Signing outgoing email with S/MIME

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

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.

Optionally, you can also provide a bundle of CA certs (PEM-encoded) to be included on each signature. This is typically an intermediate CA.

WARNING: Be mindful of the access levels for your private keys and visibility to third parties.

For Linux package 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'
   # Optional
   gitlab_rails['gitlab_email_smime_ca_certs_file'] = '/etc/gitlab/ssl/gitlab_smime_cas.crt'

2. Save the file and reconfigure GitLab for the changes to take effect.

The key must be readable by the GitLab system user (git by default).

For self-compiled installations:

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
     # S/MIME extra CA public certificates in PEM format, will be attached to signed messages
     # Optional
     ca_certs_file: /etc/pki/smime/certs/gitlab_cas.crt

2. Save the file and restart GitLab for the changes to take effect.

The key must be readable by the GitLab system user (git by default).

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

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

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

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