Download Guide to Firewalls and Network Security with Intrusion Detection and

Encryption and Firewalls
Chapter 7
Learning Objectives
Understand the role encryption plays in
firewall architecture
Know how digital certificates work and
why they are important security tools
Analyze the workings of SSL, PGP, and
other popular encryption schemes
Enable Internet Protocol Security (IPSec)
and identify its protocols and modes
Encryption
Process of encoding and decoding
information to:
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Preserve its integrity
Maintain privacy
Ensure identity of users participating in the
encrypted data session
Why Firewalls Need to Use
Encryption
Hackers take advantage of a lack of
encryption
Encryption:
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Preserves data integrity
Increases confidentiality
Is relied upon by user authentication
Plays a fundamental role in enabling VPNs
Hackers Take Advantage of a
Lack of Encryption
Hackers Take Advantage of a
Lack of Encryption
The Cost of Encryption
CPU resources and time
Bastion host that hosts the firewall should be
robust enough to manage encryption and other
security functions
Encrypted packets may need to be padded to
uniform length to ensure that some algorithms
work effectively
Can result in slowdowns
Monitoring can burden system administrator
Preserving Data Integrity
Even encrypted sessions can go wrong as a
result of man-in-the-middle attacks
Encryption can perform nonrepudiation
using a digital signature
Maintaining Confidentiality
Encryption conceals information to render it
unreadable to all but intended recipients
Authenticating Network Clients
Firewalls need to trust that the person’s claimed
identity is genuine
Firewalls that handle encryption can be used to
identify individuals who have “digital ID cards”
that include encrypted codes
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Digital signatures
Public keys
Private keys
Enabling VPNs
As an integral part of VPNs, encryption:
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Enables the firewall to determine whether the
user who wants to connect to the VPN is
actually authorized to do so
Encodes payload of information to maintain
privacy
Digital Certificates and Public
and Private Keys
Digital certificate

Electronic document that contains a digital signature
(encrypted series of numerals and characters), which
authenticates identity of person sending certificate
Keys
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Basis of digital certificates and signatures
Enable holders of digital certificates to encrypt
communications (using their private key) or decrypt
communications (using sender’s public key)
Digital Certificates
Transport encrypted codes (public and
private keys) through the firewall from one
host to another
Help ensure identity of the individual who
owns the digital certificate
Provide another layer of security in firewall
architecture
Aspects of Digital Certificates
Establishment of an infrastructure for
exchanging public and private keys
Need to review and verify someone’s digital
certificate
Difference between client- and server-based
digital certificates
The Private Key Infrastructure
Lightweight Directory Access Protocol
(LDAP)

Publicly available database that holds names of
users and digital certificates
Public-Key Infrastructure (PKI)
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Enables distribution of digital certificates and
public and private keys
Underlies many popular and trusted security
schemes (eg, PGP and SSL)
Viewing a Digital Certificate
Viewing a Digital Certificate
Types of Digital Certificates a
Firewall Will Encounter
Client-based digital certificates

Obtained by users from a Certification
Authority (CA), which issues them and vouches
for owner’s identity
Server-based digital certificates

