Other Applications of SSL - Implementing SSL/TLS Using Cryptography and PKI - page 549

Protecting Email from Eavesdroppers with S/MIME
There's no reason an email with an attachment must be a multi-part/mixed type.
If there's just one attachment and nothing else, the Content-Type of the email
header can perfectly and legitimately be the type of the attachment; the
email reader just shows nothing except an attachment with no accompanying
text. S/MIME takes advantage of this by creating an application/x-pkcs7-mime
MIME type. As you can likely guess, this is another ASN.1 encoded structure.
An S/MIME encoded email message looks like this:
Received: from smtp.receiver.com ([192.168.1.1]) by smtp.sender.com
with Microsoft SMTPSVC(6.0.3790.3959); Wed, 21 Apr 2010 12:42:48 -0500
MIME-Version: 1.0
Content-Transfer-Encoding: base64
Content-Disposition: attachment;
filename=”smime.p7m”
Content-class: urn:content-classes:message
Content-Type: application/x-pkcs7-mime;smime-type=enveloped-data;
name=smime.p7m;
smime-type=enveloped-data;
name=”smime.p7m”
Subject: This message will self-destruct in 15 seconds
Date: Wed, 21 Apr 2010 12:42:47 -0500
Message-ID: <[e-mail]>
From: “Davies, Joshua” <[e-mail]>
To: “Reader, Avid” <[e-mail]>
MIAGCSqGSIb3DQEHA6CAMIACAQAxggNGMIIBnwIBADCBhjB4MRMwEQYKCZImiZPyLGQBGRY
MRUwEwYKCZImiZPyLGQBGRYFc2FicmUxEjAQBgoJkiaJk/IsZAEZFgJhZDEWMBQGCgmSJom
ARkWBkdsb2JhbDEeMBwGA1UEAxMVU2FicmUgSW5jLiBJc3N1aW5nIENBAgo84HbtAAEABSy
CsqGSIb3DQEBAQUABIIBAKHiUib4D3g8bA1AyInu2CkcB75mgMI/Sb5mQjmMNPo7Q0ypV1n
Regarding this email:
Message body: This is a base64 encoded PKCS #7 envelope for which
the email reader software must have a legitimate certifi cate in order to
display. The body is simply an attachment.
Headers: These describe the attachment in enough detail for the receiving
email reader to interpret and decode it.
Attachment: This is named via the Content-Disposition header element
in the email message itself — in the case of S/MIME, the fi lename is impor-
tant. S/MIME dictates old DOS-style three-character fi le extensions that
indicate the type of the fi le. .p7m stands for “PKCS #7 Message.” ( .p7s , in
contrast, is a PKCS #7 signature fi le.) The fi lename itself is usually smime .
PKCS #7 is slightly more complicated to parse than the X.509 certifi cates
examined in Chapter 5 because PKCS #7 allows indefi nite-length encodings. In
other words, it follows the Canonical Encoding Rules (CER) rather than the DER