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

Andrei Popov <> Thu, 24 March 2016 17:33 UTC

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From: Andrei Popov <>
To: Wan-Teh Chang <>, Martin Thomson <>
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.



-----Original Message-----
From: TLS [] On Behalf Of Wan-Teh Chang
Sent: Thursday, March 24, 2016 12:04 AM
To: Martin Thomson <>;
Subject: Re: [TLS] Empty extensions don't go last

On Wed, Mar 23, 2016 at 10:30 PM, Martin Thomson <>; 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):

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:

Wan-Teh Chang

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