signing-milter enables you to s/mime sign an ordinary mail while passing a MTA. It is written in C and tested with the postfix MTA. It should work with any MTA implementing the milter protocol.

This repository replaces the daemontools service manager used in the original release by the Debian standard systemd service.

Install the required dependencies (if not already done):

apt-get install libcdb1 libmilter1.0.1 tinycdb

Download the current release (no Debian repository at the time of writing) for your Debian version and install the .deb file:

wget https://github.com/smeinecke/signing-milter/releases/latest/download/signing-milter_20220416-bullseye_amd64.deb
dpkg -i signing-milter_20220416-bullseye_amd64.deb

In default configuration the postfix daemon is chrooted to the spool folder located in /var/spool/postfix/. To use the socket feature of signing-milter the socket + permissions has to be configured in the /etc/default/signing-milter file:

#DISABLE_HOURLY_STATISTIK_LOGGING='yes'
#DISABLE_DAILY_STATISTIK_LOGGING='yes'
OPTIONS="-s unix:/var/spool/postfix/signing-milter/signing-milter.sock -c postfix"

Also create the folder within the postfix spool folder with the correct permissions:

mkdir -m o-rwx /var/spool/postfix/signing-milter
chown signing-milter:postfix /var/spool/postfix/signing-milter

The socket has to be configured in postfix in main.cf as new milter:

smtpd_milters = unix:signing-milter/signing-milter.sock

And reload/restart the services:

systemctl restart signing-milter
systemctl reload postfix

All certificates are configured in the /etc/signing-milter/sigingtable file.

Just add the email address + path of the pem file and use

make

to update the cdb database and trigger reloading signing-milter.

The certificates have to be readable by the signing-milter user.

Since version 20120731 signing-milter also supports intermediate certificates.

Just name your certificate with the suffix -cert+key.pem and put the intermediate + root certificate in a file suffixed by -chain.pem in the same folder.

The project is based on signing-milter.org by Andreas Schulze.