Re: [TLS] Is it possible for a client to offer TLS 1.3, but not be forced to support RSA PSS in TLS 1.2?

Martin Thomson <martin.thomson@gmail.com> Wed, 30 May 2018 05:13 UTC

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From: Martin Thomson <martin.thomson@gmail.com>
Date: Wed, 30 May 2018 15:13:42 +1000
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To: Andrey Jivsov <crypto@brainhub.org>
Cc: David Benjamin <davidben@chromium.org>, "<tls@ietf.org>" <tls@ietf.org>
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Subject: Re: [TLS] Is it possible for a client to offer TLS 1.3, but not be forced to support RSA PSS in TLS 1.2?
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On Wed, May 30, 2018 at 2:53 PM Andrey Jivsov <crypto@brainhub.org>; wrote:
> The quoted text quoted is old. The need to upgrade TLS 1.2 code if I
> support TLS 1.3 is new.

No, I'm certain we had that discussion too.

> I am curious about the scenarios when is this upgrade of TLS 1.2 to PSS
> will take place?

When people deploy TLS 1.3.  Which is happening already.  You can avoid the
need as a server because a client willing to do TLS 1.2 will probably offer
RSASSA PKCS#1 v1.5 and you can rely on that being there.  But yeah, clients
are going to have to suck it up.  Here's the text, which I think is pretty
clear:
"
Implementations that advertise support for RSASSA-PSS (which is mandatory
in TLS 1.3), MUST be prepared to accept a signature using that scheme even
when TLS 1.2 is negotiated. "