FIREWALLS & NETWORK SECURITY with Intrusion Detection and VPNs, 2 nd ed. 9 Encryption and Firewalls By Whitman, Mattord, & Austin© 2008 Course Technology

FIREWALLS & NETWORK SECURITY with Intrusion Detection and VPNs, 2nd ed.

9Encryption and Firewalls

By Whitman, Mattord, & Austin © 2008 Course Technology

Learning Objectives

Describe the role encryption plays in a firewall architecture

Explain how digital certificates work and why they are important security tools

Analyze the workings of SSL, PGP, and other popular encryption schemes

Discuss Internet Protocol Security (IPSec) and identify its protocols and modes

Firewalls & Network Security, 2nd ed. - Chapter 9 Slide 2

Firewalls and Encryption

Hackers take advantage of a lack of encryption Encryption:

– Preserves data integrity

– Increases confidentiality

– Is relied upon by user authentication

– Plays a fundamental role in enabling VPNs

Slide 3Firewalls & Network Security, 2nd ed. - Chapter 9

Hacker and an Unencrypted Packet


Hacker and an Encrypted Packet


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– Digital signatures

– Public keys

– Private keys


Enabling Virtual Private Networks (VPNs)

As an integral part of VPNs, encryption:– 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


Firewalls & Network Security, 2nd ed. - Chapter 9 11

Principles of Cryptography

Encryption: the process of converting an original message into a form that cannot be understood by unauthorized individuals

Cryptology, the science of encryption, encompasses two disciplines:– Cryptography: describes the processes involved in

encoding and decoding messages so that others cannot understand them

– Cryptanalysis: the process of deciphering the original message (plaintext) from an encrypted message (ciphertext) without knowing the algorithms and keys used to perform the encryption


Encryption Definitions

Algorithm: the mathematical formula or method used to convert an unencrypted message into an encrypted message

Cipher: the transformation of the individual components (characters, bytes, or bits) of an unencrypted message into encrypted components

Ciphertext or cryptogram: the unintelligible encoded message resulting from an encryption

Cryptosystem: the set of transformations necessary to convert an unencrypted message into an encrypted message


Encryption Definitions (continued)

Decipher: to decrypt or convert ciphertext to plaintext

Encipher: to encrypt or convert plaintext to ciphertext

Key or cryptovariable: the information used in conjunction with the algorithm to create the ciphertext from the plaintext; it can be a series of bits used in a mathematical algorithm or the knowledge of how to manipulate the plaintext

Keyspace: the entire range of values that can possibly be used to construct an individual key

Encryption Definitions (continued)

Plaintext: the original unencrypted message that is encrypted and results from successful decryption

Steganography: the process of hiding messages, usually within graphic images

Work factor: the amount of effort (usually expressed in units of time) required to perform cryptanalysis on an encoded message


Cryptographic Notation

M represents original message; C represents ciphertext; E represents encryption process; D represents the decryption process; K represents a key

So…

E(M) = C encrypting a message results in cyphertext

D(C) = M and D[E(M)] = M

E(M,K) = C specifies encrypting the message with a key; keys can be annotated K1, K2, etc. in the case of multiple keys



Common Ciphers

In encryption, the most commonly used algorithms include three functions: substitution, transposition, and XOR

In a substitution cipher, you substitute one value for another; a monoalphabetic substitution uses only one alphabet and a polyalphabetic substitution use two or more alphabets

The transposition cipher (or permutation cipher) simply rearranges the values within a block to create the ciphertext; this can be done at the bit level or at the byte (character) level


Common Ciphers (continued)

In the XOR cipher conversion, the bit stream is subjected to a Boolean XOR function against some other data stream, typically a key stream

XOR works as follows:– ‘0’ XOR’ed with ‘0’ results in a ‘0’ (0 0 = 0)– ‘0’ XOR’ed with ‘1’ results in a ‘1’ (0 1 = 1)– ‘1’ XOR’ed with ‘0’ results in a ‘1’ (1 0 = 1)– ‘1’ XOR’ed with ‘1’ results in a ‘0’ (1 1 = 0)

Simply put, if the two values are the same, you get “0”; if not, you get “1”

This process is reversible; that is, if you XOR the ciphertext with the key stream, you get the plaintext


Vernam Cipher

Also known as the one-time pad, the Vernam cipher was developed at AT&T and uses a set of characters that are used for encryption operations only one time and then discarded

The values from this one-time pad are added to the block of text, and the resulting sum is converted to text


Book or Running Key Cipher

Another method, used in the occasional spy movie, is the use of text in a book as the algorithm to decrypt a message

The key relies on two components:– Knowing which book to use

– A list of codes representing the page number, line number, and word number of the plaintext word


Symmetric Encryption

The previous methods of encryption/decryption require the same algorithm and key be used to both encipher/decipher the message

