How to differentiate between a properly encrypted message – and an opaque P7M

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Issue/Introduction

This article gives you information on how an encrypted p7m message can be readable by a digital signature policy.

Cause

 

Resolution

There are two types of signed S/MIME messages, clear signed and opaque signed.

Clear signed will have a *.p7s attachment and opaque signed will have a *.p7m attachment.

See below for further details:

https://superuser.com/questions/1333248/s-mime-signed-email-loses-attachment

 

How to differentiate between a properly encrypted message – and an opaque P7M

Looking at the email header, specifically at Content-Type. According to the above link a detached signature will have:

Content-Type: multipart/signed

and

an opaque signed will have:

Content-Type: application/pkcs7-mime;
    smime-type=signed-data;