Cryptographic Systems:
SSL/TLS, VPNs, and Kerberos
Chapter 8
Copyright 2003 Prentice-Hall
1
Figure 8-1: Cryptographic System
Phase 1:
Initial Negotiation
of Security Parameters
Client PC
Phase 2:
Mutual Authentication
Server
2
Figure 8-1: Cryptographic System
Phase 3:
Key Exchange or
Key Agreement
Client PC
Server
Phase 4:
Ongoing Communication with
Message-by-Message
Confidentiality, Authentication,
and Message Integrity
3
Figure 8-2: Major Cryptographic
Systems
Layer
Cryptographic System
Application
Kerberos
Transport
SSL/TLS
Internet
IPsec
Data Link
PPTP, L2TP (really only a tunneling system)
Physical
Not applicable. No messages are sent at this
layer—only individual bits
4
Figure 8-3: Virtual Private Network (VPN)
Site-to-Site
VPN
Protected VPN
Server Server
Internet
Corporate
Site A
Remote
Customer or
Supplier PC
VPN Protected
Server Server
Corporate
Site B
Remote
Access
VPN
Remote
Access
VPN
Remote
Corporate PC
5
Figure 8-4: SSL/TLS Operation
Applicant
(Customer Client)
Verifier
(Merchant Server)
Protects All Application Traffic
That is SSL/TLS-Aware
SSL/TLS Works at Transport Layer
6
Figure 8-4: SSL/TLS Operation
Applicant
(Customer Client)
Verifier
(Merchant Server)
1. Negotiation of Security Options (Brief)
2. Merchant Authenticates Self to Customer
Uses a Digital Certificate
Customer Authentication is Optional and Uncommon
7
Figure 8-4: SSL/TLS Operation
Applicant
(Customer Client)
Verifier
(Merchant Server)
3. Client Generates Random Session Key
Client Sends to Server Encrypted
by Public Key Encryption
4. Ongoing Communication with Confidentiality
and Merchant Digital Signatures
8
Figure 8-5: Point-to-Point Protocol (PPP) and
RADIUS for Dial-Up Remote Access
1. Login
Username
And Password
2. OK?
RADIUS
Server
Corporate
Site A
RAS 1
RAS 2
Remote
Corporate PC
Dial-Up
Connection
Public Switched
Telephone
Network
Remote
Corporate PC
Dial-Up
Connection
9
Figure 8-5: Point-to-Point Protocol (PPP) and
RADIUS for Dial-Up Remote Access
Remote
Corporate PC
3. OK
RADIUS
Server
Corporate
Site A
4. Welcome
RAS 1
RAS 2
Public Switched
Telephone
Network
Dial-Up
Connection
Remote
Corporate PC
Dial-Up
Connection
10
Figure 8-6: PPP Authentication
No Authentication
Is an Option
Server
Client
11
Figure 8-6: PPP Authentication
PAP Authentication
Authentication-Request Messages
(Send Until Response)
Authentication-Response Message
Server
Client
Poor Security: Usernames and Passwords
Are Sent in the Clear
12
Figure 8-6: PPP Authentication
CHAP Authentication
Challenge Message
Server
Response Message
Hash (Challenge Message + Secret)
Client
Server computes hash of challenge message plus secret
If equals the response message, authentication is successful
13
Figure 8-6: PPP Authentication
MS-CHAP Authentication
Challenge Message
Server
Response Message
Hash (Challenge Message + Password)
Client
CHAP, but with password as the secret.
Widely used because allows password authentication
Standard on Microsoft Windows client
Only as secure as password strength
14
Figure 8-6: PPP Authentication
EAP Authentication
Authenticate
Server
Defer authentication;
Will provide more information
Client
EAP defers authentication to a later process
Such as RADIUS authentication
15
Figure 8-7: PPP Encryption
New PPP Trailer.
Plaintext.
Original PPP Frame.
Encrypted.
New PPP Header.
Plaintext.
16
Figure 8-8: PPP on Direct Links and
Internets
PPP Frame
Connection over Direct Link
PPP Provides End-to-End Link
Applicant
(Client)
Verifier
(Server)
17
Figure 8-8: PPP on Direct Links and
Internets
PPP Frame in
IP Packet
Applicant
(Client)
PPP
Limited
to First
Data Link
(Network)
Connection over Internet
Router
Router
Verifier
(Server)
18
Figure 8-8: PPP on Direct Links and
Internets
Note:

Tunneling Places the PPP Frame in an IP
Packet, Which Delivers the Frame.

To the Receiver, Appears to be a Direct Link.