This is known as private key encryption or symmetric encryption

In this approach, the same key—a secret key—is used to encrypt and decrypt the message

Usually extremely efficient, requiring simple processing to encrypt or decrypt the message

Main challenge is getting a copy of the key to the receiver, a process that must be conducted out-of-band to avoid interception


Symmetric Encryption (continued)


The Technology of Symmetric Encryption Data Encryption Standard (DES)

– Developed in 1977 by IBM– Based on the Data Encryption Algorithm (DEA),

which uses a 64-bit block size and a 56-bit key– Federally approved standard for nonclassified data– Cracked in 1997 when developers of a new

algorithm, Rivest-Shamir-Aldeman, offered $10,000 to whomever was first to crack it

– Fourteen thousand users collaborated over the Internet to finally break the encryption

Triple DES (3DES) was developed as an improvement to DES and uses as many as three keys in succession


The Technology of Symmetric Encryption (continued)

Advanced Encryption Standard (AES)– Successor to 3DES– Based on Rijndael Block Cipher, which features

a variable block length and a key length of either 128, 192, or 256 bits

In 1998, it took a special computer designed by the Electronic Freedom Frontier more than 56 hours to crack DES; it would take the same computer approximately 4,698,864 quintillion years to crack AES


Asymmetric Encryption

Also known as public key encryption Uses two different but related keys Either key can be used to encrypt or decrypt If Key A is used to encrypt message, then only

Key B can decrypt; if Key B is used to encrypt message, then only Key A can decrypt

This technique is most valuable when one of the keys is private and the other is public

Problem: it requires four keys to hold a single conversation between two parties, and the number of keys grows geometrically as parties are added


Public Key Encryption


Digital Signatures

When asymmetric process is reversed, that the message was sent by organization owning the private key cannot be refuted (nonrepudiation)

Digital signatures: encrypted messages verified as authentic by independent facility (registry)

Digital certificate: electronic document, similar to digital signature, attached to file certifying that file is from the organization it claims to be from and has not been modified from original format

Certificate Authority (CA): agency that manages issuance of certificates and serves as electronic notary public to verify their origin and integrity


Digital Signatures (continued)


Public Key Infrastructure

Public key infrastructure (PKI) is the entire set of hardware, software, and cryptosystems necessary to implement public key encryption

Systems are based on public key cryptosystems and include digital certificates and certificate authorities

Public Key Infrastructure (continued)

Can increase an organization’s ability to protect its information assets by providing:– Authentication: digital certificates authenticate

identity of each party in an online transaction– Integrity: digital certificate asserts content signed

by the certificate has not been altered in transit– Confidentiality: keeps information confidential by

ensuring it is not intercepted during transmission– Authorization: digital certificates can replace user

IDs and passwords, enhance security, and reduce overhead

– Nonrepudiation: certificates validate actionsFirewalls & Network Security, 2nd ed. - Chapter 9 29


Hybrid Systems

Pure asymmetric key encryption not widely used except in area of certificates; instead, typically employed in conjunction with symmetric key encryption, creating a hybrid system

Hybrid process currently in use is based on Diffie-Hellman key exchange, which provides method to exchange private keys using public key encryption without exposure to third parties

In this method, asymmetric encryption is used to exchange symmetric keys, so two entities can conduct quick, efficient, secure communications based on symmetric encryption; Diffie-Hellman provided the foundation for subsequent developments in public key encryption


Hybrid Encryption


Using Cryptographic Controls

Generation of unbreakable ciphertext is possible only if proper key management infrastructure has been constructed and cryptosystems are operated and managed correctly

Cryptographic controls can be used to support several aspects of business:– Confidentiality and integrity of e-mail and its

attachments– Authentication, confidentiality, integrity, and

nonrepudiation of e-commerce transactions– Authentication and confidentiality of remote access

through VPN connections– Higher standard of authentication when used to

supplement access control systems


E-mail Security

Secure Multipurpose Internet Mail Extensions (S/MIME) builds on Multipurpose Internet Mail Extensions (MIME); adds encryption and authentication via digital signatures

Privacy Enhanced Mail (PEM) proposed by Internet Engineering Task Force (IETF) as a standard that will function with public key cryptosystems; uses 3DES and RSA for key exchanges and digital signatures

Pretty Good Privacy (PGP): uses IDEA Cipher, a 128-bit symmetric key block encryption algorithm with 64-bit blocks for message encoding; RSA for symmetric key exchange and digital signatures


Securing the Web

Secure Electronic Transactions (SET)– Developed by MasterCard and VISA in 1997 to

provide protection from electronic payment fraud– Encrypts credit card transfers with DES and uses

RSA for key exchange Secure Sockets Layer (SSL)

– Developed by Netscape in 1994 to provide security for online electronic commerce transactions

– Uses several algorithms; mainly relies on RSA for key transfer and IDEA, DES, or 3DES for encrypted symmetric key-based data transfer


