Re: [exim] Define preferred encryption algorithms

トップ ページ
このメッセージを削除
このメッセージに返信
著者: jmedard
日付:  
To: exim-users
題目: Re: [exim] Define preferred encryption algorithms
Sorry, i don't understand why you prefere blacklist to whitelist !



-----Message d'origine-----
De : Exim-users <exim-users-bounces+jmedard=amv-sa.fr@???> De la part
de Viktor Dukhovni via Exim-users
Envoyé : samedi 12 octobre 2019 19:49
À : exim-users@???
Cc : Viktor Dukhovni <exim-users@???>
Objet : Re: [exim] Define preferred encryption algorithms

> On Oct 12, 2019, at 9:36 AM, Mike Tubby via Exim-users

<exim-users@???> 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.



--
## List details at https://lists.exim.org/mailman/listinfo/exim-users
## Exim details at http://www.exim.org/
## Please use the Wiki with this list - http://wiki.exim.org/