Re: [TLS] PSS for TLS 1.3

Brian Smith <brian@briansmith.org> Mon, 23 March 2015 01:53 UTC

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Date: Sun, 22 Mar 2015 15:53:40 -1000
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From: Brian Smith <brian@briansmith.org>
To: Eric Rescorla <ekr@rtfm.com>
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Subject: Re: [TLS] PSS for TLS 1.3
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Eric Rescorla <ekr@rtfm.com> wrote:
> Obviously, if we want to move to PSS, option #2 is simplest, but
> the sentiment at the interim was to survey the WG to see whether
> there was widespread enough support for generating and verifying
> PSS to make this feasible [0].

PSS with which parameters?

My suggestion is that, if PSS is used, (SHA-256, MGF-SHA-256, 32-byte
salt) should be used with the SHA-256-based PRF and that (SHA-384,
MGF-SHA-384, 48-byte salt) should be used with the SHA-384-based PRFs.

[1] notes that there is a security advantage to using the same digest
function for the MGF as was used for digesting the signed data. It
would be a mistake to mandate support for MGF-SHA-1 in PSS signatures
in TLS 1.3, because a TLS 1.3 implementation shouldn't be need to
implement SHA-1 at all.

Some implementations are hard-coded to support only MGF-SHA-1 and 20
byte salts. It is better to require those implementations to be
updated than to require all implementations to implement SHA-1 just
for the MGF.

Cheers,
Brian

[1] https://tools.ietf.org/html/rfc3447#section-8.1