Issued by a CA to a company that issues them
to individuals
Keys
Value generated by an algorithm that can
also be processed by an algorithm to
encrypt or decrypt text
Length of the key determines how secure
the level of encryption is
Aspects of Keys That Pertain to
Firewall-Based Encryption
Public and private keys
Need to generate public keys
Need to securely manage private keys
Need to use a key server either on network
or Internet
Differences between private and public key
servers
Public and Private Keys
Private key
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Secret code generated by an algorithm
Never shared with anyone
Public key
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Encoded information generated when private
key is processed by the same algorithm
Can be exchanged freely with anyone online
A Public Key Generated by PGP
An Encrypted Communication
Session
Choosing the Size of Keys
Generating Keys
Managing Keys
Manual distribution
Use of a CA
Use of a Key Distribution Center (KDC)
Using a Key Server That Is on
Your Network
Using an Online Key Server
Analyzing Popular Encryption
Schemes
Symmetric key encryption
Asymmetric key encryption
Pretty Good Privacy (PGP)
Secure Sockets Layer (SSL)
Symmetric Encryption
Use of only one key to encrypt information,
rather than a public-private key system
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Same key is used to encrypt/decrypt a message
Both sender and recipient must have same key
Not scalable
Symmetric Key Encryption
Asymmetric Encryption
Uses only one user’s public key and private key to
generate unique session keys that are exchanged
by users during a particular session
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Only the private key must be kept secret
Scales better than symmetric encryption
Disadvantages
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Slower
Only a few public key algorithms are available (eg,
RSA and EIGamal) that are secure and easy to use for
both encryption and key exchange
Asymmetric Key Encryption
PGP
Hybrid system that combines advantages of
asymmetric (scalability) and symmetric
(speed) encryption systems
PGP
Process
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File/message is encrypted
Session key is encrypted using public key half of
asymmetric public-private key pair
Recipient of encrypted message uses his/her private key
to decode the session key
Session key is used to decode message/file
Encryption schemes used to generate public and
private key pairs
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Rivest-Shamir-Adleman (RSA) encryption
Diffie-Hellman encryption
Using PGP
Using PGP
X.509
Standard set of specifications for
assembling and formatting digital
certificates and encrypting data within them
A commonly used type of PKI
Widely used and well trusted
X.509 and PGP Compared
X.509
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Perception of trust
PGP
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Does not make use of the CA concept
Gives users ability to wipe files from hard disk
(and delete permanently)
Available both in freeware and commercial
versions
X.509 and PGP Compared
SSL
Secure way to transmit data
Uses both symmetric and asymmetric keys
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Asymmetric keys start an SSL session
Symmetric keys are dynamically generated for
the bulk of the transfer
Using Internet Protocol Security
(IPSec) Encryption
Creates a secure IP connection between two
computers
Operates under the Application layer
Transparent to users
Understanding IPSec
Set of standards and software tools that
encrypt IP connections between computers
Allows a packet to specify a mechanism for
authenticating its origin, ensuring data
integrity, and ensuring privacy
Modes of IPSec
Transport mode
Tunnel mode
Choice depends on type of network and
whether it uses NAT
Transport Mode
IPSec authenticates two computers that
establish a connection
Can optionally encrypt packets
Does not use a tunnel
Tunnel Mode
IPSec encapsulates IP packets and can
optionally encrypt them
Encrypts packet headers rather than the data
payload
Incompatible with NAT
IPSec Protocols
Authentication Header (AH)
Encapsulation Security Payload (ESP)
Authentication Header (AH)
Adds a digital signature to packets to
protect against repeat attacks, spoofing, or
other tampering
Verifies that parts of packet headers have
not been altered between client and IPSecenabled host
Incompatible with NAT
AH
Encapsulation Security Payload
(ESP)
More robust than AH;
encrypts data part of
packets as well as the
headers
Provides
confidentiality and
message integrity
Can cause problems
with firewalls that use
NAT
Components of IPSec
Two modes: transport and tunnel
Two protocols: AH and ESP
IPSec driver
Internet Key Exchange (IKE)
Internet Security Association Key Management
Protocol (ISAKMP)
Oakley
IPSec Policy Agent
Choosing the Best IPSec Mode
for Your Organization
Choosing the Best IPSec Mode
for Your Organization
ESP plus tunnel mode provides best level of
protection
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ESP conceals IP header information
Tunnel mode can both encapsulate and encrypt
packets
Enabling IPSec
Select group policy security setting for computers
that need to communicate with enhanced security
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Define at group policy level in Windows 2000
Define at local policy level if not in Windows 2000
Predefined IPSec policy levels in Windows 2000
or XP:
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Client (Respond only)
Server (Request Security)
Secure Server (Require Security)
Defining IPSec Policy at Local
Policy Level
Limitations of IPSec
If machine that runs IPSec-compliant software has
been compromised, communications from that
machine cannot be trusted
Encrypts IP connection between two machines—
not the body of e-mail messages or content of
other communications
Not an end-to-end security method
Authenticates machines, not users
Doesn’t prevent hackers from intercepting
encrypted packets
Chapter Summary
How and why encryption is used in a network
How to use encryption to complement the
firewall’s activities
Encryption applications
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PGP
SSL
IPSec
Schemes that can form part of a firewall
architecture