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Aug 19, 2010, 12:55 PM
Post #11 of 31
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On Thu, Aug 19, 2010 at 2:37 AM, Tim Starling <tstarling [at] wikimedia> wrote: > The problem with the standard key strengthening algorithms, e.g. > PBKDF1, is that they are not efficient in PHP. We don't want a C > implementation of our scheme to be orders of magnitude faster than our > PHP implementation, because that would allow bruteforcing to be more > feasible than is necessary. > > The idea I came up with is to hash the output of str_repeat(). This > increases the number of rounds of the compression function, while > avoiding tight loops in PHP code. Seems reasonable. > PHP's hash extension has been available by default since PHP 5.1.2, > and we can always fall back to using Btype hashes if it's explicitly > disabled. The WHIRLPOOL hash is supported. It has no patent or > copyright restrictions so it's not going to be yanked out of Debian or > PHP for legal reasons. It has a 512bit block size, the largest of any > hash function available in PHP, and its security goals state that it > can be truncated without compromising its properties. > > My proposed hash function is a Btype MD5 salted hash, which is then > further hashed with a configurable number of invocations of WHIRLPOOL, > with a 256bit substring taken from a MediaWikispecific location. The > input to each WHIRLPOOL operation is expanded by a factor of 100 with > str_repeat(). > > The number of WHIRLPOOL iterations is specified in the output string > as a base2 logarithm (whimsically padded out to 3 decimal digits to > allow for future universesized computers). This number can be > upgraded by taking the hash part of the output and applying more > rounds to it. A count of 2^7 = 128 gives a time of 55ms on my laptop, > and 12ms on one of our servers, so a reasonable default is probably > 2^6 or 2^7. That seems reasonable. It could probably be done a lot faster on GPUs, I guess. On Thu, Aug 19, 2010 at 4:45 AM, Daniel Kinzler <daniel [at] brightbyte> wrote: > I don't know that much about the mathematical details of hashing, but i'd like > to drop a pointer to an article if found interesting in this context: > > "Stop using unsafe keyed hashes, use HMAC" > http://rdist.root.org/2009/10/29/stopusingunsafekeyedhashesusehmac/ > > So, how does your proposal relate to HMAC? As Tim said, it doesn't  we aren't using keyed hashes, and we're only concerned about preimage attacks (not collision or secondpreimage). Preimage attacks imply secondpreimage attacks, and secondpreimage attacks imply collision attacks. Thus something that's secure against collision is also secure against preimage and secondpreimage, but a function with collision and secondpreimage attacks might have no preimage attacks. For instance, MD5 has tons of trivial collision attacks against it, but no preimage attacks (not sure about secondpreimage attacks offhand). Whirlpool has no known collision attacks, and thus no known preimage or secondpreimage attacks. On Thu, Aug 19, 2010 at 5:02 AM, Robert Rohde <rarohde [at] gmail> wrote: > Let me preface my comment by saying that I haven't studied WHIRLPOOL, > and the following may not apply to it at all. > > However, it is known that some block cypher based hashes behave poorly > when fed repeated copies of the same block. In the worst cases the > hash space is substantially truncated from its full size (which > probably is not the case for any serious cryptographic hash function). > Under less severe cases, cryptanalysis can find a new block cipher W' > such that N applications of block cipher W is the same as one > application of W'. If WHIRLPOOL is vulnerable to that kind of attack > then it would negate the effect of using str_repeat in your code. In principle, we could evade any concern like this by just using a provably secure hash function. The usual reason not to use those is that they're slow, but that's an advantage in our case. For instance (from an exercise in my cryptography course), let p be a prime, q a prime dividing p  1, and let G be the subgroup of Z_p^* of order q. Let g be a randomly chosen generator for G, and let a_1, ..., a_k be randomly chosen elements of G. If x, y_1, ..., y_k are integers in the range 1 to q, define H(x, y_1, ..., y_k) = g^x a_1^y_1 ... a_k^y_k. Then under the discrete logarithm assumption for G, it's easy to prove that H is collisionresistant. You can make similar functions based on other hard problems  http://en.wikipedia.org/wiki/Provably_secure_cryptographic_hash_function gives another example using factorization, whose correctness is probably more obvious. There are two problems with using such a function. One is that there's probably no readily available implementation, so we'd have to write our own, which might be vulnerable to sidechannel attacks. Another is that we're more worried about bruteforcing than about the algorithm being broken, so we'd have to write it in C to avoid giving one or two orders of magnitude advantage to the attacker, and then shared hosts can't use it. Perhaps the way to solve both problems is to submit the code for inclusion in future versions of the PHP hash module, after at least casual review by some cryptographers. This would require