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
