Re: [TLS] Application layer interactions and API guidance

Eric Rescorla <ekr@rtfm.com> Fri, 07 October 2016 22:04 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Fri, 7 Oct 2016 15:04:14 -0700
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To: Ilari Liusvaara <ilariliusvaara@welho.com>
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Subject: Re: [TLS] Application layer interactions and API guidance
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On Fri, Oct 7, 2016 at 3:00 PM, Ilari Liusvaara <ilariliusvaara@welho.com>;
wrote:

> On Fri, Oct 07, 2016 at 01:41:14PM -0700, Watson Ladd wrote:
> > On Fri, Sep 23, 2016 at 10:47 PM, Ilari Liusvaara
> > <ilariliusvaara@welho.com>; wrote:
> > >
> > > Also, it is very likely that 0-RTT would need its own read API, because
> > > it is pretty unlikely that existing API could be safely retrofitted
> > > or even purpose-built unified API be designed that isn't just asking
> > > for security problems via mixing 0-RTT and 1-RTT data.
> >
> > Yes. In particular I think the TLS state machine transitions need to
> > be ordered with respect to the arrival and sending of data. The
> > challenge for a multithreaded program (yes, some programs have
> > multiple threads sending and receiving at once) is making this make
> > sense, or even a single-threaded program where the TLS stack can
> > change state at times not visible to the sending thread. Maybe there
> > is some slop here, like 0-RTT can become 1-RTT on send, but this
> > raises all sorts of problems for receiving. I think we need to require
> > separation in the API.
>
> 0-RTT can't be allowed to become 1-RTT on send (unless it is auto-
> retransmit, which needs to be disableable, as sometimes that just plain
> won't work).
>

Can you explain why that is?

-Ekr



This impiles that the application always needs to explicitly signal
> the end of 0-RTT data, even just to abort sending it.
>
> > > (This is the reason one needs to be especially careful when combining
> > > dynamic client certificates with HTTP/2... Basically, it can not be
> > > done safely without coordination on application layer).
> > >
> > > One likely wants application protocol to be able to specify part of
> > > or the entiere request context and to extract that part in the other
> > > end when requesting a certificate selection. This is to allow putting
> > > part of coordination data into the request itself. However, this is not
> > > sufficient for e.g. HTTP/2 signaling, so more needs to be exchanged at
> > > application layer.
> > >
> > > And then upon completion of the authentication (either successful or
> > > explicitly rejected), signal the application with the new certificate
> > > chain and the context the request had.
> >
> > This is going to require major changes to APIs and to the HTTP/2
> > layer. It also interacts with token binding, whee! My question then
> > becomes one of what we actually need: can we assume that leakage
> > between authentication contexts over a single session is safe because
> > all contexts represent the same principal, by restricting the usages,
> > or is this overly restrictive?
>
> Yes, it is going to reuqire such changes.
>
> And you absolutely can not assume that all contexts present the same
> principal.
>
> And as I said, in multiplexed protocol like HTTP/2, you need application-
> level coordination on top. TLS just can't do that.
>
> > When we have multiple requests and replies in flight and the
> > authentication state changes, things get nasty. If we think of TLS as
> > sending a stream of events including authentication changes to the
> > application, this fits the semantics of the TLS draft as I understand
> > them to be, but does not necessarily fit what application protocols
> > want. There might be a semantic mismatch here requiring reworking of
> > one or another part.
>
> Also, some applications presumably want the events done synchronously,
> so they don't move with data reads.
>
>
> Basically, post-handshake auth is heckuva nasty problem.
>
>
> -Ilari
>
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