Fossa Team has released a beta version of Fossa Guard an extension for Chrome which enables S/MIME mailing on top of Gmail.
Fossa Guard has autonomous Compose / View dialog to avoid plain content auto-saving and provides mail signing, encryption functionality based on X.509 certificates issued by Fossa.Me service.
Attachments are supported as well though limited by 100KB each.
Certificates are associated with Google Chrome accounts and are free for public usage for 3 months (beta limitation).
Fossa Guard is supplied with a certificate enrollment (via CSR / PKCS#10) wizard and third-party trusted / endpoint certificates import / export in DER/PEM formats.
Private key resides in protected by a passphrase PKCS#12 archive on Chrome sync storage alongside with other certificates.
It guarantees synchronization between multiple user's computers.
Check out YouTube how-to videos