Yes SHA512 or even a left half SHA512 slows the attacker significantly over SHA256 as that alg has had a lot of ASIC optimization.
I personally think that SHA256 for a short lived token is still good for a while. Sent from my iPhone > On Nov 18, 2014, at 11:33 AM, Bill Mills <wmills_92...@yahoo.com> wrote: > > In this case it ceaper to add bits than use soemthing like BCrypt. > > > > On Monday, November 17, 2014 9:33 PM, John Bradley <ve7...@ve7jtb.com> wrote: > I think where we are differing is that you think looking up a precomputed > plain based on a indexed hash across some sort of media can be done faster > than 3 Giga SHA256 Hash/s. > on a small system http://thepasswordproject.com/oclhashcat_benchmarking > > I don't think the largest disk arrays can keep up with that. > > Do you have some evidence that shows that precomputing hashes would be an > effective attack against 256 bits of entropy. I agree that it would be > agains the 40 ish bits of entropy in a password. > > The likely mitigation is using PBKDF2 or BCrypt rather than SHA256, but that > would slow adoption and can be added later. > > John B. > > > > > My contention is that it can't > >> On Nov 17, 2014, at 10:27 PM, takamichi saito <sa...@cs.meiji.ac.jp> wrote: >> >> >> I agree that GPU can/may find the value on the fly. >> But, it can not find it within the session. >> The draft idea is enough against the attack with GPU. >> >> On the other, the draft idea provide ONLY one combination of hash and its >> plain. The attacker can prepare THE COMBINATION to success the attack. >> >> Adding client_ID or server_ID separate the searching space. >> Then the attacker have to find the value in each case for the attack. >> (The reason was said before.) >> >> >> (2014/11/17 13:33), John Bradley wrote: >>> The question is what is the attack. >>> >>> Any salt added needs to be communicated from the client to the AS, so we >>> must assume that the attacker has it. >>> >>> The attacker can then a) create rainbow table using the client id or >>> whatever is the known salt. Yes the attacker must create a new table >>> per client. >>> Salting is really only effective for low entropy passwords to try and >>> slow down a rainbow table attack by making the input to the hash be >>> higher than the that of the password on it's own. >>> >>> Currently attackers can generate over 4Billion SHA256 hashes per second >>> on a single GPU card. (Thank you bitcoin) >>> >>> It is faster to generate the hashes than to look them up via a index. >>> >>> If you are generating the hash in real time salting provides no >>> determent, as the salt is just included in the hash calculation. >>> >>> If the code verifier was a password rather than a 256bit random key then >>> a hash would add value against rainbow tables. >>> >>> In reality finding a collision against a salted password is much easier >>> using real time hash generation than by using rainbow tables. >>> >>> Using SHA256 with a short hash is not safe for passwords any more. >>> Something like PBES2 with at-least 200 rounds needs to be used, if you >>> want have password files from being compromised quickly if disclosed. >>> (Yes I know people are not doing that, and hence part of the reason >>> why passwords are no longer secure) >>> >>> More entropy in the code verifier adds to security eg moving to SHA512 >>> and larger verifiers, but adding a salt to SHA256 is basically a no op >>> when defending against modern attacks. >>> >>> I did originally agree with your position and wanted to HMAC the >>> client_id to defend against rainbow tables, however I am now convinced >>> that the attack has moved on so that is no more efective than a plain >>> hash over a random 256bit value. >>> >>> John B. >>> >>>> On Nov 16, 2014, at 11:06 PM, Nat Sakimura <sakim...@gmail.com >>>> <mailto:sakim...@gmail.com>> wrote: >>>> >>>> I am actually not convinced. Since the code verifier is 256bit random, >>>> adding salt does not seem to help. >>>> Salting definitely helps if len(password) << 256 bit, but ... >>>> >>>> >>>> On Mon Nov 17 2014 at 11:39:07 takamichi saito <sa...@cs.meiji.ac.jp >>>> <mailto:sa...@cs.meiji.ac.jp>> wrote: >>>> >>>> >>>> >>>> (2014/11/14 13:02), Bill Mills wrote: >>>>> Yes, "plain" is actually sufficient. The hashed value protects >>>> against >>>>> disclosure/logging threats on the server auth server and proxies >>>> perhaps >>>>> where the HTTPS is terminated somewhere other than the auth server >>>>> itself, it's not actually required for the basic >>>> functionality/security >>>>> of the mechanism. >>>> >>>> In the threat model of the SPOP scheme, a wiretap is in it. >>>> >>>> And more, the hash is not used to keep secretly in the sever/client. >>>> >>>> >>>>> >>>>> >>>>> On Thursday, November 13, 2014 7:07 PM, takamichi saito >>>>> <sa...