gwmilter

gwmilter is an email encryption gateway, designed to be deployed in front of an email server. A typical deployment will consist of a combination of an MTA with milter support, and gwmilter running on the same or separate nodes. It has been created circa 2014, and has been modernized and released as open-source in 2024.

It uses Milter API to integrate with MTAs, such as Postfix and sendmail. It supports multiple encryption protocols:

• PGP
• S/MIME
• PDF encryption

PGP and S/MIME support relies on a local installation of GnuPG for keys management.

NOTE: gwmilter only handles the encryption of emails. For decrypting the emails, you need to have the private keys and an email client that supports PGP or S/MIME, depending on your case.
For PDFs, you will need to know the configured password and have a PDF viewer that supports encryption.

IMPORTANT:

• for increased security, it is recommended that only the public part of the keys is stored for gwmilter to use.
• losing your private keys will result in the incapacity of decrypting your emails. Be sure to back up your keys.

Getting Started

• Build and run: see DEV_GUIDE.md.
• Integration environment (Docker Compose): see integrations/README.md.
• Automated end-to-end tests: see tests/README.md.

How to use

gwmilter uses a simple INI configuration file, comprised of a mandatory and reserved [general] section, and an unlimited number of encryption specification sections. The path to the INI file is specified with -c parameter (e.g. ./gwmilter -c config.ini).

The [general] section contains options pertaining to how gwmilter interacts with the system and the MTA.

The encryption specification sections can have arbitrary names (e.g. [pgp_example.com]), and specify to which recipients they will apply (match), an encryption protocol (encryption_protocol) and protocol specific options. A recipient is matched against each section's match setting. The first section to match a recipient dictates how the email will be processed for that recipient. If an email contains recipients that match multiple encryption sections (e.g. rcpt_one@example.com matches [pgp_example.com], rcpt_two@example.com matches [smime_example.com]), the original email is encrypted using the settings from the first matching section and the email is altered before returning it to the MTA. However, a copy of the original email is made for each subsequent matching section, and each copy is processed according to the matching section's settings. These emails are then re-injected over SMTP into the MTA. The recipients that do not match any section are dropped.

See config.ini and integrations/gwmilter/config.ini.template for more details.

Authors

• Claudiu Dragalina-Paraipan (drclau)