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Re: [TLS] Empty extensions don't go last

Andrei Popov <Andrei.Popov@microsoft.com> Thu, 24 March 2016 17:33 UTC

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From: Andrei Popov <Andrei.Popov@microsoft.com>
To: Wan-Teh Chang <wtc@google.com>, Martin Thomson <martin.thomson@gmail.com>
Thread-Topic: [TLS] Empty extensions don't go last
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Date: Thu, 24 Mar 2016 17:33:28 +0000
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Subject: Re: [TLS] Empty extensions don't go last
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Yes, we found this a while ago as well, and had to move extensions around.

Cheers,

Andrei

-----Original Message-----
From: TLS [mailto:tls-bounces@ietf.org] On Behalf Of Wan-Teh Chang
Sent: Thursday, March 24, 2016 12:04 AM
To: Martin Thomson <martin.thomson@gmail.com>
Cc: tls@ietf.org
Subject: Re: [TLS] Empty extensions don't go last

On Wed, Mar 23, 2016 at 10:30 PM, Martin Thomson <martin.thomson@gmail.com> wrote:
> (This is probably already known to a bunch of people, but it's 
> probably a good idea to put out there.)
>
> When deploying EMS, we recently discovered, with the help of our 
> friends at Google (who discovered this long before that) a quirk in 
> some implementations.
>
> Short story:  Don't place an empty extension at the end of your 
> ClientHello.  You will find that a small number of servers choke.

This interop problem surfaces when we added to Chrome the signed_certificate_timestamp extension for Certificate Transparency (RFC 6962):

https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fbugs.chromium.org%2fp%2fchromium%2fissues%2fdetail%3fid%3d353009%23c4&data=01%7c01%7cAndrei.Popov%40microsoft.com%7c916a8a6aa2eb4b8fd3e908d353b28e2d%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=YUuVpA6X5QhQZGOS%2bHbs30ooKE%2bW9U0W4cr6tG%2bbKEo%3d
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The change that David Benjamin made to
ssl3_CalculatePaddingExtensionLength() in his patch probably should also be considered if the padding extension (RFC 7685) is placed at the end:

https://na01.safelinks.protection.outlook.com/?url=https%3a%2f%2fcodereview.chromium.org%2f240633006%2fdiff%2f110001%2fnet%2fthird_party%2fnss%2fssl%2fssl3ext.c&data=01%7c01%7cAndrei.Popov%40microsoft.com%7c916a8a6aa2eb4b8fd3e908d353b28e2d%7c72f988bf86f141af91ab2d7cd011db47%7c1&sdata=jH5GKij0h%2boPSy4BaRvePsiN7zx2gycuSYraYDiTS4E%3d

Wan-Teh Chang

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