with a round of SHA1 in your setup, especially when it allows you to secure everyone's password immediately...

I like this idea for sites that e.g. use Microsoft ASP.NET Membership where all you can do is set a machineKey attribute about what kind of password hashing to use, and don't support any "old hash vs. new hash" alternatives for changing. Anyone know of an implementation that would do this and "drop in" to an existing ASP.NET site? Or pointers to how to approach developing this?

Don't use ASP.NET membership? I can't get over how much effort they put into MVC ASP.NET and they still carry that cludgy mess around.

Sometimes you have to deal with what you are given.

That's fair, my snark doesn't help with that, as I was in that same position about a year ago and opted to just leave it and pretend it wasn't there.

I'm sure there is a way to improve it, but I don't know how, sorry!

Why not do both then?

    1. bcrypt(SHA1(pass)) right now to secure all pws
    2. check against that, then update to bcrypt(pass) on login

That's probably the correct thing to do but since you're unlikely to ever achieve 100% migration, it doesn't help a whole lot.

The problem is that you may have to keep this code for a VERY long time (people don't log into sites for 3+ years and expect their old passwords to still work). Lugging around legacy code is never a good idea.

I agree that lugging around legacy code but whats the alternative? (Note I'm not a security expert)

AFAIK its this

1. Continue using legacy, less secure hashing algorithm

2. Upgrade your password storage scheme and carry around some extra code and/or fields in your db.

3. Crack you own users passwords and convert them

Its all kind of smelly but 2 seems like the only option (unless I'm missing an option?)

4. Expire all passwords and ask your users to come reset them. - No legacy code necessary

Regular users should have no problems if there's an accompanying blog post Non-regular users might just remember about your site/service and come back :)

5. Use bcrypt/password stretching. Store the work value alongside the password and upgrade it as people log in. To me that's not really keeping legacy code around; just an extra variable...

" … at that time, Google accepted hashes in two formats only... MD5 or SHA1."

That's, ummm, _concerning_…

I don't suppose anyone knows how securely Google are storing my gmail password? I _hope_ it's not unsalted MD5 or SHA1. (Especially since google search is probably the best general purpose md5 hash reverser most people have access to.)

Google could store passwords in plain text if they want, because they're never going to leak. I'm only partially joking.

Famous last words :)

Then at least you are only partially an idiot.

Last migration I did (maybe 12 months ago), google only accepted passwords as an sha1 hash IIRC. I was told however, that was only an intermediary step, and google runs that hash through another KDF for internal use. The sha1sum of the password is really only used for transport to their API, as an alternative to sending it in plain-text which is what your browser does when you login to google services, so I don't see it as a huge problem. Sure it could be more secure, but interoperability and keeping a relatively low barrier of entry are very important to them too.

I thought Google logon defaulted to SSL for all services.

It's still sent in plain text. Your browser doesn't do any hashing before sending your password to Google.

But you're right, as far as anybody but you and Google are concerned, it's encrypted.

There's no point in hashing the password before you send it, is there? A MITM could just as easily grab the hash and send it to Google instead.

For MITM attacks, sure. But it's a fairly portable, inexpensive way to make it more difficult to use your password with other protocols. For example, intercepting a plaintext Google password has a decent chance of making someone's bank account vulnerable.

But in that case the password has only been compromised on that one website, as opposed to every other website where it's being used (likely a non-zero number of them given the average user's password habits). I'm no security expert but I think client-side password hashing with the domain name as the salt seems like a pretty good idea, especially for sites without HTTPS logins (but it also helps in the case of a database leak even for sites with HTTPS logins). Of course, for non-HTTPS logins a network attacker could modify the HTML form code to remove the client-side hashing without the user's knowledge, but it's still at least as secure as the alternative, modulo 'false sense of security' type arguments.

Edit: never mind the last parenthetical; it pretty much wouldn't help in a database leak at all (just adds one extra hashing step to the cracking process), sorry. Still helps for non-HTTPS logins though.

Edit 2: ... though maybe if the client-side hash were something strong like bcrypt, it would help in the case of database leaks on HTTPS sites that refuse to use strong hashing on the server side for performance reasons. Sorry for the rambly disorganized post.

Right. If someone is able to infect the browser with malware, MitM your HTTPS connection, or even just load mixed HTTP/HTTPS content, then they are able to run Javascript in the login page. If they are able to run Javascript in the login page, then they are able to monitor the keystrokes as it is typed in.

This is not theoretical. This is what Tunisia did to Facebook and it's what online banking trojans (e.g. Zeus) do every day.

> Google accepted hashes in two formats only... MD5 or SHA1.

It's not surprising that they don't support importing custom formats. They support two formats to bulk-import and then surely convert to their standard from there.

Also, if that's what should be encouraged, someone might as well put together a simple framework for automating it. Basically, you hard code a list of salts and a list of hash function names (which could also be automatically generated by a tool), and it gives you a new hash function which is the composition of those other functions. There's no reason for everyone to try to do it by hand and risk messing it up in some simple way. And if there were an easy to use framework for doing it, there would be no reason for only larger sites to use it. Using a custom hash function could simply be a best practice.

But honestly, I'd be pretty happy if we could get all sites with 50,000+ users to salt and hash their passwords with any algorithm.

The PBKDF2 protocol allows you to safely brew your own password scrambler using any hash functions you choose. It is equivalent to bcrypt for common purposes, assuming the hash functions you pick are decent.

