Re: [TLS] WGLC for "Deprecating TLSv1.0 and TLSv1.1"

Benjamin Kaduk <bkaduk@akamai.com> Mon, 06 May 2019 14:39 UTC

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Date: Mon, 6 May 2019 09:39:17 -0500
From: Benjamin Kaduk <bkaduk@akamai.com>
To: Kathleen Moriarty <kathleen.moriarty.ietf@gmail.com>
Cc: "tls@ietf.org" <tls@ietf.org>
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References: <20190430234952.21F5C404C@ld9781.wdf.sap.corp> <5441930.X76MtM1CnQ@pintsize.usersys.redhat.com> <1556902416424.28526@cs.auckland.ac.nz> <20190503172022.GH4464@akamai.com> <1556904629782.23087@cs.auckland.ac.nz> <CABcZeBNKgSFYg7gm-4ZibHSzDxO9qSjM5UGQXo81Rv7_r+m9gw@mail.gmail.com> <1F7FC950-358D-4D5C-963B-B7B837AE49DA@gmail.com> <CAHbuEH4Y6PJDhoHPnCkBgsAkOhvSTHFpew3V1d9iSQs_bknYSQ@mail.gmail.com> <1556937973484.34949@cs.auckland.ac.nz> <CAHbuEH7y8gFrVjDPY2AFRH_QQQUtaB8-SqKAmgjcGO+ksM3UTw@mail.gmail.com>
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Subject: Re: [TLS] WGLC for "Deprecating TLSv1.0 and TLSv1.1"
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On Sat, May 04, 2019 at 09:00:17AM -0400, Kathleen Moriarty wrote:
> On Fri, May 3, 2019 at 10:46 PM Peter Gutmann <pgut001@cs.auckland.ac.nz>
> wrote:
> 
> > Kathleen Moriarty <kathleen.moriarty.ietf@gmail.com> writes:
> >
> > >MD5 is not discussed in the current version of RFC7525.
> >
> > I would add it, if this is guidance for general use then it should cover
> > all
> > the bases, if SHA-1 is a MUST NOT then MD5 is a REALLY REALLY REALLY MUST
> > NOT.
> >
> > (Technically SHA-1 is still safe for ephemeral signing, i.e. locations
> > where
> > an attacker can't spend arbitrary amounts of time working on precomputed
> > data,
> > which is most of TLS because of the nonces in the handshake and the fact
> > that
> > connections will quickly time out if nothing arrives, but since TLS 1.2 has
> > SHA-2 built in already there's probably little point in separating out
> > where
> > SHA-1 is safe vs. where it isn't).
> >
> 
> Sure, I agree, but needed to look through prior documents first.  Since it
> wasn't in RFC7525 as a recommendation and the minimum baseline was above
> MD5, I suspect that is why it is not mentioned.   If there is support (and
> no disagreements) the text above could be added and include SHA-1 and MD5
> MUST NOT be used.  The minimum baseline is already set above it though in
> the statement.
> 
> WG decision is appreciated on this point and proposed text for RFC 7525.
> 
> Proposed:
> 
>    When using RSA, servers SHOULD authenticate using certificates with
>    at least a 2048-bit modulus for the public key.  In addition, the use
>    of the SHA-256 hash algorithm is the minimum requirement, SHA-1 and
> MD5 MUST not be used (see [CAB-Baseline
> <https://tools.ietf.org/html/rfc7525#ref-CAB-Baseline>] for
>    more details).  Clients SHOULD indicate to servers that they request
>    SHA-256, by using the "Signature Algorithms" extension defined in
>    TLS 1.2.

We'd probably want to wordsmith it a bit more, as there's not exactly a
strict ordering on hash function strength, and "minimum requirement"
could be taken to mean "MUST use SHA-256", which is presumably not the
intent.

-Ben