What is S/MIME?

Per Microsoft, "S/MIME (Secure/Multipurpose internet Mail Extensions) is a widely accepted protocol for sending digitally signed and encrypted messages."

As you can tell from the definition above, the 2 main ingredients for S/MIME are:

  1. Digital Signatures
  2. Encryption

Using Public Key Cryptography, S/MIME guarantees data integrity and nonrepudiation. 

  • If Bob wishes to use S/MIME, then he'll need a digital certificate. This digital certificate will contain his public key. 
  • With this digital certificate, Bob can "sign" the email message with his private key. 
  • Mary can then decrypt Bob's message with Bob's public key. 
  • Mary will do the same (send her certificate to Bob) when she replies to his email, and Bob complete the same process on his end.
  • Both will now have each other's certificates for future correspondence.

 

The illustration below will help you understand how public key cryptography works. 

A diagram visualizing encryption workflow described above | 1000

Refer to this Microsoft documentation here for more information on S/MIME and steps on how to configure Office 365 to send/receive S/MIME emails.