Mailing List CGatePro@mail.stalker.com Message #105816
From: James Roman <james.roman@ssaihq.com>
Subject: Re: Error: none of client TLS cipher methods is supported
Date: Mon, 14 Sep 2015 15:18:09 -0400
To: CommuniGate Pro Discussions <CGatePro@mail.stalker.com>
X-Mailer: Apple Mail (2.2104)
I’m guessing here, but we saw some similar problems with email from some government systems. The RFC for TLS 1.0 doesn’t list AES256_SHA as a valid cipher (most likely because it had not been formalized when the RFC was published) for TLS 1.0. Several supported Linux versions shipped with OpenSSL library versions that only support up to TLS 1.0, like RedHat/Centos 5. There are also supported SUSE and Ubuntu LTS versions in the same boat. Government agencies were given a directive that they had to drop support for ciphers weaker than AES_128. We’ve seen several government agencies recompiling OpenSSL libraries for older systems to shoehorn CBC Ciphers that are only native to TLS 1.1 and 1.2 into TLS 1.0 libraries. So those systems announce TLS 1.0, but offer a limited set of ciphers that are only supported in TLS 1.1 and 1.2 only. I would not be surprised to find other organizations latching on to this hack. 

In the cases we’ve dealt with, normally these are secondary systems that may still send mail notification for applications, etc. If there were two systems, one has been upgraded to a newer OS that support TLS 1.1 and 1.2, but the second system yet to be upgraded.


On Sep 12, 2015, at 7:00 AM, CommuniGate Pro Discussions <CGatePro@mail.stalker.com> wrote:

From: Tom Rymes <trymes@rymes.com>
Subject: Re: Error: none of client TLS cipher methods is supported
Date: September 11, 2015 at 1:15:30 PM EDT


On 09/11/2015 1:07 PM, Tom Rymes wrote:
I was unable to receive an e-mail from a host today due to the following
error:

failed to accept a secure connection for DOMAIN(rymes.com). Error
Code=none of client TLS cipher methods is supported

I enabled "Weak Ciphers" and "CBC Ciphers for old TLS" in
Settings:General:Other, and that seems to have worked, implying that the
sender has an outdated installation, I presume?

I have a few questions:

1.) What are the currently recommended settings for TLS Sessions?
2.) What are my options for this sender? I believe that I can force SMTP
to not advertise TLS for certain hosts?
3.) Where can I determine what ciphers are being used to provide
evidence to the sender that they need to fix their end?

I'm replying to my own e-mail here with more information and more confusion. I found this in the documentation at https://www.communigate.com/communigatepro/PKI.html:

"CBC Ciphers for old TLS
   Select this setting if you want to support CBC-based cipher methods for SSL 3.0 and TLS 1.0 protocols. The CBC-based cipher methods are always supported for datagram (DTLS) protocols.
Weak Ciphers
   Select this setting if you want to support weak (less than 128-bit) security (cipher methods). The CBC Ciphers setting should be selected, too."

I then checked the logs, and it looks like the sender is using AES256_SHA after I enabled both of the above settings, which makes no sense to me, as it is neither CBC-based nor less than 128-bit, making me wonder why enabling those two options made a difference.

"TLS-095527(AES256_SHA) connection accepted for DOMAIN(rymes.com)"

Hopefully someone can enlighten me.

Thank you,

Tom

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