On Fri, Oct 7, 2016 at 10:03 AM, Ilari Liusvaara <ilariliusva...@welho.com> wrote:
> On Fri, Oct 07, 2016 at 09:35:40AM -0700, Eric Rescorla wrote: > > On Fri, Oct 7, 2016 at 8:26 AM, Ilari Liusvaara < > ilariliusva...@welho.com> > > wrote: > > > > > On Fri, Oct 07, 2016 at 08:01:43AM -0700, Eric Rescorla wrote: > > > > 4. I've taken a suggestion from David Benjamin to move the > negotiation > > > > of the PSK key exchange parameters out of the PSK itself and into a > > > > separate message. This cleans things up and also lets us drop the > > > > currently non-useful auth_mode parameter. > > > > > > Eeh... From the text, it seems to currently require the kex modes > > > extension if PSK extension is present. Which seems worse than useless > > > if the meaning is to get rid of the kex mode parameter from PSK > > > extension (since you will have the value anyway, but need to dig it > > > from another extension... Blech). > > > > I guess this is a matter of taste, but what convinced me was that: > > > > 1. It put all the logic on the server side. > > 2. It removed the auth mod parameter. > > > > Maybe david can say more. > > I mean if server is to accept PSK, it must now go fishing for another > extension, check that it is present and pay attention to values there. > As opposed to having the data in where it is needed. > This is a reasonable argument (and the reason I stuffed the binder here). However, David's argument was that this applied to *all* PSKs even new ones. -Ekr > > Also, didn't notice what prevents pathology like this (I presume this > > > is not allowed): > > > > > > (Assume PSK with 0RTT allowed, using AES-128-GCM-SHA256) > > > > > > ClientHello[Ciphers=CHACHA20-POLY1305-SHA256, EarlyDataIndication] > ---> > > > [0-RTT data, encrypted using AES-128-GCM-SHA256] > > > <-- ServerHello[Cipher=CHACHA20-POLY1305-SHA256] > > > <-- EncryptedExtensions[EarlyDataIndication] > > > > > > Note the record protection algorithm mismatch. > > > > > > > Yes, this is forbidden by the combination of: > > > > "The parameters for the 0-RTT data (symmetric cipher suite, > > ALPN, etc.) are the same as those which were negotiated in the connection > > which established the PSK. The PSK used to encrypt the early data > > MUST be the first PSK listed in the client's "pre_shared_key" extension." > > (though I think I just recently added cipher suite). > > > > and: > > "Any ticket MUST only be resumed with a cipher suite that is identical > > to that negotiated connection where the ticket was established." > > If 0-RTT is used with manually provisioned PSKs (might not be allowed > currently, but might be allowed soon), does that still hold? > > Also, I think it is problematic that externally provisioned PSKs can > be used with any protection with given prf-hash, while NST-provisioned > PSKs can only be used with one protection and prf-hash. > > 0-RTT requirements are separate matter, since those would apply to all. > > The original purpose of resumption-as-PSK was AFAIK to unify the two > mechanisms to simplify things. Therefore those two should be as similar > as possible. > > > > > Also, to straightforwardly prove that collision resistance of HKDF and > > > HMAC (as used) follows from collision resistance of the underlying hash > > > function, yon need to take the output to be at least the hash output > > > size. As otherwise it is not guaranteed that any collision in HKDF or > > > HMAC can be reduced into collision of the underlying hash. > > > > > > > Right. I have some text here but please feel free to suggest more. > > Yes, but the text says 256 bit output is enough. One isn't guaranteed > to be able to reduce such collision to collision of >256 bit hash. > > (In fact, if the hash is e.g. 384 bit, 256-bit collisions are extremely > unlikely to reduce). > Right. I can update. -Ekr > > > > -Ilari >
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