significant extra work, of course. Another thing to consider is if we could pick a function that's particularly inconvenient to execute on GPUs. Those are a great way for crackers to easily outdo any CPU implementation. For a basic implementation, anyway, Tim's proposal sounds like it's much better than what we have now. Fancy stuff like provably correct hash functions is probably overkill. On Thu, Aug 19, 2010 at 5:02 AM, Robert Rohde <rarohde [at] gmail> wrote: > As a complementary approach it would be nice if there was something in > Mediawiki to aid in the selection of strong passwords. Regardless of > hash function, it will still take about two billion times longer to > find one 10 character password in [AZaz09] as it does to find a 6 > character password in [az]. Even if password strength testing > algorithms were disabled on Wikipedia sites, it would still be a nice > addition to have in the Mediawiki codebase in general. I agree we should have a good system available for those who want it, and it should probably be enabled by default for sysops. However, as I've said elsewhere, these strength testing things are counterproductive for unprivileged users on open wikis. For such users, a compromised account is approximately as bad as just forgetting your password  the result in either case is you just lose access to the account. If it's compromised, it might be somewhat worse (because the attacker might prevent an email reset), but on the other hand, forgetting your password is at least a couple orders of magnitude more likely. As such, to minimize harm to users, we should encourage them to use easytoremember passwords, not strong passwords. On Thu, Aug 19, 2010 at 10:12 AM, Jonathan Leybovich <jleybov [at] yahoo> wrote: > What about using public key cryptography? Generate a keypair and use the "public" key to produce your password hashes. Store the private key offline in an underground vault just in case someday you'll need to recover the original passwords in order to rehash them. Needless to say the keypair must be entirely for internal use and not already part of some PKI system (i.e. the basis for one of Wikimedia's signed SSL certificates). If you just delete the private key, this is the same as any old hash function, but with various additional deficiencies. For instance, publickey encryption is not designed to hide length information, while hash functions are  although due to padding, this might not be a big deal. More seriously, publickey encryption schemes are invariably probabilistic, which makes them useless for our purposes. We'd have to adapt the scheme to remove any randomization. And there might be other problems. There's really no reason to go to all this trouble  people have designed and studied dedicated hash functions for a reason. On Thu, Aug 19, 2010 at 10:50 AM, Ryan Lane <rlane32 [at] gmail> wrote: > We could do a less secure, but moresecurethanpasswords alternative, > which is to use email or SMS as a one time password device. SMS is > obviously more secure than email, but would require us to ask people > for their phone numbers. SMS has loads of vulnerabilities: http://en.wikipedia.org/wiki/SMS#Vulnerabilities I don't see anyone signing up to get an email or text message every time they want to log in, either. At best, this pushes the authentication problem back to their email or SMS provider. In that case, why not just use OpenID? > We could also make a PKI infrastructure, and > allow certificate login, which is obviously safer than passwords. Not if the password is not stored on the computer and the private key is. > The real problem with any system stronger than passwords, is that it > requires a level of complexity that would be difficult for us, and > either annoying or very confusing for users. Yes, so let's not worry about it, shall we? We aren't the NSA here. On Thu, Aug 19, 2010 at 2:18 PM, Jonathan Leybovich <jleybov [at] yahoo> wrote: > In that case you could always discard the private portion of the keypair to > produce a strictly "oneway" function. And at least with this scheme you always > do have the option > of moving to 'C' regardless of whether it can accept the endproducts of B as > inputs. Plus I would wager that asymmetric ciphers will stand up to attacks far > longer than most hashing functions. As I noted above, there are hash functions whose security is provable based on the exact same assumptions used to prove security of various popular asymmetric encryption schemes. As I also noted above, there are problems with naively trying to use publickey encryption instead of hash functions. It makes more sense to just use knownsecure hash functions directly instead of trying to twist publickey encryption to our needs, if we're that worried about Whirlpool (et al.) being broken anytime soon. For what it's worth, even ancient and thoroughlybroken hash functions like MD4 don't have readilyusable preimage attacks. (MD4 was published in 1990, broken in 1995, and only in 2008 was a preimage attack published  which still requires 2^102 evaluations, instead of 2^128.) We don't have to worry much about the hash functions being broken. _______________________________________________ Wikitechl mailing list Wikitechl [at] lists https://lists.wikimedia.org/mailman/listinfo/wikitechl
