Email Security Protocols:

PEM & S/MIME

• PEM (Privacy Enhanced Mail):

Add encryption, authentication and integrity to ordinary text messages.

• MIME (Multipurpose Internet Mail Extensions):

Is a standard for encoding arbitrary data in email (images, video, etc.).

• S/MIME:
  Incorporated many principles of PEM  into MIME.

Structure of a PEM Message

PEM marks its pieces with a text string before and after the piece as:

-----BEGIN PRIVACY ENHANCED MESSAGE-----

......<data>....

-----END PRIVACY ENHANCED MESSAGE-----

The different types of pieces PEM can combine into a message are:

0. Ordinary, unsecured data.

1. Integrity-protected unmodified data (MIC-CLEAR).

2. Integrity-protected encoded data (MIC-ONLY).

3. Encoded encrypted integrity-protected data (ENCRYPTED).

Not only these types of data can be combined in a message, but they can be nested inside one another.

E.g., Alice might enclose MIC-CLEAR message from Fred in an ENCRYPTED message to Bob.

Example:

From: Alice
To: Bob
Subject: Colloquium
Date: Mon Oct 7, 2011

Dear Bob:
I would like to invite you to give a colloquium next Spring at ODU,
if you accept, let us talk about the details.
Alice

The above message may be sent in one following 3 forms:

1. MIC-CLEAR

From: Alice
To: Bob
Subject: Colloquium
Date: Mon Oct 7, 2011

-----BEGIN PRIVACY ENHANCED MESSAGE-----
Proc-Type: 4, MIC-CLEAR
Content-Type: RFC822
Originator-ID-Asymmetric:  <certificate ID>
MIC-Info: RSA-MD5, RSA, <encoded MIC>

Dear Bob:
I would like to invite you to give a colloquium next Spring at ODU,
if you accept, let us talk about the details.
Alice
-----END PRIVACY ENHANCED MESSAGE-----

2. MIC-ONLY

From: Alice
To: Bob
Subject: Colloquium
Date: Mon Oct 7, 2011

-----BEGIN PRIVACY ENHANCED MESSAGE-----
Proc-Type: 4, MIC-ONLY
Content-Type: RFC822
Originator-ID-Asymmetric: <certificate ID>
MIC-Info: RSA-MD5, RSA, <encoded MIC>

<encoded message>

-----END PRIVACY ENHANCED MESSAGE-----

3. ENCRYPTED

From: Alice
To: Bob
Subject: Colloquium
Date: Mon Oct 7, 2010

-----BEGIN PRIVACY ENHANCED MESSAGE-----
Proc-Type: 4, ENCRYPTED
Content-Type: RFC822
DEK-Info: DES-CBC, IV
Originator-ID-Asymmetric: <Originator certificate ID>
Key-Info: RSA, <encoded message key encrypted with originator public key>
MIC-Info: RSA-MD5, RSA, <encoded encrypted MIC>
Recipient-ID-Asymmetric: <Recipient certificate ID>
Key-Info: RSA, <encoded message key encrypted with recipient public key>

<encoded encrypted message using DES-CBC>

-----END PRIVACY ENHANCED MESSAGE-----

· Why we send the message key to originator?

For CC purposes and if message is returned to sender due to some error.

· Why MIC is encrypted?

Using the public-key of the Originator, a person can compute the message digest MD.
Then he can use the MD to check his guess  for the message e.g., attack or retreat.

· How to send an ENCRYPTED message to multiple recipients?

Encrypt the message key once for each recipient:

Recipient-ID-Asymmetric: <Recipient-1 certificate ID>
Key-Info: RSA, <encoded message key encrypted with recipient-1 public key>

Recipient-ID-Asymmetric: <Recipient-2 certificate ID>
Key-Info: RSA, <encoded message key encrypted with recipient-2 public key>
......

Recipient-ID-Asymmetric: <Recipient-n certificate ID>
Key-Info: RSA, <encoded message key encrypted with recipient-n public key>

PEM Encoding:

It is base-64 encoding, i.e., each 6 bits is encoded as 8-bit character in the set {A-Z, a-z, 0-9, +, /}

When PEM sees a line that begins with - it is replaced with "- ".

Thus the string in the text:
-----END PRIVACY ENHANCED MESSAGE-----

would appear as:
- ----END PRIVACY ENHANCED MESSAGE-----

Forwarding & Enclosure:

Only MIC-CLEAR and MIC-ONLY messages can be forwarded.
For ENCRYPTED messages, it must be decrypted and then re-encrypted.

Unprotected Information:

From: Alice
To: Bob
Subject: Colloquium
Date: Mon Oct 7, 2011

To protect the header information, it should be included in the text.

Secret Key Variant:

PEM can be used for both public-key and secret-key infrastructure.

A secret key between Alice and Bob can be established using out-of-band mechanism (e.g., phone, Kerberos).

Differences in S/MIME:

S/MIME is very similar to PEM.

One difference is:

boundery=----boundery marker

----boundery marker

...<Content>....

----boundery marker