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

[Andreas Walz]

>>> Eric Rescorla  11/23/16 2:18 PM >>>
> 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. 

The one benefit of checking at least the record's major version I see is that 
one
preserves means for the future to signal a record format incompatible with the
current one. Otherwise, this field is given away for arbitrary and 
non-standardized
(ab)use ...

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

[Eric Rescorla]

On Wed, Nov 23, 2016 at 3:39 AM, Andreas Walz 
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 ( 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.
> 0x.]
>

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  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  
>  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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> TLS@ietf.org
> https://www.ietf.org/mailman/listinfo/tls
>
>
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[TLS] Treatment of (legacy_record_)version field [was Re: (strict) decoding of legacy_record_version?]

[Andreas Walz]

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 (>> Benjamin Kaduk  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  
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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