Here's a question for the people here who actually know wtf they're talking about: if I choose to iterate through a set of hash functions with each pass of PBKDF2 rather than using the same one each time, what effect does that have on the entropy of the system and so on? Would it make it easier to crack, or harder?

There is no per se answer to that question. Cryptanalysis is hard, you can't assume things like commutativity and orthogonality.

This whole line of thought is at best a waste of time, and at worst dangerous.

PBKDF2-HMAC-SHA-256 is a vetted NIST approved standard with an adjustable work factor. It has been subject to professional attention for many years.

BCrypt, while not subject to nearly as much analysis nor approved by NIST, was designed by very competent cryptographers, has an adjustable work factor and is based on blowfish -- which has been subject to substantial professional attention.

Use one of the above with the highest work factor you have the processing power for and call it a day. Don't try to roll your solution.

I tend to agree, which is why my whole comment was conditional on "if it's a good idea for everyone to use custom hash functions". Basically, if we're going to have custom hash functions, we want to reduce the "roll your own" aspect of that to a minimum.

I hope nobody thought I was suggesting they actually do that. It was purely a matter of personal curiosity. Entropy is perhaps the most interesting thing in the universe.

I'm far too lazy to do anything other than slap bcrypt on it, unless there's a pressing need to do something else, which there never is.

(Not an expert) The entropy doesn't change. What you get by adding another hash function is a slightly larger die area (and thus the cost) required for brute force attacks. By using scrypt instead of PBKDF2, you can adjust the die area required for attacks in a more flexible and safe way by just giving it a different parameter.

Hash functions have different requirements than key stretching functions, but if you're interested in the security of combining cryptographic hashes, Google for "combining hash functions" and "chaining hash functions" -- there's a lot of interesting research.

>The entropy doesn't change

Yeah, that now seems obvious. The input string is the same, so the information entropy is the same. I'm struggling to think of the concept that I need here, I want to say kolmogorov complexity but I know that's wrong too.

> The PBKDF2 protocol allows you to safely brew your own password scrambler using any hash functions you choose. It is equivalent to bcrypt for common purposes, assuming the hash functions you pick are decent.

NO! Bcrypt in particular is designed to resist GPU brute forcing.

Yes, that's why I said "common purposes." If you're seriously worried about people bringing racks of GPUs to bear against you then what you're doing probably isn't "common," although I accept that this is rapidly changing.

You can calculate billions of SHA-1 hashes per second on a single $100 graphics card using standard software. That's pretty common. There's AFAIK no implementation of bcrypt for GPU.

Yes, but an opportunistic blackhat will just go and attack one of the millions of sites which don't need GPUs to crack instead. It's a margins game, like anything else.

If you're being attacked by anyone other than opportunists, you have bigger problems than your hash function. As soon as someone attacks you specifically, you're in a "trust no-one" situation, and suddenly it's time for anonymous meetings in basement carparks and the like.

I don't understand. If your "opportunistic blackhat" is willing to attack something, what are the chances that he doesn't have a pretty normal standard graphics card for this in his PC?

He probably does. What I'm saying is, why would he put that to work cracking n PBKDF2-HMAC-SHA-256 hashes per hour for n dollars per hour, when he could put it to work cracking >n MD5 hashes per hour for >n dollars?

If the answer is "my hashes protect something that is particularly valuable," then the attacker probably isn't going to hack your hash function, he's going to hack your secretary or your garbage disposal or something like that which is more effective.

Of course, in practice you should just use bcrypt anyway.

Ok, sorry for the harsh response, but i don't think that anyone is using it for "common purposes." Everyone should be using strong crypto, because it is inexpensive to do right.

I would argue that almost everyone that is storing passwords should start worrying about people bring racks of GPUs to bear against you, because it is so cheap. At 33.1 Billion MD5 hashes/s with 4 dual-linked GPUs (one machine), you can eat through all 8-digit alphanumerics very quickly for a few thousand dollars. (of course that is using PBKDF1 or less). I had done the calculations in a spreadsheet and forget how long it would take, but it is way shorter than you'd thik.

Such a toolkit would probably be a good idea.

Tie a card-carrying cryptographer to a chair until he delivers it ?

>Tie a card-carrying cryptographer to a chair until he delivers it ?

Alternatively, implement it very, very simply and put it on github, and ask card-carrying cryptographers to vet it. That one is harder on the ego though.

I'm pretty sure no cryptographer will vet it.

I read 'vet' as "tear it to bits"

Yes, exactly. After several phoenix iterations of that kind, you might have something approaching usable.

Isn't salting a canonical way to vary the output for the same input, thus reducing feasibility of precomputed attacks like rainbow tables? What are the benefits of some extra mangling, besides security through obscurity?

The point is that computers are really fast now. There's no need to precompute anything, just crack hashes on the fly. The extra mangling slows things down.

If an attacker gets access to a database of user names/hashes, a salt prevents him from simply checking the hashes against a precomputed list because each hash is totally unique.

The attacker would have to recompute all hashes for each user using their individual salt.

At least that's what I remember from Computer Security ha

Salting means that you must crack each key individually. It slows things down marginally in the long-run and at least precludes someone form using Google as your rainbow table. Have you ever tried searching for MD5 hashes of things like 'password' or 'pass123'? It's terrifying.

Looks like it wasn't clear that my question was rhetorical. My point was that some secret mangling on top of a standard hashing algorithm doesn't offer more protection than salting.

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