Over the years, I've had several clients use S/MIME to authenticate and encrypt e-mail messages. Unfortunately, encrypting anything end-to-end has problems, including those associated with scanning incoming encrypted messages, checking for data leaks, or indexing for later retrieval. When my clients turn on S/MIME, they are pretty much turning off easy e-mail scanning and retrieval.
Two weeks ago, I asked readers and vendors to submit solutions to the S/MIME problem of anti-malware scanning, indexing, and encrypted message retrieval. You didn't let me down. In fact, I got so many responses that I felt kind of dumb. But that's what I love about the computer security world: It's too big to be an expert in everything.
I knew from the start that the solution I envisioned was some sort of e-mail gateway that could handle the user's PKI asymmetric keys, performing the desired service before encrypting or decrypting the e-mail and sending it on its way. Essentially, the pathway would look something like this:
S/MIME E-mail Client <-> Anti-malware Scanner\Archiver\Data Leak Prevention <-> Encryption Gateway <-> Internet <-> Other Recipient(s)
The key question is, how would the S/MIME client and encryption gateway interact? Would the S/MIME client even know there was a change? Would the sender be required to share their S/MIME private key with the encryption gateway? How would it handle recipients on the other side?
Martijn Brinkers, author of Djigzo open source e-mail encryption gateway, sent in his solution. Djigzo can function like a normal e-mail server (called an MTA, mail transfer agent), but it can also participate in S/MIME and PDF encryption. Although the user's existing digital certificate can be imported into Djigzo, the MTA contains its own PKI digital certificate server. You can create and send new PKI digital certificates to both internal and external users, who can then install them locally so that their e-mail clients can participate in S/MIME. Djigzo supports many clients, including Outlook, Gmail, Applemail, Thunderbird, and Lotus Notes. It has fairly good documentation with lots of step-by-step screenshots.
As expected, Djigzo will automatically decrypt incoming messages being sent to internal users. Then the unencrypted message can be scanned, archived, or checked for data leaks.