@cs.meiji.ac.jp <mailto:sa...@cs.meiji.ac.jp>> wrote: >>>>> >>>>> >>>>> Sorry for my poor english. >>>>> >>>>> >>>>> 2014/11/14 10:55、Bill Mills <wmills_92...@yahoo.com >>>> <mailto:wmills_92...@yahoo.com> >>>>> <mailto:wmills_92...@yahoo.com >>>> <mailto:wmills_92...@yahoo.com>__>> のメール： >>>>> >>>>>> The whole mechanism relies on the attacker not having access >>>> to the >>>>> code_verifier or hash. It's defending against the attacker >>>> getting the >>>>> code via weakness in IPC or other such mechanism like URI >>>> handlers. How >>>>> many more bits is secure beyond 256 bits of entropy >>>> recommended? If you >>>>> want to make it longer then just make it longer, salting doesn't >>>> really >>>>> help that much. >>>>>> >>>>>> The original value or the hashed value *should* be protected >>>> by the >>>>> transport security, and if it isn't then the attacker could be >>>> stealing >>>>> the original credential used to authenticate anyway. >>>>> >>>>> Is it correct? >>>>> You mean that we don’t need to use hash itself? Only to use >>>> plain is enough? >>>>> >>>>> >>>>>> >>>>>> >>>>>> >>>>>> On Thursday, November 13, 2014 5:40 PM, takamichi saito >>>>> <sa...@cs.meiji.ac.jp <mailto:sa...@cs.meiji.ac.jp> >>>> <mailto:sa...@cs.meiji.ac.jp <mailto:sa...@cs.meiji.ac.jp>>> wrote: >>>>>> >>>>>> >>>>>> >>>>>> Hi all, >>>>>> >>>>>> I appreciate this idea, simple and powerful to achieve proof of >>>>> possession. >>>>>> But, I have some questions against the scheme. >>>>>> Sorry if these ware already discussed. >>>>>> >>>>>> I worry about using a hash function in simple way. >>>>>> I mean, a simple use of random as code_verifier may cause that >>>>> malicious client can have any code_verifier and code_challenge. >>>>>> All combinations of random and its hash can be obtained, it >>>> may not >>>>> be risk? >>>>>> >>>>>> So, we should use: >>>>>> S256 "code_challenge" = BASE64URL(SHA256("code___verifier" + >>>> “client ID”)) >>>>>> or >>>>>> S256 "code_challenge" = BASE64URL(SHA256("code___verifier" + >>>> “client >>>>> ID” + “server ID”)) >>>>>> Where, you know that client ID is client’s unique name. >>>>>> >>>>>> >>>>>> Other problem is the following, using Nat’s slide: >>>>>> http://www.slideshare.net/nat___sakimura/1112-spoppresso >>>> <http://www.slideshare.net/nat_sakimura/1112-spoppresso> >>>>> <http://www.slideshare.net/__nat_sakimura/1112-spoppresso >>>> <http://www.slideshare.net/nat_sakimura/1112-spoppresso>>. >>>>>> >>>>>> 0. Attacker prepares own code_verifier and code_challenge. >>>>>> 1. replage legitimate challenge with malicious code_challenge. >>>>>> 5. Attacker can submits own code_verifier. >>>>>> >>>>>> It may be out of the draft, I think. >>>>>> >>>>>> Best regards, >>>>>> >>>>>> >>>>>> ;; takamixhi saito >>>>>> >>>>>> _________________________________________________ >>>>>> OAuth mailing list >>>>>> OAuth@ietf.org <mailto:OAuth@ietf.org> <mailto:OAuth@ietf.org >>>> <mailto:OAuth@ietf.org>> >>>>>> https://www.ietf.org/mailman/__listinfo/oauth >>>> <https://www.ietf.org/mailman/listinfo/oauth> >>>>> >>>>> >>>>> >>>>> ;; takamixhi saito >>>>> >>>>> _________________________________________________ >>>>> OAuth mailing list >>>>> OAuth@ietf.org <mailto:OAuth@ietf.org> <mailto:OAuth@ietf.org >>>> <mailto:OAuth@ietf.org>> >>>>> https://www.ietf.org/mailman/__listinfo/oauth >>>> <https://www.ietf.org/mailman/listinfo/oauth> >>>> >>>> >>>> -- >>>> ;; takamixhi saito >>>> >>>> _________________________________________________ >>>> OAuth mailing list >>>> OAuth@ietf.org <mailto:OAuth@ietf.org> >>>> https://www.ietf.org/mailman/__listinfo/oauth >>>> <https://www.ietf.org/mailman/listinfo/oauth> >>>> >>>> _______________________________________________ >>>> OAuth mailing list >>>> OAuth@ietf.org <mailto:OAuth@ietf.org> >>>> https://www.ietf.org/mailman/listinfo/oauth >> >> >> -- >> ;; takamixhi saito >> >> _______________________________________________ >> OAuth mailing list >> OAuth@ietf.org >> https://www.ietf.org/mailman/listinfo/oauth > > > _______________________________________________ > OAuth mailing list > OAuth@ietf.org > https://www.ietf.org/mailman/listinfo/oauth _______________________________________________ OAuth mailing list OAuth@ietf.org https://www.ietf.org/mailman/listinfo/oauth
