S/MIME

S/MIME (Secure/Multipurpose Internet Mail Extensions) is a widely accepted protocol for sending digitally signed or encrypted messages. It is built into most modern mail software and interoperates between implementations, which makes it one of the most common ways to send secure email.

S/MIME relies on public-key cryptography. Each user has a pair of keys: a public key, distributed freely and used by senders to encrypt the message, and a private key, kept secret by the user and used to decrypt received messages. To use S/MIME, users must obtain a digital certificate from a trusted certificate authority (CA). The certificate carries the user’s public key and identifies the user. The private key is kept securely by the user and is never shared.

Stalwart supports S/MIME and can encrypt messages using either AES-256-CBC or AES-128-CBC symmetric encryption. AES-256-CBC provides a higher security margin at the cost of more computational work per message; AES-128-CBC uses fewer resources while still providing a strong level of security.

Obtaining a key pair

S/MIME key pairs are typically obtained by requesting a certificate from a trusted Certificate Authority (CA). Certificates are usually distributed as a PKCS #12 file (also known as a PFX file), which contains the private key paired with the public key (the S/MIME certificate). These files are password-protected so the private key remains secure.

Once the PFX file is available, it should be imported into the user’s mail client. Each mail client has a slightly different import procedure, so the client-specific instructions should be followed. Once the S/MIME certificate is installed, the public certificate can be exported (without the private key) and shared with correspondents so they can send encrypted mail.

The private key from the S/MIME certificate must always remain private and secure. Any party that gains access to the private key can impersonate the account and decrypt its messages.

Exporting certificates

Using a mail client

S/MIME public certificates, which correspondents need to send encrypted mail, can be exported from most mail clients. The exact steps vary; refer to the client’s documentation. From the client’s settings, the installed certificates can usually be viewed and exported to a file.

Using macOS

If the mail client does not offer a convenient export path, macOS users can use Keychain Access, the system’s password- and credential-management tool. Keychain Access stores certificates and keys alongside passwords; the S/MIME certificate can be located in the list of keys and exported directly.

Using OpenSSL

OpenSSL can also export the public certificate from a PFX file. This approach requires familiarity with the command line. A typical command is:

$ openssl pkcs12 -in path_to_pfx_file -clcerts -nokeys -out public_cert.pem

For legacy PKCS#12 files that fail to load, the -legacy flag is available:

$ openssl pkcs12 -legacy -in path_to_pfx_file -clcerts -nokeys -out public_cert.pem

This produces a PEM-formatted public certificate at public_cert.pem. Replace path_to_pfx_file and public_cert.pem with the actual paths and filenames.

Importing certificates

A registered certificate is represented as a PublicKey object (found in the WebUI under Account › Public Keys). An account may register a single certificate or several; registering multiple certificates is useful during key rotation, allowing a switch to a new certificate while still being able to decrypt messages encrypted with the old one.

Stalwart accepts S/MIME certificates in DER and PEM formats. DER is a binary encoding; PEM is a Base64 encoding of the same data, typically wrapped between -----BEGIN CERTIFICATE----- and -----END CERTIFICATE----- lines. A single certificate can be supplied in either format; for a bundle of multiple certificates, PEM must be used.

For example:

The certificate data is carried in the key field of the PublicKey object, alongside a required description and optional expiresAt and emailAddresses fields. Registered certificates are then referenced from AccountSettings; see the management interface for how to select a certificate and enable encryption.