Chap 8 1

Electronic mail security

Chap 8 2

Outline

Pretty good privacy S/MIME Recommended web sites

Chap 8 3

Email Security Email Security EnhancementsEnhancements

• confidentiality– protection from disclosure

• authentication– of sender of message

• message integrity– protection from modification

• non-repudiation of origin– protection from denial by sender

Chap 8 4

Pretty Good Privacy Pretty Good Privacy (PGP)(PGP)

• widely used de facto secure email• developed by Phil Zimmermann• selected best available crypto algs to

use• integrated into a single program• available on Unix, PC, Macintosh and

Amiga systems • originally free, now have commercial

versions available also

Chap 8 5

Why Is PGP Popular?

It is availiable free on a variety of platforms. Based on well known algorithms. Wide range of applicability Not developed or controlled by governmental or standards

organizations

Chap 8 6

Operational Description

Consist of five services: Authentication Confidentiality Compression E-mail compatibility Segmentation

Chap 8 7

Chap 8 8

PGP Operation – PGP Operation – AuthenticationAuthentication

1. sender creates a message2. SHA-1 used to generate 160-bit hash code

of message3. hash code is encrypted with RSA using

the sender's private key, and result is attached to message

4. receiver uses RSA or DSS with sender's public key to decrypt and recover hash code

5. receiver generates new hash code for message and compares with decrypted hash code, if match, message is accepted as authentic

Chap 8 9

PGP Operation – PGP Operation – ConfidentialityConfidentiality

1. sender generates message and random 128-bit number to be used as session key for this message only

2. message is encrypted, using CAST-128 / IDEA/3DES with session key

3. session key is encrypted using RSA with recipient's public key, then attached to message

4. receiver uses RSA with its private key to decrypt and recover session key

5. session key is used to decrypt message

Chap 8 10

PGP Operation – PGP Operation – Confidentiality & Confidentiality & Authentication Authentication

• uses both services on same message– create signature & attach to message– encrypt both message & signature– attach RSA encrypted session key

Chap 8 11

PGP Operation – PGP Operation – CompressionCompression

• by default PGP compresses message after signing but before encrypting– so can store uncompressed message &

signature for later verification– & because compression is non

deterministic

• uses ZIP compression algorithm

Chap 8 12

PGP Operation – Email PGP Operation – Email CompatibilityCompatibility

• when using PGP will have binary data to send (encrypted message etc)

• however email was designed only for text

• hence PGP must encode raw binary data into printable ASCII characters

• uses radix-64 algorithm– maps 3 bytes to 4 printable chars– also appends a CRC

• PGP also segments messages if too big

Chap 8 13

E-mail Compatibility

The scheme used is radix-64 conversion (see appendix 5B). The use of radix-64 expands the message by 33%.

24* 4/3=32

Chap 8 14

Segmentation and Reassembly

Often restricted to a maximum message length of 50,000 octets.

Longer messages must be broken up into segments. PGP automatically subdivides a message that is to large. The receiver strip of all e-mail headers and reassemble the

block.

Chap 8 15

Summary of PGP Services

Function Algorithm Used Digital Signature DSS/ SHA or

RSA/ SHA Message Encryption

CAST or I DEA or three-key triple DES with Diffi e-Hellman or RSA

Compression ZI P E-mail Compatibility

Radix-64 conversion

Segmentation -

Chap 8 16

PGP Session KeysPGP Session Keys

• need a session key for each message– of varying sizes: 56-bit DES, 128-bit

CAST or IDEA, 168-bit Triple-DES

• generated using ANSI X12.17 mode• uses random inputs taken from

previous uses and from keystroke timing of user

Chap 8 17

PGP Public & Private PGP Public & Private KeysKeys

• since many public/private keys may be in use, need to identify which is actually used to encrypt session key in a message– could send full public-key with every

message– but this is inefficient

• rather use a key identifier based on key– is least significant 64-bits of the key– will very likely be unique

• also use key ID in signatures

Chap 8 18

PGP Key RingsPGP Key Rings

• each PGP user has a pair of keyrings:– public-key ring contains all the public-

keys of other PGP users known to this user, indexed by key ID

– private-key ring contains the public/private key pair(s) for this user, indexed by key ID & encrypted keyed from a hashed passphrase

