How to read an “smime.p7m”

If you read your e-mail with a client that does not understand S/MIME encryption and someone has sent you an S/MIME encrypted message then you will see an attachment named “smime.p7m” like this one.

With Gmail you could use the “Show original” option to view the full S/MIME encoded e-mail message, cut+paste it to a text editor, save as “something.eml” and open that with Outlook Express, or similar, which understands S/MIME. Of course, you do have to have the matching private key to decrypt the message!

With other e-mail clients that do not present the option to view the unaltered e-mail, or when the “smime.p7m” attachment has been forwarded, you need a way to re-format it back into a valid S/MIME message. To do that I wrote this small program. Save the attachment to disk and feed it to “p7mfile.exe” which will format it and pass it on to Outlook Express.