Re: [TLS] tls-unique

Ilari Liusvaara <ilari.liusvaara@elisanet.fi> Thu, 08 October 2015 13:05 UTC

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Date: Thu, 8 Oct 2015 16:05:17 +0300
From: Ilari Liusvaara <ilari.liusvaara@elisanet.fi>
To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] tls-unique
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On Thu, Oct 08, 2015 at 12:04:51PM +0200, Eric Rescorla wrote:
> 
> Well, TLS 1.3 doesn't have a PRF, but instead explicitly uses HKDF.
> 
> With that said, I don't really understand the structure of your draft:
> Instead of referencing the PRF and session_hash directly, why not instead
> use RFC 5705 exporters and require the use of the session_hash extension?
> Then TLS 1.3 can just define exporters for 1.3 and we'll be done.

TLS 1.3 is different since TLS 1.3 always behaves like session_hash was
negotiated, whereas session_hash was a security fix for earlier versions.


One idea for TLS-unique for TLS 1.3: Invoke TLS-EXPORTER with:

label: "TLS 1.3 tls-unqiue"
context: No context
Length: 256

And define TLS-EXPORTER for TLS 1.3 as (this looks ugly, have some
better way at handling both context and no context cases? In original
RFC, those were different):

tmp = HKDF-Extract(label, exporter_secret)
output = HKDF-Expand(tmp, 0x01 | context, L)

or (no context case)

tmp = HKDF-Extract(label, exporter_secret)
output = HKDF-Expand(tmp, <blank>, L)


This is slightly different from other uses of HKDF. I don't mix in
session hash since exporter_secret is already Secret Nonce.


-Ilari