Chap 8 19

PGP Key ManagementPGP Key Management

• rather than relying on certificate authorities

• in PGP every user is own CA– can sign keys for users they know

directly• forms a “web of trust”

– trust keys have signed– can trust keys others have signed if

have a chain of signatures to them• key ring includes trust indicators• users can also revoke their keys

Chap 8 20

Chap 8 21

Format of PGP Message

Chap 8 22

Chap 8 23

Chap 8 24

Chap 8 25

The Use of Trust

One Key legitimacy field: (the extent the PGP will trust this public key; computed by PGP)

One or more Signature trust field: (lists of signatures on this certificates; associated with each signature is assigned a signature bit;

; sig trust bit 1 + sig trust bit 2 + .. key legitmacy bit) Owner trust field : (the degree to which this public key is trusted to

sign other certificates )

Associated with each public key in the Key ring

Chap 8 26

Chap 8 27

Chap 8 28

Revoking Public Keys

The owner issue a key revocation certificate. Normal signature certificate with a revote indicator. Corresponding private key is used to sign the certificate.

Chap 8 29

S/MIME S/MIME (Secure/Multipurpose (Secure/Multipurpose

Internet Mail Extensions)Internet Mail Extensions)• security enhancement to MIME email

– original Internet RFC822 email was text only

– MIME provided support for varying content types and multi-part messages

– with encoding of binary data to textual form

– S/MIME added security enhancements

• have S/MIME support in various modern mail agents: MS Outlook, Netscape etc

Chap 8 30

RFC 822

From: "Microsoft Outlook Express Team" <oe5@microsoft.com>

To: =?big5?B?t3MgT3V0bG9vayBFeHByZXNzIKjPpc6qzA==?=

Subject: =?big5?B?xXeq76jPpc4gT3V0bG9vayBFeHByZXNzIDU=?=

Date: Mon, 29 Sep 2003 13:02:38 +0800

Hello This is a test

Chap 8 31

Simple Mail Transfer Protocol (SMTP, RFC 822)

SMTP Limitations - Can not transmit, or has a problem with:

executable files, or other binary files (jpeg image)

“national language” characters (non-ASCII)

messages over a certain size

SMTP gateway that translates ASCII to EBCDIC do not use a consistent set of mapping

SMTP gateway to X.400 cannot handle nontextual data in X.400

Truncating lines longer than a certain length (72 to 254 characters)

Chap 8 32

Header fields in MIME

MIME-Version: Must be “1.0” -> RFC 2045, RFC 2046

Content-Type: More types being added by developers (application/word)

Content-Transfer-Encoding: How message has been encoded (radix-64)

Content-ID: Unique identifying character string. (Optional)

Content Description: Needed when content is not readable text (e.g.,mpeg) (Optional)

Chap 8 33

Chap 8 34

Chap 8 35

MIME Example (1)

From: "Microsoft Outlook Express Team" <oe5@microsoft.com>To: =?big5?B?t3MgT3V0bG9vayBFeHByZXNzIKjPpc6qzA==?=Subject: =?big5?B?xXeq76jPpc4gT3V0bG9vayBFeHByZXNzIDU=?=Date: Mon, 29 Sep 2003 13:02:38 +0800MIME-Version: 1.0Content-Type: text/html;

charset="big5"Content-Transfer-Encoding: quoted-printableX-MimeOLE: Produced By Microsoft MimeOLE V5.50.4927.1200

<HTML><HEAD><META HTTP-EQUIV=3D"Content-Type" CONTENT=3D"text/html; charset=3Dbig5"><STYLE>font{font-family:"MingLiu";font-size:9pt;color:#000000}A:hover.defaultA{color:#999900}A:hover.bodyTopLine{color:#0000FF}A:hover {color:#0033FF}:

Chap 8 36

MIME Example (2)

