Hello,
the crypto weakness of the month is named "logjam".
If you could connect to https://dhe512.zmap.io your SSL-Client /
Browser support weak crypto.
What does that mean for postfix?
We setup a postfix smtp server with
smtpd_tls_dh1024_param_file = /path/to/dh_512.pem
smtpd_tls_exclude_ciphers = ECDH
smtpd_tls_ciphers = high
smtpd_tls_protocols = TLSv1.2
and connect to that server
posttls-finger -g high -c -p TLSv1.2 $testserver
There is no warning about the weak DH key used by the server nor is
the connection rejected.
Next we replaced the RSA Key + certificate. We created a weak 512 bit
RSA key together
with a selfsigned certificate and disabled DH at all.
smtpd_tls_exclude_ciphers = DH,ECDH
Also in this case we could use ciphers up to
TLS_RSA_WITH_AES_256_GCM_SHA384 / AES256-GCM-SHA384
But also no warning about the very small RSA key.
Is it possible for the postfix smtp client to gather information about
the currently used key length
or is this hidden by the ssl library at all?
Andreas
