Signing outgoing email with S/MIME

Notification emails sent by GitLab can be signed with S/MIME for improved security.

Note: Please be aware that S/MIME certificates and TLS/SSL certificates are not the same and are used for different purposes: TLS creates a secure channel, whereas S/MIME signs and/or encrypts the message itself

Enable S/MIME signing

This setting must be explicitly enabled and a single pair of key and certificate files must be provided:

  • Both files must be PEM-encoded.
  • The key file must be unencrypted so that GitLab can read it without user intervention.
  • Only RSA keys are supported.
Note: Be mindful of the access levels for your private keys and visibility to third parties.

For Omnibus installations:

  1. Edit /etc/gitlab/gitlab.rb and adapt the file paths:

    gitlab_rails['gitlab_email_smime_enabled'] = true
    gitlab_rails['gitlab_email_smime_key_file'] = '/etc/gitlab/ssl/gitlab_smime.key'
    gitlab_rails['gitlab_email_smime_cert_file'] = '/etc/gitlab/ssl/gitlab_smime.crt'
    
  2. Save the file and reconfigure GitLab for the changes to take effect.

Note: The key needs to be readable by the GitLab system user (git by default).

For installations from source:

  1. Edit config/gitlab.yml:

    email_smime:
      # Uncomment and set to true if you need to enable email S/MIME signing (default: false)
      enabled: true
      # S/MIME private key file in PEM format, unencrypted
      # Default is '.gitlab_smime_key' relative to Rails.root (i.e. root of the GitLab app).
      key_file: /etc/pki/smime/private/gitlab.key
      # S/MIME public certificate key in PEM format, will be attached to signed messages
      # Default is '.gitlab_smime_cert' relative to Rails.root (i.e. root of the GitLab app).
      cert_file: /etc/pki/smime/certs/gitlab.crt
    
  2. Save the file and restart GitLab for the changes to take effect.

Note: The key needs to be readable by the GitLab system user (git by default).

How to convert S/MIME PKCS#12 / PFX format to PEM encoding

Typically S/MIME certificates are handled in binary PKCS#12 format (.pfx or .p12 extensions), which contain the following in a single encrypted file:

  • Public certificate
  • Intermediate certificates (if any)
  • Private key

In order to export the required files in PEM encoding from the PKCS#12 file, the openssl command can be used:

#-- Extract private key in PEM encoding (no password, unencrypted)
$ openssl pkcs12 -in gitlab.p12 -nocerts -nodes -out gitlab.key

#-- Extract certificates in PEM encoding (full certs chain including CA)
$ openssl pkcs12 -in gitlab.p12 -nokeys -out gitlab.crt