From: "=?big5?B?uOqwVL7Hqvmiug==?=" <ccchiu@mail.ncku.edu.tw>To: <Undisclosed-Recipient:;>Subject: =?big5?B?pU7C4Lx4qESudrjqOiCw6qXfquqnwKzsp96kar7HuOqkdah0pM6tuL73qHS42w=Date: Mon, 1 Mar 2004 11:49:12 +0800MIME-Version: 1.0Content-Type: multipart/mixed;

boundary="----=_NextPart_000_0010_01C3FF83.3B89A160"X-Priority: 3X-MSMail-Priority: NormalX-Mailer: Microsoft Outlook Express 6.00.2800.1158X-MimeOLE: Produced By Microsoft MimeOLE V6.00.2800.1165Return-Path: ccchiu@mail.ncku.edu.twX-OriginalArrivalTime: 01 Mar 2004 03:58:00.0296 (UTC) FILETIME=[63733E80:01C3FF41]

This is a multi-part message in MIME format.

------=_NextPart_000_0010_01C3FF83.3B89A160Content-Type: multipart/alternative;

boundary="----=_NextPart_001_0011_01C3FF83.3B8B2800":

Chap 8 37

MIME Example (2)

------=_NextPart_001_0011_01C3FF83.3B8B2800Content-Type: text/plain;

charset="big5"Content-Transfer-Encoding: quoted-printable

=B3o=ACO=B8=EA=B0T=BE=C7=AA=F9=AA=BA=A5N=C2=E0=B0T=AE=A7=AAA=B0=C8=A1A=AD=Y=B1z=A4=A3=C4@=A6=AC=A8=EC =":

=B0=EA=A5=DF=AA=EA=A7=C0=AC=EC=A7=DE=A4j=BE=C7=B8=EA=A4u=A8t=A4=CE=AD=B8=BE==F7=A8t=B8=DB=BCx=B1=D0=AEv=A1A=B8=D4=A8=A3=AA=FE=A5=F3=A1C------=_NextPart_001_0011_01C3FF83.3B8B2800Content-Type: text/html;

charset="big5"Content-Transfer-Encoding: quoted-printable

<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.0 Transitional//EN"><HTML xmlns=3D"http://www.w3.org/TR/REC-html40" xmlns:o =3D=20"urn:schemas-microsoft-com:office:office" xmlns:w =3D=20

Chap 8 38

S/MIME FunctionsS/MIME Functions• enveloped data

– encrypted content and associated keys

• signed data– (message + signed digest) base64

encoding

• clear-signed data– cleartext message + encoded signed

digest

• signed & enveloped data– nesting of signed & encrypted entities

Chap 8 39

S/MIME Cryptographic S/MIME Cryptographic AlgorithmsAlgorithms

• hash functions: SHA-1 & MD5• digital signatures: DSS & RSA• session key encryption: ElGamal &

RSA• message encryption: Triple-DES,

RC2/40 and others• have a procedure to decide which

algorithms to use

Chap 8 40

S/MIME Certificate S/MIME Certificate ProcessingProcessing

• S/MIME uses X.509 v3 certificates• managed using a hybrid of a strict

X.509 CA hierarchy & PGP’s web of trust

• each client has a list of trusted CA’s certs

• and own public/private key pairs & certs

• certificates must be signed by trusted CA’s

Chap 8 41

Certificate AuthoritiesCertificate Authorities

• have several well-known CA’s• Verisign one of most widely used• Verisign issues several types of Digital IDs• with increasing levels of checks & hence

trustClass Identity Checks Usage1 name/email check web

browsing/email2+ enroll/addr check email, subs, s/w

validate3+ ID documents e-banking/service

access

Chap 8 42

Chap 8 43

User Agent Role

S/MIME uses Public-Key Certificates - X.509 version 3 signed by Certification Authority

Functions: Key Generation - Diffie-Hellman, DSS, and RSA key-pairs.

Registration - Public keys must be registered with X.509 CA.

Certificate Storage - Local (as in browser application) for different services.

Signed and Enveloped Data - Various orderings for encrypting and signing.

Chap 8 44

User Agent Role

Example: Verisign (www.verisign.com)

Class-1: Buyer’s email address confirmed by emailing vital info.

Class-2: Postal address is confirmed as well, and data checked against directories.

Class-3: Buyer must appear in person, or send notarized documents.

Chap 8 45