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Re: [TLS] Transcript-Hash during Handshake

Andrei Popov <Andrei.Popov@microsoft.com> Thu, 23 November 2017 19:59 UTC

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From: Andrei Popov <Andrei.Popov@microsoft.com>
To: "ilariliusvaara@welho.com" <ilariliusvaara@welho.com>
CC: Peter Wu <peter@lekensteyn.nl>, "Le Van Gong, Hubert" <hubert@levangong.org>, "tls@ietf.org" <tls@ietf.org>
Thread-Topic: [TLS] Transcript-Hash during Handshake
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Date: Thu, 23 Nov 2017 19:58:56 +0000
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Subject: Re: [TLS] Transcript-Hash during Handshake
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Thanks, Ilari!

-----Original Message-----
From: ilariliusvaara@welho.com [mailto:ilariliusvaara@welho.com] 
Sent: Thursday, November 23, 2017 11:52 AM
To: Andrei Popov <Andrei.Popov@microsoft.com>
Cc: Peter Wu <peter@lekensteyn.nl>; Le Van Gong, Hubert <hubert@levangong.org>; tls@ietf.org
Subject: Re: [TLS] Transcript-Hash during Handshake

On Thu, Nov 23, 2017 at 07:42:12PM +0000, Andrei Popov wrote:
> To confirm, TLSInnerPlaintext.type and TLSInnerPlaintext.zeros are not 
> part of the handshake messages, and therefore are not included in the 
> transcript hash?

Correct. The transcript hash is also not affected by fragmentation.

E.g. in TLS 1.3, the raw finished messag fed to SHA-256 is always
14 00 00 20 <32 bytes payload>. Regardless of padding and fragmnentation (for SHA-384, that would be 14 00 00 30 <48 bytes
payload>).

(In DTLS, the header would be different and larger, but also not affected by padding and fragmentation).


-Ilari

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