Other Applications of SSL - Implementing SSL/TLS Using Cryptography and PKI - page 547

other's messages synchronously throughout the whole exchange (such as is
the case with HTTP). However, S/MIME is closely related and has all the
same pieces.
An email message, at the protocol level, looks quite a bit like an HTTP mes-
sage. It starts with a set of name-value pair headers, delimited by a single CRLF
pair, after which the message itself starts. In fact, email and HTTP share a lot
of the same name-value pair headers — email messages can have Content-
Type , Content-Transfer-Encoding , and so on. A simple email message may
look like this:
Received: from smtp.receiver.com ([192.168.1.1]) by smtp.sender.com
with Microsoft SMTPSVC(6.0.3790.3959); Fri, 13 Aug 2010 04:58:25 -0500
Subject: I'm sending an email, do you like it?
Date: Fri, 13 Aug 2010 04:58:31 -0500
Message-ID: <[e-mail]>
From: “Davies, Joshua” <[e-mail]>
To: “Reader, Avid” <[e-mail]>
Hi there, I'm sending you an email. What do you think of it?
This is a pretty simple (not to mention mundane) plaintext email. The Received
header indicates the path that the email took from sender to receiver; you can have
several such Received lines if the email was transferred over multiple relays, as
most are. The Subject is what appears in the summary area, and the remaining
headers ought to be fairly self-explanatory. The actual body of the email in this
example is the single line of text after the blank line.
Multipurpose Internet Mail Extensions (MIME)
All modern email readers, for better or for worse, support the Multipurpose
Internet Mail Extensions ( MIME ) that allow the sender of the email to declare
what are usually referred to by email reader software as attachments and which
are offered as independent downloads to the message.
An email with attachments, at the wire-level, looks like this:
Received: from smtp.receiver.com ([192.168.1.1]) by smtp.sender.com
with Microsoft SMTPSVC(6.0.3790.3959); Fri, 2 Jul 2010 08:44:43 -0500
MIME-Version: 1.0
Content-Type: multipart/mixed;
boundary=”----_=_NextPart_001_01CB19EC.B9C71780”
Subject: This email contains an attachment
Date: Fri, 2 Jul 2010 08:44:23 -0500
Message-ID: <[e-mail]>
From: “Davies, Joshua” <[e-mail]>
To: “Reader, Avid” <[e-mail]>
This is a multi-part message in MIME format.