Securing the Web (continued)

Secure Hypertext Transfer Protocol (SHTTP)– An encrypted version of HTTP– Provides secure e-commerce transactions and

encrypted Web pages for secure data transfer over the Web, using several different algorithms

Secure Shell (SSH)– Uses tunneling to provide security for remote

access connections over public networks– Provides authentication services between a

client and a server– Used to secure replacement tools for terminal

emulation, remote management, and file transfer applications



IP Security (IPSec): primary and now dominant cryptographic authentication and encryption product of IETF’s IP Protocol Security Working Group

IPSec combines several different cryptosystems:– Diffie-Hellman key exchange for deriving key material

between peers on a public network

– Public key cryptography for signing the Diffie-Hellman exchanges to guarantee the identity of the two parties

– Bulk encryption algorithms for encrypting the data

– Digital certificates signed by a certificate authority to act as digital ID cards



IPSec has two components:– The IP Security protocol itself, which specifies

the information to be added to an IP packet and indicates how to encrypt packet data

– The Internet Key Exchange, which uses asymmetric key exchange and negotiates the security associations



IPSec works in two modes of operation:– Transport mode: only IP data is encrypted—not

the IP headers themselves; allows intermediate nodes to read source and destination addresses

– Tunnel mode: entire IP packet is encrypted and inserted as payload in another IP packet

IPSec and other cryptographic extensions to TCP/IP often used to support a virtual private network (VPN), a private, secure network operated over a public, insecure network


Securing Authentication

A final use of cryptosystems is to provide enhanced and secure authentication

One approach to this issue is provided by Kerberos, which uses symmetric key encryption to validate an individual user’s access to various network resources

It keeps a database containing the private keys of clients and servers that are in the authentication domain that it supervises


Kerberos

Kerberos system knows these private keys and can authenticate one network node (client or server) to another

Kerberos also generates temporary session keys—that is, private keys given to the two parties in a conversation

Kerberos (continued)


Kerberos (continued)


Attacks on Cryptosystems

Historically, attempts to gain unauthorized access to secure communications have used brute force attacks in which the ciphertext is repeatedly searched for clues that can lead to the algorithm’s structure (ciphertext attacks)

This process, known as frequency analysis, can be used along with published frequency of occurrence patterns of various languages and can allow an experienced attacker to quickly crack almost any code if the individual has a large enough sample of the encoded text


Attacks on Cryptosystems (continued)

Occasionally, an attacker may obtain duplicate texts, one in ciphertext and one in plaintext, which enable the individual to reverse-engineer the encryption algorithm in a known-plaintext attack scheme

Alternatively, an attacker may conduct a selected-plaintext attack by sending a potential victim a specific text that they are sure the victim will forward on to others; the attacker then intercepts the encrypted message and compares it to the original plaintext



Man-in-the-middle attack: method used to intercept the transmission of a public key or even to insert a known key structure in place of the requested public key

Correlation attacks: collection of brute-force methods that attempt to deduce statistical relationships between the structure of the unknown key and the ciphertext that is the output of the cryptosystem



In a dictionary attack, the attacker encrypts every word in a dictionary using the same cryptosystem as used by the target

In a timing attack, the attacker eavesdrops during a victim’s session and uses statistical analysis of the user’s typing patterns and inter-keystroke timings to discern sensitive session information


Defending from Attacks


No matter how sophisticated encryption and cryptosystems have become, however, they have retained the same flaw that the first systems contained thousands of years ago: if you discover the key, that is, the method used to perform the encryption, you can determine the message

Thus, key management is not so much the management of technology but rather the management of people


Chapter Summary

Encryption: process of rendering information unreadable to all but the intended recipients; purpose is to preserve the integrity and confidentiality of information and/or make the process of authenticating users more effective

Firewalls use encryption both to provide protection for data in transit and to help keep firewall secure

Encryption of data incurs costs since it requires processing time to encrypt and decrypt the data being protected

Chapter Summary (continued)

Cryptology: science of encryption Cryptography: complex process of making and

using codes Applying concealing techniques in encryption and

decoding ciphertext is called decryption Process used to decrypt data when the process

and/or keys are unknown is called cryptanalysis Cryptographic controls: techniques and tools used

to implement cryptographic protections; used to secure e-mail, Web access, Web applications, file transfers, remote access procedures like VPNs


Chapter Summary (continued)

Cryptographic control systems often subject to attack

Many methods of attack have evolved– Brute computational approaches

– Use of weaknesses often found in implementation of cryptographic controls

Some attacks attempt to inject themselves between the parties of a secured communication channel

Other attacks combine multiple brute-force approaches into one correlation attack


Documents

FIREWALLS & NETWORK SECURITY with Intrusion Detection and VPNs, 2 nd ed. 9 Encryption and Firewalls By Whitman, Mattord, & Austin© 2008 Course Technology