Re: [Lurk] [TLS] TLS 1.2 and sha256

David Benjamin <davidben@chromium.org> Mon, 11 June 2018 21:53 UTC

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From: David Benjamin <davidben@chromium.org>
Date: Mon, 11 Jun 2018 17:52:55 -0400
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To: Daniel Migault <daniel.migault@ericsson.com>
Cc: tls <tls@ietf.org>, LURK BoF <lurk@ietf.org>
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Subject: Re: [Lurk] [TLS] TLS 1.2 and sha256
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In both TLS 1.2 and TLS 1.3, SHA-256 isn't hardcoded per se. It's a
function of the cipher suite you negotiate (and also, separately, the
signature algorithm you negotiate). That said, in practice, both are pretty
solidly dependent on SHA-256. Most options involve it. AES-128-GCM and
ChaCha20-Poly1305 are currently paired with SHA-256 while only AES-256-GCM
is paired with SHA-384.

We could certainly define new cipher suites for either of TLS 1.2 and TLS
1.3 as needed. But defining a new cipher suite for TLS 1.2 doesn't
magically deploy it for all existing TLS 1.2 servers. Those servers must
deploy new code, at which point updating your TLS library to include it
would also pull in TLS 1.3 anyway (or whatever the latest TLS version is by
then).

So I think there will likely be no point in bothering with TLS 1.2
allocations at that point. More options means more combinatorial complexity
for implementations, which means more our rather limited collective
resources in this space get even more thinly spread.

David

On Mon, Jun 11, 2018 at 5:25 PM Daniel Migault <daniel.migault@ericsson.com>;
wrote:

> Hi,
>
> TLS 1.2 uses sha256 as the prf hash function. When sha256 will not be
> considered secured, I am wondering if we can reasonably envision
> deprecating sha256 for TLS 1.2 or if TLS 1.2 will at that time be
> deprecated in favor of TLS 1.X X>= 3 ?
>
> In other words, I am wondering how much we can assume TLS 1.2 is
> associated to sha256.
>
> Yours,
> Daniel
>
>
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