On 10/11/2019 2:55 AM, Jeremy Harris via Exim-users wrote:
The openssl_options are fed to the SSL_CTX_set_options() interface
(via some fairly-obvious processing). The tls_require_ciphers is
fed to SSL_CTX_set_cipher_list().
http://exim.org/exim-html-current/doc/html/spec_html/ch-encrypted_smtp_connections_using_tlsssl.html#SECTreqciphssl
talks about order of the list of ciphers, which to me implies that
the library uses that order as a preference.
--
Cheers,
Jeremy
@Jeremy:
Apparently IBM agrees with your interpretation:
https://www.ibm.com/support/knowledgecenter/en/SSB23S_1.1.0.15/gtpc2/cpp_ssl_ctx_set_cipher_list.html
Note what IBM support says: "You must specify the ciphers in order of
preference from highest to lowest."
This thread has given me a much deeper understanding of how to manage
cipher negotiation in exim. As a result of this thread I have adopted
Viktor's setting for tls_require_ciphers. (Thanks Viktor)
Following is the cipher list result I see on CentOS 7.7.1908
with openssl 1:1.0.2k-19.el7:
[root@localhost ~]#openssl ciphers
'DEFAULT:!EXPORT:!LOW:!MEDIUM:!kECDH:!kDH:!aDSS:!PSK'|tr : '\n'
ECDHE-RSA-AES256-GCM-SHA384
ECDHE-ECDSA-AES256-GCM-SHA384
ECDHE-RSA-AES256-SHA384
ECDHE-ECDSA-AES256-SHA384
ECDHE-RSA-AES256-SHA
ECDHE-ECDSA-AES256-SHA
DHE-RSA-AES256-GCM-SHA384
DHE-RSA-AES256-SHA256
DHE-RSA-AES256-SHA
DHE-RSA-CAMELLIA256-SHA
AES256-GCM-SHA384
AES256-SHA256
AES256-SHA
CAMELLIA256-SHA
ECDHE-RSA-AES128-GCM-SHA256
ECDHE-ECDSA-AES128-GCM-SHA256
ECDHE-RSA-AES128-SHA256
ECDHE-ECDSA-AES128-SHA256
ECDHE-RSA-AES128-SHA
ECDHE-ECDSA-AES128-SHA
DHE-RSA-AES128-GCM-SHA256
DHE-RSA-AES128-SHA256
DHE-RSA-AES128-SHA
DHE-RSA-CAMELLIA128-SHA
AES128-GCM-SHA256
AES128-SHA256
AES128-SHA
CAMELLIA128-SHA
My previous setting (last visited about 4 years ago) resulted in a list
more than double the length of this, with some ciphers considered very
weak included. Although, TLS connections (both directions) typically
result in a TLS1.2 connection using one of the top ciphers in the list.
I also tried adding '@STRENGTH' to the setting but found it produced the
exact same order. Does exim add that, or does openssl automatically sort
by strength?
Phil Carroll
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