Sorry, i don't understand why you prefere blacklist to whitelist !
-----Message d'origine-----
De : Exim-users <exim-users-bounces+jmedard=amv-sa...@exim.org> De la part
de Viktor Dukhovni via Exim-users
Envoyé : samedi 12 octobre 2019 19:49
À : exim-users@exim.org
Cc : Viktor Dukhovni <exim-us...@dukhovni.org>
Objet : Re: [exim] Define preferred encryption algorithms
> On Oct 12, 2019, at 9:36 AM, Mike Tubby via Exim-users
<exim-users@exim.org> wrote:
>
> # OWASP Widest Compatibility (List C)
> tls_require_ciphers =
> TLS_AES_256_GCM_SHA384:TLS_CHACHA20_POLY1305_SHA256:TLS_AES_128_GCM_SH
> A256:DHE-RSA-AES256-GCM-SHA384:DHE-RSA-AES128-GCM-SHA256:ECDHE-RSA-AES
> 256-GCM-SHA384:ECDHE-RSA-AES128-GCM-SHA256:DHE-RSA-AES256-SHA256:DHE-R
> SA-AES128-SHA256:ECDHE-RSA-AES256-SHA384:ECDHE-RSA-AES128-SHA256:ECDHE
> -RSA-AES256-SHA:ECDHE-RSA-AES128-SHA:DHE-RSA-AES256-SHA:DHE-RSA-AES128
> -SHA
Explicit lists like this are still a bad idea. With the noop TLS 1.3 names
removed, the above is a proper subset of the more sensible:
DEFAULT:!EXPORT:!LOW:!MEDIUM:!kECDH:!kDH:!aDSS:!kRSA
(for comparison also add the optional exclusions !SRP
and !PSK, as they're not enabled without explicit
additional configuration and callbacks in the application)
which for no good reason leaves out the ECDSA ciphers (and there are some
systems with just ECDSA certs):
ECDHE-ECDSA-AES128-GCM-SHA256
ECDHE-ECDSA-AES128-SHA
ECDHE-ECDSA-AES128-SHA256
ECDHE-ECDSA-AES256-GCM-SHA384
ECDHE-ECDSA-AES256-SHA
ECDHE-ECDSA-AES256-SHA384
ECDHE-ECDSA-CHACHA20-POLY1305
and all TLSv1.2 CHACHA ciphers:
ECDHE-ECDSA-CHACHA20-POLY1305
DHE-RSA-CHACHA20-POLY1305
ECDHE-RSA-CHACHA20-POLY1305
Your list also leaves out all RSA key exchange ciphers, which is still IMHO
premature for SMTP. You'll not be able to complete a handshake with systems
that don't do server-side DHE/ECDHE forward-secrecy. It is somewhat safer
to disable these just in your SMTP acceptor, but not such a good idea to do
in the SMTP transport. Therefore I'd like to suggest
instead:
DEFAULT:!EXPORT:!LOW:!MEDIUM:!kECDH:!kDH:!aDSS
>From which you can subtract anything any stray code points you're sure you
don't like. Avoid cipher whitelists, use blacklists instead.
--
Viktor.
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