Allows organization to continue using existing
PPP-based security such as encryption and
authentication
19
Figure 8-9: Point-to-Point Tunneling
Protocol (PPP)
IP Protocol 47 (GRE) Data Connection
Local
ISP Access
(Not Secure)
RADIUS
Server
PPTP
RAS
Corporate
Site A
Internet
TCP Port 1723
Supervisory
Connection
(Vulnerable)
ISP
PPTP
Access
Concentrator
Remote
Corporate
PC
20
Figure 8-9: Point-to-Point Tunneling
Protocol (PPP)
New: Not in Book
Direct connection between PC
And RADIUS Server
IP Protocol 47 (GRE) Data Connection
RADIUS
Server
PPTP
RAS
Corporate
Site A
Internet
TCP Port 1723
Supervisory
Connection
(Vulnerable)
Remote
Corporate
PC
21
Figure 8-10: PPTP Encapsulation for
Data Frames
Encapsulated
Original
IP Packet
Enhanced General
Routing
Encapsulation
(GRE) Header;
Information About
Encapsulated
Packet
New IP Header;
Protocol=47;
IP Destination
Address Is That of
Remote Access
Server
22
Figure 8-11: Layer 2 Tunneling Protocol
(L2TP)
Internal
Server
DSL Access
Multiplexer
(DSLAM)
with L2TP
L2TP
RAS
L2TP Tunnel
Local
Network
Client
Running
PPP
DSL
Carrier Network
Note: L2TP does not provide security. It provides only tunneling.
L2TP recommends the use of IPsec for security.
23
Figure 8-12: IPsec Operation: Tunnel
and Transport Modes
Transport Mode
Site
Network
Extra
Software
Required
Security
in Site
Network
Secure Connection
Secure on
the Internet
Site
Network
Security
in Site
Network
Extra
Software
Required
24
Figure 8-12: IPsec Operation: Tunnel
and Transport Modes
Tunnel Mode
Site
Network
No
Extra
Software
IPsec
Server
No
Security
in Site
Network
Tunneled
Connection
Secure on
the Internet
IPsec
Server
Site
Network
No
Security
in Site
Network
No
Extra
Software
25
Figure 8-12: IPsec Operation: Tunnel
and Transport Modes
Transport Mode
Destination IP Address
Is Actual Address;
Vulnerable to Scanning
Orig. IP
Hdr
IPsec
Hdr
Protected Packet
Data Field
IPsec
Hdr
Protected
Original Packet
Tunnel Mode
Destination IP Address
Is IPsec Server Address
Host IP Address
Is not Revealed
New IP
Hdr
26
Figure 8-13: IPsec ESP and AH
Protection
Confidentiality
Encapsulating
Security
Payload
IP
Header
Protocol = 50
ESP
Header
Protected
ESP
Trailer
Authentication and Message Integrity
Protocol = 51
Authentication
Header
IP
Header
Authentication
Header
Protected
Authentication and Message Integrity
No Confidentiality
27
Modes and Protections
ESP
Confidentiality
Authentication
Integrity
AH
Authentication
Integrity
Possible
Possible
Tunnel Mode
Possible
(IPsec Gateway to
Gateway)
Possible
Transport Mode
(End-to-End)
28
Figure 8-14: IPsec Security
Associations
2. Security Association (SA)
for Transmissions from A to B
Party A
3. Security Association (SA)
For Transmission from B to A
(Can Be Different Than
A to B SA)
Party B
1. List of
Allowable
Security
Associations
1. List of
Allowable
Security
Associations
IPsec Policy Server
29
Figure 8-15: Establishing IPsec
Security Associations Using IKE
Internet Key Exchange
Security Association
UDP Port 500
Party A
Party B
IPsec SAs
First establish IKE association and
protected session
Then create IPsec SAs within the
Protection of the IKE session.
30
Figure 8-16: Key-Hashed Message
Authentication Codes (HMACs)
Shared Key
Original Plaintext
Hashing with MD5, SHA1, etc.
HMAC
Key-Hashed Message Authentication Code (HMAC)
Appended to Plaintext Before Transmission
HMAC
Original Plaintext
Note: There is no
encryption; only
hashing
31
Figure 8-16: Key-Hashed Message
Authentication Codes (HMACs)
Receiver Redoes the HMAC Computation
On the Received Plaintext
Shared Key
Received Original Plaintext
Hashing with same algorithm.
Computed HMAC
Received HMAC
If computed and received HMACs are the same,
The sender must know the key and so is authenticated
32
Figure 8-17: Kerberos Authentication
System
Kerberos Server
Key Distribution Center
(K)
Abbreviations:
A = Applicant
V = Verifier
K = Kerberos Server
Applicant (A)
Verifier (V)
33
Figure 8-17: Kerberos Authentication
System
Kerberos Server
Key Distribution Center
(K)
1. Request for
Ticket-Granting
Ticket
2. Response:
TGT*, Key nA**
Applicant (A)
*TGT (Ticket-Granting
Ticket) is encrypted in a
way that only K can
decrypt. Contains
information that K
will read later.
**Key nA (Network
Login Key for A) is
encrypted with A’s
Master Key (Key mA).
In future interactions
with K, A will use nA
Verifier (V)
to limit the master
key’s exposure.
34
Figure 8-18: Kerberos Ticket-Granting
Service: Part 1
Kerberos Server
Key Distribution Center
(K)
1. Request Ticket
for V; TGT;
Authenticator*
encrypted with
Key nA
2. Response:
Key AV** encrypted
with Key nA;
Service Ticket
Applicant (A)
*Authenticator is A’s
IP address, user name,
and time stamp. This
authenticator is encrypted
with Key nA to prove that
A sent it.
**Key AV is a
symmetric session
key that A will use
with V.
Verifier (V)
35
Figure 8-19: Kerberos Ticket-Granting
Service: Part 2
*Authenticator (Auth)
encrypted with Key AV.
Kerberos Server
Key Distribution Center
(K)
**Service Ticket contains
Key AV encrypted with the
Verifier’s master key, Key mV.
3. Request for Connection:
Auth*; Service Ticket**
5. Ongoing Communication with Key AV
Applicant (A)
Verifier (V)
4. V decrypts Service Ticket;
Uses Key AV to test Auth
36
Figure 8-20: Placement of Firewalls
and Cryptographic Servers
Filtered
by
Firewall
Internet
Cryptographic
Firewall Server
Internal
Host
Can
Read
Decrypted
Packets
Not
Filtered
by
Firewall
Open to
Attack
Firewall
Cryptographic
Server
Filtered by
Firewall
Creates
Holes for
Cryptographic
Systems
Internal
Host
37