Re: [kitten] draft-hansen-scram-sha256 and incorporating session hashing for channel binding
Nico Williams <nico@cryptonector.com> Thu, 28 May 2015 20:37 UTC
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Date: Thu, 28 May 2015 15:36:56 -0500
From: Nico Williams <nico@cryptonector.com>
To: Simon Josefsson <simon@josefsson.org>
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Subject: Re: [kitten] draft-hansen-scram-sha256 and incorporating session hashing for channel binding
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On Thu, May 28, 2015 at 10:01:42PM +0200, Simon Josefsson wrote: > > On Thu, May 28, 2015 at 05:11:22PM +0200, Simon Josefsson wrote: > > > > > You then go on to say: "Personally, I would prefer to change to > > > > > another mandatory channel binding that is secure for all TLS > > > > > versions." > > > > > > > > This is not really appropriate here because it's the applications > > > > that need to do this, and we can't say anything here about this > > > > that will force them to. > > > > > > > A reference to TLS-SESSION-HASH of the same level (i.e., normative > > > > or informative) as RFCs 5246 and 5929 would be nice. > > > > > > I believe that what is required is > > > > > > 1) scram-sha256 has a normative reference to tls-session-hash; or > > > > > > 2) tls-session-hash uses an Update: that makes it applicapable to > > > all TLS versions, and that it is clarified (if not already the > > > case) that tls-session-hash must be used; or > > > > And RFC5929. > > That's not strictly necessary, is it? 5929 refer to TLS, so if > tls-session-hash update TLS specs, 5929 indirectly refer too > tls-session-hash. At least that is how I would interprete it. But > I'm all for making things explicit. Less not-strictly-necessary than the change you're proposing to SCRAM... After all, it'd be useful if anyone reading RFC5929 saw that there's a security consideration when using TLS w/o the session hash (namely: that tls-unique is insecure when used with connections made by resuming sessions). > > My vote is for (2). I don't mind (1) in addition, but I want (2). > > There is precedent for that for TLS, several of the security fixes > Update:'s all TLS RFCs. Yes. > > > 3) scram-sha256 uses a new channel binding that is secure with or > > > without tls-session-hash. > > > > We disagree as to (3). This is advice we can give to apps in the > > security considerations section, not something we can force a SASL or > > GSS mechanism to do because API-wise the mechanism doesn't get a > > choice. > > I don't understand this... The mechanism is not in a position to use any one CB type or other. The application chooses the CB type. > > > I believe 1) and 2) would be worse than 3) for the next ~5 years or > > > so, and things being equal after that. SASL libraries/applications > > > rarely have any influence over TLS internals, but they directly > > > influence the channel binding used. Using another channel binding > > > for [...] > > > > No, they don't. Certainly not GSS ones, and the SASL implementations > > I'm familiar with don't either. > > > > E.g., GSS_Init_sec_context() doesn't get a handle to a channel to bind > > to, it only gets the channel binding as already extracted by the app. > > Worse, GSS_Init_sec_context() doesn't even get the channel binding > > type. > > ...and I don't understand any of this. I don't understand how you can't understand it. GSS_Init_sec_context() doesn't have access to the TLS (or whatever) channel, and therefore the mechanism can't be involved in getting the channel binding. > A new channel binding in SCRAM-SHA256 would work exactly the same way > as tls-unique works in SCRAM-SHA1 today. > > In my SASL library, the app has to provide the tls-unique channel > binding (either upfront or in a callback) when SCRAM-SHA1 is used. If > I would implement SCRAM-SHA256, and assuming that document referred > directly to tls-unique-prf, the app would need to supply the > tls-unique-prf data upfront or in a callback when SCRAM-SHA256 is > used. I don't see any problem with this. The channel binding type is > hardcoded with the SASL mechanism name. The CB type is NOT a part of the mechanism name. The CB type name is prefixed to the CB data in the SASL/GS2 case (but not in the non-SASL GSS case). The best SCRAM-SHA256 could do is detect that tls-unique is used in the SASL/GS2 case and fail. Nico --
- [kitten] AD sponsoring draft-hansen-scram-sha256 Stephen Farrell
- Re: [kitten] AD sponsoring draft-hansen-scram-sha… Peter Saint-Andre - &yet
- Re: [kitten] AD sponsoring draft-hansen-scram-sha… Tony Hansen
- Re: [kitten] AD sponsoring draft-hansen-scram-sha… Peter Saint-Andre - &yet
- Re: [kitten] AD sponsoring draft-hansen-scram-sha… Simon Josefsson
- Re: [kitten] [saag] AD sponsoring draft-hansen-sc… Simon Josefsson
- Re: [kitten] [saag] AD sponsoring draft-hansen-sc… Alexey Melnikov
- Re: [kitten] [saag] AD sponsoring draft-hansen-sc… Dave Cridland
- Re: [kitten] AD sponsoring draft-hansen-scram-sha… Simon Josefsson
- Re: [kitten] [saag] AD sponsoring draft-hansen-sc… Martin Thomson
- Re: [kitten] [saag] AD sponsoring draft-hansen-sc… Sam Whited
- Re: [kitten] [saag] AD sponsoring draft-hansen-sc… Stephen Farrell
- Re: [kitten] [saag] AD sponsoring draft-hansen-sc… Tony Hansen
- Re: [kitten] [saag] AD sponsoring draft-hansen-sc… Tony Hansen
- [kitten] draft-hansen-scram-sha256 and the hash i… Tony Hansen
- [kitten] draft-hansen-scram-sha256 and incorporat… Tony Hansen
- Re: [kitten] draft-hansen-scram-sha256 and the ha… Dave Cridland
- Re: [kitten] draft-hansen-scram-sha256 and the ha… Alexey Melnikov
- Re: [kitten] draft-hansen-scram-sha256 and the ha… Tony Hansen
- Re: [kitten] draft-hansen-scram-sha256 and the ha… Simon Josefsson
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Simon Josefsson
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Alexey Melnikov
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Simon Josefsson
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Alexey Melnikov
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Tony Hansen
- Re: [kitten] [saag] AD sponsoring draft-hansen-sc… Karthikeyan Bhargavan
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Simon Josefsson
- Re: [kitten] [saag] AD sponsoring draft-hansen-sc… Simon Josefsson
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Nico Williams
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Nico Williams
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Simon Josefsson
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Simon Josefsson
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Stephen Farrell
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Nico Williams
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Tony Hansen
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Nico Williams
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Simon Josefsson
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Tony Hansen
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Nico Williams
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Simon Josefsson
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Simon Josefsson
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Nico Williams
- Re: [kitten] draft-hansen-scram-sha256 and incorp… Tony Hansen