gmailでメールを暗号化して送付できるgmail s/mime

Gmailでs/mimeを使って暗号化してメールが送れるfirefoxのエクステンションがGmail S/MIME。署名の認証は今のところサポートしていない。

Gmail S/MIME provides Firefox users with S/MIME support for Gmail. S/MIME support is integrated directly into Google's Gmail web interface. Reading and sending of encrypted mail is supported. Verification of signatures is not currently supported. There are some interoperability and performance issues which I plan to iron out in later releases.

http://richard.jones.name/google-hacks/gmail-smime/gmail-smime.html