Re: [TLS] Treatment of (legacy_record_)version field [was Re: (strict) decoding of legacy_record_version?]

Eric Rescorla <ekr@rtfm.com> Wed, 23 November 2016 13:18 UTC

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From: Eric Rescorla <ekr@rtfm.com>
Date: Wed, 23 Nov 2016 05:17:52 -0800
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To: Andreas Walz <andreas.walz@hs-offenburg.de>
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Subject: Re: [TLS] Treatment of (legacy_record_)version field [was Re: (strict) decoding of legacy_record_version?]
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On Wed, Nov 23, 2016 at 3:39 AM, Andreas Walz <andreas.walz@hs-offenburg.de>;
wrote:

> Dear all,
>
> bringing up this thread again ....
>
> In the course of studying the way TLS implementations treat the "version"
> (or "legacy_record_version") field in the record header, we were wondering
> (please excuse if we missed some arguments here from past discussions):
>
> (1) What is an implementation (in particular when receiving the first
> bytes over a new connection) supposed to do if the record's version field
> signals a protocol version the implementation does not support? I
> understand that, at this stage, enforcing a specific value (e.g. 0x0301
> according to the TLSv1.3 draft) is detrimental to interoperability.
> However, if that field bears any meaning (in either TLSv1.3 or previous
> versions), what is it? I would expect this field is supposed to allow
> signaling a potentially non-backward compatible record format
> (inauspiciously interfering with a receiver disregarding the record
> version). Provided this field isn't treated as an enum, what about
> checking/enforcing at least the major version as BoringSSL does (as far as
> I know)? In any case, I would propose to be very clear about this in the
> text (my sense was that there is some work in progress, but I couldn't find
> anything). In implementations (<TLSv1.3), we found all kinds of
> interpretations.
>

In general, it should ignore it. It's going to become increasingly common
to have this be a version you don't support given the recommendation to use
0301 and the ongoing deprecation of TLS 1.0. I think it would be fine to
sanity check the major version, but I'm not sure what would be gained by
requiring this.



> (2) What is an implementation (up to TLSv1.2, as the TLSv1.3 spec is
> rather clear about that) supposed to use for the record's protocol version
> field before a version has been agreed upon (e.g. when sending an alert
> after receiving an unparsable ClientHello)? My best guess would be to set
> it to the lowest (TLS) protocol version that uses the same record format
> (probably 0x0301). However, we observe several servers which, in such
> cases, answer with an alert with weird record protocol version values, e.g.
> 0x0000.]
>

Yes, this seems like a reasonable procedure. Not sure how to tell TLS 1.2
impls what to do at this point, though.

-Ekr


>
> Thanks and Cheers,
> Andi
>
> ___________________________________
>
> Andreas Walz
> Research Engineer
> Institute of reliable Embedded Systems and Communication Electronics
> (ivESK)
> Offenburg University of Applied Sciences, 77652 Offenburg, Germany
>
>
>
> >>> Benjamin Kaduk <bkaduk@akamai.com>; 11/10/16 5:22 PM >>>
> On 11/08/2016 06:25 PM, Martin Thomson wrote:
>
> On 9 November 2016 at 05:59, Brian Smith <brian@briansmith.org>; <brian@briansmith.org>; wrote:
>
> This isn't a pervasively shared goal, though. It's good to let the browsers
> police things if they want, but I think a lot of implementations would
> prefer to avoid doing work that isn't necessary for interop or security.
>
> If you permit someone to enforce it, then that is sufficient.  I don't
> think that we should ever force someone to enforce these sorts of
> things (as you say, sometimes strict enforcement isn't cheap or even
> desirable).
>
>
> Agreed.  We should probably change the text a bit, though, as right now
> readers can get two different readings depending on whether they go for a
> strict decode_error (or illegal_parameter?) since the struct doesn't match
> the definition, or follow the "MUST be ignored for all purposes".
>
> -Ben
>
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