Re: Disable weak TLS
- From: per@xxxxxxxxxxxx (Per Hedeland)
- Date: Fri, 28 Sep 2007 22:50:34 +0000 (UTC)
In article <1i54nze.1qsyacc7a4xusN%hugo@xxxxxxxxx> hugo@xxxxxxxxx (Hugo
Villeneuve) writes:
<decourl@xxxxxxxxx> wrote:
Hi,
I have a server which advertises STARTTLS and accepts TLS if the
client wishes or accepts in-the-clear otherwise. I need to be able to
disallow negotiation of weak (56 bit or below) encryption without
enforcing a TLS requirement (in-the-clear should still be fine).
Appreciate any info as the FAQ recommends this group for advanced
TLS info.
I'd go with Per, you have a strange requirement.
By default, most use the best encryption available. Does your log really
list 56 bit encryption entries?
If a client can only use 56 bit encryption, I doubt he will fail back
nicely to no starttls if you disable low grade encryption. You'll
probably have to add "Srv_Features:computername S" entries to the access
file to disable STARTTLS for certain clients.
Looking at the code, if you recompile sendmail with -D_FFR_TLS_1, you
will be able to add an "O CipherList=HIGH" options to your sendmail.cf
file to select the encryptions methods availble to the TLS engine. (See
the openssl ciphers manual page for how to write the CipherList=
string.)
Well, while we're giving out solutions without understanding the
problem:-), it can also be worth pointing to the STARTTLS section in
cf/README, where you at least in principle can figure out that you can
suppply an extension to the tls_client ruleset via LOCAL_TLS_CLIENT, and
therein require that $&{cipher_bits} is 0 or unset or > 56. This could
at least be used to give a meningful error message, that just might be
displayed by the MUA - you're very unlikely to get that if the SSL
handshake fails due to disjunct cipher lists.
I have not tested this. FFR means For Future Release. Or experimental.
Or "you're on your own".
I haven't looked at that code (nor tried it), but this should be safer
than most FFRs, since it's very trivial - sendmail just needs to pass
the CipherList string as-is to OpenSSL. (Though admittedly there are
many mistakes that can be made in OpenSSL usage.:-)
This page explains it with siteconfig and m4 cf exemples (the proper way
to do things):
http://sial.org/howto/sendmail/cipherlist/
Hm, I would hardly call RC4-MD5 "weak" though, especially not in this
context. Connect to your bank with your DES3- and AES-capable browser
and check what you get...:-)
--Per Hedeland
per@xxxxxxxxxxxx
.
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- From: decourl
- Re: Disable weak TLS
- From: Hugo Villeneuve
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